Phase Formation in the System of Triple Phosphates Sr–M2+–Ln3+ (M2+ = Zn2+, Mg2+, Mn2+; Ln3+ = Eu3+, Tb3+)

The phase formation in the system of triple phosphates Sr–M²⁺–Ln³⁺ (M²⁺ = Zn²⁺, Mg²⁺, Mn²⁺; Ln³⁺ = Eu³⁺, Tb³⁺) is considered. Specific features of the strontiowhitlockite phase formation and the isomorphism in the series of phosphates Sr\(_{{9-x}}\)Mn_xTb(PO₄)₇, Sr\(_{{9-x}}\)Mg_xEu(PO₄)₇, and Sr\(_{{9-x}}\)Zn_xEu(PO₄)₇ (0 ≤ x ≤ 1.0), obtained by solid-phase synthesis, have been studied. It is shown that a continuous series of solid solutions of these compositions cannot be formed. Crystallization of phases with strontiowhitlockite-type structures is observed for only stoichiometric phosphates Sr₈MgEu(PO₄)₇ and Sr₈ZnEu(PO₄)₇. Crystal chemical analysis of the possibility of strontiowhitlockite phase formation in the studied series is performed. It is shown that samples with the strontiowhitlockite structure crystallize in the centrosymmetric space group (sp. gr. R\(\bar {3}\)m), in contrast to the mother structure of mineral whitlockite and its calcium-based synthetic analogues. The conditions for the formation of phosphates with the stronciowhitlockite structure are indicated. The photoluminescence properties have been investigated. It is shown that the samples exhibit stable photoluminescence in the red-orange region due to the emission of Eu³⁺ cations, whereas a quenching effect is observed in the Sr\(_{{9-x}}\)Mn_xTb(PO₄)₇ series.