Vibrational behavior of guest anions in the li3AsO4 Structure

We have investigated the vibrational behavior of the ν3 mode of guest anions ((PO4)3−, (SO4)2−) forming a dilute solid solution with the two polymorphic species of Li3AsO4. For the (PO4)3−anion, the ν3 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 triplet components, suggesting a close association between the (SO4)2− anion and the vacancy; the multiplet observed for the HT polymorph is justified by the existence of two structurally different types of vacancies. Charge compensation by the (SiO4)4− ion leads to a complex spectrum indicating the existence of two types of structurally different (SO4)2− anions, in relation to two structurally different mechanisms of charge compensation. © 1995 Academic Press, Inc.