# Robustness of topological states to lattice instability in non-symmorphic topological insulator KHgSb

## TL;DR

This study investigates the lattice stability of the non-symmorphic topological insulator KHgSb, revealing a lattice distortion below 150 K and identifying a more stable structure that preserves the topological hourglass fermions.

## Contribution

It demonstrates that KHgSb's assumed space group is unstable and proposes a lower-energy structure that maintains the topological properties, supported by experimental and theoretical analysis.

## Key findings

- Lattice distortion occurs below 150 K in KHgSb.
- The stable structure is space group P6_3mc, not P6_3/mmc.
- KHgSb is highly sensitive to temperature and laser power, decomposing under high conditions.

## Abstract

We report a polarized Raman scattering study of non-symmorphic topological insulator KHgSb with hourglass-like electronic dispersion. Supported by theoretical calculations, we show that the lattice of the previously assigned space group $P6_3/mmc$ (No. 194) is unstable in KHgSb. While we observe one of two calculated Raman active E$_{2g}$ phonons of space group $P6_3/mmc$ at room temperature, an additional A$_{1g}$ peak appears at 99.5 ~cm$^{-1}$ upon cooling below $T^*$ = 150 K, which suggests a lattice distortion. Several weak peaks associated with two-phonon excitations emerge with this lattice instability. We also show that the sample is very sensitive to high temperature and high laser power, conditions under which it quickly decomposes, leading to the formation of Sb. Our first-principles calculations indicate that space group $P6_3mc$ (No. 186), corresponding to a vertical displacement of the Sb atoms with respect to the Hg atoms that breaks the inversion symmetry, is lower in energy than the presumed $P6_3/mmc$ structure and preserves the glide plane symmetry necessary to the formation of hourglass fermions.