Anisotropic lattice compression and pressure-induced electronic phase transitions in ${\mathrm{Sr}}_2\mathrm{IrO}{}_4$

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 ${\kappa }_c(P,T)\equiv -(1/c)(dc/dP)$ 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 $P=13.7\pm 0.5$ 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.

Figure 1

Raw x-ray powder diffraction profiles for several pressures at $T=20$ K ($\lambda =0.61986$ Å). The Miller indices of selected reflections are indicated.

Figure 2

Comparison between observed (open symbols) and calculated (red lines) x-ray powder diffraction profiles for selected pressures and $T=20$ K. The differences are also shown (blue lines). The expected Bragg peak positions for the tetragonal $I{4}_1/acd$ space group are shown in short vertical bars. The spurious peaks identified by * are due to the rhenium gasket.

Figure 3

Pressure dependence of the tetragonal lattice parameters $a$ (a), $c$ (b), unit cell volume $V$ (c), and $c/a$ ratio (d) at $T=20$ K. The red solid line in (c) is a fitting according to the Murnaghan equation of state (see text). The blue dashed line in (d) is a second-order polynomial curve that captures the behavior below 30 GPa, and is shown as a guide to the eyes.

Figure 4

Pressure dependence of the $a$-axis compressibility ${\kappa }_a\equiv -(1/a)(da/dP)$ and the $c$-axis compressibility ${\kappa }_c\equiv -(1/c)(dc/dP)$ at $T=20$ K (a) and room temperature (b). The data in (b) were extracted from the results of Ref. [20]. The solid and dashed lines are guides to the eyes.

Figure 5

Temperature dependence of the tetragonal elongation ($c/a$ ratio) on warming under constant $P=13.7\pm 0.5$ GPa.