Structural Features of Phosphates Ca_9 – xM_xDy(PO₄)₇ (M = Zn²⁺ or Mg²⁺)

Abstract—Phosphate solid solutions based on the whitlockite family Ca_9 – xM_xDy(PO₄)₇ (M = Zn²⁺ or Mg²⁺) have been obtained. The samples have been investigated by a combination of the methods, such as X‑ray diffraction analysis, second optical harmonic generation, and luminescence spectroscopy. It is established that the unit-cell symmetry change (R3c → $R\bar {3}c$) in the solid-solution series with an increase in the degree of substitution Ca²⁺ → M²⁺; the concentration dependence is determined. Phase transitions of the ferro- and antiferroelectric types, as well as their temperature positions, have been determined by dielectric spectroscopy. The synthesized samples exhibit strong luminescence under 350-nm excitation with the resulting radiation in the white region in color coordinates.