New mixed-valence oxides of bismuth: Bi1-xYxO1.5+δ (x=0.4)

Abstract

New bismuth mixed-valence compounds Bi _1-x Y _x O _{1.5+ δ} (x=0.4, 0<δ≤0.16) have been synthesized by introducing excess oxygens into an anion-deficient fluorite type crystal Bi _0.6 Y _0.4 O _1.5 by heating at 703 K under high oxygen partial pressure up to 300 atm. Values of δ were determined by iodometric titration and thermogravimetric analysis. A distinct red shift of the absorption edge from 2.7 to ca. 1.7 eV was observed on going from δ=0 to 0.16. These compounds showed semiconductive properties with an activation energy of 0.9 eV, ruling out the possibility that the introduction of excess oxygen causes the formation of a single-valence Bi ion. These optical and electrical properties demonstrated the formation of a mixed-valence state between Bi ³⁺ 6s ² and Bi ⁵⁺ 6s ⁰ ; gap-narrowing was explained by charge-transfer absorption.