# Pressure-Induced Structural Phase Transition in Ho2Ce2O7 Oxide

## TL;DR

This study shows that Ho2Ce2O7 undergoes an irreversible structural change under high pressure, transitioning from a cubic to a hexagonal structure.

## Contribution

The discovery of a pressure-induced irreversible structural transition in Ho2Ce2O7 with coexisting metastable phases is newly reported.

## Key findings

- A phase transition in Ho2Ce2O7 begins at 23.8 GPa and involves cation disordering and coordination changes.
- Two phases coexist above the transition threshold: the original cubic phase and a metastable hexagonal phase.
- The high-pressure phase remains stable even after decompression, indicating an irreversible transition.

## Abstract

The structural evolution of Ho2Ce2O7 under high pressure was systematically investigated using synchrotron X-ray diffraction (up to 31.5 GPa) and Raman spectroscopy (up to 41.7 GPa). At ambient pressure, the compound adopts a common C-type cubic rare earth oxide structure (space group Ia-3). A pressure-induced phase transition was observed to commence at 23.8 GPa, characterized by a gradual structural evolution that persisted through the maximum experimental pressure of 31.5 GPa. This transition involves cation disordering accompanied by coordination environment modifications. High-pressure X-ray diffraction analysis reveals the coexistence of two distinct phases above the transition threshold: the parent cubic phase (Ia-3) and a metastable hexagonal phase (R-3c). Notably, the high-pressure phase configuration persists upon complete decompression to ambient conditions, demonstrating the irreversible nature of this pressure-induced structural transition.