R. Zurmuhlen et al., 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)/, Journal of applied physics, 77(10), 1995, pp. 5351-5364