Mechanism of ionic polarizability, bond valence, and crystal structure on the microwave dielectric properties of disordered Li10MTi13O32 (M = Zn, Mg) spinels

Abstract

The influence mechanism of the microwave dielectric properties of disordered spinels (Fdm) Li₁₀MTi₁₃O₃₂ (M = Zn, Mg) was investigated via Rietveld structural refinement, ionic polarizability, bond valence, P–V–L theory, Raman spectroscopy, and DC conductivity. For Li₁₀ZnTi₁₃O₃₂ and Li₁₀MgTi₁₃O₃₂, promising microwave dielectric properties of ε_r = 28.23 ± 0.3 and 29.23 ± 0.3, Q × f = 35 800 ± 500 GHz and 32 100 ± 500 GHz (at ∼7.5 GHz), and τ_f = −17.06 ± 2.0 ppm per °C and −11.05 ± 2.0 ppm per °C, respectively, were obtained at 980 °C. Bond valences reveal that almost all cations are rattling, weakening the bond strengths and widening the molecular dielectric polarizability. The expansion structures also result in τ_f values closer to zero and lower Q × f values. The bond ionicity and lattice energy of the Ti–O bonds are much greater than those of other bonds, indicating that the Ti–O bond is a major contributor to ε_r and Q × f. Moreover, the DC conductivity clarified that the Q × f values of the disordered spinels Li₁₀ZnTi₁₃O₃₂ and Li₁₀MgTi₁₃O₃₂ are not as high as other ordered spinels because of the transport of Li⁺ ions in their structures.