Polarons, oxygen vacancies, and hydrogen in Ba_xSr₁-_xTiO₃

The role of hydrogen in strontium titanate and barium strontium titanate is analysed, for both ceramics and single crystals. New infrared and second-harmonic generation (SHG) experiments show the relationship between hydrogen and oxygen vacancies. The infrared data exhibit strong absorption in the 3000-6000 cm^-1 region due to polarons. The SHG data exhibit rather strong signals at room temperature (nominally forbidden). These data are correlated with the temperature dependence of activation enthalpies from Steinsvik (Oslo), and it is found that the latter are characterised by a universal exponent of 7/4 for the dependence upon vacancy concentration, in agreement with the predictions for a cubic lattice in percolation theory.