# Doping driven structural distortion in the bilayer iridate (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$

## TL;DR

This study uses neutron diffraction and optical second harmonic generation to reveal how electron doping causes a structural symmetry lowering and creates distinct Ir sites in bilayer iridate Sr3Ir2O7.

## Contribution

It provides detailed experimental evidence of doping-induced structural distortion and symmetry lowering in (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$, clarifying the nature of the phase transition.

## Key findings

- Structural symmetry lowers from C2/c to P2_1/c below T_S.
- Two unique Ir sites emerge in the distorted phase.
- Doping modifies oxygen octahedral rotations and tilting.

## Abstract

Neutron single crystal diffraction and rotational anisotropy optical second harmonic generation data are presented resolving the nature of the structural distortion realized in electron-doped (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ with $x=0.035$ and $x=0.071$. Once electrons are introduced into the bilayer spin-orbit assisted Mott insulator Sr$_3$Ir$_2$O$_7$, previous studies have identified the appearance of a low temperature structural distortion and have suggested the presence of a competing electronic instability in the phase diagram of this material. Our measurements resolve a lowering of the structural symmetry from monoclinic $C2/c$ to monoclinic $P2_1/c$ and the creation of two unique Ir sites within the chemical unit cell as the lattice distorts below a critical temperature $T_S$. Details regarding the modifications to oxygen octahedral rotations and tilting through the transition are discussed as well as the evolution of the low temperature distorted lattice as a function of carrier substitution.