Structural investigation of KxBa1−xGa2−xGe2+xO8 solid solutions using the X-ray Rietveld method

The KxBa1−xGa2−xGe2+xO8 (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe3O8 (space group P21/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T–O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P21/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.