Polymorphism and cation transport properties in arsenates Na3M2(AsO4)3 (M = Al, Cr, Fe, Ga)
1986; Elsevier BV; Volume: 18-19; Linguagem: Inglês
10.1016/0167-2738(86)90167-0
ISSN1872-7689
AutoresF. d'Yvoire, M. Pintard-Scrépel, E. Bretey,
Tópico(s)Chemical Synthesis and Characterization
ResumoThe crystal chemistry and ion transport properties of the arsenates Na3M2(AsO4)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 Na3+3xFe2−x(AsO4)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 LiNO3 and AgNO3 ; exchanges with KNO3 are slower. The conductivity of vitreous NaAlAs is also reported. The results are compared with those previously obtained with Na3M2(PO4)3 and discussed in terms of structure-ion transport relationship.
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