Phase evolution, phase transition, raman spectra, infrared spectra, and microwave dielectric properties of low temperature firing (K _0.5xBi_1-0.5x)(Mo_xV_1-x)O₄ ceramics with scheelite related structure

In the present work, the (K_0.5xBi_1-0.5x)(Mo _xV_1-x)O₄ ceramics (0≤x ≤ 1.00) were prepared via the solid state reaction method and sintered at temperatures below 830°C. At room temperature, the BiVO₄ scheelite monoclinic solid solution was formed in ceramic samples with x < 0.10. When x lies between 0.1-0.19, a BiVO₄ scheelite tetragonal phase was formed. The phase transition from scheelite monoclinic to scheelite tetragonal phase is a continuous, second order ferroelastic transition. High temperature X-ray diffraction results showed that this phase transition can also be induced at high temperatures about 62°C for x = 0.09 sample, and has a monoclinic phase at room temperature. Two scheelite tetragonal phases, one being a BiVO ₄ type and the other phase is a (K,Bi)_1/2MoO₄ type, coexist in the compositional range 0.19 < x < 0.82. A pure (K,Bi)_1/2MoO₄ tetragonal type solid solution can be obtained in the range 0.82 ≤ x ≤ 0.85. Between 0.88 ≤ x ≤ 1.0, a (K,Bi)_1/2MoO₄ monoclinic solid solution region was observed. Excellent microwave dielectric performance with a relative dielectric permittivity around 78 and Qf value above 7800 GHz were achieved in ceramic samples near the ferroelastic phase boundary (at x = 0.09 and 0.10).