Complex pressure-temperature structural phase diagram of honeycomb iridate Cu$_2$IrO$_3$

TL;DR

This study maps the complex pressure-temperature phase diagram of Cu₂IrO₃, revealing structural transitions, dimer formations, and stacking fault healing, which are crucial for understanding its potential quantum spin liquid state.

Contribution

It provides the first detailed pressure-temperature structural phase diagram of Cu₂IrO₃, highlighting temperature-dependent phase transitions and structural stability insights.

Findings

Ir-Ir dimer formation below 8 GPa

Discontinuous interplanar distance reduction at 15 GPa at room temperature

Different high-pressure structures at room and low temperatures

Abstract

$\mathrm{Cu_2IrO_3}$ is among the newest layered honeycomb iridates and a promising candidate to harbor a Kitaev quantum spin liquid state. Here, we investigate the pressure and temperature dependence of its structure through a combination of powder x-ray diffraction and x-ray absorption fine structure measurements, as well as $ab$-$initio$ evolutionary structure search. At ambient pressure, we revise the previously proposed $C2/c$ solution with a related but notably more stable $P2_1/c$ structure. Pressures below 8 GPa drive the formation of Ir-Ir dimers at both ambient and low temperatures, similar to the case of $\mathrm{Li_2IrO_3}$. At higher pressures, the structural evolution dramatically depends on temperature. A large discontinuous reduction of the Ir honeycomb interplanar distance is observed around 15 GPa at room temperature, likely driven by a collapse of the O-Cu-O dumbbells. At 15 K, pressures beyond 20 GPa first lead to an intermediate phase featuring a continuous reduction of the interplanar distance, which then collapses at 30 GPa across yet another phase transition. However, the resulting structure around 40 GPa is not the same at room and low temperatures. Remarkably, the reduction in interplanar distance leads to an apparent healing of the stacking faults at room temperature, but not at 15 K. Possible implications on the evolution of electronic structure of $\mathrm{Cu_2IrO_3}$ with pressure are discussed.