Chemical bond characteristics, Raman spectra, and microwave dielectric properties of garnet ceramics Ca3B2ZnGa2O12 (B = Nb, Sb)
Two cubic garnet ceramics, Ca 3 B 2 ZnGa 2 O 12 ( B = Nb, Sb), were synthesized via the solid-state approach, where Zn 2+ and Ga 3+ jointly occupied the tetrahedra ( C -site), and Nb 5+ (or Sb 5+ ) fully occupied the octahedra ( B -site). Ca 3 Nb 2 ZnGa 2 O 12 ceramic sintered at 1390 °C possessed a relatively larger ε r , τ f , and Q × f value of 16.57, −44.0 ppm/°C, and 39,200 GHz compared to ε r = 9.04, Q × f = 25,300 GHz, and τ f = −61.2 ppm/°C for Ca 3 Sb 2 ZnGa 2 O 12 fired at 1270 °C. Bond valence, ionic polarizability, packing fraction, Raman spectroscopy , and the P–V-L chemical bond theory were used to investigate the intrinsic factors affecting microwave dielectric properties . Compared with the measured permittivity ε r , the theoretical permittivity ε r (C-M) of Ca 3 Nb 2 ZnGa 2 O 12 is underestimated, and Ca 3 Sb 2 ZnGa 2 O 12 shows the opposite trend, which is mainly related to the under- and over-bonding in the structure. The packing fraction of Ca 3 Nb 2 ZnGa 2 O 12 is slightly lower than that of Ca 3 Sb 2 ZnGa 2 O 12 , but it does not lead to the low Q × f value of Ca 3 Nb 2 ZnGa 2 O 12 . The variation in Q × f was related to relative density, weak short-range order (SRO), and lattice energy. The minus sign and magnitude of τ f for both ceramics were determined by their large positive τ αm values.
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