Strong influence of the thermodynamic path in the high pressure properties of intercalated honeycomb iridate Cu₂IrO₃

The search for Kitaev quantum spin liquids in J_eff = 1/2 honeycomb iridates has recently focused on intercalated systems, in which H or a transition metal separates the Ir honeycomb layers. One such system, Cu₂IrO₃, has been shown to display a remarkably complex high-pressure structural phase diagram¹, with room temperature or 10 K compression leading to distinct phase transitions. To investigate the metastability of these phases, we studied the electronic and crystal structure of Cu₂IrO₃ using x-ray spectroscopy and scattering measurements taken through distinct thermodynamic paths. Pressurizing Cu₂IrO₃ at low temperature leads to a Cu to Ir electron transfer above 30 GPa. However, this state is not reached by pressurizing at room temperature followed by a nearly isobaric cool down. On the other hand, compression at 10 K followed by isobaric warming, preserves the charge transferred state up to at least room temperature, thus demonstrating that neither are a simple metastable phase. These results show that the properties of compressed Cu₂IrO₃ are highly dependent on its pressure/temperature history.

¹Fabbris et al., Phys. Rev. B 104, 014102 (2021)