Impurity band conduction in reduced samples of bismuth iron molybdate

Abstract

An experimental investigation of the electrical resistance of n-type bismuth iron molybdate (Bi3FeMo2012) pellets at temperatures from 90 to 620 K has been made in an attempt to detect impurity band conduction near room temperature. The pellets were chemically reduced by exposing them to 13.2% methanol vapour in a nitrogen carrier gas inside an oven at 620 K. Lightly reduced samples yield an activation energy of 360 meV over the temperature region of 300 - 620 K which represents the donor energy level below the conduction band edge. A change in this activation energy over the temperature region of 150 to 300 K suggests the adsorption of water vapour or oxygen which tends to trap out carriers and suppress the pellet’s conductance. For the more heavily reduced samples a near temperature independent resistance, in the temperature range of 90 to 300 K, suggests a metallic like conduction through an impurity band. A lowering of the activation energy, with increasing reduction, from 16 meV down to 1 meV suggests a transition from hopping to metallic conduction in the impurity band formed by the oxygen vacancy donor states. As the oxygen vacancy donor concentration increases the temperature region dominated by conduction through the impurity band expands to higher temperature. X-ray powder diffraction measurements reveal the formation of Bi, Bi203 or MoO3 as well as MoO2 during the reduction sequence, but have been excluded as an explanation of the metallic like behavior observed in the temperature region of 90 to 300 K due to the small amounts observed.