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

The Ca 9 Y(VO 4 ) 7-x (PO 4 ) x (x = 0, 1, 3, 4) solid solution whitlockite-like single crystals have been grown by the Czochralski method from mixtures of single phase Ca 9 Y(VO 4 ) 7 and Ca 9 Y(PO 4 ) 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 R 3 c . At least one of them, Ca 9 Y(VO 4 ) 7 , demonstrates ferroelectric behaviour with phase transition at 942 °C. Unlike the others, Ca 9 Y(VO 4 ) 3 (PO 4 ) 4 crystallizes in centrosymmetric whitlockite-like structure with space group R 3 ¯ 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 H v and fracture toughness K 1c of solid solution single crystals are also determined.