Phase evolution, phase transition, and microwave dielectric properties of scheelite structured xBi(Fe _1/3Mo _2/3)O ₄-(1-x) BiVO ₄ (0.0 ≤ x ≤ 1.0) low temperature firing ceramics

In the present work, the xBi(Fe _1/3Mo _2/3)O ₄-(1-x)BiVO ₄ (0.0 ≤ x ≤ 1.0) ceramics were prepared via the solid state reaction method. All the ceramics can be densified at low sintering temperatures around 820 °C. At room temperature, the BiVO ₄ type scheelite monoclinic solid solution was formed in ceramic samples with a composition of x ≤ 0.10. When x lies between 0.1 and 0.7, a BiVO ₄ scheelite tetragonal phase is formed at room temperature. In the range 0.7 ≤ x < 0.9, the ceramic samples were found to be composites consisting of BiVO ₄ type tetragonal and Bi(Fe _1/3Mo _2/3)O ₄ type monoclinic scheelite phases, and when x ≥ 0.9, the Bi(Fe _1/3Mo _2/3)O ₄ type monoclinic scheelite solid solution was formed. In the BiVO ₄ type monoclinic solid solution region, the phase transition to tetragonal phase was studied by in situ Raman and Far-Infrared spectroscopies and by thermal expansion analysis. All of these methods indicated that the phase transition temperature almost linearly decreased from 255 °C for pure BiVO ₄ to about -9 °C for x = 0.1 sample. High performance microwave dielectric properties with a high permittivity of about 74.8, high Qf values above 11500 GHz, and a small temperature coefficient of resonant frequency within +20 ppm per °C in a wide temperature range of 20-140 °C can be obtained in the composite ceramic sample with 60 mol% x = 0.10 composition and 40 mol% x = 0.02 composition. The xBi(Fe _1/3Mo _2/3)O ₄-(1-x)BiVO ₄ (0.0 ≤ x ≤ 1.0) ceramics might provide useful candidate materials for microwave integrated capacitive devices, such as filters, antennas, etc.