Identification of Fe⁴⁺, Fe⁵⁺ and Fe⁴⁺-V₀ Photochromic Absorption Bands in SrTiO₃

Abstract

SrTiO₃Aℓ exhibits photochromism due to the presence of iron as a minority impurity. The wavelength dependence of the optical absorption due to Fe⁵⁺ agrees with the wavelength dependence of the bleaching of Fe⁵⁺ EPR.¹ After the decay of the Fe⁵⁺ and an initial decay of Fe³⁺ EPR, the intensity of the latter is enhanced with the same wavelength dependence as the optical absorption due to Fe⁴⁺. During these processes, a hole center is also created. This correlation of the optical absorption with the EPR bleaching and enhancement curves, shows that the photochromic absorption bands must be due to electrpn transfer from the O²⁻ valence states to the Fe⁵+ and Fe⁴⁺ ions. The bands occur at 2.09 and 2.82 eV for Fe⁴⁺, and 1.99 and 2.53 eV for Fe⁵⁺; they show a shift to higher energies as the charge state of the impurity decreases.² A weak EPR, spectrum axially symmetric around <100>, with g ^e_ll ∼ 8, g ^e_⊥ ∼ 0 was observed in Aℓ and Mg-doped crystals. This spectrum is due to Fe⁴⁺-V_O. The angular dependence of the resonance fields is given by

$$H\cdot 4gn\cdot \cos \delta ={{\left[{{\left(hv\right)}^{2}}-{{a}^{2}}\right]}^{1/2}}$$

similar to Mn³⁺-V_O.3 From X- and K-band measurements, g_ll = 2.007 ± 0.001, a = 0.1504 ± 0.0008 cm⁻¹. Optical charge transfer of valence O²⁻ electrons to Fe⁴⁺-V_O, converts the center to well-known Fe³⁺-V_O; the respective EPR quenching and creation rates are identical in the investigated energy range 1.85 – 3.07 eV, this allowed a determination of the charge transfer bands.⁴ Fe⁴⁺-V_O is a shallow acceptor and observed only in low Fermi-level SrTiO₃ at temperatures below 77°K.

Text not available. Details will be published in refs. 2 and 4.