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.