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Crystal chemistry and luminescence properties of red-emitting CsGd1−xEux(MoO4)2 solid-solution Cite this: Dalton Trans., 2014, 43, 9669 phosphors†

Pinglu Shi,a Zhiguo Xia,*a,b Maxim S. Molokeevc and Victor V. Atuchind,e,f

Scheelite related alkali-metal rare-earth double molybdate compounds with a general formula of ALn-

(MoO4)2 can ﬁnd wide application as red phosphors. The crystal chemistry and luminescence properties

of red-emitting CsGd1−xEux(MoO4)2 solid-solution phosphors have been evaluated in the present paper. A detailed analysis of the structural properties indicates the formation of isostructural scheelite-type

CsGd1−xEux(MoO4)2 solid-solutions over the composition range of 0 ≤ x ≤ 1. The photoluminescence emission (PL) and excitation (PLE) spectra, and the decay curves were measured for this series of com- Received 31st January 2014, pounds. The critical doping concentration of Eu3+ is determined to be x = 0.6 in order to realize the Accepted 28th March 2014 maximum emission intensity. The emission spectra of the as-obtained CsGd(1−x)Eux(MoO4)2 phosphors DOI: 10.1039/c4dt00339j show narrow high intensity red lines at 592 and 615 nm upon excitation at 394 or 465 nm, revealing great www.rsc.org/dalton potential for applications in white light-emitting diode devices.

double molybdates and tungstates, with the general formula of 1. Introduction + + + ALn(MO4)2 (A is an alkali-metal cation or Tl ,Ag or Cu ), Ln is White light-emitting diodes (LEDs) have the potential to a rare earth metal cation or Y3+ and M is Mo6+ or W6+), owing become the next generation of solid-state illumination lighting to their good optical properties, stability and relatively simple due to their unique properties, such as high reliability, long preparation. Accordingly, many double molybdate- and tung- lifetime, low energy consumption, environmentally friendly state-based phosphors have been investigated, such as NaY- 1–3 3+ 5 6 3+ 7 nature, and so on. The most convenient way to generate (MoO4)2:Eu , NaEu0.96Sm0.04(MoO4)2, NaLa(MoO4)2:Eu , 8 3+ 9 white light is through the combination of an InGaN-based KEu(MoO4)2, NaLu(WO4)2:Eu , and so on. This series of Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21. blue LED chip with a broad-band yellow phosphor Y3Al5O12: compounds, ALn(MO4)2, possess different crystal structures – Ce3+. However, one of the challenges is that this method has a depending on a specific cation combination.10 18 For a host low color-rendering index (CRI) due to the deficiency of the lattice such as this one, all of the rare earth ions occupy the 3+ 4 red emission from the Y3Al5O12:Ce phosphor. In order to low symmetry sites surrounded by oxygen atoms. When the overcome this shortcoming, a highly-efficient red-emitting special red-emitting rare earth ions, such as Eu3+,Sm3+ or phosphor is essential. Therefore, much attention has been Pr3+, are introduced to the host, the doping ions insert into paid to Eu3+-activated scheelite-related alkali-metal rare-earth the existing Ln3+ sites. However, the Ln3+ sites are of low symmetry, and as a result, the compounds doped with Eu3+ can 5 –7 show a strong red emission from the D0 F2 transition at nearly 613 nm. Furthermore, these phosphors exhibit an aSchool of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China intense absorption at 394 and 465 nm, which meets the appli- bSchool of Materials Sciences and Engineering, University of Science and Technology cation for the near-UV/blue LED chip well. Beijing, Beijing 100083, China. E-mail: [email protected]; Fax: +86-10-8237-7955; Since Cs+ has the largest effective ion radius in the alkali- Tel: +86-10-8237-7955 metal group, it is expected to produce new red-emitting phos- cLaboratory of Crystal Physics, Kirensky Institute of Physics, SB RAS, phors with high-efficiency and high quenching concentrations Krasnoyarsk 660036, Russia dLaboratory of Optical Materials and Structures, Institute of Semiconductor Physics, in the CsLn(MO4)2 compounds. Regrettably, the CsLn(MO4)2 SB RAS, Novosibirsk 630090, Russia compounds are poorly investigated, and the structural infor- – eFunctional Electronics Laboratory, Tomsk State University, Tomsk 634050, Russia mation has only been reported for a few crystals.19 23 In this fLaboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, study, CsGd1−xEux(MoO4)2 solid-solution phosphors are pro- Novosibirsk 630090, Russia †Electronic supplementary information (ESI) available. See DOI: 10.1039/ posed and designed, and the crystal chemistry and lumine- c4dt00339j scence properties of the red-emitting CsGd1−xEux(MoO4)2

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phosphors are evaluated in detail. The eﬀective radii of Gd3+ and Eu3+ ions are very close and continuous tuning of structural and spectroscopic parameters can be reasonably sup-

posed in the quasi-binary system CsGd(MoO4)2–CsEu(MoO4)2. It should be noticed, however, that the existence of CsGd-

(MoO4)2 molybdate has long been known, and some thermal – and magnetic properties were observed in several studies.24 27

Contrary to that, the properties of CsEu(MoO4)2 are unknown in the literature.

2. Experimental

A series of CsGd1−xEux(MoO4)2 (x = 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0) samples were synthesized using conventional

solid-state reactions. The starting materials, Cs2CO3 (99.9%), Fig. 1 XRD patterns of CsGd1−xEux(MoO4)2 samples (x = 0, 0.05, 0.10, Gd2O3 (99.99%), MoO3 (99.95%) and Eu2O3 (99.99%) were 0.20, 0.40, 0.60, 0.80 and 1.0) and JCPDS 49-419 CsGd(MoO4)2 as a weighed according to the stoichiometric ratio and mixed reference. homogeneously in an agate mortar. A 0.5% excess of Cs2CO3 was added to be used as a self-flux. After grinding, the mix- tures were calcined in air at 950 °C for 6 h in a muffle furnace the x values were constrained in accordance to nominal com- to form the final product.28 Subsequently, the as-obtained positions. All of the metal atoms were refined using isotropic samples were ground into powders and the phase composition thermal parameters. One thermal parameter, however, was was controlled. used for all oxygen atoms. Correction for the preferred orien- The X-ray powder diffraction (XRD) data were collected on a tation was implemented on the (100) plane. The refinement SHIMADZU model XRD-6000 diffractometer equipped with a was stable and gave low R-factors. The main processing and linear detector and Cu-Kα radiation (λ = 0.15406 nm) source refinement parameters are summarized in Table 1. As an operating at 40 kV and 30 mA. The photoluminescence emis- example, the experimental (dots) and theoretical (lines) X-ray sion (PL) and excitation (PLE) spectra were measured using a diffraction patterns of CsGd0.4Eu0.6(MoO4)2 are shown in Hitachi F-4600 spectrometer with a 150 W Xe lamp as the exci- Fig. 2. The crystal structure is illustrated in the inset in Fig. 2. tation source under the working voltage of 500 V. The lumine- The CsGd1−xEux(MoO4)2,0≤ x ≤ 1 solid-solutions crystallize scence decay curves were recorded on a spectrofluorometer in space group P2/c exhibiting a layered structure. The stacking (HORIBA, JOBIN YVON FL3-21) and a 460 nm pulse laser of alternating layers: –Gd–(MoO )–Cs–(MoO )– runs along the radiation (nano-LED) was used as the excitation source.29 4 4 crystallographic a-axis. Each layer is parallel to the (100) clea- Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21. The temperature-dependent luminescence properties were vage plane. All of the rare earth ions and Cs+ ions are located measured on the same spectrophotometer combined with a in the eight-coordinated sites surrounded by oxygen atoms. self-made computer-controlled electrical furnace. The dependencies of the unit cell parameters on x are shown in Fig. 1S–4S.† The dependence of the unit cell volume, V,onx

in CsGd1−xEux(MoO4)2 is shown in Fig. 3. It is found that the 3. Results and discussion CsGd1−xEux(MoO4)2 compounds are isostructural with the random substitution of Gd/Eu atoms in the same Ln3+ sites.

The XRD patterns of CsGd1−xEux(MoO4)2 (x = 0, 0.05, 0.1, 0.2, Furthermore, the atom coordinates and thermal parameters, 0.4, 0.6, 0.8, 1.0) are displayed in Fig. 1. All of the samples with as well as the main bond-lengths of the CsGd1−xEux(MoO4)2 different Gd/Eu content present similar diffraction patterns solid-solution compounds are listed in Tables S1 and S2.† without impurity peaks. All of the diffraction peaks are Based on the Rietveld refinement results, the unit cell indexed by a single monoclinic phase, and the peak positions volume is proportional to x, which also confirmed that the 30 ’ agree well with the JCPDS card 41-419 CsGd(MoO4)2. Consid- solutions are isostructural and obey Vegard s rule. As is ering the ion sizes and valence states, Eu3+ ions are most likely evident from Fig. 1S and 2S,† the variations of cell parameters 3+ occupying the Gd sites. The structure of CsGd1−xEux(MoO4)2 a and b are relatively small. Comparatively, the drastic increase 3+ 3+ is isostructural to CsDy(MoO4)2. The atomic coordinates were of parameter c takes place upon substitution of Gd by Eu † β used to determine the structure of the series of CsGd1−x- ions as shown in Fig. 3S. The monoclinic angle decreases † Eux(MoO4)2 solid-solutions. All refinements and data proces- with the increase of x and remains close to 90° (Fig. 4S ). sing were performed using the program TOPAS 4.2. The con- Thus, it can be concluded that the increase of V in CsGd1−x- 3+ 3+ centration of Gd was not determined because Gd and Eu Eux(MoO4)2 solid-solutions is caused predominantly by unit ions have similar atomic scattering functions, and therefore cell elongation parallel to the c-axis. It is interesting to con-

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Table 1 Main reﬁnement parameters of CsGd1−xEux(MoO4)2 phosphors with diﬀerent chemical compositions

Formula CsEu(MoO4)2 CsGd0.2Eu0.8(MoO4)2 CsGd0.4Eu0.6(MoO4)2 CsGd0.6Eu0.4(MoO4)2 CsGd0.8Eu0.2(MoO4)2 CsGd(MoO4)2 Space group P2/c Cell parameters a = 9.5255(1) Å a = 9.5256(2) Å a = 9.5247(1) Å a = 9.5264(2) Å a = 9.5261(2) Å a = 9.5289(2) Å b = 5.0886(2) Å b = 5.0884(2) Å b = 5.0862(2) Å b = 5.0837(2) Å b = 5.0810(2) Å b = 5.0823(2) Å c = 8.0901(1) Å c = 8.0881(9) Å c = 8.077(1) Å c = 8.0702(8) Å c = 8.0623(6) Å c = 8.0563(7) Å β = 90.868(6)° β = 90.853(5)° β = 90.823(6)° β = 91.085(6)° β = 91.154(6)° β = 91.246(6)° V = 392.09(4) Å3 V = 391.97(5) Å3 V = 391.22(5) Å3 V = 390.76(4) Å3 V = 390.15(3) Å3 V = 390.06(4) Å3 −3 Dx (g cm ), Mr Dx = 5.122 Dx = 5.133 Dx = 5.152 Dx = 5.167 Dx = 5.184 Dx = 5.194 Mr = 604.76 Mr = 605.82 Mr = 606.86 Mr = 607.93 Mr = 608.98 Mr = 610.03 Number of 588/88 587/99 587/99 586/99 586/99 585/90 reflections/ number of parameters R-Factors, % Rwp = 4.59 Rwp = 4.11 Rwp = 4.41 Rwp = 4.39 Rwp = 3.90 Rwp = 4.56 Rp = 3.58 Rp = 3.23 Rp = 3.36 Rp = 3.26 Rp = 3.00 Rp = 3.42 RB = 1.28 RB = 0.92 RB = 1.02 RB = 1.19 RB = 0.82 RB = 1.39 Rexp = 2.60 Rexp = 2.50 Rexp = 2.49 Rexp = 2.58 Rexp = 2.49 Rexp = 2.41 χ2 = 1.77 χ2 = 1.64 χ2 = 1.77 χ2 = 1.70 χ2 = 1.56 χ2 = 1.89

octupole coordination.31 Furthermore, the averaged dimen-

sions of (Gd,Eu)O8 square antiprisms should increase with the increase of x. In the a and b directions, the general intume-

scences of the (Gd,Eu)O8 square antiprisms can be partly dam- pened through deformation and rotation of other metal polyhedra, which results in a little variation of cell parameters

a and b. In the c direction, however, the (Gd,Eu)O8 square antiprisms are strictly linked by edges to chains and dampening is

impossible. Therefore, the variation of the (Gd,Eu)O8 square antiprisms with the increase of x is transmitted to the increase of parameter c. It is known from available structural information that ALn-

(MoO4)2 (A = Li, Na, Ag, Cu, K, Cs, Rb, Tl) compounds may Fig. 2 Experimental and calculated XRD patterns, and Rietveld diffe- crystallize in different space groups than P2/c, such as Pccm, rence plot of CsGd0.4Eu0.6(MoO4)2 at room temperature. Inset shows Pbcn and I4 /a.30 Recently, it has been found that ALn(MoO ) the crystal structure. 1 4 2 molybdates can be classified using a quite simple parameter, 3+ p = RLn/RA, where RLn is the radius of the Ln ion and RA is the radius of the A ion.17 A related diagram is shown in Fig. 4. Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21. In the considered molybdates, CsGd1−xEux(MoO4)2, parameter p increases in the range of 1.632–1.652 upon the increase of x, and therefore the compounds appeared close to the border between the sectors of P2/c and Pccm space groups. This is very interesting because the local symmetry of the Ln site in P2/c is 2, but the local symmetry of the Ln site in Pccm is 222. The structural difference could lead to a difference in the physical

properties. However, all of the CsGd1−xEux(MoO4)2 com-

Fig. 3 Dependence of unit cell volume, V,onx in CsGd(1−x)Eux(MoO4)2 molybdates.

sider a possible atomic mechanism of this strongly anisotropic + + + + Fig. 4 Different structure fields in ALn(MoO4)2 (A = Li ,Na,K,Cu, ff + + + + variation of the structural parameters. The e ective ionic Rb ,Ag,Cs,Tl) molybdates. CsGd(1−x)Eux(MoO4)2 compounds are radius of Eu3+ is a little bigger than that of the Gd3+ ion in the marked by the arrow.

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pounds, 0 ≤ x ≤ 1, crystallize in P2/c. This helps us to precisely tungstate CsLu(WO4)2, however, in comparison with the molyb- define the lines in Fig. 4 between the sectors related to P2/c dates, the monoclinic angle β is greater and two oxygen ions and Pccm space groups and noticeably expand the region of are pushed closer to the tungsten atom. This results in the

P2/c phases in the diagram. appearance of [WO6] octahedra, as shown in Fig. 5, instead of

The developed diagram shown in Fig. 4 is valuable for MoO4 tetrahedra present in the molybdates. Another specific crystal symmetry predictions. For example, the hypothetical feature of ALn(WO4)2 tungstates is a wide spread of monoclinic

solid-solutions CsSm1−xEux(MoO4)2 and CsPm1−xEux(MoO4)2, crystals with space group C2/c even for comparatively small A containing the Sm3+ and Pm3+ ions with effective ion radii cations, which is forbidden in the molybdates. Indeed, well 3+ 34 35 lower than that of Eu with coordination number (CN) = 8, can known laser hosts, such as KEu(WO4)2, RbGd(WO4)2, TlSm 31 36 37 be considered. Then, in compounds CsSm1−xEux(MoO4)2 and (WO4)2, KGd(WO4)2 and others can be considered. There- 6+ 6+ CsPm1−xEux(MoO4)2,thevalueofp ranges from 1.652 to 1.603 fore, in spite of Mo and W ions possessing similarly and from 1.652 to 1.592 upon the decrease of x,respectively. effective ion radii, they show different behavior in binary crys- Thus, in accordance with the diagram shown in Fig. 4, the binary tals and structural relations should be specified individually.

molybdates should appear in the sector of space group Pccm. The emission spectra of CsGd1−xEux(MoO4)2 phosphors in Another interesting subject is the substitution of Mo6+ by the red spectral range from 580 to 660 nm under excitation at W6+ in binary molybdates because the solutions produced are 394 and 465 nm are shown in Fig. 6a and 6b, respectively. The promising for the creation of eﬀective red phosphors.32 As for insets indicate the trend in the emission intensities as a func- the tungsten oxides with a general composition of CsLn- tion of the Eu3+ concentration. The emission intensities 3+ (WO4)2, only the CsLu(WO4)2 structure has been reported up to increase with the increasing Eu content up to x = 0.6 and now.33 This compound crystallizes in the P2/c space group, then decreases thereafter. This indicates that the phosphors which is similar to the related Cs-containing molybdates. In have a high critical value of Eu3+ quenching concentration.38 The emission spectra exhibit a strong peak centering at 5 –7 615 nm corresponding to the D0 F2 electric dipole transition of the Eu3+ ions. The weak emission peak around 592 nm is 5 7 7 assigned to the magnetic dipole transition of D0– F1. From 3+ the structural analysis, the Eu site in the CsGd(MoO4)2 host lacks inversion symmetry. According to the Judd–Ofelt theory,39 in this case the electric dipole emission is strong and the magnetic dipole transition is relatively weak.1,3,40 Our experimental results also confirm this point. Other emission 5 7 peaks between 650 and 710 nm corresponding to D0– F3 and 5 –7 3+ D0 F4 transitions of the Eu ion are very weak and can not be clearly observed. The excitation spectrum monitored at 615 nm, emission

spectrum under 394 nm excitation of CsGd0.4Eu0.6(MoO4)2, Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21. and energy level diagram of Eu3+ illustrating the two spectra, are displayed in Fig. 7. The excitation spectrum reveals two regions: the broad absorption band extending from 215 nm to

Fig. 5 Crystal structure of CsLu(WO4)2 with WO6 octahedra. 350 nm with a center at 297 nm and several narrow excitation

Fig. 6 PL spectra of samples CsGd1−xEux(MoO4)2 (x = 0.10, 0.20, 0.40, 0.60, 0.80 and 1.0) at 394 nm (a), 465 nm (b) excitation, the insets show the dependence of PL intensity on the Eu3+ doping concentration.

Dalton Transactions Paper

3+ Fig. 7 PLE (λem = 615 nm, blue line) and PL (λex = 394 nm, black line) spectra with the energy level diagram of Eu in the CsGd0.4Eu0.6(MoO4)2 phosphor at RT.

peaks above 350 nm. The wide excitation band mainly origi- sition. Furthermore, the UV-Vis reflectance spectrum of the 2−– 6+ nated from the charge transfer transition of O Mo [ligand CsGd0.4Eu0.6(MoO4)2 phosphor was measured and is shown in to metal charge transfer (LMCT)]. Above this, the LMCT of Fig. 8. The broad band absorption band below 350 nm should − – O2 –Eu3+ contributed to the band.41 43 The LMCT band position be ascribed to the LMCT band. Except for this, several sharp can be estimated using the empirical formula by Jørgensen: absorption lines corresponding to the f–f transitions of Eu3+ ions are detected. Especially, some observed excitation and 1 1 ﬃ Ect ðcm Þ¼½χoptðXÞχoptðMÞ 30 000 cm ð1Þ absorption lines at 394 and 465 nm can echo the e cient absorption of the phosphor meeting the possible application χ where opt(X) is the optical electronegativity of the anion, for the near-UV/blue LED chip. 3+ 5 7 which is equal to the Pauling electronegativity value, 3.2; The lifetime of the Eu D0– F2 emission was measured at ﬀ 3+ χopt(M) is the optical electronegativity of the central metal ion, di erent Eu concentrations. The normalized decay curves χ 44 χ 42 with opt(Mo) = 2.101 and opt(Eu) = 1.74. From expression recorded for the CsGd(1−x)Eux(MoO4)2 phosphors under exci- − (1), the LMCT of O2 –Mo6+ is around ∼303 nm and the tation at 460 nm are shown in Fig. 9. They fit well with the − O2 –Eu3+ LMCT is at ∼228 nm. Clearly, the estimations agree first-order exponential decay formula: well with the experimental result. The sharp excitation lines ð Þ¼ þ ð =τÞðÞ between 350 nm and 550 nm are attributed to the f–f tran- I t I0 A exp t 2 sitions of Eu3+. Moreover, some other small excitation peaks where I and I0 are the luminescence intensity at time t and 0, were also detected. The excitation spectrum demonstrates that A is a constant, and τ represents the characteristic decay time Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21. the CsGd0.4Eu0.6(MoO4)2 phosphor can absorb near-UV/blue for exponential component.45 The decay curves indicate that light well. For the present compound, the intensity of the f–f transitions is higher than that of the charge transfer tran-

Fig. 8 UV-Vis reﬂectance spectrum of the CsGd0.4Eu0.6(MoO4)2 Fig. 9 The decay curves of the CsGd1−xEux(MoO4)2 samples (x = 0.10, phosphor. 0.20, 0.40, 0.60, 0.80 and 1.0) at room temperature.

Paper Dalton Transactions

all Eu3+ ions occupy the same atom environment, which is consistent with the description of the crystal structure above.4 On the basis of the results fitting a single exponential decay,

the lifetime values of CsGd1−xEux(MoO4)2 phosphors were calculated to be 0.579, 0.581, 0.626, 0.633, 0.539 and 0.475 ms at different the Eu concentrations. The lifetime values initially increase and reach a maximum at the Eu3+ content equal to 0.6. Then, the values rapidly decrease upon increasing the Eu3+ content. Obviously, the variation of the luminescence decay curves is the same as that of the emission spectra with the variation of the Eu3+ concentration. The concentration quenching effect assigned to the energy transfer is now veri- fied in more detail. When the Eu3+ doping concentration increases from 0.1 to 0.6, more and more luminescence sites 3+ in the CsGd1−xEux(MoO4)2 host are occupied by Eu . However, upon Eu3+ doping above the critical point, the high concen- Fig. 11 A ln[I0/IT) − 1] vs.1/kT activation energy graph for the thermal 3+ tration of Eu will enlarge the ratio of ions approaching in the quenching of the CsGd0.4Eu0.6(MoO4)2 phosphor. host. As a result, both the energy transfer rate between Eu3+ and Eu3+ and the probability of energy transfer to killer sites environment, electron–phonon interaction is dominant. (such as defects) increases. Consequently, the lifetimes are 3+ shortened and the emission intensities decrease as the Eu3+ Initially, the Eu luminescence center in the excited state is 46 activated by phonon interactions which are released through a concentration grows. 5 7 crossover between the D0 excited state and Fj ( j =1,2,3,4) The thermal stability of the phosphor is an important 47 aspect when it is used in high-powered LEDs. The emission ground state. Thus, the emission intensity decreases as the temperature increases. In order to clarify the thermal quench- spectra measured for CsGd0.4Eu0.6(MoO4)2 under 394 nm exci- ff ing behavior, the activation energy was calculated through the tation at di erent temperatures between 25 and 250 °C are 48 shown in Fig. 10. The inset shows the temperature dependence Arrhenius equation: of the luminescence quenching. The emission intensity of the I I ¼ 0 ð3Þ phosphor decreases as the temperature increases up to 250 °C. T ΔE 1 þ c exp Obviously, when the temperature increased to 150 °C, the kT corresponding PL intensity dropped to 81.3% of its initial

value. This result indicates that the CsGd0.4Eu0.6(MoO4)2 phos- where I0 and IT are the luminescence intensities of phor has a relatively good thermal quenching eﬀect. The CsGd0.4Eu0.6(MoO4)2 at room temperature and at the tested thermal quenching phenomenon can be explained using the temperature, respectively; c is a constant; k is the Boltzmann − − configurational coordinate diagram. In a high-temperature constant (8.617 × 10 5 eV K 1) and ΔE is the activation energy Published on 31 March 2014. Downloaded by Michigan State University 22/09/2014 02:15:21.

Fig. 10 The PL spectra (λex = 394 nm) of the CsGd0.4Eu0.6(MoO4)2 phosphor at diﬀerent temperatures in the range of 25–250 °C. The inset Fig. 12 The CIE chromaticity coordinate diagram of the CsGd0.4Eu0.6- shows the dependence of the PL intensity on temperature. (MoO4)2 red phosphors.

Dalton Transactions Paper

for thermal quenching. According to eqn (3), the plot of ln[(I0/ 6 Z. L. Wang, H. B. Liang, L. Y. Zhou, J. Wang, M. L. Gong − 128 IT) 1] vs.1/kT yields a straight line which is illustrated in and Q. Su, J. Lumin., 2008, , 147. Fig. 11. The activation energy, ΔE, can be obtained from the 7 M. Yang, H. P. You, Y. C. Jia, H. Qiao, N. Guo and slope of the plot. The ΔE value is calculated to be 0.329 eV Y. H. Song, CrystEngComm, 2011, 13, 4046. which is comparatively high. 8 T. Wu, Y. F. Liu, Y. N. Lu, L. Wei, H. Gao and H. Chen, Fig. 12 exhibits the CIE chromaticity coordinate of the phos- CrystEngComm, 2013, 15, 2761. phor, and the value is (0.665, 0.335), which is located in the 9 Z. J. Wang, J. P. Zhong, H. B. Liang and J. Wang, Opt. deep red region. The color point is near the edge of the CIE Mater. Express, 2013, 3, 418. diagram, showing that the phosphor has a high color purity. 10 S. V. Borisov and R. F. Klevtsova, Kristallografiya, 1968, 13, 517. 11 P. V. Klevtsov and R. F. Klevtsova, J. Struct. Chem., 1977, 18, 4. Conclusions 419. 12 V. V. Atuchin and B. I. Kidyarov, J. Korean Cryst. Growth In summary, a series of red-emitting phosphors composed of Cryst. Technol., 2002, 12, 323. CsGd(1−x)Eux(MoO4)2 (x = 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) 13 V. V. Atuchin, O. D. Chimitova, T. A. Gavrilova, were prepared by conventional solid-state synthesis. The M. S. Molokeev, S. J. Kim, N. V. Surovtsev and crystal structure of CsGd(MoO4)2 and CsEu(MoO4)2 end pro- B. G. Bazarov, J. Cryst. Growth, 2011, 318, 683. ducts is defined for the first time. The XRD patterns indicate 14 G. Banoît, J. Véronique, A. Arnaud and G. Alain, Solid State that the obtained materials crystallize in a layered structure Sci., 2011, 13, 460. 3+ 3+ and Eu ions enter into the Gd sites. The crystal structures 15 V. V. Atuchin, V. G. Grossman, S. V. Adichtchev, of the CsGd(1−x)Eux(MoO4)2 solutions were clarified from N. V. Surovtsev, T. A. Gavrilova and B. G. Bazarov, Opt. ff powder X-ray di raction data as a function of composition. Mater., 2012, 34, 812. The luminescence properties of these compounds have been 16 M. Maczka, A. G. S. Filho, W. Paraguassu, P. T. C. Freire, reported. Under excitation of 394 and 465 nm, the systems J. M. Filho and J. Hanuza, Prog. Mater. Sci., 2012, 57, 1335. 5 –7 show a strong red emission at 615 nm due to the D0 F2 tran- 17 O. D. Chimitova, V. V. Atuchin, B. G. Bazarov, sition. Both the emission intensities and the lifetimes vary M. S. Molokeev and Z. G. Bazarova, Proc. SPIE-Int. Soc. Opt. 3+ with the change in Eu content in the host lattice. Further- Eng., 2013, 8771, 8771A. ffi more, the phosphors can e ciently absorb near-UV/blue light, 18 V. V. Atuchin, O. D. Chimitova, S. V. Adichtchev, making them candidates for potential red phosphors for near- B. G. Bazarov, T. A. Gavrilova, M. S. Molokeev, N. V. Surovtsev UV/blue LED applications. and Z. G. Bazarova, Mater. Lett.,2013,106, 26. 19 M. V. Bobkova, I. 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