Spectroscopic study of Bi xEu 1−xVO 4 and Bi yGd 1−yVO 4 mixed oxides

Abstract

The mixed oxides Bi x Eu 1− x VO 4 and Bi y Gd 1− y VO 4 crystallize in a zircon-type structure, for 0 < x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6 s 2 of Bi 3+ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi x Eu 1− x VO 4 spectra are ascribed to intra-configurational 4 f – 4 ftransitions of the Eu 3+ ion. The broad absorption shift in BiLnVO 4 (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO 4 lattice, is ascribed to charge transfer processes in a Bi-VO 4 center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi 3+. The concept of an internal pressure of Bi 3+ ions is also used to explain the broad A-band shifts.