LiBC by polarized Raman spectroscopy: Evidence for lower crystal symmetry ?

TL;DR

This study uses polarized Raman spectroscopy on LiBC crystals to investigate their lattice vibrations, revealing discrepancies with assumed symmetry and suggesting lower crystal symmetry than previously thought.

Contribution

It provides experimental evidence indicating that LiBC has lower crystal symmetry than the commonly assumed D6h, based on Raman selection rules and observed modes.

Findings

Observed Raman modes incompatible with D6h symmetry.

Forbidden modes disappeared after thermal treatment.

B-C bond stretching mode shows small damping.

Abstract

The paper presents polarized Raman scattering study on a few-micron-size crystallite of LiBC with natural faces. The experiment on as grown sample has revealed a four lattice modes with frequencies at 1276 cm^-1, 830 cm^-1, 546 cm^-1 and 170 cm^-1, respectively. The number of observed Raman lines and their selection rules are incompatible with the assumed D6h symmetry. The modes at 1276 cm^-1 and 170 cm^-1 correspond to the expected Raman active modes. In contrast with the superconducting compound MgB2, the B-C bond stretching mode (at 1276 cm^-1) has rather small damping. The two "forbidden" modes (at 830 cm^-1 and 546 cm^-1) disappeared after subsequent thermal treatment.