Preparation and crystal chemistry of some tetrahedral Li3PO4-type compounds

The crystal chemistry of Li3PO4, Li3VO4 and Li3AsO4 are compared. All three have an isostructural low phase, designated βII, and an isostructural high phase, γII, but in Li3VO4 and Li3AsO4 the high-low transformation proceeds reversibly through one or more transitional phases some of which can be quenched to ambient. The crystal chemistry of derivative Li3PO4 phases, including Li2MgSiO4, Li2ZnSiO4, Li2CoSiO4, Li2MgGeO4 and Li2ZnGeO4 is compared and the occurrence of high, low, and of distorted high and low phases is correlated with the temperature of preparation and rate of cooling. The derivative Li3PO4 phases show extensive or complete mutual solubility not only with each other, but with Li3PO4, with M2XO4 compounds (M = Zn2+, Mg2+; X = Ge4+, Si4+) and also with Li4XO4 compounds (X = Ge4+, Si4+). The sequence of phase transformations encountered on heating or cooling is quite sensitive to the stoichiometry of the derivative phases.