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

The crystal lattice of Sr2IrO4 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 kappa(c)(P, T) -(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 kappa(a)(P, T) is nearly temperature independent and shows a linear behavior with P. The anomaly in 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 +/- 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(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.