# Pressure-induced formation of rhodium zigzag chains in the honeycomb rhodate Li$_2$RhO$_3$

## TL;DR

This study investigates how pressure affects the crystal structure of Li₂RhO₃, revealing the formation of rhodium zigzag chains at high pressure, which is supported by experimental diffraction data and theoretical calculations.

## Contribution

It demonstrates pressure-induced formation of rhodium zigzag chains in Li₂RhO₃ and explains the structural transition through anisotropic pressure effects.

## Key findings

- Zigzag chains form at pressures above 14 GPa.
- Structural phase mixture occurs between 6.5 GPa and 14 GPa.
- The zigzag pattern is reproducible under anisotropic pressure conditions.

## Abstract

We use powder x-ray diffraction to study the effect of pressure on the crystal structure of the honeycomb rhodate Li$_2$RhO$_3$. We observe low-pressure ($P$$<$$P_{c1}$ = 6.5 GPa) and high-pressure ($P$$>$$P_{c2}$ = 14 GPa) regions corresponding to the monoclinic $C2/m$ symmetry, while a phase mixture is observed at intermediate pressures. At $P$$>$$P_{c2}$, the honeycomb structure becomes distorted and features short Rh--Rh bonds forming zigzag chains stretched along the crystallographic $a$ direction. This is in contrast to dimerized patterns observed in triclinic high-pressure polymorphs of $\alpha$-Li$_2$IrO$_3$ and $\alpha$-RuCl$_3$. Density-functional theory calculations at various pressure conditions reveal that the observed rhodium zigzag-chain pattern is not expected under hydrostatic pressure but can be reproduced by assuming anisotropic pressure conditions.