Nonstoichiometric microwave dielectric ceramics [(Na_0.5-xBi_0.5+x/3)_0.5Ca_0.5]MoO₄ with low sintering temperatures

In this work, [(Na_0.5-xBi_0.5+x/3)_0.5Ca_0.5]MoO₄ (x = ±0.03, ±0.06, ±0.09, ±0.12) microwave dielectric ceramics prepared by the solid-state reaction method are investigated. All the samples can be sintered well below 800℃. The sintered ceramics show a scheelite structure without any secondary phase, indicating that a solid solution is formed in nonstoichiometric [(Na_0.5-xBi_0.5+x/3)_0.5Ca_0.5]MoO₄. While x value increases from -0.12 to 0.12, the relative permittivity rises from 16.7 to 21.0, TCF value is improved from -21 ppm/℃ to +1 ppm/℃, and Q × f value varies in the range of 17,000 GHz and 34,000 GHz. The Raman analysis reveals that one of the external modes is attributed to be the main factor affecting the performances. When x = 0.09 and 0.12, high performance microwave dielectric ceramics can be well densified at low sintering temperatures (750−775℃) with relative permittivities of 20.9–21.0, improved Q × f values of 31,400−33,000 GHz, and near-zero temperature coefficients of resonate frequency (|TCF| ≈ ±2 ppm/℃).