Pressure-induced iso-structural phase transitions in pyrochlore iridates \(A_2\hbox {Ir}_2\hbox {O}_7\) (\(A\! \,=\,\)Pr, Gd, Dy and Er)

Abstract

We investigated pressure-induced changes in pyrochlore iridates (\(A_2\hbox {Ir}_2\hbox {O}_7\), \(A=\) Pr, Gd, Dy and Er) using Raman spectroscopy and X-ray diffraction. All four compounds exhibited a pressure-driven isostructural transition associated with the rearrangement of \(\hbox {IrO}_6\) octahedra in the pyrochlore lattice. Interestingly, the critical pressure (\(P_\textrm{c}\)) for this transition correlates inversely with the A-site cation radius, \(P_\textrm{c}\) decreased from \(\sim \)10.8 GPa (Er, smallest cation) to \(\sim \)7.5 GPa (Pr, largest cation). Additionally, the bulk modulus systematically decreases with increasing ionic radius of A-site across the \(A_2\hbox {Ir}_2\hbox {O}_7\) series. High-pressure Raman spectroscopy revealed anomalous decrease of the linewidth with increasing pressure for Ir–O (\(T_{2g}^{4}\)) and Ir–O–Ir (\(A_{1g}\) and \(E_g\)) vibrational modes, suggesting a suppression of electron–phonon coupling due to enhanced electronic bandwidth under pressure.