Polarons, oxygen vacancies, and hydrogen in BaxSr1-xTiO3

The role of hydrogen in strontium titanate and barium strontium titanate is analysed, for both ceramics and single crystals. New infrared and second-h armonic generation (SHG) experiments show the relationship between hydrogen and oxygen vacancies. The infrared data exhibit strong absorption in the 3 000-6000 cm(-1) region due to polarons. The SHG data exhibit rather strong signals at room temperature (nominally forbidden). These data are correlate d with the temperature dependence of activation enthalpies from Steinsvik ( Oslo), and it is found that the latter are characterised by a universal exp onent of 7/4 for the dependence upon vacancy concentration, in agreement wi th the predictions for a cubic lattice in percolation theory. (C) 2001 Else vier Science Ltd. All rights reserved.