Structural features of phosphates Ca$_{9-x}$M$_{x}$Dy(PO$_{4}$)$_{7}$ ($M$ = Zn$^{2+}$, Mg$^{2+}$)

Phosphate solid solutions based on the whitlockite family Ca$_{9-x}$M$_{x}$Dy(PO$_{4}$)$_{7}$ ($M$ = Zn$^{2+}$ or Mg$^{2+}$) 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 ($R$3$c$ $\to$ $R\bar 3c$) in the solid-solution series with an increase in the degree of substitution Ca$^{2+}$ $\to$ M$^{2+}$ 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.