Novel laser crystals in Ca9Y(VO4)7-x(PO4)x mixed system

- Ca9Y(VO4)7-x(PO4)x solid solution single crystals were grown by the Czochralski method.
- The Ca9Y(VO4)3(PO4)4 centrosymmetric structure was described in details.
- The characterization Ca9Y(VO4)7-x(PO4)x solid solution single crystals was done.

The Ca9Y(VO4)7-x(PO4)x (x = 0, 1, 3, 4) solid solution whitlockite-like single crystals have been grown by the Czochralski method from mixtures of single phase Ca9Y(VO4)7 and Ca9Y(PO4)7 starting materials produced by solid-state synthesis. Real compositions of solid solution crystals has been determined and analyzed. It is found that crystals with x = 0, 1, 4 exist with polar structure at room temperature, space group R3c. At least one of them, Ca9Y(VO4)7, demonstrates ferroelectric behaviour with phase transition at 942 °C. Unlike the others, Ca9Y(VO4)3(PO4)4 crystallizes in centrosymmetric whitlockite-like structure with space group R3¯c confirmed by single-crystal X-ray structure analysis. Similar to other whitlockite-family phosphates and vanadates, its crystal structure is described as a disordered structural version of usual whitlockites, corresponding to their high-temperature paraelectric phases. The transmission and luminescence spectra of solid solution crystals are reported, the hardness Hv and fracture toughness K1c of solid solution single crystals are also determined.