The structural properties of the hexagonal multiferroic $h\text{−}{\mathrm{HoMnO}}_3$ under high pressure have been explored using synchrotron x-ray diffraction and x-ray absorption spectroscopy in diamond anvil cells. The structure was found to undergo a pressure-induced phase transition at $\sim 24$ GPa to a rhombohedrally distorted superstructure, which is isostructural to the oxygen-loaded h-R${\mathrm{MnO}}_{3+\delta }$ ($R=\mathrm{Y},\mathrm{Dy},\mathrm{Ho},\mathrm{Er}$; $\delta \approx 0.28)$ phases found in the same systems. The driving force behind the phase transition is the highly compressible $\mathit{ab}$ plane which facilitates a gradual charge disproportionation of Mn(III) with pressure. We speculate this stabilizes the spin-liquid phase due to ferromagnetic coupling between neighboring Mn(II)/Mn(IV) and Mn(III). In addition, we demonstrate that the structural behavior is highly susceptible to nonhydrostatic conditions and the choice of pressure medium should be carefully made.

• Received 25 January 2023
• Accepted 6 April 2023

DOI:https://doi.org/10.1103/PhysRevB.107.134115