Rattling effects on microwave dielectric properties of Ca3TiBGe3O12 (B = Mg, Zn) garnets
Herein two cubic garnet ceramics Ca 3 TiBGe 3 O 12 (B = Mg, Zn) were prepared with optimal sintering temperatures of 1300 ℃ and 1200 ℃, respectively. Both ceramics exhibited promising microwave dielectric properties as well as rather low coefficient of thermal expansion ( α L ) of ε r = 11.95, Q×f = 83,000 GHz, τ f = −34.1 ppm/℃ and α L = 3.9 ppm/℃ for Ca 3 TiMgGe 3 O 12 , and ε r = 11.80, Q×f = 79,000 GHz, τ f = −42.9 ppm/℃ and α L = 4.0 ppm/℃ for Ca 3 TiZnGe 3 O 12 . Bond valence calculation reveals that Ca 2+ at the A-site is slightly compressed, Ge 4+ at C-site is rattling, and compressed Mg 2+ /Zn 2+ coexists with rattling Ti 4+ in the B-site. The large positive deviations (17.22% for Ca 3 TiMgGe 3 O 12 and 10.90% for Ca 3 TiZnGe 3 O 12 ) between the porosity corrected ε r (Corr) (12.32 and 12.41) and calculated ε r (C-M) (10.51 and 11.19) by the C-M equation were observed, which might be attributed to the combination of rattling effects in B-site and C-site. The relatively stronger rattling effect in Ca 3 TiMgGe 3 O 12 leads to a higher ε r and a τ f closer to zero than those of Ca 3 TiZnGe 3 O 12 . Besides, the Q×f values of Ca 3 TiBGe 3 O 12 (B = Mg, Zn) ceramics are mainly affected by relative density, packing fraction, and Raman mode (A 1 g ).
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