Effect of lattice-induced strain on microwave dielectric polarization and properties: A case study of Ga-based garnet

In microwave dielectric materials, the phenomenon that the measured permittivity (er) is inconsistent with the calculated permittivity (er(C-M)) often occurs. In the present work, the intrinsic factors affecting the microwave dielectric properties of Ga-based garnet Ca3B1.5Ga3.5O12 (B = Nb, Ta) are studied by bond valence and the Phillips-Van Vechten-Levine (P-V-L) methods, and the relationship between lattice-induced strain effect and dielectric polarization is elucidated. Since the ionic polarizability of Nb5+ is lower than that of Ta5+, the er(C-M) values of Ca3Nb1.5Ga3.5O12 (CNG) and Ca3Ta1.5Ga3.5O12 (CTG) are 11.06 and 12.17, respectively. However, this is the opposite trend of the measured values (15.00 and 12.62). The global instability index (GII) is equal to 0.013 and 0.044 v.u., respectively. Both the indexes, bond strain index (BSI) and GII, are related to the entire crystal structure. A stronger lattice-induced compressive strain and a higher degree of covalency lead to a lower measured er, a higher Qxf, a higher temperature coefficient of permittivity ze, and a larger negative zf.