X-ray scattering study of pyrochlore iridates: Crystal structure, electronic, and magnetic excitations

Abstract

We have investigated the structural, electronic, and magnetic properties of the pyrochlore iridates ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ using a combination of resonant elastic x-ray scattering, x-ray powder diffraction, and resonant inelastic x-ray scattering (RIXS). The structural parameters of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ have been examined as a function of temperature and applied pressure, with a particular emphasis on regions of the phase diagram where electronic and magnetic phase transitions have been reported. We find no evidence of crystal symmetry change over the range of temperatures ($\ensuremath{\sim}6$ to 300 K) and pressures ($\ensuremath{\sim}0.1$ to 17 GPa) studied. We have also investigated the electronic and magnetic excitations in single-crystal samples of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ using high-resolution Ir ${\mathrm{L}}_{3}$-edge RIXS. In spite of very different ground state properties, we find that these materials exhibit qualitatively similar excitation spectra, with crystal field excitations at $\ensuremath{\sim}3--5$ eV, spin-orbit excitations at $\ensuremath{\sim}0.5--1$ eV, and broad low-lying excitations below $\ensuremath{\sim}0.15$ eV. In single-crystal samples of ``Eu-rich'' ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ (found to possess an actual stoichiometry of ${\mathrm{Eu}}_{2.18}{\mathrm{Ir}}_{1.82}{\mathrm{O}}_{7.06})$ we observe highly damped magnetic excitations at $\ensuremath{\sim}45$ meV, which display significant momentum dependence. We compare these results with recent dynamical structure factor calculations.