Polymorphism of Li₃AsO₄ and structure refinement of its low-temperature form

Abstract

The polymorphism of Li₃AsO₄ has been studied by DTA and thermodiffraction with X-rays and neutrons. Only one reversible transition is observed: α(LT) ← β(HT) at 1018 K. The structure of the α form was determined by neutron powder diffraction using Rietveld refinement; all cations are located inside anionic tetrahedra as in wurtzite. The α ← β 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 β form.

Key Words:

• Arsenates
• lithium arsenate
• tetrahedral structures