Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics

Abstract

Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ. In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒfalls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 Å and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.