Comment on “Tuning of Photoluminescence and Local Structures of

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Comment on “Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu2+ phosphors” hen et al. published an article entitled “Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu2+ phosphors”.1 The paper contains data on the research of solid solutions in the system of xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu2+ and their luminescent properties. The authors assumed that Sr2Ca(PO4)2 and Ca10Li(PO4)7 are isostructural and belong to the structural type of β-Ca3(PO4)2, SG R3c. The authors of the article do not cite any references to justify this assumption and do not analyze the structural data for Sr2Ca(PO4)2 and Ca10Li(PO4)7. In these systems the authors have claimed the existence of continuous solid solutions with 0 ≤ x ≤ 1 in spite of the nonlinear change of unit cell parameters (a and c) and volume (V) in dependence of x (Figure 1a, Chen et al.1). The authors relate such change of unit cell parameters to the presence of vacancy (□) at the M4 position when the substitution proceeds according to the scheme 2Li+ = Me2+ + □. In order to prove this assumption the authors have used a formula which connects the sizes of cations and vacancies. Being the author of publications on the crystal structures of Sr2Ca(PO4)22 and Ca10Li(PO4)7,3 I would like to make the following remarks on the experimental data of the article by Chen et al.1 The Sr2Ca(PO4)2 (Sr7Ca3.5(PO4)7, Z = 6) compound is crystallized in the R3̅m space group (a = 10.6619 Å, c = 19.4815 Å, Z = 10.5,2 Figure 1b,d), while the Ca10Li(PO4)7 compound is crystallized in the R3c space group (a = 10.4203 Å, c = 37.389 Å, Z = 6,3 Figure 1a,c). According to the structural analysis data, the compound Ca10Li(PO4)73 is isostructural to β-Ca3(PO4)2 (a = 10.439 Å, c = 37.375 Å, Z = 21; Ca10.5(PO4)7, Z = 6).4 Calcium cations completely occupy the M1, M2, M3, and M5 positions, while lithium cations completely occupy the M4 position. In these structures lithium cations statistically occupy two close positions M(41) and M(42). We have proved that the occupation of M(41) and M(42) positions by lithium cations change in the temperature range of 77−300 K. In the system of Ca3(PO4)2−Sr3(PO4)22 for compositions with 0 ≤ x ≤ 10/7, solid solutions are formed isostructural to β-Ca 3 (PO 4 ) 2 (phase I, SG R3c); for compositions with 13/7 ≤ x ≤ 16/7 solid solutions of Ca3−xSrx(PO4)2 are formed with the structure derivative of βCa3(PO4)2 (phase II, SG R3̅m) and twice decreasing unit cell parameter “c”. Sr2Ca(PO4)2 (x = 14/7) is located within the field with 13/7 ≤ x ≤ 16/7. Compositions with 10/7 < x < 13/ 7 are two-phase, and X-ray patterns of phase I and phase II differ very little. Only weak reflections with odd indices l (for example, 223, 131, and 315) disappear during the transition from phase I (SG R3c) to phase II (SG R3̅m) (Figure 2). The reflections with odd indices l are also visible in the X-ray pattern of Ca10Li(PO4)7 and are absent in the XPRD pattern of Sr2Ca(PO4)2 (Figure 2). The data on the second-harmonic generation (SHG) indicate that in Ca10Li(PO4)7 (I2ω/I2ω(SiO2) = 1.4) there is no center of

C

© 2017 American Chemical Society

Figure 1. ab projection of the Ca10Li(PO4)7 (a, c) and Sr2Ca(PO4)2 (b, d) structures. Columns A and B are indicated.

symmetry. In Sr2Ca(PO4)2 the negligible background of the SHG signal (I2ω/I2ω(SiO2) = 0.022) is evidence of the center of symmetry. Thus, phases Sr2Ca(PO4)2 and Ca10Li(PO4)7 are not completely isostructural as they belong to different Received: February 14, 2017 Revised: March 9, 2017 Published: March 31, 2017 3800

DOI: 10.1021/acs.chemmater.7b00625 Chem. Mater. 2017, 29, 3800−3802

Comments

Chemistry of Materials

position. However, numerous previous studies have proved that europium cations Eu3+/Eu2+ are located only in M1, M2, and M3 positions.5,6 It should be noted that in their earlier paper while describing Sr1.75Ca1.25(PO4)2:Eu2+ the authors claimed that Eu2+ cations were located in M1, M2, M3, and M4 positions.7 In conclusion, we can note that Chen et al.1 have studied a complicated system, in which the extreme compositions have very close XPRD patterns and structures. Only the analysis of weak hkl indices with odd indices l makes it possible to choose a special group and properly interpret the properties of solid solutions, such as luminescence. The cited data on Eu2+ luminescence in the xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7 system cannot be doubted as it is really possible to conduct tuning of photoluminescence by changing the composition of solid solutions. The presence of two phases in samples does not influence luminescent properties. Nevertheless the luminescent properties of xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7 studied by Chen et al.1 are new, are correct, and do not cause problems. At the same time the interpretation of luminescent properties from the point of view of the obtained structural data is not correct. The experimental data on solid solution structures (9000− 15000 counts, see Supporting Information in ref 1) are insufficient for revealing subtle peculiarities of their structure. For this reason the authors obtained abnormal P−O distances (1.33 and 1.77 Å) in the PO4 tetrahedra. Usually P−O distances in the PO4 tetrahedra lay in the range 1.50−1.58 Å. The obtained XPRD patterns could be used only to define the lattice parameters of the Le Bail method.

Figure 2. Full (a) and partial (b) XPRD patterns for the Sr2Ca(PO4)2 (1), SrCa2(PO4)2 (2) (JCPDS 52-0467), and Ca10Li(PO4)7 (3) (JCPDS 45-0550). Reflections with identical indices are marked by broken lines.

Bogdan I. Lazoryak*

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symmetry groups, the first with center of symmetry and the second without center of symmetry. According to the Gibbs rule in equilibrium conditions, between two different phases should be a two-phase field. Such a two-phase field has been described for the solid solution of Ca3−xSrx(PO4)22 for the phase diagram of Ca3(PO4)2 (SG R3c)−Ca5/7Sr16/7(PO4)2 (SG R3m ̅ ). This system completely corresponds to the phase diagram of Sr2Ca(PO4)2 (SG R3̅m)−Ca10Li(PO4)7:Eu2+ (SG R3c) studied by Chen et al.1 The presence of doped cations Eu2+ does not influence the structure of the initial phases. In reality, in the system of Sr2Ca(PO4)2−Ca10Li(PO4)7 there are three fields: (1) solid solutions on the basis of Sr2Ca(PO4)2 (phase II, SG R3̅m), (2) solid solutions on the basis of Ca10Li(PO4)7 (phase I, SG R3c), and (3) two-phase field of Sr2Ca(PO4)2 + Ca10Li(PO4)7. The presence of the two-phase region in the system Sr2Ca(PO4)2−Ca10Li(PO4)7:Eu2+ is the true reason for nonlinear changing of the unit cell parameters, revealed by Chen et al.1 High values of R-factors (about 10%) observed during the refinement of solid solution structures of Ca3−xSrx(PO4)2 (see Supporting Information in ref 1) is related to two-phase samples and incorrect choice of a special group. In Figure 2(b1)1 there are data on joint occupation of the M4 position by calcium cations, strontium, and lithium. It should be noted that by the Rietveld it is difficult to define the occupation of a light atom (lithium) on the background of two heavy atoms (calcium and strontium). If we take into account that the lithium cation splits into two positions in Ca10Li(PO4)7,2 it becomes obvious that it is impossible to define the occupation of the M4 position by lithium cations. The authors of the paper claim that Eu2+ cations are located in the M4

Department of Chemistry, Moscow State University, 119992 Moscow, Russia

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bogdan I. Lazoryak: 0000-0003-1952-5555 Notes

The author declares no competing financial interest.

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REFERENCES

(1) Chen, M.; Xia, Z.; Molokeev, M. S.; Wang, T.; Liu, Q. Tuning of photoluminescence and local structures of substituted cations in xSr2Ca(PO4)2-(1-x)Ca10Li(PO4)7:Eu2+ phosphors. Chem. Mater. 2017, 29, 1430−1438. (2) Belik, A. A.; Izumi, F.; Stefanovich, S. Yu.; Malakho, A. P.; Lazoryak, B. I.; Leonidov, I. A.; Leonidova, O. N.; Davydov, S. A. Polar and centrosymmetric phases in solid solutions Ca3‑xSrx(PO4)2 (0 ≤ x ≤ 16/7). Chem. Mater. 2002, 14, 3197−3205. (3) Morozov, V. A.; Belik, A. A.; Kotov, R. N.; Presnyakov, I. A.; Khasanov, S. S.; Lazoryak, B. I. Crystal structures of double calcium and alkali metal phosphates Ca10M(PO4)7 (M = Li, Na, K). Crystallogr. Rep. 2000, 45, 13−20 (translated from Kristallograf iya 2000, 45, 19− 26). (4) Dickens, B.; Schroeder, L. W.; Brown, W. E. Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca3(PO4)2. I. The crystal structure of pure β-Ca3(PO4)2. J. Solid State Chem. 1974, 10, 232−248. (5) Benhamou, R. A.; Bessiere, A.; Wallez, G.; Viana, B.; Elaatmani, M.; Daoud, M.; Zegzouti, A. New insight in the structure − luminescence relationships of Ca9Eu(PO4)7. J. Solid State Chem. 2009, 182, 2319−2325. 3801

DOI: 10.1021/acs.chemmater.7b00625 Chem. Mater. 2017, 29, 3800−3802

Comments

Chemistry of Materials (6) Ding, X.; Wang, Y. Novel orange light emitting phosphor Sr9(Li,Na,K)Mg(PO4)7:Eu2+ excited by NUV light for white LEDs. Acta Mater. 2016, 120, 281−291. (7) Ji, H.; Huang, Z.; Xia, Z.; Molokeev, M. S.; Atuchin, V. V.; Fang, M.; Huang, S. New yellow-emitting whitlockite-type structure Sr1.75Ca1.25(PO4)2:Eu2+ phosphor for near-UV pumped white lightemitting devices. Inorg. Chem. 2014, 53, 5129−5135.

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DOI: 10.1021/acs.chemmater.7b00625 Chem. Mater. 2017, 29, 3800−3802