Optical study of the vibrational and dielectric properties of BiMnO3

TL;DR

This study investigates the vibrational and dielectric properties of BiMnO3 using infrared and Raman spectroscopy, revealing its centrosymmetric structure and temperature-dependent dielectric behavior without phonon softening or magnetic transition effects.

Contribution

It provides the first detailed infrared and Raman spectroscopic analysis of BiMnO3, clarifying its centrosymmetric structure and dielectric properties across temperatures.

Findings

BiMnO3 has a C2/c centrosymmetric structure at all temperatures above 10 K.

The dielectric permittivity increases from 18.5 to 45 as temperature decreases from 300 K to 10 K.

Infrared phonons show no softening and are unaffected by the ferromagnetic transition.

Abstract

BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also ferroelectric (FE) due to a non-centro-symmetric C2 structure, until diffraction data indicated that its space group is the centro-symmetric C2/c. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with C2/c at any T > 10 K, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at Tc.