Synthesis and Crystal Structure of the Short Lnsb2o4br Series (Ln = Eu–Tb) and Luminescence Properties of Eu3+-Doped Samples

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Article Synthesis and Crystal Structure of the Short LnSb2O4Br Series (Ln = Eu–Tb) and Luminescence Properties of Eu3+-Doped Samples

Felix C. Goerigk 1, Veronica Paterlini 2, Katharina V. Dorn 2 , Anja-Verena Mudring 2,* and Thomas Schleid 1,*

1 Institute for Inorganic Chemistry, University of Stuttgart, Pfaﬀenwaldring 55, D-70569 Stuttgart, Germany; [email protected] 2 Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden; [email protected] (V.P.); [email protected] (K.V.D.) * Correspondence: [email protected] (A.-V.M.); [email protected] (T.S.)

Received: 2 October 2020; Accepted: 23 November 2020; Published: 27 November 2020

Abstract: Pale yellow crystals of LnSb2O4Br (Ln = Eu–Tb) were synthesized via high temperature solid-state reactions from antimony sesquioxide, the respective lanthanoid sesquioxides and tribromides. Single-crystal X-ray diffraction studies revealed a layered structure in the monoclinic space group P21/c. In contrast to hitherto reported quaternary lanthanoid(III) halide oxoantimonates(III), in LnSb2O4Br the lanthanoid(III) cations are exclusively coordinated by oxygen atoms in the form of 13 square hemiprisms. These [LnO8] − polyhedra form layers parallel to (100) by sharing common edges. 1 3 All antimony(III) cations are coordinated by three oxygen atoms forming ψ -tetrahedral [SbO3] − units, which have oxygen atoms in common building up meandering strands along [001] according 1 [ v t ]– to SbO2/2O1/1 (v = vertex-sharing, t = terminal). The bromide anions are located between two ∞{ } layers of these parallel running oxoantimonate(III) strands and have no bonding contacts with the Ln3+ cations. Since Sb3+ is known to be an efficient sensitizer for Ln3+ emission, photoluminescence studies were carried out to characterize the optical properties and assess their suitability as light phosphors. 3+ Indeed, for both, GdSb2O4Br and TbSb2O4Br doped with about 1.0–1.5 at-% Eu efficient sensitization 3+ of the Eu emission could be detected. For TbSb2O4Br, in addition, a remarkably high energy transfer from Tb3+ to Eu3+ could be detected that leads to a substantially increased Eu3+ emission intensity, rendering it an efficient red light emitting material.

Keywords: lanthanoid compounds; halide oxoantimonates(III); structure elucidation; luminescence characterization; solid-state synthesis; single-crystal X-ray diﬀraction

1. Introduction Lanthanoid compounds containing complex oxoanions are an interesting material class for application as light phosphors because the valence 4f -states of lanthanoids typically give rise to photoluminescence characteristic for the respective lanthanoid. However, direct photoexcitation into 4f -levels is an ineﬃcient way to stimulate photoluminescence, because of the forbidden character of these transitions. For all-inorganic materials this problem can be circumvented through charge-transfer sensitization or by energy transfer (ET) from other metal cations, which are all allowed processes. Both processes are possible in lanthanoid(III) oxopnictogenates(III/V) [1–4], such as oxoantimonates, which are readily available through high-temperature solid-state reaction of the respective binary starting materials, i.e., the respective lanthanoid(III) and pnictogen(III) sesquioxides (Ln2O3 and Pn2O3). Moreover, the lanthanoid(III) oxoarsenates(III) or -antimonates(III) generally feature a quite high

Crystals 2020, 10, 1089; doi:10.3390/cryst10121089 www.mdpi.com/journal/crystals Crystals 2020, 10, 1089 2 of 23 stability in water and air. Varying the host lattice and the lanthanoid(III) dopant in these systems reveals the possibility to fine-tune the selective emission properties in a wide range [2,5,6]. For example, current 3+ 3+ research on Y[PO4] co-doped with Gd and Sb revealed that antimon(III) cations are appropriate to activate gadolinium(III) emission in the ultraviolet (UV-B) range [7]. This inspired us to synthesize lanthanoid(III) containing oxoantimonates(III) to have materials at hand equipped with intrinsic Sb3+ cations as sensitizer for Ln3+ cations and therefore being suitable as efficient luminescent material class. The structural diversity of lanthanoid(III) oxopnictogenates(III) is quite large and varies not only with the lanthanoid, but also on the pnictogen (As or Sb). Structural complexity can be enlarged through the incorporation of halide anions. For ternary lanthanoid(III) oxoarsenates(III) the compositions LnAsO3 (Ln = La, Ce) and Ln2As4O9 (Ln = Pr, Nd, Sm) are known [8–11]. The Ln[AsO3] representatives crystallize either with the α-Pb[SeO3]- or the K[ClO3]-type structure exhibiting coordination numbers of eight or nine for the lathanoid(III) cations and feature isolated ψ1-tetrahedral 3– [AsO3] anions without any bridging As–O–As contacts as the dominating structural feature. These anions occur as discrete triangular pyramids with stereochemically active lone pairs at the As3+ 4– 4– cations. In contrast, the oxoarsenates Ln2As4O9 display both [As2O5] pyroanions and cyclic [As4O8] 1 3– units as condensation products of ψ -tetrahedral [AsO3] entities according to Ln4[As2O5]2[As4O8]. Up to now, no other further ternary lanthanoid(III) oxoarsenates(III) are known. The addition of fluxes, such as alkali-metal halides, to the oxide reaction mixtures in order to improve crystallization and reduce the reaction temperature often results in the formation of different halide-anion containing quaternary compounds, enriching the structural diversity. So far, three different compositions for lanthanide oxoarsenates(III) containing trivalent lanthanoids are known, i.e., Ln5 3[AsO3]4, Ln3OX[AsO3]2 and Ln3X2[As2O5][AsO3]. Similar to their halide-free congeners, × 1 3 4 they have in common ψ -tetrahedral [AsO3] − anions, and also feature pyroanionic [As2O5] − groups. In contrast to them, because of the presence of halide anions, mixed oxide-halide coordination polyhedra around the Ln3+ cations occur, providing coordination numbers as high as seven, eight or nine [12–18]. With antimony as the heavier congener of arsenic, lanthanoid(III) oxoantimonates(III) with three different compositions are known: LnSb5O12 (Ln = La), Ln3Sb5O12 (Ln = Pr, Nd, Sm–Gd and Yb) 3+ 5+ and LnSbO3 (Ln = Ce). While LaSb5O12 contains both Sb and Sb cations with coordination 3– 7– numbers of three and six, respectively, featuring pyramidal [SbO3] as well as octahedral [SbO6] anions, Ln3Sb5O12 and CeSbO3 include antimony(III) cations exclusively. In the Ln3Sb5O12 series, coordination numbers of three and four for the Sb3+ cations with oxygen atoms are present in 3– 1 5– 1 the shape of vertex-connected [SbO3] ψ -tetrahedra and [SbO4] ψ -square pyramids, while the unique central lanthanoid(III) cation is coordinated eightfold by oxygen atoms. In cubic CeSbO3, coordination numbers of 8 for cerium and 6 for antimony are reported with identical interatomic distances (d(Ce-O) = d(Sb-O) = 234 pm). However, both crystallographically independent oxygen-atom sites have to be only occupied by about 85% to realize trivalent antimony, so the true crystal structure of CeSbO3 remains puzzling [19–23]. Compared to the oxoarsenates(III), analogous compounds with the heavier congener antimony synthesized under halide-flux conditions seemed to be restricted to the formula type Ln5X3[SbO3]4 (tetragonal, P4/ncc for X = F; monoclinic, P2/c for X = Cl) [24,25]. For the heavier lanthanoids, divergent oxoantimonate(III) halides with the composition LnSb2O4X are encountered, which exhibit completely different structural features in their crystal structures known so far and can be interpreted as derivatives of the rare-earth metal(III) (RE) oxobismuthate(III) halides (REBi2O4X with RE = Y, Pr, Nd and Sm-Lu for X = Cl-I) [26–29]. For lanthanum being the largest rare-earth metal(III) cation, an antimony(III)- and bromide-containing crystal structure with the formula (La0.75Sb0.25)OBr related to LaOBr (PbFCl-type structure, tetragonal, P4/nmm) is also reported, which can be understood as heavily Sb3+-doped 3+ LaOBr [25]. Not in this compound, but in all the LnSb2O4X cases, the Ln cations have only contact to 13– oxygen atoms resulting in square [LnO8] square prisms. In Ln1+nSb2-nO4X with Ln = Nd, Sm and X = Cl, Br as the only known members, tetragonal crystal structures (P4/mmm) containing anionic layers 5– ψ1 2 [ v ] of vertex-sharing square pyramidal [SbO4] units (square -pyramids) according to SbO4/2 − ∞{ } Crystals 2020, 10, 1089 3 of 23

(v = vertex-sharing) are observed. The halide anions have little or no contact to the Ln3+ cations, but settle with far away Sb3+ cations at positions, where four of them encircle their stereochemically active lone pairs. However, no knowledge of further representatives and their crystal structures for the heavier lanthanoid(III) systems Ln-Sb-O-X was available up to now. In this work, we report on the single-crystal synthesis of the short LnSb2O4Br series (Ln = Eu–Tb) crystallizing in a novel structure type featuring layered polyhedra around the central lanthanoid(III) cations without any contact to the bromide anions. With the crystal structure at hand, photoluminescence measurements of bulk GdSb2O4Br were performed to investigate the luminescence 3+ processes caused by Sb . With this understanding, the photoluminescence investigations of EuSb2O4Br 3+ 3+ and TbSb2O4Br as well as doped GdSb2O4Br:Eu and TbSb2O4Br:Eu were undertaken to examine the consequence of incorporating multiple luminescent cations. The article discusses both the relevant excitation and emission spectra and deals with possible energy transfer mechanisms of the involved ions investigated with lifetime measurements to reach an understanding about the major luminescence processes taking place and the special role of Sb3+ cations in the crystal structure as intrinsic sensitizers and possible eﬃcient alternative to the forbidden direct excitation via 4f -4f transitions of the Ln3+ ions.

2. Materials and Methods

2.1. Synthesis of LnSb2O4Br Representatives

The lanthanoid(III) oxoantimonate(III) bromides LnSb2O4Br (Ln = Eu–Tb) were synthesized via solid-state reactions from sesquioxides with anhydrous lanthanide tribromides (Ln2O3: ChemPur, Karlsruhe, Germany, 99.9%) at elevated temperatures. About 1.0 mg (0.0028 mmol) of Eu2O3 was applied for doping and 54 mg (0.154 mmol) for the pure europium compound, gadolinium sesquioxide (Gd2O3: 54 mg, 0.153 mmol) or in case of Ln = Tb a mixture of terbium metal (Tb: ChemPur, Karlsruhe, Germany, 99.9%: 8 mg, 0.050 mmol) and terbium(III,IV) oxide (Tb4O7: ChemPur, Karlsruhe, Germany, 99.9%: 49 mg, 0.066 mmol) for in-situ synproportionation of Tb2O3, lanthanoid(III) tribromide (EuBr3, GdBr3 and TbBr3, Sigma-Aldrich, Taufkirchen, Germany, 99.99%: 61 mg, 0.153 mmol) and antimony sesquioxide (Sb2O3: ChemPur, Karlsruhe, Germany, 99.9%: 134 mg, 0.460 mmol) served as starting materials according to Equation (1). Eutectic mixtures of cesium bromide (CsBr: ChemPur, Karlsruhe, Germany, 99.9%: 530 mg, 2.490 mmol) and sodium bromide (NaBr: Merck, Darmstadt, Germany, suprapur: 127 mg, 1.234 mmol) were used as ﬂux.

CsBr + NaBr Ln O + LnBr + 3 Sb O 3 LnSb O Br (Ln = EuTb) (1) 2 3 3 2 3 → 2 4 The reactions were carried out in evacuated fused silica ampoules (Quarz- und Glasbläserei Müller, Berlin-Adlershof, Germany; inner diameter: 10 mm, wall thickness: 1 mm, length: 60 mm). The starting materials were transferred into the ampoules inside of an argon-filled glove box (Glovebox Systemtechnik, GS Mega E-Line, Malsch, Germany) and sealed under dynamic vacuum. The multi-stage heating program shown in Figure A1 was applied employing a Nabertherm L 9/12 muffle furnace 1 (Nabertherm, Lilienthal, Germany). Generally, cooling rates of 2–3 ◦C h− were chosen. An increase of 1 the rates to 5 ◦C h− upwards resulted in a significant decrease of the obtained crystal size. After cooling to room temperature, the samples were washed with about 250 mL demineralized water and dried subsequently in a drying oven at 80 ◦C for at least 6 h. First visual inspection with a stereo microscope of the powder samples revealed a homogeneous, pale yellow powder with square tabular shaped crystals of the target compound. However, using crossed-polarized light indicated the tendency for the formation of aggregated and twinned crystals.

2.2. Single-Crystal and Powder X-ray Diffraction For single-crystal X-ray diffraction (SCXRD) measurements, crystals of adequate size were isolated under a stereomicroscope and fixed with grease into a glass capillary (outer diameter: 0.1 mm, wall Crystals 2020, 10, 1089 4 of 23 thickness: 0.01 mm). The diffraction experiments were performed on a Stoe StadiVari four-circle diffractometer (Stoe and Cie, Darmstadt, Germany) with Eulerian cradle and Mo-Kα radiation at room temperature. For data collection and integration, Stoe X-Area 1.86 (2018) was used [30]. The crystal structures of EuSb2O4Br and TbSb2O4Br were solved in the space group P21/c through direct methods and subsequently refined using the ShelX-1997 program package [31,32]. Powder X-ray diffraction (PXRD) experiments were performed on a Stoe Stadi-P diffractometer with monochromatic Cu-Kα radiation (λ = 154.06 pm) in transmission geometry and a Stoe linear PSD detector with a range of 5◦/2θ. As monochromator, a curved germanium single crystal with (111) as diffractive face was used. About 5 mg of the slightly crushed sample were fixed between two layers of an amorphous adhesion tape. Since it was not possible to grow single crystals of GdSb2O4Br in sufficient quality for single-crystal X-ray diffraction analysis, a Le Bail profile fit of the powder data for GdSb2O4Br to obtain the lattice constants with the Fullprof suite was performed [33–36]. EuSb2O4Br was used as the starting model.

2.3. Electron-Beam Microprobe Analysis Scanning electron images as well as energy- and wavelength-dispersive X-ray spectrometry were performed on a Cameca SX-100 microanalyzer (Cameca, Gennevilliers, France) equipped with one energy-dispersive X-ray spectrometer (EDXS, ThermoScientific UltraDry, Waltham, MA, USA) and five wavelength-dispersive X-ray spectrometers (WDXS). Crystals of sufficient size were placed on a conductive carbon pad (Plano G3357, Wetzlar, Germany) and afterwards coated with a thin carbon layer to prevent surface charging. An acceleration voltage of 20 kV was chosen to improve the intensities of the emission lines of the heavier elements. Quantification of an europium(III)-doped sample of TbSb2O4Br was performed by using Eu[PO4], Tb[PO4], InSb and CsBr for instrument calibration. The oxygen content was calculated using the individual valences of all present ions. As matrix correction, the Pouchou and Pichoir algorithm “PaP” as implemented in the Cameca PeakSight 6.2 package was applied [37–39].

2.4. Photoluminescence Investigations Photoluminescence measurements were carried out at room temperature on a Fluorolog-3 modular spectrofluorometer (Horiba JobinYvon, France), equipped with a double excitation monochromator, a single emission monochromator (HR320), and a R928P PMT detector. A continuous xenon lamp (450 W) was used as the excitation source for steady state measurements. For lifetime acquisition, the time-correlated single photon counting (TCSPC) technique was used, with a xenon microsecond-pulsed lamp for excitation. For both steady state and lifetimes measurements, the samples were measured as powders in front field geometry, by putting the sample holder plane perpendicular to the incoming ray, and collecting the emitted light at 22.5◦ from the incident light.

3. Results and Discussion

3.1. Crystal-Structure Description

All members of the LnSb2O4Br series (Ln = Eu–Tb) crystallize in the monoclinic space group 3+ P21/c with four formula units per unit cell. The lanthanoid(III) cations (Ln ) occupy only one crystallographically unique position exhibiting a coordination number of eight. Being coordinated 13– exclusively by oxygen atoms, distorted [LnO8] anti- or hemiprisms are formed (Figure1). The observed lanthanoid(III)-oxygen bond lengths fall into the range between 227 and 258 pm, which are similar to those found in the respective sesquioxides (Eu2O3 (Sm2O3- or B-type: 224–262 pm and bixbyite- or C-type: 230–239 pm), Gd2O3 (Sm2O3- or B-type: 215–270 pm and bixbyite- or C-type: 228–239 pm) and Tb2O3 (Sm2O3- or B-type: 218–273 pm and bixbyite- or C-Type: 13– 227–239 pm) [40–45]. These [LnO8] polyhedra are edge-connected by common oxygen atoms Crystals Crystals2020, 10 2020, x FOR, 10, xPEER FOR PEERREVIEW REVIEW 55 of of 23 23 Crystals 2020, 10, 1089 5 of 23 – spreading out parallel to the (100) plane according to the Niggli formula {[𝐿𝑛O/]–} (e = edge- spreading out parallel to the (100) plane according to the Niggli formula {[𝐿𝑛O/] } (e = edge- sharing, Figure 2). sharing, Figureto 2). form inﬁnite layers spreading out parallel to the (100) plane according to the Niggli formula 2 [ e ]5– LnO8/2 (e = edge-sharing, Figure2). ∞{ }

Figure 1. Coordination sphere of the Ln3+ cations in the crystal structure of the LnSb2O4Br series (Ln = 3+ Figure 1. Coordination sphere of the Ln cations in the crystal structure of the LnSb2O4Br series Eu‒Tb) built up by eight oxygen atoms in the shape of a square [LnO8]13‒ hemiprism. (Ln = Eu–Tb) built up by eight oxygen3+ atoms in the shape of a square [LnO ]13 hemiprism. Figure 1. Coordination sphere of the Ln cations in the crystal structure8 of− the LnSb2O4Br series (Ln =

Eu‒Tb) built up by eight oxygen atoms in the shape of a square [LnO8]13‒ hemiprism.

2 [ e ]5– – Figure 2. Inﬁnite layer of the composition LnO8/2 in the crystal structure of the LnSb2O4Br series Figure 2. Infinite layer of the composition∞ { {[𝐿𝑛O/} ] } in the crystal structure of the LnSb2O4Br 13 (Ln = Eu–Tb) as viewed along [100] showing edge-connected [LnO8] − hemiprisms spreading out 8 13‒ series (Lnparallel = Eu‒ toTb) the as (100) viewed plane. along [100] showing edge-connected [LnO ] hemiprisms spreading out parallel to the (100) plane. – Figure 2. Infinite layer of the composition {[𝐿𝑛O/] } in the crystal structure of the LnSb2O4Br The Sb3+ cations occupy two different crystallographic positions. Both (Sb1)3+ and (Sb2)3+ are series (Ln = Eu‒Tb) as viewed along [100] showing edge-connected [LnO8]13‒ hemiprisms spreading surrounded by three oxygen atoms each in the shape of [SbO3]3– ψ1-tetrahedra (Figure 3). These out parallel to the (100) plane. pyramidal units share common oxygen atoms and build up infinite strands of vertex-connected [SbO3]3– ψ1-tetrahedra with alternating (Sb1)3+ and (Sb2)3+ cations meandering along the c axis (Figure 3+ 3+ 3+ The Sb cations occupy two –different crystallographic positions. Both (Sb1) and (Sb2) are 4) according to {[SbO/O/] } (v = vertex-sharing, t = terminal). The different bonding situation is surrounded by three oxygen atoms each in the shape of [SbO3]3– ψ1-tetrahedra (Figure 3). These also reflected well by the interatomic Sb3+–O2– distances. While the terminal oxygen atoms reside at pyramidaldistances units of shareabout 193common pm to theoxygen central atoms Sb3+ cations, and build the linking up infinite ones exhibit strands noticeable of vertex-connected longer values 3 3– 1 3+ 3+ [SbO ]with ψ -tetrahedra 205–212 pm. with These alternating different values (Sb1) are and also (Sb2) illustrated cations in Figure meandering 4 for the along example the ofc axis TbSb (Figure2O4Br – 4) accordingand all tolone {[SbOpairs present/O/ ]at} the(v =Sb vertex-sharing,3+ cations are pointing t = terminal). into the sameThe different direction bondingalong [100]. situation is also reflected well by the interatomic Sb3+–O2– distances. While the terminal oxygen atoms reside at distances of about 193 pm to the central Sb3+ cations, the linking ones exhibit noticeable longer values with 205–212 pm. These different values are also illustrated in Figure 4 for the example of TbSb2O4Br and all lone pairs present at the Sb3+ cations are pointing into the same direction along [100].

Crystals 2020, 10, 1089 6 of 23

The Sb3+ cations occupy two different crystallographic positions. Both (Sb1)3+ and (Sb2)3+ 3– 1 are surrounded by three oxygen atoms each in the shape of [SbO3] ψ -tetrahedra (Figure3). These pyramidal units share common oxygen atoms and build up infinite strands of vertex-connected 3– 1 3+ 3+ [SbO3] ψ -tetrahedra with alternating (Sb1) and (Sb2) cations meandering along the c axis 1 [ v t ]– (Figure4) according to SbO2/2O1/1 (v = vertex-sharing, t = terminal). The different bonding ∞{ } situation is also reflected well by the interatomic Sb3+–O2– distances. While the terminal oxygen atoms reside at distances of about 193 pm to the central Sb3+ cations, the linking ones exhibit noticeable Crystals 2020, 10, x FOR PEER REVIEW 6 of 23 longer values with 205–212 pm. These different values are also illustrated in Figure4 for the example of 3+ TbSbCrystals2O 20204Br, 10 and, x FOR all lone PEER pairs REVIEW present at the Sb cations are pointing into the same direction along6 [100].of 23

(b) (a) (b) (a) Figure 3. The first and second coordination spheres of the two crystallographically different Sb3+ 3+ Figurecations 3. (Thea): Sb1, ﬁrst and(b): secondSb2) in coordinationthe crystal structure spheres of of the the two Ln crystallographicallySb2O4Br series (Ln = di Euﬀerent‒Tb) Sbwithcations vertex- Figure 3. The first and second coordination spheres of the two crystallographically different Sb3+ (aconnected): Sb1, (b): [SbO Sb2)3] in3‒ theψ1-tetrahedra crystal structure as dominant of the LnstructuralSb2O4Br feature. series ( Ln = Eu–Tb) with vertex-connected cations (a): Sb1, (b): Sb2) in the crystal structure of the LnSb2O4Br series (Ln = Eu‒Tb) with vertex- 3 1 [SbO3] − ψ -tetrahedra as dominant structural feature. connected [SbO3]3‒ ψ1-tetrahedra as dominant structural feature.

3 1 Figure 4. Network of vertex-connected [SbO3] − ψ -tetrahedra in the crystal structure of the LnSb2O4Br Figure 4. Network of vertex-connected [SbO3]3‒ ψ1-tetrahedra in the crystal structure of the LnSb2O4Br 1 [ v t ]– series (Ln = Eu–Tb) building up meandering strands of the composition SbO2/2O1/1 – along [001]. series (Ln = Eu‒Tb) building up meandering strands3‒ 1 of the composition∞ { {[SbO/O/] }} along [001]. TheFigure given 4. Network interatomic of vertex-connected Sb–O distances (in [SbO pm)3] are ψ -tetrahedra valid for TbSb in the2O4 crystalBr. structure of the LnSb2O4Br The given interatomic Sb‒O distances (in pm) are valid for TbSb2O4Br. – series (Ln = Eu‒Tb) building up meandering strands of the composition {[SbO/O/] } along [001]. InThe valentinite given interatomic and senarmontite, Sb‒O distances the two(in pm) natural are valid crystalline for TbSb modifications2O4Br. of Sb2O3, the antimony(III)- oxygenIndistances valentinite feature and values senarmontite, of 198–202 the pm two and thereforenatural crystalline fall in between modifications the range ofof our Sb2 refinedO3, the antimony(III)-oxygen distances feature values of 198–202 pm and therefore fall in between the range distancesIn valentinite for the LnSb and2O4 Brsenarmontite, series with regards the two to the natural differences crystalline of the linking modifications and the terminal of Sb2O oxygen3, the antimony(III)-oxygenof our refined distances distances for the featurLnSb2Oe values4Br series of 198–202with regards pm and to the ther differencesefore fall in of between the linking the and range the terminal oxygen atoms [46,47]. Consequently, the sharing of an oxygen atom between two of our refined distances for the LnSb2O4Br series with regards to the differences of the linking and the terminalantimony(III) oxygen cations atoms leads [46,47]. to an expansionConsequently, of the the respective sharing bond of anlength oxygen by at atomleast 10between pm. two antimony(III)The influence cations of leads the lone to an electron expansion pair of of the a pnictogen(III)respective bond cation length has by recently at least been10 pm. reported for comparableThe influence lanthanoid(III) of the lone oxopnict electronogenates(III), pair of a pnictogen(III) such as the lanthacationnoid(III) has recently halide been oxoarsenates(III) reported for comparablewith the compositions lanthanoid(III) Ln oxopnict5Cl3[AsOogenates(III),3]4 and Ln3X 2such[As2O as5][AsO the lantha3] [12,14].noid(III) While halide the oxoarsenates(III) title compounds feature one-dimensional infinite strands of corner-connected [SbO3]3– units, the latter only display with the compositions Ln5Cl3[AsO3]4 and Ln3X2[As2O5][AsO3] [12,14]. While the title compounds isolated [AsO3]3– ψ1-tetrahedra or doubles of them in vertex-condensed [As2O5]4– pyroanions. In these feature one-dimensional infinite strands of corner-connected [SbO3]3– units, the latter only display condensed units, the bridging oxygen atoms also show the longest distances to the arsenic(III) isolated [AsO3]3– ψ1-tetrahedra or doubles of them in vertex-condensed [As2O5]4– pyroanions. In these condensedcations, much units, like the in thebridging monoclinic oxygen members atoms ofalso the show series the LnSb longest2O4Br, wheredistances the bondto the lengths arsenic(III) to the cations, much like in the monoclinic members of the series LnSb2O4Br, where the bond lengths to the

Crystals 2020, 10, 1089 7 of 23 atoms [46,47]. Consequently, the sharing of an oxygen atom between two antimony(III) cations leads to an expansion of the respective bond length by at least 10 pm. The inﬂuence of the lone electron pair of a pnictogen(III) cation has recently been reported for comparable lanthanoid(III) oxopnictogenates(III), such as the lanthanoid(III) halide oxoarsenates(III) with the compositions Ln5Cl3[AsO3]4 and Ln3X2[As2O5][AsO3][12,14]. While the title compounds 3– feature one-dimensional inﬁnite strands of corner-connected [SbO3] units, the latter only display 3– 1 4– isolated [AsO3] ψ -tetrahedra or doubles of them in vertex-condensed [As2O5] pyroanions. In these condensed units, the bridging oxygen atoms also show the longest distances to the arsenic(III) cations, much like in the monoclinic members of the series LnSb2O4Br, where the bond lengths to 3– the corner-connecting oxygen atoms of the [SbO3] groups fall into a longer interval than the short terminal ones. CrystalsComparing 2020, 10, x theFOR structuralPEER REVIEW features of the short LnSb2O4Br (Ln = Eu–Tb) series with7 of 23 the crystal 4+ structure of the long Na2Ln3Cl3[TeO3]4 series (Ln = La–Nd, Sm–Lu) featuring Te as cation isoelectronic corner-connecting oxygen atoms of the [SbO3]3– groups fall into a longer interval than the short to Sb3+ reveals several similarities [48–50]. The lanthanoid(III) cations also exhibit an eightfold terminal ones. 13 coordinationComparing sphere the exclusively structural features by oxygen of the short atoms Ln buildingSb2O4Br (Ln up = Eu [Ln‒Tb)O8 ]series− hemiprisms with the crystal and thus no 3+ 2 e 5– 4+[ ] Ln structure-X− bonds of exist.the long These Na2Ln hemiprisms3Cl3[TeO3]4 series build (Ln up = layers La‒Nd, of Sm the‒Lu) composition featuring Te Lnas Ocation8/2 parallel ∞{ } to theisoelectronic (001) plane. to Sb Furhtermore,3+ reveals several the similarities Te4+ cations [48–50]. are The coordinated lanthanoid(III) by cations three oxygenalso exhibit atoms an forming eightfold2 coordination sphere exclus1 ively by oxygen atoms building up [LnO8]13‒ hemiprisms4+ 2 and4 + [TeO3] − anions in the shape of ψ -tetrahedra, but in this case, without any Te –O −–Te contacts. thus no Ln3+‒X‒ bonds exist. These hemiprisms build up layers of the composition {[𝐿𝑛O ]–} The halide anions X– are only connected to the sodium cations located in between / two layers of parallel to the (001) plane. Furhtermore, the Te4+ cations are coordinated by three oxygen atoms 13 [LnO ] hemiprisms.2‒ 1 4+ 2‒ 4+ forming8 − [TeO3] anions in the shape of ψ -tetrahedra, but in this case, without any Te ‒O ‒Te contacts.The crystallographically The halide anions X– are unique only connected bromide to anionsthe sodium do ca nottions exhibit located anyin between bonding two layers contacts to the 3+ 3+ – Ln ofcations, [LnO8]13‒since hemiprisms. the shortest Ln Br separations amount to 422 pm. The bromide anions are The crystallographically unique bromide··· anions do not exhibit13– any bonding contacts to the Ln3+ located between planes formed by edge-connected [LnO8] polyhedra. The antimony(III) cations 3+ – connectedcations, to since these the oxygen shortest atomsLn ···Br show separations distances amount of 318–320 to 422 pm. pm The up bromide to 366–367 anions pm are to located the neighboring between planes formed by edge-connected [LnO8]13– polyhedra. The antimony(III) cations connected bromide anions (Figure5). Comparing these values with ternary antimony oxide bromides seems to these oxygen atoms show distances of 318–320 pm up to 366–367 pm to the neighboring bromide 3+ appropriate.anions (Figure In Sb 5).4 OComparing5Br2, a well-known these values with ternary ternary antimony(III) antimony oxide oxide bromides bromide, seems appropriate. the Sb cations also showInonly Sb4O5 shortBr2, a well-known interatomic ternary distances antimony(III) to the oxide oxygen bromide, atoms the with Sb3+ cations a major also contribution show only short to chemical bondinginteratomic (189–251 distances pm), whileto the oxygen the bromide atoms with anions a major are partially contribution even to morechemical removed bonding away (189–251 from the Sb3+ cationspm), with while distances the bromide of 302–366anions are pm partially [51]. Almosteven more the removed same holdsaway from for a the second Sb3+ cations antimony(III) with oxide distances of 302–366 pm [51]. Almost the same holds for a3+ second2– antimony(III) oxide bromide with 3+ – bromide with the composition Sb8O11Br2, where Sb –O distances of 190–257 pm and Sb Br the composition Sb8O11Br2, where Sb3+–O2– distances of 190–257 pm and Sb3+···Br– distances of 314–396 ··· distances of 314–396 pm occur [52,53]. Therefore, in both these ternary compounds and LnSb O Br pm occur [52,53]. Therefore, in both these ternary compounds and LnSb2O4Br (Ln = Eu‒Tb), the Sb3+‒ 2 4 3+ 3+ (Ln =BrEu–Tb),‒ bond lengths the Sb illustrate–Br− thebond tendency lengths of Sb illustrate3+ cations theto be tendency coordinated of by Sb bromidecations anions to be with coordinated far by bromidegreater anions interatomic with distances far greater compared interatomic to the oxygen distances atoms. compared to the oxygen atoms.

FigureFigure 5. Coordination 5. Coordination sphere sphere of of the the crystallographically crystallographically uniqueunique bromide anions anions in in the the crystal crystal structure of the LnSb O Br series (Ln = Eu–Tb) with shortest interatomic distances of d(Sb Br) = 318–320 pm structure2 of 4the LnSb2O4Br series (Ln = Eu‒Tb) with shortest interatomic distances of d(Sb···Br)··· = 318 ‒ and320 no bondingpm and no contacts bonding contacts to any Ln to 3any+ cation. Ln3+ cation.

In Figure 6, a section of the crystal structure with indication of the cell edges, coordination polyhedra around the lanthanoid(III) cations and the refined anisotropic displacement ellipsoids is shown. The lattice parameters for all members of the LnSb2O4Br series (Ln = Eu‒Tb) are listed in Table 1, while the crystallographic data as well as the positional atomic parameters and selected interatomic distances for EuSb2O4Br and TbSb2O4Br can be found in Tables A1–A3. In the case of GdSb2O4Br, it was also possible to synthesize very flat, square platelets of crystals with the desired composition, however, the quality of the obtained single crystals revealed to be too poor for a trustworthy structure

Crystals 2020, 10, 1089 8 of 23

In Figure6, a section of the crystal structure with indication of the cell edges, coordination polyhedra around the lanthanoid(III) cations and the refined anisotropic displacement ellipsoids is shown. The lattice parameters for all members of the LnSb2O4Br series (Ln = Eu–Tb) are listed in Table1, while the crystallographic data as well as the positional atomic parameters and selected interatomic distances for EuSb2O4Br and TbSb2O4Br can be found in Tables A1–A3. In the case of GdSb2O4Br, it was Crystalsalso possible 2020, 10, tox FOR synthesize PEER REVIEW very flat, square platelets of crystals with the desired composition, however,8 of 23 the quality of the obtained single crystals revealed to be too poor for a trustworthy structure refinement. refinement. Instead, it was possible to refine the unit-cell parameters of GdSb2O4Br both via single- Instead, it was possible to refine the unit-cell parameters of GdSb2O4Br both via single-crystal X-ray crystaldiffraction X-ray as diffraction well as powder as well X-ray as powder diffraction X-ray methods diffraction (Section methods 3.2). (Section 3.2).

Figure 6. Section of the crystal structure of the LnSb O Br series (Ln = Eu–Tb) viewed along [010] Figure 6. Section of the crystal structure of the LnSb2O44Br series (Ln = Eu‒Tb) viewed along [010] emphasizing the unit-cell edges and [LnO ]13 polyhedra (dark red) utilizing the reﬁned anisotropic emphasizing the unit-cell edges and [LnO88]13‒ −polyhedra (dark red) utilizing the refined anisotropic displacementdisplacement parameters parameters with with an an ellipsoidal ellipsoidal probability probability of of 95%. 95%.

Table 1. Lattice parameters for the three members of the LnSb2O4Br series (Ln = Eu–Tb) as determined Tableby means 1. Lattice of powder parameters X-ray for diff theractometry. three members of the LnSb2O4Br series (Ln = Eu‒Tb) as determined by means of powder X-ray diffractometry. Compound EuSb2O4Br GdSb2O4Br TbSb2O4Br Compound EuSb2O4Br GdSb2O4Br TbSb2O4Br Crystal system monoclinic Crystal system monoclinic Space group P21/c (no. 14) a/pmSpace group 895.69(2)P21/c 895.67(2) (no. 14) 894.56(2) b/pma/pm 791.90(2)895.69(2) 895.67(2) 789.27(2) 894.56(2) 785.84(2) c/pmb/pm 790.46(2)791.90(2) 789.27(2) 788.44(2) 785.84(2) 784.25(2) β/◦ c/pm 91.743(1)790.46(2) 788.44(2) 91.681(1) 784.25(2) 91.625(1) β/° 91.743(1) 91.681(1) 91.625(1) 3.2. Powder X-ray Diffraction 3.2. PowderFor phase X-ray analyses Diffraction of the obtained and water-washed product mixtures of all three systems, PXRD measurementsFor phase analyses were performed. of the obtained Figure A2and shows water-wa theshed diffractogram product mixtures of EuSb2 Oof4 Brall andthree TbSb systems,2O4Br, PXRDrespectively. measurements While good were accordance performed. among Figure the positionsA2 shows of thethe simulateddiffractogram and measuredof EuSb2O reflections4Br and TbSbis observed,2O4Br, respectively. a remarkable While variance good ofaccordance the relative among reflection the positions intensities of the occurs. simulated This kindand measured of texture reflectionseffects can is be observed, attributed a remarkable to the extreme variance platelet of habitthe relative of the reflection crystals (Figure intensit 8).ies Intensive occurs. This crushing kind of of texturethe powder effects samples can be and attributed subsequent to the diff extremeractionexperiments platelet habit showed of the ancrystals improvement (Figure of8). the Intensive relative crushingintensities of towardsthe powder the theoreticalsamples and values, subsequent however, diffraction strong reflex experiments broadening showed was observed.an improvement Therefore, of thethe samplesrelative wereintensities only slightlytowards crushed the theoretical to avoid thesevalues, broadenings. however, strong reflex broadening was observed. Therefore, the samples were only slightly crushed to avoid these broadenings. Since it was not possible to yield single crystals of GdSb2O4Br with sufficient quality for a reliable structure determination, the lattice parameters were refined from powder X-ray diffraction methods. For comparison, single crystals of GdSb2O4Br with unit-cell parameters of a = 895.54(6) pm, b = 788.91(5) pm, c = 787.67(5) pm and β = 91.725(3)° were observed. The experimental and simulated pattern as well as the plot of the calculated deviation and the Bragg positions are given in Figure 7. The refined cell parameters for GdSb2O4Br are also given in Table 1 and are in high accordance with the values deriving from the single-crystal measurement as well as with the expected ones due to the lanthanoid contraction.

Crystals 2020, 10, 1089 9 of 23

Since it was not possible to yield single crystals of GdSb2O4Br with sufficient quality for a reliable structure determination, the lattice parameters were refined from powder X-ray diffraction methods. For comparison, single crystals of GdSb2O4Br with unit-cell parameters of a = 895.54(6) pm, b = 788.91(5) pm, c = 787.67(5) pm and β = 91.725(3)◦ were observed. The experimental and simulated pattern as well as the plot of the calculated deviation and the Bragg positions are given in Figure7. The refined cell parameters for GdSb2O4Br are also given in Table1 and are in high accordance with the values deriving from the single-crystal measurement as well as with the expected ones due to the Crystals 2020, 10, x FOR PEER REVIEW 9 of 23 Crystalslanthanoid 2020, 10 contraction., x FOR PEER REVIEW 9 of 23

FigureFigure 7. 7. LeLe Bail Bail profile proﬁle matching matching of of GdSb GdSb22OO4Br4Br showing showing the the measured measured pattern pattern (blue), (blue), the the simulated simulated Figure 7. Le Bail profile matching of GdSb2O4Br showing the measured pattern (blue), the simulated patternpattern (red), (red), the the deviation deviation plot plot (green (green),), and and the the reflex reﬂex positions (black). pattern (red), the deviation plot (green), and the reflex positions (black). 3.3.3.3. Electron-Probe Electron-Probe Microanalysis Microanalysis 3.3. Electron-Probe Microanalysis Ln Ln ToTo evaluate evaluate the the crystal morphology of of the three LnSb2OO44BrBr representatives representatives with with Ln == EuEu–Tb,‒Tb, To evaluate the crystal morphology of the three LnSb2O4Br representatives with Ln = Eu‒Tb, scanningscanning electronelectron images images were were recorded. recorded. In Figure In Figure8, representative 8, representative micrographs micrographs of all three of compounds all three scanning electron images were recorded. In Figure 8, representative micrographs of all three compoundsillustrating theillustrating size and the crystal size habitand crystal of the synthesizedhabit of the crystalssynthesized are given. crystals A prominentare given. A feature prominent for all compounds illustrating the size and crystal habit of the synthesized crystals are given. A prominent featurethree compounds for all three is compounds the formation is the of platelet-shapedformation of platelet-shaped crystals. crystals. feature for all three compounds is the formation of platelet-shaped crystals.

(a) (b) (c) (a) (b) (c)

FigureFigure 8. 8. ScanningScanning electron electron images of EuSb2O44BrBr ( (aa),), GdSb 22OO44BrBr ( (bb),), and and TbSb TbSb22OO44BrBr (c (c) )illustrating illustrating Figure 8. Scanning electron images of EuSb2O4Br (a), GdSb2O4Br (b), and TbSb2O4Br (c) illustrating thethe platelet platelet crystal crystal habit habit with with typical typical edge edge lengths lengths of 50 of‒ 50–150150 µmµ form the for theeuropium europium compound compound and 10 and‒ the platelet crystal habit with typical edge lengths of 50‒150 µm for the europium compound and 10‒ 3510–35 µm µform the for gadolinium the gadolinium and andterbium terbium representatives. representatives. 35 µm for the gadolinium and terbium representatives.

AllAll members members of of the the LnLnSbSb2O2O4Br4Br series series (Ln (Ln = Eu= Eu–Tb)‒Tb) were were characterized characterized with with energy-dispersive energy-dispersive X- All members of the LnSb2O4Br series (Ln = Eu‒Tb) were characterized with energy-dispersive X- rayX-ray spectroscopy spectroscopy to toverify verify the the purity purity and and validate validate the the absence absence of of unintended unintended elements elements in in the the gained gained ray spectroscopy to verify the purity and validate the absence of unintended elements in the gained crystals.crystals. As As shown shown in in Figure Figure 9,9, each compound exhi exhibitsbits the emission lines of the intended elements crystals. As shown in Figure 9, each compound exhibits the emission lines of the intended elements andand no no additional additional ones ones are are pres presentent in in the the examined examined samples. samples. and no additional ones are present in the examined samples. For an europium(III)-doped sample of TbSb2O4Br as representative of the series, quantitative For an europium(III)-doped sample of TbSb2O4Br as representative of the series, quantitative measurements of the single crystals were performed. This was of special interest, since previous measurements of the single crystals were performed. This was of special interest, since previous structure studies about the crystal structures in the quaternary system Sm‒Sb‒O‒X (X = Cl and Br) structure studies about the crystal structures in the quaternary system Sm‒Sb‒O‒X (X = Cl and Br) revealed a mixed-Ln3+/Sb3+ occupancy of the Sb3+ position and, therefore, an incoherency with the revealed a mixed-Ln3+/Sb3+ occupancy of the Sb3+ position and, therefore, an incoherency with the theoretical formula “SmSb2O4X” [26]. For this new structure type in the quaternary system Ln‒Sb‒ theoretical formula “SmSb2O4X” [26]. For this new structure type in the quaternary system Ln‒Sb‒ O‒Br with similarities regarding structural features and synthesis methods, it seemed appropriate to O‒Br with similarities regarding structural features and synthesis methods, it seemed appropriate to apply a second method to confirm the determined composition of at least one representative with the apply a second method to confirm the determined composition of at least one representative with the formula LnSb2O4Br (Ln = Eu‒Tb). Moreover, the method was applied to determine the degree of formula LnSb2O4Br (Ln = Eu‒Tb). Moreover, the method was applied to determine the degree of doping in TbSb2O4Br, since it is not possible to refine the europium(III) content in a terbium(III) host doping in TbSb2O4Br, since it is not possible to refine the europium(III) content in a terbium(III) host lattice by means of X-ray diffraction methods. lattice by means of X-ray diffraction methods.

Crystals 2020, 10, x FOR PEER REVIEW 10 of 23

The results of the quantitation using wavelength-dispersive spectrometry are compiled in Table A4. When comparing the experimental values to the expected of TbSb2O4Br doped with 1.5 at-% Eu3+ regardingCrystals 2020 , 10the, 1089 total amount of lanthanoid(III) cations, the measurements validate formula10 of 23 TbSb2O4Br:Eu3+ with the desired doping content.

FigureFigure 9. 9. Energy-dispersiveEnergy-dispersive X-ray X-ray spectra of the LnSb2OO44BrBr series series with with LnLn == EuEu–Gd‒Gd at at an an acceleration acceleration voltagevoltage of of 20 20 kV kV with with assignment assignment of the relevant emission lines.

3.4. PhotoluminescenceFor an europium(III)-doped Spectroscopy sample of TbSb2O4Br as representative of the series, quantitative measurements of the single crystals were performed. This was of special interest, since previous Antimony(III) compounds are known to show an excitation band in the ultraviolet range (UV) structure studies about the crystal structures in the quaternary system Sm–Sb–O–X (X = Cl and Br) with a maximum around 270 nm corresponding to the partially forbidden 1S0 → 3P1 electronic revealed a mixed-Ln3+/Sb3+ occupancy of the Sb3+ position and, therefore, an incoherency with the transition. The Sb3+ emission 3P1 → 1S0 is known to occur over a broad range (from 300 to 650 nm), and theoretical formula “SmSb O X”[26]. For this new structure type in the quaternary system Ln-Sb-O-Br often overlaps the excitation2 range4 of trivalent lanthanoid cations, such as Eu3+ and Tb3+, rendering it with similarities regarding structural features and synthesis methods, it seemed appropriate to apply a potentially a suitable sensitizer for lanthanoid emission [54,55]. The luminescence of Sb3+ in second method to confirm the determined composition of at least one representative with the formula LnSb2O4Br representatives with Ln = Eu‒Tb can be best investigated in GdSb2O4Br as Gd3+ only emits LnSb2O4Br (Ln = Eu–Tb). Moreover, the method was applied to determine the degree of doping in in the UV (typically around 310 nm) due to a transition from the 6P7/2 to 8S7/2 levels. However, even TbSb2O4Br, since it is not possible to refine the europium(III) content in a terbium(III) host lattice by efficient sensitization of Gd3+ has been reported in YPO4:Sb3+, Gd3+ [7]. The excitation spectrum of means of X-ray diffraction methods. GdSb2O4Br monitored at 455 nm shows a band with a maximum at 257 nm, which can be assigned to The results of the quantitation using wavelength-dispersive spectrometry are compiled in the the partially forbidden 1S0 → 3P1 electronic transition of Sb3+ (Figure 10). Table A4. When comparing the experimental values to the expected of TbSb2O4Br doped with 1.5 at-% Eu3+ regarding the total amount of lanthanoid(III) cations, the measurements validate formula 3+ TbSb2O4Br:Eu with the desired doping content.

3.4. Photoluminescence Spectroscopy Antimony(III) compounds are known to show an excitation band in the ultraviolet range (UV) with a maximum around 270 nm corresponding to the partially forbidden 1S 3P electronic transition. 0 → 1 The Sb3+ emission 3P 1S is known to occur over a broad range (from 300 to 650 nm), and often 1 → 0 overlaps the excitation range of trivalent lanthanoid cations, such as Eu3+ and Tb3+, rendering it 3+ potentially a suitable sensitizer for lanthanoid emission [54,55]. The luminescence of Sb in LnSb2O4Br 3+ representatives with Ln = Eu–Tb can be best investigated in GdSb2O4Br as Gd only emits in the UV 6 8 (typically around 310 nm) due to a transition from the P7/2 to S7/2 levels. However, even eﬃcient 3+ 3+ 3+ sensitization of Gd has been reported in YPO4:Sb , Gd [7]. The excitation spectrum of GdSb2O4Br monitored at 455 nm shows a band with a maximum at 257 nm, which can be assigned to the the partially forbidden 1S 3P electronic transition of Sb3+ (Figure 10). 0 → 1

CrystalsCrystals 2020, 202010, x, 10FOR, x FOR PEER PEER REVIEW REVIEW 11 of11 23 of 23 Crystals 2020, 10, 1089 11 of 23

Figure 10. Excitation (λem = 455 nm) and emission spectra (λex = 257 nm) of GdSb2O4Br. Figure 10. Excitation (λem = 455 nm) and emission spectra (λex = 257 nm) of GdSb2O4Br. UponFigure excitation 10. Excitation into 257 (λem nm, = 455 a broadnm) and band emission is visible spectra in the (λ exemission = 257 nm) spectrum of GdSb 2inO4 Br.the region Upon excitation into 257 nm, a broad band is visible in the emission spectrum in the region between between 400 and 500 nm (Figure 10), with a maximum at 455 nm, corresponding to the 3transition1 400 and 500 nm (Figure 10), with a maximum at 455 nm, corresponding to the transition P1 S0. Upon3P1→ 1S excitation0. This band into in the 257 blue nm, region a broad of the band electrom is viagneticsible spectrumin the emission occurs for spectrum all the investigated in the→ region This band in the blue region of the electromagnetic spectrum occurs for all the investigated compounds betweencompounds 400 and upon 500 nmexcitation (Figure in 10), 1S 0with → 3Pa 1 maxi(insetmum in Figureat 455 11,nm, left). corresponding For TbSb2O to4Br:Eu the 3+transition and upon excitation in 1S 3P (inset in Figure 11, left). For TbSb O Br:Eu3+ and GdSb O Br:Eu3+ its 3 1 3+ 0 1 3+2 4 2 4 P1→GdSb S0. This2O4Br:Eu band inits theintensity →blue regionis negligible of the compared 3electrom+ agneticto the Eu spectrum transitions occurs (Figure for 11,all theleft) investigatedand less3+ intensity is negligible3+ compared to the Eu transitions (Figure 11, left) and less intense than Tb intense than Tb transitions in 1the0 →terbium3 1 derivatives (inset in Figure 11, left), suggesting2 4 the3+ compoundstransitions upon in the excitation terbium derivatives in S (inset P in(inset Figure in 11 ,Figure left), suggesting 11, left). the For presence TbSb ofO anBr:Eu energy and presence of3+ an energy transfer (ET) from Sb3+ to Ln3+. The different3+ contribution of the Sb3+ emission GdSbtransfer2O4Br:Eu (ET) its from intensity Sb3+ to Lnis 3negligible+. The diﬀerent compared contribution to the of theEu Sb transitions3+ emission (blue(Figure region) 11, left) compared and less (blue region) compared to the Eu3+ emission (red region) as well as Tb3+ role, are demonstrated by the intenseto thethan Eu 3Tb+ emission3+ transitions (red region) in the as wellterbium as Tb 3+derivativesrole, are demonstrated (inset in Figure by the chromaticity 11, left), suggesting coordinates the chromaticity coordinates calculated for TbSb2O4Br:Eu3+ and GdSb2O4Br:Eu3+ (λex = 257 nm) (Figure 11, presencecalculated of an forenergy TbSb transferO Br:Eu (ET)3+ and from GdSb Sb3+O toBr:Eu Ln3+3. +The(λ different= 257 nm) contribution (Figure 11, right of the). InSb order3+ emission to right). In order to explain2 4 this difference, a deep2 4 investigationex on the luminescence properties of both 3+ 3+ 3+ (blue explainregion) this compared di3+ﬀerence, to athe deep Eu investigation emission3+ (red on theregion) luminescence as well as properties Tb role, of are both demonstrated TbSb2O4Br:Eu by the TbSb2O4Br:Eu and3 GdSb+ 2O4Br:Eu was performed. chromaticityand GdSb coordinates2O4Br:Eu wascalculated performed. for TbSb2O4Br:Eu3+ and GdSb2O4Br:Eu3+ (λex = 257 nm) (Figure 11, right). In order to explain this difference, a deep investigation on the luminescence properties of both TbSb2O4Br:Eu3+ and GdSb2O4Br:Eu3+ was performed.

3+ 3+ Figure 11. Left: emission spectra of TbSb2O4Br:Eu3+ (green line) and GdSb2O43+Br:Eu (blue line) Figure 11. Left: emission spectra of TbSb2O4Br:Eu (green line)3+ and GdSb2O4Br:Eu (blue line) at λex = at λex = 257 nm. Inset: comparison of emission in the Sb region for all the compounds upon the 257 nm. Inset: comparison of emission in the Sb3+ region for all the compounds upon the same same excitation. These spectra were normalized for the respective Sb3+ emission maximum of each excitation. These spectra were normalized for the respective Sb3+ emission maximum of each compound. Right: Commission internationale de l’éclairage (CIE) diagram (CIE 1931) for TbSb2O4Br, compound. Right3+ : Commission internationale de l’écla3+ irage (CIE) diagram (CIE 1931) for TbSb2O4Br, TbSb2O4Br:Eu , GdSb2O4Br, and GdSb2O4Br:Eu at λex = 257 nm. TbSb2O4Br:Eu3+, GdSb2O4Br, and GdSb2O4Br:Eu3+ at λex = 257 nm. Figure 11. Left: emission spectra of TbSb2O4Br:Eu3+ (green line) and GdSb2O4Br:Eu3+ (blue line) at λex = 257 nm. Inset: comparison of emission in the Sb3+ region for all the compounds upon the same excitation. These spectra were normalized for the respective Sb3+ emission maximum of each compound. Right: Commission internationale de l’éclairage (CIE) diagram (CIE 1931) for TbSb2O4Br,

TbSb2O4Br:Eu3+, GdSb2O4Br, and GdSb2O4Br:Eu3+ at λex = 257 nm.

Crystals 2020, 10, x FOR PEER REVIEW 12 of 23

Excitation spectra of EuSb2O4Br, GdSb2O4Br:Eu3+, and TbSb2O4Br:Eu3+ are shown in Figure 12. For EuSb2O4Br, when monitoring at 611 nm, the most intense band appears at 464 nm, which belongs to the set of 7F0 → 5D2 hypersensitive dielectric dipole transitions of Eu3+. The other excitation lines correspond to 7F0 → 5L6 at 393 nm, 7F0 → 5D1 and 7F1 → 5D1 at 525 and 534 nm of Eu3+, respectively. All the aforementioned transitions can also be observed in the two Eu3+-doped compounds, GdSb2O4Br:Eu3+ and TbSb2O4Br:Eu3+. Moreover, in both the Eu3+-doped compounds, two broad bands can be detected in the UV region. In addition to the band around 250 nm, a second broad band at higher wavelength can be observed, which is attributed to the 2p (O2−) → 4f (Eu3+) charge transfer which frequently is observed in Eu3+ oxide compounds [6,56–58]. Due to the eight-fold coordination of the lanthanide cations in GdSb2O4Br:Eu3+ and TbSb2O4Br:Eu3+ and to their similar radii, both compounds are supposed to have charge transfer bands in the region around 260–270 nm, similarly to Gd2Zr2O7:Eu3+ and LaPO4:Eu3+ [57]. Thus in the ultraviolet (UV) region, an overlap between the aforementioned 1S0 → 3P1 transition of Sb3+ and the Eu3+ charge transfer band occurs. For GdSb2O4Br:Eu3+, the charge transfer band is the most intense. Conversely, it is negligible in the neat CrystalsEu-compound.2020, 10, 1089 12 of 23 By monitoring the Eu3+ emission at 611 nm in TbSb2O4Br:Eu3+, besides the characteristic 3+ interconfigurational f–f transitions of Eu , albeit with3+ comparatively weak3+ relative intensity, the Excitation spectra of EuSb2O4Br, GdSb2O4Br:Eu , and TbSb2O4Br:Eu are shown in Figure 12. bands in the UV region of light belonging to Eu3+-O2- charge transfer and the Sb3+ 1S0 → 3P1 transitions For EuSb O Br, when monitoring at 611 nm, the most intense band appears at 464 nm, which belongs can be seen.2 4 In addition, transitions characteristic for Tb3+ are present which points to Tb3+ → Eu3+ to the set of 7F 5D hypersensitive dielectric dipole transitions of Eu3+. The other excitation lines sensitization.0 → 2 correspond to 7F 5L at 393 nm, 7F 5D and 7F 5D at 525 and 534 nm of Eu3+, respectively. 0 → 6 0 → 1 1 → 1

3+ Figure 12. Left: excitation spectra of Eu 3+-containing LnSb2O4Br compounds (Ln = Eu–Tb), monitored Figure 12. Left: excitation spectra of Eu -containing Ln3+ Sb2O4Br compounds (Ln = Eu–Tb), monitored at λem = 611 nm. Right: excitation spectra of Tb -based compound TbSb2O4Br, monitored at at λem = 611 nm. Right: excitation spectra of Tb3+-based compound TbSb2O4Br, monitored at λem = 541 λ = 541 nm. emnm. All the aforementioned transitions can also be observed in the two Eu3+-doped compounds, Excitation spectra monitored at the Tb3+ emission for TbSb2O4Br as well as TbSb2O4Br:Eu3+ are GdSb O Br:Eu3+ and TbSb O Br:Eu3+. Moreover, in both the Eu3+-doped compounds, two broad shown2 4 in Figure 12 (right). 2Both4 materials show the characteristic bands of Tb3+ which can be assigned bands can be detected in the UV region. In addition to the band around 250 nm, a second broad to the 7F6 → 5D3 and 7F6 → 5D4 transitions. The presence of Tb3+ peaks in the excitation spectrum of band at higher wavelength can be observed, which is attributed to the 2p (O2 ) 4f (Eu3+) charge − → transfer which frequently is observed in Eu3+ oxide compounds [6,56–58]. Due to the eight-fold 3+ 3+ coordination of the lanthanide cations in GdSb2O4Br:Eu and TbSb2O4Br:Eu and to their similar radii, both compounds are supposed to have charge transfer bands in the region around 260–270 nm, 3+ 3+ similarly to Gd2Zr2O7:Eu and LaPO4:Eu [57]. Thus in the ultraviolet (UV) region, an overlap 1 3 3+ 3+ between the aforementioned S0 P1 transition of Sb and the Eu charge transfer band occurs. 3+ → For GdSb2O4Br:Eu , the charge transfer band is the most intense. Conversely, it is negligible in the neat Eu-compound. 3+ 3+ By monitoring the Eu emission at 611 nm in TbSb2O4Br:Eu , besides the characteristic interconﬁgurational f –f transitions of Eu3+, albeit with comparatively weak relative intensity, 3+ 2 3+ 1 the bands in the UV region of light belonging to Eu –O − charge transfer and the Sb S0 3 3+ → P1 transitions can be seen. In addition, transitions characteristic for Tb are present which points to Tb3+ Eu3+ sensitization. → 3+ 3+ Excitation spectra monitored at the Tb emission for TbSb2O4Br as well as TbSb2O4Br:Eu are shown in Figure 12 (right). Both materials show the characteristic bands of Tb3+ which can be assigned to the 7F 5D and 7F 5D transitions. The presence of Tb3+ peaks in the excitation spectrum 6 → 3 6 → 4 Crystals 2020, 10, x FOR PEER REVIEW 13 of 23

TbSb2O4Br:Eu3+ when detecting the Eu3+ emission indicates the presence of the Tb3+ → Eu3+ energy transfer. When excited at 283 and 464 nm,, corresponding to 1S0 → 3P1 of Sb3+, charge transfer band, 7F0 → 5L6 and 7F0 → 5D2 of Eu3+, respectively (Figure 13), all Eu3+-containing compounds show emission from the 5D0 level of Eu3+ transitions. The low site symmetry of Eu3+ in the compound series increases the number of observable transitions [6]. The hypersensitive (or electric dipole) band 5D0 → 7F2 with five sublevels, and the magnetic dipole transition 5D0 → 7F1 with all three possible sublevels can be observed. The dominance of the 5D0 → 7F2 transition in the emission spectra relative to the 5D0 → 7F1 Crystalstransition2020, 10 is, 1089compatible with the location of Eu3+ in a low-symmetry site [59]. The asymmetry ratio,13 of 23 defined as (5D0 → 7F2)/(5D0 → 7F1), is a parameter that indicates the distortion of Eu3+ site with respect

to an inversion centre.3+ As expected, the asymmetry3+ ratio is significantly larger than 1 indicating3+ a 3+ of TbSb2O4Br:Eu when detecting the Eu emission indicates the presence of the Tb Eu low-symmetry site, in agreement with a site symmetry of 1 for Ln3+ in LnSb2O4Br [59,60]. → energyVery transfer. weak peaks corresponding to emission from higher levels are also visible: at 582 nm, When excited at 283 and 464 nm„ corresponding to 1S 3P of Sb3+, charge transfer band, attributable to the forbidden transition 5D1 → 7F0 and at 579 nm,0 →that probably1 corresponds to the 5D1 7F 5L and 7F 5D of Eu3+, respectively (Figure 13), all Eu3+-containing compounds show →0 7F3 transition6 and0 others2 even weaker in the 560–576 nm region [5]. The intensity of the 5D0 → 7F4 → 5→ 3+ 3+ emissiontransition from is higher the D than0 level the ofmagn Eu etictransitions. dipole band, The as already low site foun symmetryd in lanthanoid(III) of Eu in theorthoborates compound serieswith increasesCs sites symmetry the number [61] of or observable in the polyoxometalate transitions [6 ].Na The9[EuW hypersensitive10O36] · 14 H2 (orO with electric a distorted dipole) D band4d 5D 7F with ﬁve sublevels, and the magnetic dipole transition 5D 7F with all three possible symmetry0 → 2 [62]. 0 → 1 sublevels can be observed.

Ln3+ Figure 13. Left: emission spectra upon excitation at the charge-transfer band of the3+ matrix Figure 13. Left: emission spectra upon excitation at the charge-transfer band of3+ the7 Ln matrix5 (λex = (λex = 283 nm). Right: emission spectra excited in the characteristic Eu F0 D2 transiton 283 nm). Right: emission spectra excited in the characteristic Eu3+ 7F0 → 5D2 transiton →(λex = 464 nm). (λex = 464 nm).

3+ 3+ Due to the full Eu concentration,5 7 a partial quenching of Eu emission occurs in EuSb2O54Br. This7 The dominance of the D0 F2 transition in the emission spectra relative to the D0 F1 is confirmed not only by the short→ lifetime of 5D0 → 7F2 detected upon excitation at 464 nm (10.7 →µs), transition is compatible with the location of Eu3+ in a low-symmetry site [59]. The asymmetry ratio, but also by the absence of the transitions from levels higher than 5D0. As can be observed in the defined as (5D 7F )/(5D 7F ), is a parameter that indicates the distortion of Eu3+ site with respect excitation spectrum0 → 2 (Figure0 → 12,1 left) by detecting the most intense Eu3+ emission at 611 nm, no to an inversion centre. As expected, the asymmetry ratio is significantly larger than 1 indicating a 3+ low-symmetry site, in agreement with a site symmetry of 1 for Ln in LnSb2O4Br [59,60]. Very weak peaks corresponding to emission from higher levels are also visible: at 582 nm, attributable to the forbidden transition 5D 7F and at 579 nm, that probably corresponds to the 1 → 0 5D 7F transition and others even weaker in the 560–576 nm region [5]. The intensity of the 1 → 3 5D 7F transition is higher than the magnetic dipole band, as already found in lanthanoid(III) 0 → 4 orthoborates with Cs sites symmetry [61] or in the polyoxometalate Na [EuW O ] 14 H O with a 9 10 36 · 2 distorted D4d symmetry [62]. 3+ 3+ Due to the full Eu concentration, a partial quenching of Eu emission occurs in EuSb2O4Br. 5 7 This is confirmed not only by the short lifetime of D0 F2 detected upon excitation at 464 nm → 5 (10.7 µs), but also by the absence of the transitions from levels higher than D0. As can be observed Crystals 2020, 10, 1089 14 of 23

Crystals 2020, 10, x FOR PEER REVIEW 14 of 23 in the excitation spectrum (Figure 12, left) by detecting the most intense Eu3+ emission at 611 nm, noexcitation excitation transitions transitions are are present present in inthe the UV UV region, region, and and for for this this reason, reason, the the sample sample does does not not show any emission upon excitation at 257 nm and was not includedincluded in thethe CIECIE (Commission(Commission internationale de l’l’éclairage)éclairage) diagram.diagram. Nevertheless,Nevertheless, upon upon excitation excitation in in the the most most intense intense excitation excitation transition transition at 464 at nm,464 correspondingnm, corresponding to the 7toF the5 D7F0transition → 5D2 transition of Eu3+, EuSb of EuO 3+Br, showsEuSb2O the4Br characteristicshows the luminescencecharacteristic 0 → 2 2 4 featuresluminescence of Eu3 +features. This means of Eu that3+. This energy means migration that energy between migration ions in the between bulk is ions not completely in the bulk eﬃ iscient not uponcompletely excitation efficient at lower upon energy. excitation at lower energy. 3+3+ The emissionemission spectrumspectrum of of the the Tb Tb -based-based materials materials upon upon excitation excitation with withλex λ=ex 378= 378 nm nm are are shown shown in 2 4 5 5 4 7→ 7 56 5 4 →7 7 5 5 4 →7 7 4 5 5 4 → 7 3 Figurein Figure 14. 14. The The spectrum spectrum of TbSb of TbSb2O4BrO showsBr showsD4 D F6 ,FD, 4D F 5F, ,DD4 FF4,, D4 F3 transitions,transitions, → →3+3+ → → 2 4 33++ resulting overalloverall inin anan impressionimpression of of green green light. light. In In the the Eu Eu-doped-doped compound, compound, TbSb TbSb2OO4Br:EuBr:Eu ,, emission fromfrom EuEu33++ areare dominant dominant whilst those for Tb33++ areare hardly hardly visible. visible. Together Together with the presence of TbTb33++ excitation bands by monitoring thethe luminescenceluminescence ofof EuEu33++,, the strongstrong EuEu33++ emission at the expense ofof TbTb33++ emission, indicates the presence ofof notablenotable TbTb33++ → Eu33++ energyenergy transfer. transfer. Moreover, Moreover, → this can explain the red shift in the CIE diagram with respect to GdSb2O4Br:Eu3+3+: in TbSb2O4Br:Eu33++, this can explain the red shift in the CIE diagram with respect to GdSb2O4Br:Eu : in TbSb2O4Br:Eu , Sb33++ → EuEu3+3+ isis mediated mediated by by Tb Tb3+3+, ,that that in in turn turn transfers transfers almost almost all the excitationexcitation lightlight toto Eu Eu3+3+,, shifting → the final emission towards the red, while in GdSb2O4Br:Eu33++ some Sb33++ contribution is still present, the ﬁnal emission towards the red, while in GdSb2O4Br:Eu some Sb contribution is still present, giving a combination ofof bothboth redred andand blueblue contributions.contributions.

3+ 7 5 Figure 14.14. Tb 3+-based-based materials materials upon excitation atat 378378 nm,nm, correspondingcorresponding to to the the F7F6 → 5DD3 transition 6 → 3 of Tb3+3+. .The The inset inset in inthe the bottom bottom part part of th ofe the figure ﬁgure shows shows a magnification a magniﬁcation of Tb of3+ Tb emission3+ emission in the in Eu the3+- 3+ 3+ 3+ Eudoped-doped compound compound TbSb2 TbSbO4Br2 emphasizingO4Br emphasizing the less the intense less intense Tb transitions. Tb transitions.

Crystals 2020, 10, 1089 15 of 23

Analysis of the luminescence decays were performed in order to better investigate the interactions (i.e., radiative and non-radiative processes) involved between the emitting cations, such as energy transferCrystals 2020 (ET), 10 and, x FOR concentration PEER REVIEW quenching. 15 of 23 3+ 1 3 The Eu emission (611 nm), upon excitation with λex = 257 nm (corresponding to the S0 P1 3+ ex 1 0 →→3 1 electronicThe Eu transition emission of Sb (6113+) wasnm), probed upon excitation in order to with allow λ for = 257 a comparison nm (corresponding of the optical to the properties S P of electronic transition3+ of Sb3+) was probed3+ in order to allow for a comparison of the optical properties GdSb2O4Br:Eu and TbSb2O4Br:Eu (see also the CIE diagram, Figure 11, right). 3+ 3+ of GdSb2O4Br:Eu and TbSb2O4Br:Eu (see also the CIE diagram, Figure 11, right). 3+ Upon this excitation, an intensity rise time can be seen for both the decay curves of GdSb2O4Br:Eu Upon this excitation,3+ an intensity 3rise+ time can be seen for both the decay curves of and TbSb2O4Br:Eu , indicating that Eu -emitting levels are fed by energy transfer processes. GdSb2O4Br:Eu3+ and TbSb2O4Br:Eu3+, indicating that Eu3+-emitting levels are fed by energy transfer The corresponding lifetime values were calculated as a weighted average of the two components [63]. processes. The corresponding lifetime values were calculated as a weighted average of the two As previously discussed, a difference between excitation spectra of the two materials is noticeable components [63]. As previously discussed, a difference between excitation spectra of the two when monitoring the Eu3+ emission at 611 nm: In TbSb O Br:Eu3+ the main peak occurs at 488 nm, 3+ 2 4 3+ materials is noticeable when monitoring the Eu emission3 at+ 611 nm: In TbSb2O4Br:Eu the main peak followed by the peak at 378 nm, both belonging to Tb , while the bands3+ in the UV region have occurs at 488 nm, followed by the peak at 378 nm, both belonging3+ to Tb , while the bands in the UV3+ a minor importance. This demonstrates the major role of Tb in sensitizing3+ the emission of Eu , region have a minor importance. This demonstrates the major role of Tb in sensitizing3+ 3+ the emission allowing a color more shifted towards the red. By contrast, in GdSb2O4Br:Eu , Eu emission can of Eu3+, allowing a color more shifted towards the red. By contrast, in GdSb2O4Br:Eu3+, Eu3+ emission only be sensitized through Sb3+ energy transfer. This is related to the greater role played by the can only be sensitized through Sb3+ energy transfer. This is related to the greater role played by the blue component in the final emission, as shown in the different color coordinates in the CIE diagram. blue component in the final emission, as shown in the different color coordinates in the CIE diagram. (see Figure 11, right). (see Figure 11, right). Moreover, the lifetimes of both doped materials, TbSb O Br:Eu3+ and GdSb O Br:Eu3+, Moreover, the lifetimes of both doped materials, TbSb2O24Br:Eu4 3+ and GdSb2O4Br:Eu2 4 3+, are aredependent dependent on the on theexcitation excitation wavelength, wavelength, as an as example, an example, a comparison a comparison between between the decays the decays of of GdSb O Br:Eu3+ upon excitation in the 1S 3P electronic transition of Sb3+ at 257 nm vs. GdSb2O42Br:Eu4 3+ upon excitation in the 1S0 → 3P1 electronic0 → 1 transition of Sb3+ at 257 nm vs. excitation in excitation in the Eu3+ transition at 7F 5L at 393 nm is shown in Figure 15 (right). the Eu3+ transition at 7F0 → 5L6 at 393 nm0 → is shown6 in Figure 15 (right).

TbSb O Br:Eu3+ TbSb O Br:Eu3+ 2 4 2 4 3+ 3+ GdSb2O4Br:Eu GdSb2O4Br:Eu λ λ = 464 nm ex = 257 nm ex λ = 611 nm λ em em = 611 nm Intensity / a.u. Intensity Intensity / a.u. Intensity

0246 0 5 10 15 Time / ms Time / ms

Figure 15. Lifetime comparison upon excitation at 257 nm between the Eu3+ emission monitored at Figure 15. Lifetime comparison upon excitation at 257 nm between the Eu3+ emission monitored at λem = 611 nm in the different matrices (GdSb2O4Br and TbSb2O4Br, left); upon excitation at 464 nm em = 611 nm in the different matrices (GdSb2O4Br and TbSb2O4Br, left); upon excitation at 464 nm λ 3+ (middle); and for GdSb2O4Br:Eu upon excitation at λex = 257 nm and λex = 393 nm (right). All the (middle); and for GdSb2O4Br:Eu3+ upon excitation at λex = 257 nm and λex = 393 nm (right). All the intensity decays are reported on a logarithm (log) intensity scale. intensity decays are reported on a logarithm (log) intensity scale. Eu3+ lifetime corresponding to the main emission at 611 nm was also evaluated upon excitation 3+ Eu lifetime corresponding to the7 main 5emission at3+ 611 nm was also evaluated3+ upon excitation in the most intense band at 464 nm ( F0 D2 of Eu ). In TbSb2O4Br:Eu , a single exponential in the most intense band at 464 nm (7F0 → 5D→2 of Eu3+). In TbSb2O4Br:Eu3+, a single exponential intensity intensity decay was observed and an excited state lifetime of about 0.8 ms could be calculated. decay was observed and an excited state lifetime of about 0.8 ms could be calculated. The single3 + The single exponential behavior reflects the occupation of only one crystallographic3+ site by Eu . exponential behavior reflects the occupation of only one crystallographic3+ site by Eu . The observed The observed lifetime is similar to the value observed3+ for Eu in zirconia samples [56], as well as lifetime is similar to the value observed for Eu in zirconia samples [56], as well as with antimony–3+ with antimony–germanate–silicate glasses [63]. However, for excitation of GdSb2O4Br:Eu with germanate–silicate glasses [63]. However, for excitation of GdSb2O4Br:Eu3+ with 464 nm, the decay 464 nm, the decay curve needed to be fitted bi-exponential (Figure 15, middle and fitted components in curve needed to be fitted bi-exponential (Figure 15, middle and fitted components in Table A5), 3+ Tablepointing A5), to pointing two different to two Eu diff3+erent emitting Eu species,emitting either species, with either different with environment, different environment, potentially potentially Eu3+ in Eu3+ in the bulk and Eu3+ as the surface (note that it was not possible to obtain single crystals of the bulk and Eu3+ as the surface (note that it was not possible to obtain single crystals of GdSb2O4Br GdSbof sufficient2O4Br ofquality sufficient for single quality crystal for single X-ray crystal diffraction X-ray (SCXRD) diffraction analysis). (SCXRD) analysis). The efficiency of Tb3+ Eu3+ was evaluated through lifetime analysis, by using the formula: The efficiency of Tb3+ →→ Eu3+ was evaluated through lifetime analysis, by using the formula: –1 ηET = 1 – τDA τD 1 (2) ηET = 1 τDA τD− (2) where τDA is the lifetime of the donor (Tb3+) in presence− of the acceptor (Eu3+), and τD is the lifetime of the donor in the undoped material. Thus, the emission decays corresponding to the Tb3+ emission from the 5D4 level were acquired by exciting into the most intense excitation band, namely 483 nm (corresponding to the 7F6 → 5D4 transition), for both TbSb2O4Br and TbSb2O4Br:Eu3+ (Figure 16). The undoped sample shows a double exponential decay. The total lifetime of 0.25 ms can be calculated as a weighted average of the two

Crystals 2020, 10, 1089 16 of 23

3+ 3+ where τDA is the lifetime of the donor (Tb ) in presence of the acceptor (Eu ), and τD is the lifetime of the donor in the undoped material. 3+ 5 Thus, the emission decays corresponding to the Tb emission from the D4 level were acquired 7 5 by exciting into the most intense excitation band, namely 483 nm (corresponding to the F6 D4 Crystals 2020, 10, x FOR PEER REVIEW 3+ → 16 of 23 transition), for both TbSb2O4Br and TbSb2O4Br:Eu (Figure 16). The undoped sample shows a double exponential decay. The total lifetime of 0.25 ms can be calculated as a weighted average of the two 3+ componentscomponents as reported as reported by byZm Zmojdaojda et et al. al. [63], [63], while while thethe EuEu3+-doped-doped sample sample shows shows a value a value of 12.3 of 12.3µs, µs, fittedﬁtted by a bysingle a single exponential exponential decay. decay. The The ηηETET obtainedobtained from thesethese values values is is around around 95%. 95%.

3+ FigureFigure 16. Lifetimes 16. Lifetimes of ofterbium terbium based based compounds. compounds. TheThe Tb Tb3+emission emission at atλem λem= 541= 541 nm nm is monitored is monitored 3+ upon excitation at λexc = 483 nm for both the Eu3+ -doped and the undoped TbSb O Br samples. upon excitation at λexc = 483 nm for both the Eu -doped and the undoped TbSb2 2O4 4Br samples. 4. Conclusions 4. Conclusions LnSb2O4Br series (Ln = Eu–Tb) presents a new material class of quaternary lanthanoid(III) oxoantimonate(III)LnSb2O4Br series ( halides.Ln = Eu‒ WithTb) presents respect toa new the hithertomaterial reported class of quaternary quaternary samarium(III) lanthanoid(III) oxoantimonate(III)oxoantimonate(III) halides. halides, theWith presented respect compounds to the showhitherto structural reported similarities, quat suchernary as thesamarium(III) eightfold oxoantimonate(III)oxygen-coordination halides, sphere the of thepresented lanthanoid(III) compound cations,s butshow no mixed-occupied structural simi siteslarities, for the Lnsuch3+ and as the 3+ eightfoldSb cations.oxygen-coordination Besides La5F3[SbO sphere3]4, the of reported the lanthanoid(III) compounds can cations, be viewed but asno a mixed-occupied new composition typesites for in the quaternary systems Ln(III)–Sb(III)–O–X without mixed-occupation sites so far. As important the Ln3+ and Sb3+ cations. Besides La5F3[SbO3]4, the reported compounds can be viewed as a new structural features, the lanthanoid(III) cations are coordinated exclusively by eight oxygen atoms in composition type in the13- quaternary systems Ln(III)‒Sb(III)‒O‒X without mixed-occupation sites so the shape of [LnO8] hemiprisms. As second structural feature worth mentioning, the antimony(III) far. As important structural features, the lanthanoid(III) cations are3- coordinated exclusively by eight cations build up meandering strands of vertex-connected [SbO3] anions and, therefore, show a 13‒ oxygenremarkable atoms in di theﬀerence shape to of the [Ln alreadyO8] reportedhemiprisms. lanthanoid(III) As second halide struct oxopnictogenates(III)ural feature worth mentioning, such as the antimony(III) cations build up meandering strands of vertex-connected [SbO3]3‒ anions and, La5F3[SbO3]4 or Ln3X2[As2O5][AsO3], where mostly isolated or vertex-shared [PnO3] − anions as therefore,dimers areshow present. a remarkable difference to the already reported lanthanoid(III) halide oxopnictogenates(III) such as La5F3[SbO3]4 or Ln3X2[As2O5][AsO3], where mostly isolated or vertex- shared [PnO3]3‒ anions as dimers are present. Whilst the bulk europium compound, EuSb2O4Br, is not suitable as luminescent material, both GdSb2O4Br and TbSb2O4Br doped with Eu3+ show strong photoluminescence thanks to Sb3+ which is acting as a blue-emitting species and sensitizer for lanthanide. TbSb2O4Br:Eu additionally benefits form strong energy transfer from Tb3+ to Eu3+. Since Sb3+ is emitting in the blue, Tb3+ in the green and Eu3+ in the red white-light emitting materials can be envisioned in this class of compound.

Author Contributions: F.C.G. synthesized the title compounds LnSb2O4Br (Ln = Eu‒Tb). F.C.G. and T.S. solved their crystal structures. Funds provided to T.S. funded the chemicals, the materials, the scientific equipment and the infrastructure for synthesis, single-crystal and powder X-ray diffraction measurements as well as electron- probe microanalysis. F.C.G. investigated the LnSb2O4Br series (Ln = Eu‒Tb) via electron-beam X-ray microprobe techniques. V.P. performed the photoluminescence measurements with the scientific equipment provided by support to A.-V.M. V.P. and K.V.D. were supported by fellowships provided by the Göran Gustafsson prize in Chemistry, Royal Swedish Academy of Sciences, to A.-V.M. V.P. and A.-V.M. interpreted the the data. F.C.G., V.P., K.V.D. and A.-V.M. wrote the initial paper, A.-V.M. and T.S. reviewed it carefully. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the State of Baden-Württemberg (Stuttgart).

Crystals 2020, 10, 1089 17 of 23

Whilst the bulk europium compound, EuSb2O4Br, is not suitable as luminescent material, 3+ 3+ both GdSb2O4Br and TbSb2O4Br doped with Eu show strong photoluminescence thanks to Sb which is acting as a blue-emitting species and sensitizer for lanthanide. TbSb2O4Br:Eu additionally beneﬁts form strong energy transfer from Tb3+ to Eu3+. Since Sb3+ is emitting in the blue, Tb3+ in the green and Eu3+ in the red white-light emitting materials can be envisioned in this class of compound.

Author Contributions: F.C.G. synthesized the title compounds LnSb2O4Br (Ln = Eu–Tb). F.C.G. and T.S. solved their crystal structures. Funds provided to T.S. funded the chemicals, the materials, the scientific equipment and the infrastructure for synthesis, single-crystal and powder X-ray diffraction measurements as well as electron-probe microanalysis. F.C.G. investigated the LnSb2O4Br series (Ln = Eu–Tb) via electron-beam X-ray microprobe techniques. V.P. performed the photoluminescence measurements with the scientific equipment provided by support to A.-V.M., V.P. and K.V.D. were supported by fellowships provided by the Göran Gustafsson prize in Chemistry, Royal Swedish Academy of Sciences, to A.-V.M., V.P. and A.-V.M. interpreted the the data. F.C.G., V.P., K.V.D. and A.-V.M. wrote the initial paper, A.-V.M. and T.S. reviewed it carefully. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the State of Baden-Württemberg (Stuttgart). Acknowledgments: The authors would like to thank Falk Lissner for the single-crystal X-ray diffraction measurements. A.-V.M. acknowledges the Royal Swedish Academy of Sciences for support through the Göran Gustafsson prize in Chemistry and Stockholm University. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A

Table A1. Crystallographic data of the end-members of the LnSb2O4Br series (Ln = Eu and Tb).

Formula EuSb2O4Br TbSb2O4Br Crystal system monoclinic Space group P21/c (no. 14) Lattice constants, a/pm 895.69(6) 895.37(6) b/pm 791.82(5) 786.14(5) c/pm 790.38(5) 785.09(5) β/ ◦ 91.817(3) 91.638(3) Formula units, Z 4 –3 Calculated density, Dx/g cm 6.394 6.569 3 · –1 Molar volume, Vm/cm mol 84, 36 83, 17 · Diffractometer StadiVari (STOE, four-circle diffractometer) Wavelength (λ/pm) 71.07 (Mo-Kα) F(000) 928 936 2θmax/◦ 63.58 63.72 hkl range ( hmax, kmax, lmax) 13, 11, 11 12, 11, 11 ± ± ± Observed reflections 19853 12173 Unique reflections 1840 1438 Absorption coefficient, µ/mm–1 27.68 29.52 Absorption correction numerical (Stoe X-Shape 2.21) Rint/Rσ 0.076/0.041 0.078/0.045 R /R with |F | 4σ(F ) 0.055/0.034 0.047/0.033 1 1 O ≥ O wR2/GooF 0.082/1.047 0.071/1.019 Structure determination and refinement Program package ShelX-1997 [31,32] Extinction coefficient, ε/pm–3 0.00012(9) 0.00022(9) Residual electron density, +1.91/ 1.78 +2.02/ 1.95 ρ/e– 10–6 pm–3 − − CCDC number CSD-2016635 CSD-2016636 Crystals 2020, 10, 1089 18 of 23

Table A2. Fractional atomic coordinates and coeﬃcients of the equivalent isotropic displacement

parameters of EuSb2O4Br (top) and TbSb2O4Br (bottom). All atoms occupy the general Wyckoﬀ site 4e.

2 Atom x/a y/b z/c Ueq/pm Eu 0.48857(4) 0.23760(5) 0.50274(5) 104(1) Sb1 0.78149(6) 0.05812(7) 0.75756(7) 108(1) Sb2 0.21659(6) 0.00677(7) 0.79543(7) 103(1) O1 0.6332(6) 0.0021(7) 0.5856(7) 147(12) O2 0.3642(6) 0.1758(7) 0.7481(7) 123(11) O3 0.6656(6) 0.0086(7) 0.9703(7) 112(11) O4 0.6686(6) 0.2105(7) 0.2594(7) 107(10) Br 0.02117(11) 0.23521(12) 0.50388(12) 205(2) Tb 0.48990(4) 0.23728(5) 0.50185(5) 85(1) Sb1 0.78039(6) 0.05548(7) 0.75626(7) 92(2) Sb2 0.21888(6) 0.00810(7) 0.79380(7) 90(1) O1 0.6321(6) 0.0028(7) 0.5804(7) 128(14) O2 0.3675(6) 0.1777(7) 0.7465(7) 111(13) O3 0.6617(6) 0.0069(7) 0.9696(7) 100(13) O4 0.6686(6) 0.2102(7) 0.2597(7) 118(13) Br 0.01915(12) 0.23617(11) 0.50403(11) 188(2)

TableA3. Selected interatomic distances (d/pm, e.s.d.: 0.5 pm) for EuSb O Br (left) and TbSb O Br (right). ± 2 4 2 4

Contact EuSb2O4Br TbSb2O4Br Ln–O1 228.7 226.6 Ln–O10 235.2 231.4 Ln–O2 231.9 228.7 Ln–O20 237.1 235.4 Ln–O3 256.4 253.0 Ln–O30 257.7 254.9 Ln–O4 255.8 252.9 Ln–O40 258.4 257.7

Sb1–O1 192.2 193.3 Sb1–O3 204.5 204.5 Sb1–O4 209.3 209.7

Sb2–O2 192.6 192.7 Sb2–O3 210.3 212.0 Sb2–O4 205.7 204.0

Br–Sb1 328.8 327.9 Br–Sb1‘ 329.7 328.6 Br–Sb1“ 360.6 358.7 Br–Sb1“‘ 360.8 358.8 Br–Sb2 318.4 318.3 Br–Sb2‘ 337.5 337.0 Br–Sb2“ 344.8 343.9 Br–Sb2“‘ 367.2 366.2 Crystals 2020, 10, 1089 19 of 23

3+ Table A4. Quantitative electron-beam microprobe analysis of TbSb2O4Br doped with 1.5 at-% Eu using wavelength-dispersive spectrometry and the Pouchou and Pichoir (PaP) matrix correction algorithm. The oxygen content was calculated stoichiometrically.

Ion Emission Line Content (wt-%) Normalized Content (at-%) Theoretical Content (at-%) 3+ Tb Lα 28.0(8) 12.3(3) 12.31 3+ Eu Lα 0.43(7) 0.20(3) 0.19 3+ Sb Lα 43.2(7) 25.0(4) 25.00 Br− Kα 14.4(7) 12.6(6) 12.50 2 O − – 11.6(9) 49.9(9) 50.00 Crystals 2020, 10, x FOR PEER REVIEW 19 of 23 Crystals 2020, 10, x FOR PEER REVIEW 19 of 23

FigureFigureFigure A1. A1.A1. AppliedApplied Applied multi-stage multi-stage multi-stage furnacefurnace furnace program program program for for the for synthesisthe the synthesis synthesis of single of of crystalssingle single crystals withcrystals the with compositionwith the the compositioncomposition Ln SbLn2SbO42BrO4 Br(Ln ( Ln= Eu = Eu‒Tb).‒Tb). LnSb2O4Br (Ln = Eu–Tb).

(a)( a) (b()b )

FigureFigure A2.A2. MeasuredMeasured ((blueblue)) andand correspondingcorresponding simulatedsimulated (red(red)) powder powder X-ray X-ray pattern pattern of of EuSb EuSbO2OBr4Br (a) Figure A2. Measured (blue) and corresponding simulated (red) powder X-ray pattern of EuSb2O44Br and(a) and TbSb TbSbO Br2O4 (Brb). ( Whileb). While a high a high accordance accordance of the of measured the measur anded calculated and calculated reflection reflection positions positions is given, (a) and TbSb22O44Br (b). While a high accordance of the measured and calculated reflection positions is given,textureis given, texture eff textureects effects probably effects probably dueprobably to thedue due crystal to tothe the habitcrysta crysta generatel habitl habit generate visiblegenerate divisible ffvisibleerences di differences infferences the relative in in the the intensities.relative relative intensities. intensities.

Crystals 2020, 10, 1089 20 of 23 Crystals 2020, 10, x FOR PEER REVIEW 20 of 23

TableTable A5. ComponentsComponents and and the the average average time time of the of multi-exponential the multi-exponential decays reporteddecays inreported the manuscript. in the manuscript.For the single-exponential For the single-exponential decays, the decays, ﬁtted lifetime the fitted is reportedlifetime is as repo onlyrted the as average only the value. average value.

SampleSample λex/λnmex/nm λem/λemnm/nm A A11 ττ11//s A2A2 τ2/sτ2/s τavgτavg//s s TbSb2O4Br:Eu3+ 3+ 257 611 34.11 3.08 × 10−3 65.89 1.09 × 103 −3 2.3 × 103 −3 TbSb2O4Br:Eu 257 611 34.11 3.08 10− 65.89 1.09 10− 2.3 10− × × × 3 −3 464464 611 611 0.80.810 × −10 × 6 483483 541 541 12.312.310 × −10−6 × 3+ 3+ −3 3 −3 3 −3 GdSbGdSb2O42Br:EuO4Br:Eu 257257 611 611 22.88 22.88 4.124.12 × 1010− 77.1277.12 1.041.0410 ×− 10 3.53.510 × −10 × 3 × 4 × 3 464464 611 611 33.45 33.45 2.872.87 × 1010−−3 66.5566.55 7.747.7410 ×− 10−4 2.12.110 × −10−3 × 4 × 3 × 3 393 611 64.87 9.06 10 −4 35.13 4.65 10 −3 3.6 10 −3 393 611 64.87 9.06× × 10− 35.13 4.65× ×− 10 3.6× × −10 4 4 3 TbSbTbSb2O4BrO Br 483 483 541 541 22.54 22.54 1.39 × 1010−4 74.4674.46 2.742.7410 × 10−4 0.250.2510 × 10−3 2 4 × − × − × −

3+ TbSb O Br:Eu3+ TbSb O Br:Eu3+ TbSb2O4Br:Eu 2 4 2 4 fit fit fit λ λ = 464 nm λ = 483 nm ex = 257 nm ex ex λ λ = 611 nm λ = 541 nm em = 611 nm em em Intensity / a.u. / Intensity Intensity / a.u. / Intensity Intensity / a.u. Intensity

0 5 10 15 20 02468100 5 10 15 20 ms Time / ms Time / ms

GdSb O Br:Eu3+ 3+ GdSb O Br:Eu3+ 2 4 GdSb2O4Br:Eu 2 4 fit fit fit λ λ λ = 393 nm ex = 257 nm ex = 464 nm ex λ λ λ = 611 nm em = 611 nm em = 611 nm em Intensity / a.u. Intensity / a.u. Intensity /a.u.

0 102030010200 102030 Time / ms Time / ms Time / ms

TbSb2O4Br fit λ ex = 483 nm λ em = 541 nm

Intensity / a.u.

012345 Time / ms FigureFigure A3. A3. IntensityIntensity decay decay curves curves as as measured measured (black) (black) and and fitted ﬁtted decay decay curves curves (red). (red).

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