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

New bismuth mixed-valence compounds Bi1-xYxO1.5+delta(x = 0.4, 0 < delta less than or equal to 0.16) have been synthesized by introducing excess oxygens into an anion-deficient fluorite type crystal Bi0.6Y0.4O1.5 by heating at 703 K under high oxygen partial pressure up to 300 atm. Values of delta 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 delta = 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 Bi3+ 6s(2) and Bi5+ 6s(0); gap-narrowing was explained by charge-transfer absorption.