Centrosymmetric and acentric whitlockite-type phosphates and vanadates among Ca3(VO4)2-Ca3(PO4)2-Y2O3 compositions

Abstract

Identifications and studies of whitlockite-type phases for the first time are fulfilled in the cation-anion mixed phosphate-vanadate ferroelectric systems Ca10.5(V1-x P x O4)7, Ca9Y(V1-x O4)7, Ca10.5-1.5x Y x (VO4)7 and Ca10.5-1.5x Y x (PO4)7. These systems are the sections of a more extensive multi-component system Ca3(VO4)2-Ca3(PO4)2-Y2O3, but are all built on the basis of well-known individual compounds of the type of mineral whitlockite. The solid solutions formed between them are also of ferroelectric whitlockite-type, but some compositions are not ferroelectric, and according to the nonlinear optical phase analysis possess centrosymmetric structure. Ferroelectric phase transitions were detected in the region of 1000-1300 K by the method of dielectric spectroscopy. A precision X-ray study of some compositions of solid solutions using synchrotron radiation revealed a significant deviation in the cationic-anionic arrangement from being completely disordered. Partial ordering of structural tetrahedral groups of PO43- and VO43- determines a significant increase in nonlinear optical activity in the mixed anions whitlockite-type compounds.