Polymorphism and cation transport properties in arsenates Na 3M 2(AsO 4) 3 (M = Al, Cr, Fe, Ga)

Abstract

The crystal chemistry and ion transport properties of the arsenates Na 3M 2(AsO 4) 3 (designated by NaMAs) have been investigated. In agreement with the data of Schwarz and Schmidt, NaCrAs and NaFeAs have been found dimorphic, with a low-temperature garnet-like form (I). The high temperature form (II) is rhombohedral, about 10 % less compact than I. II-NaFeAs forms a range a solid solutions Na 3+3 x Fe 2− x ( AsO 4) 3 with 0 < x ≲ 0.25. NaAlAs and NaGaAs crystallize only in the II form; NaAlAs undergoes on cooling, near 44°C, a transition β⇌α associated with a slight monoclinic distortion. The I and II forms of NaMAs display very different cation transport properties : I-NaFeAs has a low conductivity (∼ 10 −6 Ω −1 cm −1 at 300°C) and is a poor cation exchanger. II-NaFeAs has a conductivity three orders of magnitude higher and exhibits, as well as II-NaAlAs, fast ion exchange reactions with molten LiNO 3 and AgNO 3 ; exchanges with KNO 3 are slower. The conductivity of vitreous NaAlAs is also reported. The results are compared with those previously obtained with Na 3M 2(PO 4) 3 and discussed in terms of structure-ion transport relationship.