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-Nb 2 O 5 )-xGeO 2 were investigated systematically. The phase composition and phase transformation of (1-x) (3ZnO-Nb 2 O 5 )-xGeO 2 ceramics were confirmed by XRD refinement, SEM micrographs and EDS analysis. All sintered samples were composed of Zn 3 Nb 2 O 8 and ZnNb 2 O 6 phases. Phase transition between Zn 3 Nb 2 O 8 and ZnNb 2 O 6 was closely related the introduction of GeO 2 . When the doping content was low (x = 0–0.007), the phase transformation from ZnNb 2 O 6 to Zn 3 Nb 2 O 8 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 Ge 4+ , 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 ZnO 6 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-0.91Nb 2 O 5 -0.09GeO 2 sintered at 1150 °C for 6 h had excellent microwave dielectric properties: ε r  = 21.3, Q f  = 99,140 GHz, τ f  = −54 ppm/°C, which, to our best knowledge, the best performance with relative lower sintering temperature. [ABSTRACT FROM AUTHOR]