  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^*$$\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 $\Delta_{OP}$ 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 $T^*$ in the temperature evolution of
infrared-active mode frequencies suggesting the presence of significant
spin-phonon coupling in the system.
