Studies on the defect structure of α-Nb 2O 5

Abstract

It is known that α-Nb 2O 5 exists as a metal-excess semiconductor. In order to further study the manner in which the excess metal is present in the lattice, the oxygen pressure dependence and the temperature dependence of electrical conductivity have been re-examined using α-Nb 2O 5 single crystals. An oxygen pressure range of one atm to 10 −6 atm and a temperature range of 600°C–1350°C were investigated. The dependence of conductivity on the pressure of oxygen is found to follow σ ∼ p o 2 − 1 n . The coefficient n is equal to four at temperatures above 800°C and is not well defined at the lower temperature. From the temperature dependence of the conductivity at a constant oxygen pressure it is found that, at high temperatures, the apparent activation energy is 1.4 eV, whereas below 800°C it is 0.4 eV. The change in lattice parameter as a function of deviation from stoichiometric composition has also been determined. The results obtained at high temperatures agrees with the previous findings on the sintered specimens that the predominant equilibrium defects in nonstoichiometric α-Nb 2O 5 are oxygen vacancies. The behavior found at low temperatures is associated with the surface reaction controlled conduction. From quenching experiments, the effective diffusion constant of oxygen vacancies in α-Nb 2O 5 has been found to obey the empirical relation, D = 4⋯7 × 10 −3 exp − 1⋯2 kT cm 2/sec.