Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7

TL;DR

This paper reports the experimental observation of a ferroelectric-like phase transition in the metallic compound Cd2Re2O7, providing evidence for a class of materials known as ferroelectric metals.

Contribution

It provides the first experimental evidence of a ferroelectric metal, specifically in Cd2Re2O7, supported by elastic moduli measurements and first-principles calculations.

Findings

Cd2Re2O7 undergoes a cubic-to-tetragonal phase transition.

The transition is consistent with ferroelectric metal behavior.

Lattice instabilities involve O ion motions with Eu symmetry.

Abstract

A class of materials known as ``ferroelectric metals'' was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)], but to date no examples of this class have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ``ferroelectric metal'' in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a two-fold degenerate mode with Eu symmetry, and that motions of the O ions forming the O octahedra dominate the energetics of the transition.