NASICON-TYPE MATERIALS - NA3SC2(PO4)3, NA3CR2(PO4)3, NA3FE2(PO4)3 - NA+-NA+ CORRELATIONS AND PHASE-TRANSITIONS

The thermal behavior of Nasicon type materials Na//3M//2(PO//4)//3 (M equals Sc, Cr, Fe) is examined. Three types of phases are found. a) A high temperature superionic rhombohedral gamma phase, with only local order, which is the same for all three M elements, b) An intermediate phase in which the average lattice is either rhombohedral or monoclinic. This intermediate phase corresponds to highly correlated Na//2** plus type ions while Na//1** plus ions are strongly disordered. c) A low temperature alpha phase which is different for M equals Cr or Fe and corresponds to an ordering of the Na//1 ions. For M equals Sc it is shown that powders tend to Fe alpha type correlations while single crystals tend to Cr alpha type one because of strong strains. The transitions are sharp for M equals Fe (and Sc powders), much smoother for M equals Cr and diffuse for M equals Sc crystals. They are discussed in terms of ion transfer.