Pressure dependence of the structure and electronic properties of Sr3Ir2O7

TL;DR

This study investigates how pressure influences the structure and electronic properties of Sr3Ir2O7, revealing a phase transition at 54 GPa and a pressure-induced insulator-metal transition around 20 GPa, using x-ray diffraction and density functional theory.

Contribution

It provides the first detailed analysis of pressure-induced structural and electronic changes in Sr3Ir2O7, including a novel high-pressure phase and insights into the insulator-metal transition mechanism.

Findings

Structural phase transition at 54 GPa with volume collapse

Insulator-metal transition around 20 GPa driven by bandwidth increase

High-pressure phase exhibits metallic behavior due to band overlap

Abstract

We study the structural evolution of Sr$_3$Ir$_2$O$_7$ as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry, and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+$U$+spin orbit) to optimize the crystal structure, and to elucidate the electronic and magnetic properties of Sr$_3$Ir$_2$O$_7$ at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO$_6$ octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at $\sim$20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra, and strongly overlapping $t_{2g}$ and $e_g$ bands. The $t_{2g}$ bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr$_3$Ir$_2$O$_7$.