Robust dielectric properties of B-site size-disordered hexagonal Ln2CuTiO6 (Ln = Y, Dy, Ho, Er, and Yb)

Abstract

Hexagonal Ln2CuTiO6 (Ln = Y, Dy, Ho, Er, and Yb) exhibits a rare combination of interesting dielectric properties, in the form of relatively large dielectric constants (ϵ′>30), low losses, and extremely small temperature and frequency dependencies, over large ranges of temperature and frequency [Choudhury et al., Appl. Phys. Lett. 96, 162903 (2010) and Choudhury et al., Phys. Rev. B 82, 134203 (2010)], making these compounds promising as high-k dielectric materials. The authors present a brief review of the existing literature on this interesting class of oxides, complimenting it with spectroscopic data in conjunction with first-principles calculation results, revealing a novel mechanism underlying these robust dielectric properties. These show that the large size differences in Cu2+ and Ti4+ at the B-site, aided by an inherent random distribution of CuO5 and TiO5 polyhedral units, frustrates the ferroelectric instability, inherent to the noncentrosymmetric P63 cm space group of this system, and gives rise to the observed relatively large dielectric constant values. Additionally, the phononic contributions to the dielectric constant are dominated primarily by mid-frequency (>100 cm−1) polar modes, involving mainly Ti4+ 3d0 ions. In contrast, the soft polar phonon modes with frequencies typically less than 100 cm−1, usually responsible for dielectric properties of materials, are found to be associated with non-d0 Cu2+ ions and to contribute very little, giving rise to the remarkable temperature stability of dielectric properties of these compounds.