Structural dependence of the microwave dielectric properties of Cr³⁺-substituted ZnGa₂O₄ spinel ceramics: Crystal distortion and vibration mode studies

Zn[Cr_xGa_(1-x)]₂O₄ (abbreviated Cr-ZGO) ceramics with x = 0-0.15 were synthesized by the conventional solid state method. The correlation between the microwave dielectric properties and the changes in the crystal structure and cation distribution of Cr-ZGO as a function of Cr³⁺ substitution for Ga³⁺ has been systematically investigated. The addition of Cr³⁺ promotes the densification of the ceramics and results in a normal spinel structure with Cr³⁺ occupying an octahedral site. With an increase in the Cr³⁺ content from 0 to 15 mol%, the ϵ_r of the ceramics increases, while the τ_f remains almost unchanged and the Q × f values ramp up to a maximum at 1 mol% Cr³⁺ addition and then ramp down continuously. 1Cr-ZGO exhibits the best microwave dielectric properties, with ϵ_r, Q × f, tanδ and τ_f being 9.88 (@9.9 GHz), 111470 GHz, 8.87, and -74 ppm °C^-1, respectively. The Q × f value was increased by almost 40% compared with the pure ZGO ceramics (∼80000 GHz). The enhancement of ϵ_r and Q × f values of Cr-ZGO can be attributed to the better bulk density, but further analysis by Rietveld refinement, Raman spectroscopy and FT-IR spectroscopy elucidated that the octahedral tilting in the spinel structure and the degree of short-range cation distribution are other governing factors. The effect of Cr³⁺ substitution for Ga³⁺ on the octahedral distortion and cation distribution in the spinel structure has been discussed in detail.