IMPURITY BAND CONDUCTANCE THROUGH OXYGEN VACANCY DONOR STATES IN BISMUTH IRON MOLYBDATE

Under chemical reduction at 620 K in an atmosphere of methanol vapor i n a nitrogen carrier gas, bismuth iron molybdate [Bi3FeO4(MoO4)(2)] wi ll give up lattice oxygen and form highly mobile vacancies. These vaca ncies act as donors and sit about 0.4 eV below the conduction band edg e: The band gap is 2.7 eV. Because of the ability to remove up to 1% o f the lattice oxygen and still maintain crystallographic stability, it is relatively easy to produce high donor carrier densities (up to 10( 20) cm(-3)). Under these conditions, and taking into account that this is a wide gap semiconductor with well compensated deep donor levels, it is possible to measure an electrical conductivity dominated by carr ier movement through the impurity band. Both phonon assisted hopping a nd the transition to metallic impurity conduction have been observed. The crossover temperature from conduction dominated by the impurity ba nd to conduction dominated by the conduction band was seen to increase from 125 to 380 K with increasing oxygen depletion. (C) 1996 American Institute of Physics.