STRUCTURE OF BA(Y1 2(+3)TA1/2(+5))O-3 AND ITS DIELECTRIC-PROPERTIES IN THE RANGE 10(2)-10(14)-HZ, 20-600-K/

The goal of this work was to understand the correlation between micros copic material parameters and the dielectric function of candidate mat erials for applications in the microwave frequency range. The structur e and dielectric properties of Ba2+(Y1/23+Ta1/25+)O-3 (BYT), a typical representative of the Ba(B1/23+B1/25+)O-3 complex perovskite family, has been investigated from 10(2) to 10(14) Hz and from 20 to 600 K. At T-c=253+/-1 K, BYT undergoes an equitranslational improper ferroelast ic, second-order phase transition, characterized by the tilting of the oxygen octahedra. The space group symmetry changes from <Fm(3)over ba r m>, in the high temperature phase, to I4/m below T-c. The existence of an intermediate temperature region (T-c-40<T<T-c) has been observed , where the compound exhibits structural and dielectric properties dif ferent from those in the well-defined high (T>T-c) and low (T< T-c-40 K) temperature phases. Infrared reflectivity (10(12)-10(14) Hz) and su bmillimeter transmission (10(11)-3X10(12) Hz) measurements yield diele ctric losses which are believed to be mainly of intrinsic origin (one- and two-phonon absorption). Comparing a theory of two-phonon differen ce absorption processes, due to thermally activated polar branches, wi th the loss measured at 400-1400 GHz, the intrinsic loss can be extrap olated to lower frequencies. At 10 GHz the extrapolated value is about 1/4 of the loss actually measured in a BYT resonator. Nonpolar phonon s, including the soft branch, which have not been considered for the e xtrapolation procedure, are partially responsible. The temperature dep endencies indicate the soft branch to be of considerable importance fo r intrinsic losses. Oxygen vacancies can be excluded as an extrinsic l oss source, as sintering and annealing in N-2, air, and O-2 had no mea surable influence on the loss at 10 GHz.