Sb-doping effects on twin-shift option and microwave dielectric properties of Ba8CoNb6O24 eight-layer hexagonal perovskite ceramics

Abstract

B-site deficient eight-layer hexagonal perovskites possess in general high quality factors and large positive temperature coefficients of resonant frequency (τf). Recently we successfully lowered τf of shifted perovskite Ba8CoNb6O24 down to near zero through one-third Ta5+ substitution but raised the cost of the materials. In this work, Ba8CoNb6O24 was doped with cheaper elements Sb5+ on Nb5+ sites using conventional solid-state reactions, and near-zero τf of 0.66(2) ppm/°C, modest εr of 27.99(3), Q × f of 1.016(6) × 104 GHz, and significantly reduced cost of the ceramics were achieved on the composition Ba8CoNb5.4Sb0.6O24. The lower polarizability of Sb5+ than Nb5+ results in smaller εr and therefore τf values. About 15 % Sb5+ substitution for Nb5+ in Ba8CoNb6-xSbxO24 dramatically transforms the shifted structure to twinned structure, in contrast with 50 % Ta5+ substitution in the Ta5+ case. This is ascribed to reduced SOJT effects and ionic size of d10 Sb5+ compared with d° Nb5+ and Ta5+.