Structural Chemistry of Halide Including Thallides A8tl11x1− N

crystals

Article Structural Chemistry of Halide including Thallides A8Tl11X1−n (A = K, Rb, Cs; X = Cl, Br; n = 0.1–0.9)

Stefanie Gärtner 1,2,* ID , Susanne Tiefenthaler 1, Nikolaus Korber 1 ID , Sabine Stempfhuber 2 and Birgit Hischa 2 1 Institute of Inorganic Chemistry, University of Regensburg, 93040 Regensburg, Germany; [email protected] (S.T.); [email protected] (N.K.) 2 Central Analytics, X-ray Crystallography Dept., University of Regensburg, 93040 Regensburg, Germany; [email protected] (S.S.); [email protected] (B.H.) * Correspondence: [email protected]; Tel.: +49-941-943-4446

Received: 23 July 2018; Accepted: 7 August 2018; Published: 10 August 2018

Abstract: A8Tl11 (A = alkali metal) compounds have been known since the investigations of Corbett et al. in 1995 and are still a matter of current discussions as the compound includes one extra 7− electron referred to the charge of the Tl11 cluster. Attempts to substitute this additional electron by incorporation of a halide atom succeeded in the preparation of single crystals for the lightest triel homologue of the group, Cs8Ga11Cl, and powder diffraction experiments for the heavier homologues also suggested the formation of analogous compounds. However, X-Ray single crystal studies on A8Tl11X to prove this substitution and to provide a deeper insight into the inﬂuence on the thallide substructure have not yet been performed, probably due to severe absorption combined with air and moisture sensitivity for this class of compounds. Here, we present single crystal X-Ray structure analyses of the new compounds Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5Rb3Tl11Cl0.5, Cs5.7K2.3Tl11Cl0.6 and K4Rb4Tl11Cl0.1. It is shown that a (partial) incorporation of halide can also be indirectly determined by examination of the Tl-Tl distances, thereby the newly introduced cdd/cdav ratio allows to evaluate 7− the degree of distortion of Tl11 clusters.

Keywords: thallide; intermetallics; single crystal; X-ray structure analysis

1. Introduction Naked cluster anions of the main group elements are well-known for group 14 and 15 elements in solid-state [1–4]. Most of these compounds can be described in terms of the Zintl-Klemm concept [5–7] by formally transferring the valence electrons of the electropositive element to the electronegative under formation of salt-like structures, so called polyanionic salts. Homoatomic group 14 or 15 element 4− polyanions are known since Zintl himself in 1930 stated the existence of Pb9 during potentiometric titrations in liquid ammonia solutions [5]. In contrast, the existence of naked group 13 element clusters is not self-evident due to lower values for the electron affinity of group 13 elements which results in a predominantly metallic character of the analogous compounds [3,8]. The first naked thallium cluster was described in 1967 by Hansen and Smith in the binary solid-state compound Na2Tl [9], which contains Tl4 tetrahedra with a calculated formal charge of −8 by assuming complete electron transfer. These tetrahedral assemblies are related to the structures of ATt (A: alkali metal, Tt: group 14 element) [10–12] and white phosphorus due to their formal iso-(valence)-electronic character. The 7− largest (empty) thallide cluster is represented by the Tl11 cluster which is present in binary materials A8Tl11 [13,14] and A15Tl27 [15] (A = K, Rb, Cs). The A8Tr11 (Tr = group 13 element) structure type was first described in 1991 for the lighter homologue indium in K8In11 [16], of which the crystal structure proved the presence of a naked, pentacapped trigonal prismatic shaped In11 cluster, which was

Crystals 2018, 8, 319; doi:10.3390/cryst8080319 www.mdpi.com/journal/crystals Crystals 2018, 8, 319 2 of 10 assigned a charge of −7. Additionally, one extra-electron per formula unit is present, being responsible for the metallic character. The additional electron, referred to the charge of −7 of the cluster anion, is not necessary for the stability of the clusters [17] and can be replaced by halide atoms, which are located on a −3 void (Wyckoff position 6b) at the origin of the unit cell resulting in a diamagnetic character of the compounds. Halide incorporation was proven for the lighter homologue of the group, Cs8Ga11Cl by X-ray single crystal structure analysis [18]. Powder diffraction experiments suggested the formation of the heavier homologues Rb8Ga11Cl, Cs8Ga11X (X = Br, I), Rb8In11Cl, Cs8In11Cl, Cs8Tl11X (X = Cl, Br, I). Recently, continuative studies on halides A8Tr11 (Tr = Ga, In) have been reported [19]. However, the formation of Rb8Tl11Cl was termed as doubtful due to the lack of a signiﬁcant change in the lattice constants compared to the binary phase Rb8Tl11, which also is a common problem for the remaining halide including thallides of this structure family. Therefore, well-resolved single crystal X-ray diffraction studies should provide a deeper insight into the involvement and the role of halide in A8Tl11X compounds. Thereby, we concentrated on the heavier alkali metals K, Rb and Cs as for sodium no experimental evidence of Tl11 clusters is reported. The questions we wanted to answer were: (1) How does the geometry of the thallide cluster change on halide incorporation; (2) Is there a Rb8Tl11Cl? (3) How do mixed cation sites affect the amount of halide incorporation? In Section3 (Results), we report on the ﬁrst single crystal X-Ray structure determination of halide including thallides, Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5Rb3Tl11Cl0.5, Cs5.7K2.3Tl11Cl0.6 and K3.98Rb4.02Tl11Cl0.1. Subsequently, (Section4, Discussions), the crystal structures are investigated according to the questions listed above.

2. Materials and Methods All compounds have been synthesized via a stoichiometric approach using high temperature solid state techniques. Cesium and rubidium were produced by the reduction of the corresponding alkali metal halide with elemental calcium [20] and distilled twice, potassium was segregated for purification. Thallium lumps have been stored under inert atmosphere and were used without further purification. The starting materials were enclosed in tantalum crucibles (for stoichiometric approaches see AppendixA) which were subsequently placed in quartz glass ampoules and sealed under argon atmosphere. The same temperature program was used for all compounds: Heating to 700 ◦C with a heating rate of 50 ◦C/h, holding for 24 h, cooling to room temperature with a cooling rate of 3 ◦C/h to allow for crystallization. All compounds are very sensitive towards moisture and oxygen and degeneration of the crystals was observed (gassing) in dried mineral oil within few hours. Suitable single crystals for X-ray structure analysis were isolated in dried mineral oil and mounted on a Rigaku SuperNova (Rigaku Polska Sp. Z o. o. Ul, Wroclaw, Poland) (Mo-source, Eos detector) using MiTeGen loops. Thereby, the transfer needed to be very quick as the crystals started to decompose as soon as the mineral oil film became too thin. Once being placed on the diffractometer in the nitrogen stream at 123 K the crystals remained stable and data collection was possible. Powder diffraction samples were measured in sealed capillaries (0.3–0.5 mm) on a Powder on a STOE Stadi P diffractometer (STOE, Darmstadt, Germany) (monochromatic Mo-Kα1 radiation λ = 0.70926 Å) equipped with a Dectris Mythen 1 K detector.

3. Results

All compounds crystallize in the K8In11 structure type (rhombohedral, spacegroup R−3c) and especially for the mixed alkali metal compounds many of the crystals happened to form typical “multicrystals”. Due to the presence of reverse/obverse twinning a R(obv) ﬁlter was applied during data reduction [21]. The materials naturally possess very high absorption coefﬁcients (µ > 60 mm−1), therefore small single crystals have been chosen for the X-ray analyses. However, the data sets Crystals 2018, 8, 319 3 of 10 still suffer from severe absorption effects which could be reduced by carefully applying numerical absorption correction [21]. Thereby, the adjustment of the correct shape played a dominant role. Table1 lists the data for the structure determination. For the chloride including compounds two additional, unresolved but several times reproduced residual electron density peaks (≈1.5 Å beside the chlorine atom, ≈2.2 Å beside cesium; along the c-axis) are present, which we attribute to unresolved absorption effects as this direction is along the thinnest direction of the plate like crystals. For the bromine including compound this effect is not as dominant as for the chlorine including ones but still is observed.

Table 1. Crystal data and structure reﬁnement details.

Compound Cs8Tl11Cl0.80 Cs8Tl11Br0.92 Cs5.13Rb2.87Tl11Cl0.49 Cs5.67K2.33Tl11Cl0.60 K3.98Rb4.02Tl11Cl0.1 CSD number * 434541 434540 434539 434538 1856564 Mr [g·mol−1] 3339.71 3385.20 3192.52 3114.02 2751.04 Crystal system Trigonal Trigonal Trigonal Trigonal Trigonal Space group R-3c R−3c R-3c R-3c R-3c a [Å] 10.4691 (4) 10.5608 (3) 10.3791 (5) 10.3291 (9) 10.0948 (4) b [Å] 10.4691 (4) 10.5608 (3) 10.3791 (5) 10.3291 (9) 10.0948 (4) c [Å] 53.297 (3) 53.401(2) 52.437 (3) 51.909 (5) 51.0274 (18) α [◦] 90 90 90 90 90 β [◦] 90 90 90 90 90 γ [◦] 120 120 120 120 120 V [Å3] 5058.8 (5) 5157.9 (4) 4892.0 (5) 4796.3 (9) 4503.3 (4) Z 6 6 6 6 6 F(000) 8068.0 8180.0 7726.0 7544.0 6703.0 −3 ρcalc [g·cm ] 6.578 6.539 6.502 6.469 6.087 µ [mm−1] 60.902 60.745 64.052 61.908 65.822 2θ-range for data 7.59 to 58.982 7.558 to 69.266 7.694 to 54.202 7.758 to 54.198 7.91 to 69.18 collection [◦] Reﬂections 79177/1473 12097/2374 3174/1194 3394/1176 5040/1934 collected/independent Data/restraints/parameters 1473/0/36 2374/0/36 1194/0/34 1176/0/34 1934/1/37 Goodness-of-ﬁt on F2 1.244 1.136 1.089 1.148 1.033 R1 = 0.0280 R1 = 0.0242 R1 = 0.0392 R1 = 0.0456 R1 = 0.0242 Final R indices [I > 2σ(I)] wR2 = 0.0619 wR2 = 0.0528 wR2 = 0.0928 wR2 = 0.0996 wR2 = 0.0461 R1 = 0.0309 R1 = 0.0280 R1 = 0.0466 R1 = 0.0566 R1 = 0.0292 R indices (all data) wR2 = 0.0629 wR2 = 0.0541 wR2 = 0.0970 wR2 = 0.1034 wR2 = 0.0478 Rint 0.0497 0.0385 0.0446 0.0449 0.0280 Largest diff. peak/hole 2.96/−1.38 1.83/−3.38 4.82/−2.29 3.62/−2.21 1.48/−1.67 [e·Å−3] * Further details of the crystal structure investigation(s) may be obtained from The Cambridge Crystallographic data centre CCDC on quoting the deposition number CSD-xxxxxx or the the deposition number CCDC-xxxxxxx at https://www.ccdc.cam.ac.uk/structures/?

With only cesium being present in Cs8Tl11Cl0.8 and Cs8Tl11Br0.9 we obtained phase pure materials according to the powder diffraction pattern of the bulk material (Figure1; reﬁned cell contstants at room temperature: Cs8Tl11Cl0.8: a = 10.566 (5) Å, c = 53.67 (3) Å, R−3c; Cs8Tl11Br0.9: a = 10.613 (3) Å, c = 53.680 (19) Å). The well-crystallized Cs8Tl11X crystals and the resulting good quality single crystal diffraction data allowed the splitting of one alkali metal position according to the site occupancy factor (s.o.f.) of the halide atom (see Section 4.3). For the mixed alkali metal compounds, we always additionally observed less reduced A15Tl27 phases as a side product. This observation became reasonable when we determined the NE value (number of electrons per thallium atom) which sums up to a value of 8/11 = 0.72 for A8Tl11, 15/27 = 0.55 for A15Tl27 and 7/11 = 0.63 for A8Tl11X. The formation of less reduced A15Tl27 were completely comprehensible if the higher reduced A8Tl11Xx (x << 1) phases would have formed when the halide content was signiﬁcantly less than 1, because the overall degree of reduction was given by the stoichiometric approach for A8Tl11X. If less halide was incorporated, this is according to a higher degree of reduction and consequently, a less reduced phase was formed in addition. The remaining halide re-crystallized as (mixed) AX, of which we also could observe single crystals. The crystals for the mixed alkali metal compounds A8Tl11Xx happened to form multicrystals together with A15Tl27 Crystals 2018, 8, x FOR PEER REVIEW 3 of 10

Table 1 lists the data for the structure determination. For the chloride including compounds two additional, unresolved but several times reproduced residual electron density peaks (≈1.5 Å beside the chlorine atom, ≈2.2 Å beside cesium; along the c-axis) are present, which we attribute to unresolved absorption effects as this direction is along the thinnest direction of the plate like crystals. For the bromine including compound this effect is not as dominant as for the chlorine including ones but still is observed.

Table 1. Crystal data and structure refinement details.

b [Å] 10.4691 (4) 10.5608 (3) 10.3791 (5) 10.3291 (9) 10.0948 (4)

c [Å] 53.297 (3) 53.401(2) 52.437 (3) 51.909 (5) 51.0274 (18) α [°] 90 90 90 90 90 β [°] 90 90 90 90 90 γ [°] 120 120 120 120 120 V [Å3] 5058.8 (5) 5157.9 (4) 4892.0 (5) 4796.3 (9) 4503.3 (4) Z 6 6 6 6 6 F(000) 8068.0 8180.0 7726.0 7544.0 6703.0 ρcalc [g·cm−3] 6.578 6.539 6.502 6.469 6.087 μ [mm−1] 60.902 60.745 64.052 61.908 65.822 2θ-range for data 7.59 to 58.982 7.558 to 69.266 7.694 to 54.202 7.758 to 54.198 7.91 to 69.18 collection [°] Reflections 79177/1473 12097/2374 3174/1194 3394/1176 5040/1934 collected/independent Data/restraints/paramete 1473/0/36 2374/0/36 1194/0/34 1176/0/34 1934/1/37 rs Goodness-of-fit on F2 1.244 1.136 1.089 1.148 1.033 R1 = 0.0280 R1 = 0.0242 R1 = 0.0392 R1 = 0.0456 R1 = 0.0242 Final R indices [I > 2σ(I)] wR2 = 0.0619 wR2 = 0.0528 wR2 = 0.0928 wR2 = 0.0996 wR2 = 0.0461 R1 = 0.0309 R1 = 0.0280 R1 = 0.0466 R1 = 0.0566 R1 = 0.0292 R indices (all data) wR2 = 0.0629 wR2 = 0.0541 wR2 = 0.0970 wR2 = 0.1034 wR2 = 0.0478 Rint 0.0497 0.0385 0.0446 0.0449 0.0280 Largest diff. peak/hole 2.96/−1.38 1.83/−3.38 4.82/−2.29 3.62/−2.21 1.48/−1.67 [e·Å−3] * Further details of the crystal structure investigation(s) may be obtained from The Cambridge CrystalsCrystallographic2018, 8, 319 data centre CCDC on quoting the deposition number CSD-xxxxxx or the the 4 of 10 deposition number CCDC-xxxxxxx at https://www.ccdc.cam.ac.uk/structures/?

and for the reported single crystals (except K4Rb4Tl11Cl0.1) the data quality was worse compared With only cesium being present in Cs8Tl11Cl0.8 and Cs8Tl11Br0.9 we obtained phase pure materials to Cs Tl X phases. Therefore, the splitting of the alkali metal position could only be observed for according8 11 to the powder diffraction pattern of the bulk material (Figure 1; refined cell contstants at K4Rb4Tl11Cl0.1, for the remaining mixed alkali compounds splitting positions could not be reasonably room temperature: Cs8Tl11Cl0.8: a = 10.566 (5) Å, c = 53.67 (3) Å, R−3c; Cs8Tl11Br0.9: a = 10.613 (3) Å, c = introduced. In these cases, we only reﬁned the s.o.f. of the halide (see Section 4.3). 53.680 (19) Å).

Crystals 2018, 8, x FOR PEER REVIEW(a) (b) 4 of 10

Figure 1. Measured (a) and calculated (c) powder diffraction patterns of Cs8Tl11Cl0.8; Measured (b) Figure 1. Measured (a) and calculated (c) powder diffraction patterns of Cs8Tl11Cl0.8; Measured (b) and and calculated (d) powder diffraction patterns of Cs8Tl11Br0.9 (diffractograms generated by the calculated (d) powder diffraction patterns of Cs8Tl11Br0.9 (diffractograms generated by the program program STOE WinXPOW adapted from [22], with permission from © 1887 STOE. STOE WinXPOW [22]).

The well-crystallized Cs8Tl11X crystals and the resulting good quality single crystal diffraction 4. Discussions data allowed the splitting of one alkali metal position according to the site occupancy factor (s.o.f.) of the4.1. halide HowDoes atom the (see Geometry Sectionof 4.3). the Thallide Cluster Change on Halide Incorporation? For the mixed alkali metal compounds, we always additionally observed less reduced A15Tl27 All A Tl and A Tl X compounds include Tl 7− clusters, which are best described as a very phases as a8 side11 product.8 11 Thisx observation became reasonable11 when we determined the NE value compressed, ﬁvefold-capped trigonal prism (Figure2). Three symmetry independent thallium atoms (number of electrons per thallium atom) which sums up to a value of 8/11 = 0.72 for A8Tl11, 15/27 = are located on three different Wyckoff positions of space group R-3c: Tl1(12c; 3-fold rotational axis), 0.55 for A15Tl27 and 7/11 = 0.63 for A8Tl11X. The formation of less reduced A15Tl27 were completely Tl2 (36f ; general position) and Tl3 (18e; 2-fold rotational axis) build a cluster consisting of 11 Tl atoms comprehensible if the higher reduced A8Tl11Xx (x << 1) phases would have formed when the halide contentwith point was group significantlyD3. The less deviations than 1, frombecause point the group overallD3h degreeare very of small reduction and are was represented given by bythe a distortion of the height of the trigonal prism built by Tl3-atoms. This distortion is also reﬂected in stoichiometric approach for A8Tl11X. If less halide was incorporated, this is according to a higher degreethe distances of reduction of Tl2–Tl3 and consequently, (d(Tl2–Tl3): = a cd)less as reduced there are phase two was crystallographic formed in addition. independent The remaining distances halidepresent re-crystallized (d(Tl2–Tl3) = as d(Tl2–Tl3#5); (mixed) AX, d(Tl3#3–Tl2) of which we =also d(Tl3#2–Tl2)). could observe The single degree crystals. of distortion The crystals decreases for with increasing similarity of the capping distances (cd). the mixed alkali metal compounds A8Tl11Xx happened to form multicrystals together with A15Tl27 and for the reported single crystals (except K4Rb4Tl11Cl0.1) the data quality was worse compared to Cs8Tl11X phases. Therefore, the splitting of the alkali metal position could only be observed for K4Rb4Tl11Cl0.1, for the remaining mixed alkali compounds splitting positions could not be reasonably introduced. In these cases, we only refined the s.o.f. of the halide (see Section 4.3).

4. Discussions

4.1. How Does the Geometry of the Thallide Cluster Change on Halide Incorporation?

All A8Tl11 and A8Tl11Xx compounds include Tl117− clusters, which are best described as a very compressed, fivefold-capped trigonal prism (Figure 2). Three symmetry independent thallium atoms are located on three different Wyckoff positions of space group R-3c: Tl1(12c; 3-fold rotational axis), Tl2 (36f; general position) and Tl3 (18e; 2-fold rotational axis) build a cluster consisting of 11 Tl atoms with point group D3. The deviations from point group D3h are very small and are represented by a distortion of the height of the trigonal prism built by Tl3-atoms. This distortion is also reflected in the distances of Tl2–Tl3 (d(Tl2–Tl3): = cd) as there are two crystallographic independent distances present (d(Tl2–Tl3) = d(Tl2–Tl3#5); d(Tl3#3–Tl2) = d(Tl3#2–Tl2)). The degree of distortion decreases with increasing similarity of the capping distances (cd).

Crystals 2018, 8, 319 5 of 10 Crystals 2018, 8, x FOR PEER REVIEW 5 of 10

(a) (b)

Figure 2. Two perspectives (a) side view; (b) top view show the distortion of the trigonal prism in the Figure 2. Two perspectives (a) side view; (b) top view show the distortion of the trigonal prism in 7− Tl11 cluster7− which results in the point group D3 for the cluster; Symmetry operations for the the Tl11 cluster which results in the point group D3 for the cluster; Symmetry operations for the generation of equivalent atoms: #1: 1/3 + x − y, 2/3 − y, 7/6 − z; #2: 1/3 + y, −1/3 + x, 7/6 − z; #3: 1 − y, x − generation of equivalent atoms: #1: 1/3 + x − y, 2/3 − y, 7/6 − z; #2: 1/3 + y, −1/3 + x, 7/6 − z; #3: y, z; #4: 1 – x + y, 1 − x, z; #5: 4/3 − x, 2/3 – x + y, 7/6 − z. 1 − y, x − y, z; #4: 1 – x + y, 1 − x, z; #5: 4/3 − x, 2/3 – x + y, 7/6 − z.

In Tables 2 and 3 the distances as well as the distortion angles are listed and the dependence on the Inamount Tables of2 andhalide3 the incorporation distances as is well clearly as the evident. distortion In contrast, angles are the listed height and of thethe dependencetrigonal prism on(Tl3–Tl3) the amount as well of halideas the incorporationdistance of the is capping clearly evident.atom Tl2 Into contrast,the mean the plane height built of by the Tl3 trigonal atoms prism[d(Tl2-plane) (Tl3–Tl3) = as0.5 well Å in as all the compounds] distance of do the not capping significantly atom Tl2 change. to the Based mean planeon these built observations by Tl3 atoms we [d(Tl2-plane) = 0.5 Å in all compounds] do not signiﬁcantly change. Based on these observations introduced a cdd/cdav ratio (cdd: capping distance difference; cdav: average capping distance; we(Equation introduced (1)) awhich cdd/cd allowedav ratio for (cdd: a quick capping estimation distance of difference;the degree cdof avdistortion.: average The capping dependence distance; of (Equation (1)) which allowed for a quick estimation of the degree of distortion. The dependence of the the cdd/cdav ratio on the amount of halide is conspicuous and therefore allows for the evaluation of cdd/cdthe involvementav ratio on of the halide amount atoms of halide by solely is conspicuous analyzing the and distances therefore between allows forheavy the atom evaluation positions. of the involvement of halide atoms by solely analyzing the distances between heavy atom positions. − cdd cd21 cd Table 2. Selected distances in [Å] (numbering= scheme according to,cd1values ≤ cd2 taken from [1,15]), tilt angle cd+ cd (1) and cdd/cdav value for K Tl andcd Rb Tl . 21 8 11 8av 11  2 Atom 1 Atom 2 K8Tl11 Rb8Tl11 Tl2 Tl3 3.0476 (4) 3.060 Table 2. Selected distancesTl2 in [Å] (numberingTl33 scheme3.1396 a (4)ccording to, values 3.157 taken from [1,15]), tilt 1 angle and cdd/cdav valueTl1 for K8Tl11 andTl3 Rb8Tl11. 3.1304 (4) 3.147 Tl3 Tl33 3.2054 (7) 3.219 TiltAtom [◦] 1 Atom 2 4.69 K8Tl (2)11 Rb8Tl11 4.90 cdd/cdTl2av [%] Tl3 3.0476 2.97 (4) 3.060 3.12 Tl2 Tl33 3.1396 (4) 3.157 Table 3. Selected distances in [Å]Tl1 (numbering Tl3 scheme1 3.1304 according (4) to),3.147 tilt angle and cdd/cdav value for Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5RbTl33Tl11 Cl0.5 andTl33 Cs5.73.2054K2.3Tl11 (7)Cl0.6 and3.219 K4Rb 4Tl11Cl0.1. Tilt [°] 4.69 (2) 4.90 Atom 1 Atom 2 Cs Tl Cl Cs Tl Br Cs Rb Tl Cl Cs K Tl Cl K Rb Tl Cl 8 11 0.8 cdd/cd8 11 av 0.9[%] 5 3 2.9711 0.5 3.125.7 2.3 11 0.6 4 4 11 0.1 Tl2 Tl3 3.0656 (4) 3.0743 (2) 3.0605 (6) 3.0554 (7) 3.0564 (2) 3 Tl2Table 3.Tl3 Selected 3.0632distances (4) in [Å] 3.0766 (numbering (2) scheme 3.0896 according (6) to), tilt 3.0656 angle (4) and cdd/cd 3.1298av value (3) for Tl1 Tl31 3.0894 (4) 3.1006 (2) 3.1049 (7) 3.0884 (8) 3.1274 (3) Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5Rb3Tl11Cl0.5 and Cs5.7K2.3Tl11Cl0.6 and K4Rb4Tl11Cl0.1. Tl3 Tl33 3.2019 (11) 3.2102 (4) 3.2025 (11) 3.1873 (11) 3.2104 (4) ◦ AtomTilt 1 [ ] Atom 2 0.12 Cs8 (2)Tl11Cl0.8 0.069Cs8Tl (7)11Br0.9 Cs 0.945Rb (2)3Tl11Cl0.5 Cs 0.345.7K (5)2.3Tl11Cl0.6 K 2.3524Rb4 (7)Tl11Cl0.1 cdd/cd [%] 0.08 0.07 0.95 0.32 2.38 Tl2 av Tl3 3.0656 (4) 3.0743 (2) 3.0605 (6) 3.0554 (7) 3.0564 (2) Tl2 Tl33 3.0632 (4) 3.0766 (2) 3.0896 (6) 3.0656 (4) 3.1298 (3) Tl1 Tl31 3.0894 (4) 3.1006 (2) 3.1049 (7) 3.0884 (8) 3.1274 (3) | − | 3 cdd cd2 cd1 Tl3 Tl3 3.2019 (11) 3.2102= (4) 3.2025; cd1 (11)≤ cd 2 3.1873 (11) 3.2104 (4)(1) cdav cd2+cd1 Tilt [°] 0.12 (2) 0.069 (7) 2 0.94 (2) 0.34 (5) 2.352 (7) cdd/cdav [%] 0.08 0.07 0.95 0.32 2.38

Crystals 2018, 8, 319 6 of 10 Crystals 2018, 8, x FOR PEER REVIEW 6 of 10

4.2.4.2. Is There aa RbRb88Tl11Cl?Cl?

DespiteDespite numerousnumerous effortsefforts wewe diddid notnot succeedsucceed inin producingproducing crystalscrystals ofof RbRb88TlTl1111Cl ofof sufﬁcientsufficient qualityquality forfor aa reliablereliable determinationdetermination of halidehalide incorporationincorporation directly from the electron density maps. TheThe incorporationincorporation ofof halidehalide cannotcannot bebe completelycompletely ruled out at thisthis pointpoint asas therethere isis somesome residualresidual electronelectron densitydensity atat WyckoffWyckoff positionposition 66bb accordingaccording toto thethe positionposition ofof thethe halidehalide atom in the previously discusseddiscussed compounds.compounds. The The s.o.f. s.o.f. for for a chlorine a chlorine atom atom at this at position this position reﬁned refined to a value to ofa 0.08.value However, of 0.08. ◦ theHowever, cdd/cd theav ratiocdd/cd ofav 2.4% ratio comparedof 2.4% compared to 3.0% (Kto 83.0%Tl11) (K and8Tl11 3.1%) and (Rb 3.1%8Tl 11(Rb) and8Tl11) a and tilt a angle tilt angle of 2.4 of ◦ ◦ (K2.4°8Tl (K11:8Tl 4.711: 4.7°,, Rb8 RbTl118Tl:11 4.9: 4.9°)) are are very very similar similar to to the the values values found found in in K4 RbK4Rb4Tl411TlCl11Cl0.10.1and and suggestsuggest aa minimalminimal involvementinvolvement ofof chloride.chloride. Therefore,Therefore, wewe assumeassume thatthat RbRb88TlTl1111Cl does exist, but the amount ofof incorporatedincorporated chlorinechlorine isis lessless thanthan 10%,10%, whichwhich alsoalso isis inin lineline withwith thethe statedstated observationsobservations ofof CorbettCorbett etet al.al. from powder diffractiondiffraction experiments.experiments.

4.3.4.3. How Do Mixed Cation SitesSites AffectAffect thethe AmountAmount ofof HalideHalide Incorporation?Incorporation? ItIt needsneeds toto bebe emphasizedemphasized thatthat forfor thethe preparationpreparation ofof allall compoundscompounds thethe samesame stoichiometricstoichiometric approachapproach waswas employedemployed andand thethe dependencedependence ofof thethe amountamount ofof halidehalide incorporationincorporation onon thethe cesiumcesium contentcontent isis conspicuous.conspicuous. Therefore, thethe cationcation positionspositions neededneeded toto bebe examinedexamined moremore inin detail.detail. ThereThere areare twotwo differentdifferent cationcation positionspositions inin thethe asymmetricasymmetric unitunit correspondingcorresponding toto WyckoffWyckoff positionposition 3636ff forfor A1 and Wyckoff position 12c for A2. For Cs Tl X the A2 position showed the previously mentioned A1 and Wyckoff position 12c for A2. For Cs88Tl1111X the A2 position showed the previously mentioned splitting.splitting. ByBy takingtaking this this splitting splitting as as well well as freeas free s.o.f. s.o.f. values values for thefor halidethe halide (later (l fixedater fixed at the at s.o.f. the values.o.f. forvalue Cs2A) for Cs2A) into account, into account, a significantly a significantly improved improved model model could becould refined be refined (Figure (Figure3). 3).

Figure 3. Introduction of split positions and free s.o.f. values for the halide in Cs8Tl11Br0.9 results in an Figure 3. Introduction of split positions and free s.o.f. values for the halide in Cs8Tl11Br0.9 results in an improvedimproved modelmodel (residual (residual electron electron density density maps, maps generated, generated by Olex2 by Olex2 [23]). adapted from [23], with permission from © 2009 Wiley.

For Cs5Rb3Tl11Cl0.5 and Cs5.7K2.3Tl11Cl0.6 the position of A1 is mixed occupied by both alkali For Cs5Rb3Tl11Cl0.5 and Cs5.7K2.3Tl11Cl0.6 the position of A1 is mixed occupied by both alkali metals metals and the s.o.f. values for cesium on the mixed position are very similar to the s.o.f. values of and the s.o.f. values for cesium on the mixed position are very similar to the s.o.f. values of the halide the halide position. The position of A2 is only occupied by the heavier alkali metal cesium, which is position. The position of A2 is only occupied by the heavier alkali metal cesium, which is in in accordance with the observations of Corbett et al. for the binary A8Tl11 phases [14]. In summary, accordance with the observations of Corbett et al. for the binary A8Tl11 phases [14]. In summary, this this would mean a favored halide incorporation when cesium is present on both crystallographically would mean a favored halide incorporation when cesium is present on both crystallographically independent alkali metal positions. To prove this assumption, we investigated the system K-Rb-Tl in a independent alkali metal positions. To prove this assumption, we investigated the system K-Rb-Tl in stoichiometric approach to produce K4Rb4Tl11Cl which resulted in crystals of K4Rb4Tl11Cl0.1 (besides a stoichiometric approach to produce K4Rb4Tl11Cl which resulted in crystals of K4Rb4Tl11Cl0.1 (besides the side products (K,Rb)15Tl27 and (K,Rb)Cl). Careful investigation of the data of K4Rb4Tl11Cl0.1 the side products (K,Rb)15Tl27 and (K,Rb)Cl). Careful investigation of the data of K4Rb4Tl11Cl0.1 showed the splitting of the A2 position, whereby convergence of the refinement was achieved when showed the splitting of the A2 position, whereby convergence of the refinement was achieved when A1 and one splitting position are mixed occupied by Rb and K. The second splitting position Rb2A is A1 and one splitting position are mixed occupied by Rb and K. The second splitting position Rb2A is exclusively occupied by Rb. The overall s.o.f. for A2 was fixed at unity using a SUMP restraint. At the exclusively occupied by Rb. The overall s.o.f. for A2 was fixed at unity using a SUMP restraint. At max. peak of the residual electron density a chlorine atom was placed of which the s.o.f. refined at the max. peak of the residual electron density a chlorine atom was placed of which the s.o.f. refined 0.103 (13) and was fixed according to the s.o.f. of Rb2A (Figure4). at 0.103 (13) and was fixed according to the s.o.f. of Rb2A (Figure 4).

Crystals 2018, 8, x FOR PEER REVIEW 7 of 10 Crystals 2018, 8, 319 7 of 10 Crystals 2018, 8, x FOR PEER REVIEW 7 of 10

Figure 4. Introduction of split positions and free s.o.f. values for the halide in K4Rb4Tl11Cl0.1 results in

an improved model (residual electron density maps, generated by Olex2 adapted from [23], with Figurepermission 4. Introduction from © 2011 ofof splitsplit Olex2 positionspositions Wiley. andand free free s.o.f. s.o.f. values values for for the the halide halide in in K K4Rb4Rb4Tl4Tl1111ClCl0.10.1 resultsresults in an improvedimproved modelmodel (residual(residual electron electron density density maps, maps, generated generated by by Olex2 Olex2 [23 ]).adapted from [23], with permissionThe resulting from © coordination 2011 Olex2 Wiley. sphere of the halide is best described as distorted cubic, where the longerThe resultingdistances coordination are along the sphere room of diagonal the halide of is the best cubic described arrangement as distorted from cubic, the halide where to the the The resulting coordination sphere of the halide is best described as distorted cubic, where the longerposition distances of A2. are This along distance the room shortens diagonal significantly of the cubic for X-A2 arrangement by introducing from the split halide positions to the position(same s.o.f. longer distances are along the room diagonal of the cubic arrangement from the halide to the of A2.as halide), This distance resulting shortens in a less signiﬁcantly distorted forcubic X-A2 arrangement by introducing (Figure split 5). positions This cubic (same arrangement s.o.f. as halide), greatly position of A2. This distance shortens significantly for X-A2 by introducing split positions (same s.o.f. resultingresembles in a the less coordination distorted cubic of the arrangement halide in the (Figure CsCl5). structure This cubic type arrangement (d(Cs-Cl) = greatly 3.573 Å; resembles d(Cs-Br) = as halide), resulting in a less distorted cubic arrangement (Figure 5). This cubic arrangement greatly the3.718 coordination Å). The distances of the halide within in thethe CsClthe distorted structure cubic type arrangements (d(Cs-Cl) = 3.573 as well Å; d(Cs-Br) as the s.o.f. = 3.718 values Å). for The the resembles the coordination of the halide in the CsCl structure type (d(Cs-Cl) = 3.573 Å; d(Cs-Br) = distanceshalide/split within position the the in distorted Cs8Tl11X cubic (X = arrangementsCl or Br) and as K well4Rb4Tl as11 theCl0.1 s.o.f. are listed values in for Tables the halide/split 4 and 5 lists 3.718 Å). The distances within the the distorted cubic arrangements as well as the s.o.f. values for the positiondistances in Cs 8asTl 11wellX (X =as Cl s.o.f. or Br) values and K4 Rbof4 Tlthe11Cl mixed0.1 are listedoccupied in Tables sites4 and within5 lists distancesCs5Rb3Tl11 asCl well0.5 and halide/split position in Cs8Tl11X (X = Cl or Br) and K4Rb4Tl11Cl0.1 are listed in Tables 4 and 5 lists asCs s.o.f.5.7K values2.3Tl11Cl of0.6. the mixed occupied sites within Cs5Rb3Tl11Cl0.5 and Cs5.7K2.3Tl11Cl0.6. distances as well as s.o.f. values of the mixed occupied sites within Cs5Rb3Tl11Cl0.5 and Cs5.7K2.3Tl11Cl0.6.

(a) (b)

Figure 5. Distorted cubic arrangement around the halide (a); respectively void (b). Cs1: x, y, z; Cs1#1: Figure 5. Distorted cubic(a) arrangement around the halide (a); respectively(b) void (b). Cs1: x, y, z; Cs1#1: 1 − y, x − y, z; Cs1#2: 1−x + y, 1 − x. z; Cs1#3: 4/3 − x, 2/3 − y, 2/3 − z; Cs1#4: 1/3 + y, 2/3 – x + y, 273 − z; 1 − y, x − y, z; Cs1#2: 1 − x + y, 1 − x, z; Cs1#3: 4/3 − x, 2/3 − y, 2/3 − z; Cs1#4: 1/3 + y, 2/3 – x + y, FigureCs1#5: 5. Distorted1/3 + x − y, cubic -1/3 arrangement+ x, 2/3 − z; Cs2A/B: around x, the y, halidez; Cs2A/B#1: (a); respectively 4/3 − x, 2/3 void − y, (2/3b). −Cs1: z. x, y, z; Cs1#1: 273 − z; Cs1#5: 1/3 + x − y, −1/3 + x, 2/3 − z; Cs2A/B: x, y, z; Cs2A/B#1: 4/3 − x, 2/3 − y, 2/3 − z. 1 − y, x − y, z; Cs1#2: 1−x + y, 1 − x. z; Cs1#3: 4/3 − x, 2/3 − y, 2/3 − z; Cs1#4: 1/3 + y, 2/3 – x + y, 273 − z; Cs1#5:Table 1/3 4. + Distancesx − y, -1/3 in+ x, [Å] 2/3 within − z; Cs2A/B: the distorted x, y, z; Cs2A/B#1:cubic arrangement 4/3 − x, 2/3 around − y, 2/3 the − z. halide/void and s.o.f. Table 4. Distances in [Å] within the distorted cubic arrangement around the halide/void and s.o.f. values for the halide/Cs2A (split position) in Cs8Tl11X (X = Cl or Br) and K4Rb4Tl11Cl0.1. valuesTable 4. for Distances the halide/Cs2A in [Å] within (split position)the distorted in Cs cubic8Tl11 Xarrangement (X = Cl or Br) around and K 4theRb 4halide/voidTl11Cl0.1. and s.o.f. Position 1–Position 2 Cs8Tl11Br0.9 Cs8Tl11Cl0.8 Position 1–Position 2 K4Rb4Tl11Cl0.1 values for the halide/Cs2A (split position) in Cs8Tl11X (X = Cl or Br) and K4Rb4Tl11Cl0.1. Position 1–PositionCs2A-X1 2 Cs8Tl11 3.990Br0.9 (2) Cs8Tl 3.99111Cl (9)0.8 Position Rb2A-X1 1–Position 2 K4Rb 3.804Tl (2)11Cl 0.1 PositionCs2A-X1Cs2B-void 1–Position 2 3.990 Cs8Tl (2)4.38811Br0.9 Cs 3.9918Tl4.35411Cl (9)0.8 PositionK2B/Rb2B-void Rb2A-X11–Position 2 K4Rb4.0964Tl 3.8011Cl (2)(3)0.1 Cs2B-voidCs2A-X1Cs1-X1/void 4.388 3.990 3.6705 (2) (4) 3.991 3.5876 4.354 (9) (7) K1/Rb1-X1/voidK2B/Rb2B-void Rb2A-X1 3.80 3.5994 4.096 (2) (3)(9) Cs1-X1/void 3.6705 (4) 3.5876 (7) K1/Rb1-X1/void 3.5994 (9) Cs2B-voids.o.f. (X1/Cs2A) 4.388 0.924 (6) 4.354 0.76 (2) K2B/Rb2B-void s.o.f. (X1/Rb2A) 4.096 0.103 (3) (13) s.o.f. (X1/Cs2A)Cs1-X1/void 0.924 3.6705 (6) (4) 3.5876 0.76 (2)(7) K1/Rb1-X1/void s.o.f. (X1/Rb2A) 3.5994 0.103 (9) (13) s.o.f. (X1/Cs2A) 0.924 (6) 0.76 (2) s.o.f. (X1/Rb2A) 0.103 (13)

CrystalsCrystals2018 2018, 8,, 3198, x FOR PEER REVIEW 8 of8 of 10 10

TableTable 5. Distances5. Distances in [Å]in [Å] within within the the cubic cubic arrangement arrangement around around the halide/voidthe halide/void in Cs in5 CsRb53RbTl113TlCl110.5Cl0.5and and CsCs5.75.7KK2.32.3TlTl1111ClCl0.60.6.. The The s.o.f. values forfor CsCs atat thethe mixedmixed occupied occupied A1 A1 site site resemble resemble the the s.o.f. s.o.f. values values for for thethe halide halide (numbering (numbering scheme scheme according according to to Figure Figure5). 5).

Position 1–Position 2 Cs5Rb3Tl11Cl0.5 Cs5.7K2.3Tl11Cl0.6 Position 1–Position 2 Cs5Rb3Tl11Cl0.5 Cs5.7K2.3Tl11Cl0.6 A2-X1/void 4.099 (2) 4.002 A2-X1/void 4.099 (2) 4.002 A1-X1/voidA1-X1/void 3.6160 3.6160 (13) (13) 3.5492 3.5492 (2) (2) s.o.f. (A1s.o.f. = Cs)(A1 = Cs) 0.521 0.521 (12) (12) 0.612 0.612 (9) (9) s.o.f. (X1)s.o.f. (X1) 0.50 0.50 (4) (4) 0.60 0.60 (4) (4)

TheThe same same stoichiometric stoichiometric approach approach to to produce produce the the hitherto hitherto presented presented compounds compounds also also was was employed by using solely potassium. Here, we only observed well crystallized halide-free K8Tl11 and employed by using solely potassium. Here, we only observed well crystallized halide-free K8Tl11 K15Tl27 phases. The previously stated stability of the halide including A8Tr11X phases might not and K15Tl27 phases. The previously stated stability of the halide including A8Tr11X phases might not exclusivelyexclusively be causedbe caused by the by effect the ofeffect charge of balancecharge duebalance to halide due incorporationto halide incorporation but by the stabilization but by the ofstabilization a preferably of heavier a preferably halide atomheavier in ahalide distorted atom cubic in a arrangementdistorted cubic including arrangement cesium including preferentially cesium (Figurepreferentially6). If less (Figure (or no) 6). cesium If less is (or involved, no) cesium then lessis involved, (or even then no) halideless (or will even be incorporated.no) halide will If be rubidiumincorporated. is involved If rubidium as the is heaviest involved alkali as the metal, heavie thenst thealkali amount metal, of then incorporated the amount halide of incorporated seems to halide seems to be limited to approximately 10%. In return, the Tl11 clusters themselves seem to be limited to approximately 10%. In return, the Tl11 clusters themselves seem to tolerate any charge − − betweentolerate 7 any− and charge 8−. between 7 and 8 .

Figure 6. The unit cell of Cs8Tl11Br0.9 shows the two characteristic components: Tl11 clusters and the distortedFigure 6. cubic The arrangement unit cell of Cs around8Tl11Br the0.9 shows halide the atom. two characteristic components: Tl11 clusters and the distorted cubic arrangement around the halide atom.

AuthorAuthor Contributions: Contributions:Conceptualization, Conceptualization, S.G.; S.G.; Methodology, Methodology, S.G., S.G., S.T., S.T., B.H., B.H., S.S.; S.S.; Validation, Validation, S.G.; S.G.; Formal Formal Analysis,Analysis, S.G., S.G., S.S., S.S., B.H.; B.H.; Investigation, Investigation, S.G., S.G., S.T.; S.T.; Resources, Resources, N.K.; N.K.; Writing-Original Writing-Original Draft Draft Preparation, Preparation, S.G., S.T.; S.G., Writing-ReviewS.T.; Writing-Review & Editing, & Editing, S.G.; Visualization, S.G.; Visualization, S.G., S.T.; S.G., Supervision, S.T.; Supervision, S.G.; Project S.G.; Administration,Project Administration, S.G. S.G. Funding: This work was supported by the German Research Foundation (DFG) within the funding programme Funding: This work was supported by the German Research Foundation (DFG) within the funding programme Open Access Publishing. Open Access Publishing. Acknowledgments: The authors thank Marc Schlosser (working group of A. Pfitzner) for collecting the powder diffractionAcknowledgment: patterns and The Dr. authors Michael thank Bodensteiner Marc Schlosser (X-ray (working Structure grou department)p of A. Pfitzner) for discussions for collecting concerning the powder the refinementdiffraction of patterns (K,Rb)8Tl and11Cl Dr.0.1 .Michael Bodensteiner (X-ray Structure department) for discussions concerning the Conflictsrefinement of Interest: of (K,Rb)The8Tl11 authorsCl0.1. declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

Crystals 2018, 8, 319 9 of 10

Appendix Stoichiometric approaches Cs8Tl11Cl0.8: 0.445 g Cs (3.3 mmol), 1.076 g Tl (5.3 mmol Tl) and 0.081 g CsCl (0.51 mmol) Cs8Tl11Br0.9: 0.413 g Cs (3.1 mmol), 0.998 g Tl (4.9 mmol Tl) and 0.091 g CsBr (0.44 mmol) Cs5Rb3Tl11Cl0.5: 0.246 g Cs (1.9 mmol), 0.128 g Rb (1.5 mmol), 1.124 g Tl (5.5 mmol) and 0.061 g RbCl (0.5 mmol) Cs5.7K2.3Tl11Cl0.6: 0.264 g Cs (2 mmol), 0.058 g K (1.5 mmol), 1.117 g Tl (5.5 mmol) and 0.037 g KCl (0.5 mmol) K3.98Rb4.02Tl11Cl0.1: 0.154 g Rb (1.8 mmol), 0,052 g K (1.3 mmol), 1.0134 g Tl (5 mmol) and 0.034 g KCl (0.45 mmol)

References

1. Gärtner, S.; Korber, N. Polyanions of group 14 and group 15 elements in alkali and alkaline earth metal solid state compounds and solvate structures. In Zintl Ions Principles and Recent Developments, Fässler, T.F., Ed.; Springer: Berlin, Germany, 2011; pp. 25–56. 2. Gärtner, S.; Korber, N. Chapter 1.09-Zintl anions. In Comprehensive Inorganic Chemistry II, 2nd ed.; Reedijk, J., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, The Netherlands, 2013; pp. 251–267. 3. Corbett, J.D. Polyanionic clusters and networks of the early p-element metals in the solid state: Beyond the zintl boundary. Angew. Chem. Int. Ed. 2000, 39, 670–690. [CrossRef] 4. Scharfe, S.; Kraus, F.; Stegmaier, S.; Schier, A.; Fässler, T.F. Zintl ions, cage compounds, and intermetalloid clusters of group 14 and group 15 elements. Angew. Chem. Int. Ed. 2011, 50, 3630–3670. [CrossRef][PubMed] 5. Zintl, E.; Goubeau, J.; Dullenkopf, W. Salzartige Verbindungen und intermetallische Phasen des Natriums in ﬂüssigem Ammoniak. Z. Phys. Chem. 1931, 154, 1–46. [CrossRef] 6. Zintl, E.; Dullenkopf, W. Über den Gitterbau von NaTl und seine Beziehung zu den Strukturen des β-Messings. Z. Phys. Chem. 1932, B16, 195–205. [CrossRef] 7. Eisenmann, B.; Cordier, G. Structural patterns of homo-and heteronuclear anions in zintl phases and related intermetallic compounds and concepts for their interpretation. In Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S.M., Ed.; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1996; pp. 61–137. 8. Guloy, A.M. Polar intermetallics and zintl phases along the zintl border. In Inorganic Cemistry in Focus III; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2006.

9. Hansen, D.A.; Smith, J.F. Structure and bonding model for Na2Tl. Acta Cryst. 1967, 22, 836–845. [CrossRef] 10. Marsh, R.E.; Shoemaker, D.P. The crystal structure of NaPb. Acta Cryst. 1953, 197–205. [CrossRef] 11. Busmann, E. Die kristallstrukturen von KSi, RbSi, KGe, RbGe und CsGe. Z. Anorg. Allg. Chem. 1961, 313, 90–106. [CrossRef] 12. Lorenz, C.; Gärtner, S.; Korber, N. Ammoniates of zintl phases: Similarities and differences of binary phases

A4E4 and their corresponding solvates. Crystals 2018, 8, 276. [CrossRef] 13. Blase, W.; Cordier, G.; Muller, V.; Haussermann, U.; Nesper, R.; Somer, M. Preparation and crystal-structures

of Rb8In11,K8Tl11, and Rb8Tl11 band-structure calculations on K8In11. Z. Naturforsch. B 1993, 48, 754–760. [CrossRef] 7− 14. Dong, Z.-C.; Corbett, J.D. A8Tl11 (A = K, Rb, or Cs) Phases with Hypoelectronic Tl11 cluster anions: Syntheses, structure, bonding and properties. J. Cluster Sci. 1995, 6, 187–201. [CrossRef]

15. Dong, Z.C.; Corbett, J.D. A15Tl27 (A = Rb,Cs): A structural type containing both isolated clusters and condensed layers based on the tl-11 fragment. Syntheses, structure, properties, and band structure. Inorg. Chem. 1996, 35, 1444–1450. [CrossRef][PubMed]

16. Sevov, S.C.; Corbett, J.D. A remarkable hypoelectronic indium cluster in K8In11. Inorg. Chem. 1991, 30, 4875–4877. [CrossRef] 17. Wang, F.; Wedig, U.; Prasad, D.; Jansen, M. Deciphering the chemical bonding in anionic thallium clusters. J. Am. Chem. Soc. 2012, 134, 19884–19894. [CrossRef][PubMed]

18. Henning, R.W.; Corbett, J.D. Cs8Ga11, a new isolated cluster in a binary gallium compound. A family of valence analogues A8Tr11X: A = Cs, Rb; Tr = Ga, In, Tl; X = Cl, Br, I. Inorg. Chem. 1997, 36, 6045–6049. [CrossRef][PubMed] Crystals 2018, 8, 319 10 of 10

19. Falk, M.; El Addad, A.; Röhr, C. Crystal and electronic structure of alkali triele halogenides of the K8In11-type structure. In Proceedings of the 25th Annual Conference of the German Crystallographic Society, Karlsruhe, Germany, 27–30 March 2017. 20. Hackspill, L. Sur quelques properiétés des métaux alcalins. Helv. Chim. Acta 1928, 11, 1003–1026. [CrossRef] 21. Agilent. Crysalis pro. v.39.37b ed.; Agilent Technologies Ltd.: Yarnton, UK, 2014. 22. Computer Program: STOE WinXPOW; STOE & Cie 2000 Darmstadt: Darmstadt, Germany, 2011. 23. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. Olex2: A complete structure solution, reﬁnement and analysis program. J. Appl. Crystallogr. 2009, 42, 339–341. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).