Comment on “Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+ phosphors”

Substituted Cations in xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu phosphors” C et al. published an article entitled “Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu phosphors”. The paper contains data on the research of solid solutions in the system of xSr2Ca(PO4)2−(1 − x)Ca10Li(PO4)7:Eu 2+ 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 The authors of the article do not cite any references to justify this assumption and do not analyze the structural data for Sr2Ca(PO4)2 and Ca10Li(PO4)7.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.). 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 = Me + □. 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)2 2 and Ca10Li(PO4)7, 3 I would like to make the following remarks on the experimental data of the article by Chen et al. 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, 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, Figure 1a,c). According to the structural analysis data, the compound Ca10Li(PO4)7 3 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)2 2