POLYMORPHISM OF LI3ASO4 AND STRUCTURE REFINEMENT OF ITS LOW-TEMPERATURE FORM

The polymorphism of Li3AsO4 has been studied by DTA and thermodiffraction with X-rays and neutrons. Only one reversible transition is observed: alpha(LT) --> beta(HT) at 1018 K. The structure of the alpha form was determined by neutron powder diffraction using Rietveld refinement; all cations are located inside anionic tetrahedra as in wurtzite. The alpha --> beta transition consists of cationic re-ordering within the tetrahedral sites of an oxygen hexagonal close packed (hcp) arrangement. The orthorhombic cell is conserved but with two modifications: doubling of the b parameter caused by the new cationic order and an increase of the c parameter resulting from Coulombic repulsions between Li+ cations located in edge-sharing tetrahedra of the beta form.