Investigation of the phase transformation and its effects on the microwave performance in 3ZnO-Nb2O5-GeO2 system

The phase composition, crystal structure and microwave dielectric properties of (1-x) (3ZnO-Nb2O5)-xGeO2 were investigated systematically. The phase composition and phase transformation of (1-x) (3ZnO-Nb2O5)-xGeO2 ceramics were confirmed by XRD refinement, SEM micrographs and EDS analysis. All sintered samples were composed of Zn3Nb2O8 and ZnNb2O6 phases. Phase transition between Zn3Nb2O8 and ZnNb2O6 was closely related the introduction of GeO2. When the doping content was low (x = 0-0.007), the phase transformation from ZnNb2O6 to Zn3Nb2O8 occurred, which was discovered for the first time. To clarify the phase transition, the local charge density and band structures were calculated using the CASTEP program. With the substitution of Ge4+, part of the electrons transferred from the oxygen atom to the middle of 'niobium-oxygen' and 'zinc-oxygen'. According, the covalency of 'niobium-oxygen' and 'zinc-oxygen' was enhanced. The shrinkage of ZnO6 octahedron resulted in a decrease in the coordination number of zinc atoms. At last, the relationships between phase transition, crystal structure and the microwave performance were established. The system of 2.73ZnO-0.91Nb2O5-0.09GeO2 sintered at 1150 °C for 6 h had excellent microwave dielectric properties: εr = 21.3, Qf = 99,140 GHz, τf = −54 ppm/°C, which, to our best knowledge, the best performance with relative lower sintering temperature.