Rattling effects on microwave dielectric properties of Ca3TiBGe3O12 (B = Mg, Zn) garnets

Abstract

Herein two cubic garnet ceramics Ca3TiBGe3O12 (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 (αL) of εr = 11.95, Q×f = 83,000 GHz, τf = −34.1 ppm/℃ and αL = 3.9 ppm/℃ for Ca3TiMgGe3O12, and εr = 11.80, Q×f = 79,000 GHz, τf = −42.9 ppm/℃ and αL = 4.0 ppm/℃ for Ca3TiZnGe3O12. Bond valence calculation reveals that Ca2+ at the A-site is slightly compressed, Ge4+ at C-site is rattling, and compressed Mg2+/Zn2+ coexists with rattling Ti4+ in the B-site. The large positive deviations (17.22% for Ca3TiMgGe3O12 and 10.90% for Ca3TiZnGe3O12) between the porosity corrected εr(Corr) (12.32 and 12.41) and calculated εr(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 Ca3TiMgGe3O12 leads to a higher εr and a τf closer to zero than those of Ca3TiZnGe3O12. Besides, the Q×f values of Ca3TiBGe3O12 (B = Mg, Zn) ceramics are mainly affected by relative density, packing fraction, and Raman mode (A1 g).