VIBRATIONAL BEHAVIOR OF GUEST ANIONS IN THE LI3ASO4 STRUCTURE

We have investigated the vibrational behavior of the v(3) mode of gues t anions ((PO4)(3-), (SO4)(2-)) forming a dilute solid solution with t he two polymorphic species of Li3AsO4. For the (PO4)(3-)anion, the v(3 ) mode is represented by a more or less resolved triplet, as expected from the low site symmetry and the small distortion of the host anion. But a much more complex vibrational behavior is exhibited by the (SO4 )(2-) anion, whose different charge must be chemically compensated for , either by a Li+ vacancy, or by another suitable anion such as (SiO4) (4-). Compensation by a vacancy leads to a large splitting of the trip let components, suggesting a close association between the (SO4)(2-) a nion and the vacancy; the multiplet observed for the HT polymorph is j ustified by the existence of two structurally different types of vacan cies. Charge compensation by the (SiO4)(4-) ion leads to a complex spe ctrum indicating the existence of two types of structurally different (SO4)2- anions, in relation to two structurally different mechanisms o f charge compensation. (C) 1995 Academic Press, Inc.