Low-Temperature Sintering and Microwave Dielectric Properties of CuxZn1−xTi0.2Zr0.8Nb2O8 Ceramics with the Aid of LiF

M 2+ N 4+ Nb 2 O 8 -type ceramics (where M = Mg, Ca, Mn, Co, Ni, Zn and N = Ti, Zr) are essential for satellite communication and mobile base stations due to their medium relative permittivity ( ε r ) and high quality factor ( Q × f ). Although ZnTi 0.2 Zr 0.8 Nb 2 O 8 ceramic exhibits impressive microwave dielectric properties, including an ε r of 29.75, a Q × f of 107,303 GHz, and a τ f of −24.41 ppm/°C, its sintering temperature of 1150 °C remains a significant barrier for integration into low-temperature co-fired ceramic (LTCC) technologies. To overcome this limitation, a strategy involving the partial substitution of Zn 2+ with Cu 2+ and the addition of LiF as a sintering aid was devised for ZnTi 0.2 Zr 0.8 Nb 2 O 8 . The dual impact of Cu 2+ partial substitution and LiF as a sintering enhancer facilitated the successful sintering of Cu 0.3 Zn 0.7 Ti 0.2 Zr 0.8 Nb 2 O 8 ceramics at a reduced temperature of 950 °C using the conventional solid-state reaction method. These ceramics exhibited excellent microwave dielectric properties. Notably, Cu 0.3 Zn 0.7 Ti 0.2 Zr 0.8 Nb 2 O 8 ceramic with 40 mol% LiF addition demonstrated optimal microwave dielectric properties without any reaction with a silver electrode at a sintering temperature of 950 °C, yielding ε r = 32, Q × f = 45,543 GHz, and τ f = −43.5 ppm/°C.