The electrical conductance of SrFeO2.5+x thin films

Thin films of the non-stoichiometric perovskite SrFeO^sub 2.5+x^ have been grown by the pulsed excimer laser deposition technique onto sapphire substrates. The electrical conductance properties of the thin films have been determined in a series of experiments done both isothermally and with programmed temperature changes from ambient to 490°C and under O^sub 2^/N^sub 2^ atmospheres with oxygen concentrations in the range from 100 ppm to 100%. Over these ranges of temperature and oxygen partial pressure a wide range of oxygen stoichiometry in SrFeO^sub 2.5+x^ occurs (approximately 0 < x < 0.5), which includes all four known phases in the SrFeO^sub 2.5 + x^ + O^sub 2^ system. The experimentally measured values for the activation energy of conduction, ε^sub A^, for SrFeO^sub 2.5+x^ films at temperatures 100 < T < 200°C are in the range 0.30 < ε^sub A^ < 0.47 eV under oxygen at partial pressures 0.001 0.001 atm, the oxygen stoichiometries of the films change during the programmed temperature ramps. For these conditions, the values δε^sub A^/δ T exhibit minima/maxima in the temperature range 250 < T < 320^sup °^C which are interpreted as being due to the onset of the order-disorder phase transition from the cubic to the tetragonal and orthorhombic ordered phases of SrFeO^sub 2.5+x^ with oxygen stoichiometry in the range 0.08 < x < 0.38. The SrFeO^sub 2.5+x^ thin films have application as oxygen sensing materials, and a relationship between conductance and oxygen sensitivity, S^sub ox^, has been derived. The values of S^sub ox^ for SrFeO^sub 2.5+x^ thin films increases by more than an order of magnitude for compositions close to the lower stoichiometric limit where the principal phase conversion is between the cubic perovskite and the brownmillerite forms.[PUBLICATION ABSTRACT]