CORRELATING THE CRYSTAL STRUCTURE AND THE PHASE TRANSITIONS WITH THE DIELECTRIC PROPERTIES OF KxBa1-xGa2-xGe2+xO8 SOLID SOLUTIONS

KxBa1-xGa2-xGe2+xO8 (0 <= x <= 1) solid solutions exist in two structural modifications: a low-temperature-stable phase with a pseudo-orthorhombic P2(1)/a symmetry and a high-temperature-stable phase with a monoclinic C2/m symmetry. The P2(1)/a phase is necessary for high Qxf values. Therefore, the synthesis of the pure P2(1)/a-structure for KxBa1-xGa2-xGe2+xO8 ceramics is essential for microwave dielectric applications. In this work, the KxBa1-xGa2-xGe2+xO8 solid solutions were studied in terms of their crystal structure, phase transitions, sintering behavior and dielectric properties. The solid-state reaction between the reagents resulted in the P2(1)/a phase for the compositions 0 <= x <= 0.67 and in the C2/m phase for the compositions 0.9 <= x <= 1. However, the C2/m powders undergo a C2/m double right arrow P2(1)/a phase transformation after prolonged annealing at a temperature that is 50-80 degrees C below the lowest C2/m double right arrow P2/m transition temperature of the particular composition. The sintering of the P2(1)/a powders was also restricted to a critical temperature range in order to avoid the formation of the C2/m phase. The thermal history and the processing conditions were found to influence the C2/m double right arrow P2(1)/a phase transformation by complex mechanisms. The well-sintered KxBa1-xGa2-xGe2+xO8 (x=0.9 and 0.67) ceramics with the P2(1)/a structure showed a good combination of a low permittivity (epsilon(r)=5.6-6), high Qxf-values (96700-134000 GHz), a small temperature coefficient of resonant frequency (similar to 20 ppm/degrees C) and a low sintering temperature (910-970 degrees C).