Anisotropic lattice compression and pressure-induced electronic phase transitions in Sr2IrO4

Abstract

The crystal lattice of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is investigated with synchrotron x-ray powder diffraction under hydrostatic pressures up to $P=43$ GPa and temperatures down to 20 K. The tetragonal unit cell is maintained over the whole investigated pressure range, within our resolution and sensitivity. The $c$-axis compressibility ${\ensuremath{\kappa}}_{c}(P,T)\ensuremath{\equiv}\ensuremath{-}(1/c)(dc/dP)$ presents an anomaly with pressure at ${P}_{1}=17$ GPa at fixed $T=20$ K that is not observed at $T=300$ K, whereas ${\ensuremath{\kappa}}_{a}(P,T)$ is nearly temperature independent and shows a linear behavior with $P$. The anomaly in ${\ensuremath{\kappa}}_{c}(P,T)$ is associated with the onset of long-range magnetic order, as evidenced by an analysis of the temperature dependence of the lattice parameters at fixed $P=13.7\ifmmode\pm\else\textpm\fi{}0.5$ GPa. At fixed $T=20$ K, the tetragonal elongation $c/a(P,T)$ shows a gradual increment with pressure and a depletion above ${P}_{2}=30$ GPa that indicates an orbital transition and possibly marks the collapse of the ${J}_{\text{eff}}=1/2$ spin-orbit-entangled state. Our results support pressure-induced phase transitions or crossovers between electronic ground states that are sensed, and therefore can be probed, by the crystal lattice at low temperatures in this prototype spin-orbit Mott insulator.