Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCr$_2$S$_4$ from first principles

TL;DR

This study uses first-principles calculations to analyze the dielectric properties and phonon stability of ferromagnetic CdCr$_2$S$_4$, finding no evidence of polar mode softening as the cause of its anomalous dielectric behavior.

Contribution

First-principles calculations of phonons and dielectric response in CdCr$_2$S$_4$ reveal stability of infrared-active modes and challenge the soft-mode explanation for dielectric anomalies.

Findings

Infrared-active phonons are stable in CdCr$_2$S$_4$

Calculated dielectric constant matches experiments

Anomalous dielectric behavior is not due to polar mode softening

Abstract

We have studied the dielectric properties of the ferromagnetic spinel CdCr$_2$S$_4$ from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr$_2$S$_4$ is not due to the softening of a polar mode. We suggest further experiments to clarify this point.