STRUCTURE AND PROPERTIES OF COMPLEX PEROVSKITE MICROWAVE CERAMIC DIELECTRIC RESONATORS. H. Sreemooolanadhan, R. Ratheesh, M. T. Sebastian, P. Mohanan, Electronic Ceramics, Regional Research Laboratory, Thiravananthapuram-695 0l9, India ^*Electronics Department, Cochin Univ. of Sci. & Tech., Thrikkakara-682 022, India

Ceramic materials with high dielectric constant and low loss called `dielectric resonators (DR)' have become indispensable components for both active and passive devices in modern microwave systems. Ba(B'_1/2 Nb_1/2)O₃ [B' = La, Pr, Nd, Sm, In, Tb, Y, Gd, Er] complex perovskites have been prepared as a microwave dielectric ceramic resonator material. They have high [[epsilon]]_r(30-45), low loss and very small temperature variation of resonant frequency in the microwave range. Several earlier workers reported that these materials have cubic symmetry with (NH₄)₃FeF₆ structure. Recent studies show that these materials have a cubic symmetry at high temperature and transforms to one with a lower symmetry (tetragonal, orthorhombic or monoclinic) on cooling. The mechanism of the transition and its relation to the tolerance factor, difficulties of X-ray diffraction to find the transition and the correct symmetry and dielectric properties of these materials are discussed.