Fluorite-related iridate Pr${}_3$IrO${}_7$: Crystal growth, structure, magnetism, thermodynamic, and optical properties

Spin-orbit coupling in heavy 5$d$ metal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr${}_3$IrO${}_7$. Single crystals of Pr${}_3$IrO${}_7$ have been grown by the KF flux method. Structural analysis shows that Pr${}_3$IrO${}_7$ crystallizes in an orthorhombic phase with $Cmcm$ symmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation between $M_{ZFC}$ and $M_{FC}$, however, a weak hump in $M(T)$ is observed at $T^{\ast }$$\sim$10.4~K. The specific heat data reveal two maxima at $\sim$253 K and $\sim$4.8 K. The optical conductivity ${\sigma }_1(\omega )$ spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gap ${\mathrm{\Delta }}_{OP}$ of size $\sim$500~meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition ($\sim$ 253 K). An anomaly is observed at around $T^{\ast }$ in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin-phonon coupling in the system.