Influence of Germanium Substitution on the Crystal Chemistry and Dielectric Properties of Mg2SnO4

The effects of Ge4+ substitution on the microwave dielectric properties of inverse spinel Mg2SnO4 ceramics were systematically investigated. A series of Mg2(Sn1−xGex)O4 (x = 0.00–0.05) ceramics were synthesized via solid-state reaction and sintered at 1450–1600 °C. X-ray diffraction confirmed single-phase inverse spinel structures (Fd-3 m) for compositions up to x = 0.03, while minor MgSnO3 secondary phases appeared at x = 0.05. Rietveld refinement revealed a linear decrease in lattice parameter from 8.6579 Å (x = 0) to 8.6325 Å (x = 0.05), consistent with Vegard’s law for the substitution of smaller Ge4+ (0.53 Å, Shannon ionic radius, octahedral coordination) for Sn4+ (0.69 Å, Shannon ionic radius, octahedral coordination) in octahedral sites. Optimal dielectric properties were achieved at x = 0.03 sintered at 1550 °C; the dielectric constant (εr) increased from 7.6 to 8.0, while the quality factor (Qf) improved by 19% from 56,200 to 67,000 GHz, which is attributed to reduced phonon scattering from Ge-induced lattice contraction. The temperature coefficient of resonant frequency (τf) remained stable (−64 to −68 ppm/°C) across all compositions. Property degradation at x = 0.05 correlated with the onset of Ge4+ solubility limit and MgSnO3 formation. These results demonstrate that controlled Ge4+ substitution effectively enhances the microwave dielectric performance of Mg2SnO4 ceramics for communication applications.