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Optical Measurements on Heavily Reduced Bismuth Iron Molybdate: Evidence for Vacancy-Induced Phase Transitions

Volume 46, Number 12 (Dec. 1992) Page 1898-1903

Sears, W.M.; Keeler, W.J.

It is known that the conductance of a bismuth iron molybdate pellet is greatly affected by the stoichiometry (number of oxygen vacancies) of the compound. The number of these vacancies is determined by the amount of reducing vapor (such as ethanol), which tends to increase their number and the amount of oxygen, which tends to decrease it, in the atmosphere. For this reason the compound can be used as a chemical sensor. Earlier measurements of the conductance in a highly reducing methanol vapor atmosphere, determined by measuring the current passing between two gold wires embedded in a pressed pellet, have shown three well-defined stages in the conductance, which are related to solid-state phase transitions occurring on the surface of the pellet granules. In this paper we further examine the nature of these transitions using infrared, Raman, and photoluminescence spectroscopy to follow the structural changes that occur as greater amounts of oxygen are removed. These changes are seen as shifts in relative scattering intensity as well as frequency.