Electric and magnetic polarizabilities of hexagonal Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb)

TL;DR

This study explores the magnetic and dielectric properties of hexagonal Ln2CuTiO6 compounds, revealing a rare combination of high dielectric constants with weak temperature dependence, due to structural effects suppressing ferroelectricity.

Contribution

It demonstrates that size mismatch between Cu and Ti suppresses ferroelectricity, resulting in stable dielectric properties, supported by first-principles calculations.

Findings

Most compounds show low-temperature antiferromagnetic transition.

Large dielectric constants with low losses and weak temperature dependence.

Dielectric stability is linked to vibrational mode contributions.

Abstract

We investigated the rare-earth transition metal oxide series, Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb), crystallizing in the hexagonal structure with non-centrosymmetric P63cm space group for possible occurrences of multiferroic properties. Our results show that while these compounds, except Ln=Y, exhibit a low temperature antiferromagnetic transition due to the ordering of the rare-earth moments, the expected ferroelectric transition is frustrated by the large size difference between Cu and Ti at the B-site. Interestingly, this leads these compounds to attain a rare and unique combination of desirable paraelectric properties with high dielectric constants, low losses and weak temperature and frequency dependencies. First-principles calculations establish these exceptional properties result from a combination of two effects. A significant difference in the MO5 polyhedral sizes for M = Cu and M = Ti suppress the expected co-operative tilt pattern of these polyhedra, required for the ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against any temperature variation. Changes in the temperature stability of the dielectric constant amongst different members of this series are shown to arise from changes in relative contributions from soft polar modes.