DIELECTRIC-SPECTROSCOPY OF BA(B'B-1/2''(1/2))O-3 COMPLEX PEROVSKITE CERAMICS - CORRELATIONS BETWEEN IONIC PARAMETERS AND MICROWAVE DIELECTRIC-PROPERTIES .2. STUDIES BELOW THE PHONON EIGENFREQUENCIES (10(12)-10(12) HZ)

Dielectric spectroscopy in the submillimeter, millimeter, microwave, and radio frequency range has been performed between 300 and 600 K (for some cases below 300 K) on nine Ba(B1/2'B1/2 ')O3 complex perovskite ceramic compounds. The real part of the permittivity ε' decreases linearly with the increasing tolerance factor t<1 approaching unity. It is insensitive to imperfections in the ceramic, such as impurities, vacancies, etc., and entirely determined by polar lattice vibrations. Its temperature dependence is influenced by the presence of a structural phase transition observed in six of the investigated compounds. It is shown that the imaginary part of the permittivity ε' in the submillimeter range is mainly of intrinsic origin. The ε'(f) dependences were fitted applying a microscopic theory using polar-phonon parameters that have been determined in the phonon resonance region by infrared reflection spectroscopy (Part I). The theory allows the extrapolation of minimum intrinsic loss due to polar-phonon contributions down to the microwave region. The difference between the extrapolated and measured loss at 10 GHz is due to other intrinsic and extrinsic contributions gaining importance at lower frequencies. The submillimeter measurements reveal a systematic loss decrease with the tolerance factor approaching unity (optimal packing), suggesting the ionic size to be of importance for intrinsic loss. A fourth power dependence of loss on permittivity has been found which compares well with the theoretically expected dependence. The contribution of two-phonon difference absorption processes due to the nonpolar soft branch influences the microwave loss as evidenced in particular by ε'(T) measurements. In the case of Nd- and Gd-containing compounds losses related to the paramagnetic subsystem are believed to be the origin of increasing loss with decreasing temperature at 10 GHz. © 1995 American Institute of Physics.