Robustness of the pyrochlore structure in rare-earth A2Ir2O7 iridates and pressure-induced structural transformation in IrO2

Abstract

A comprehensive study of the structural properties of the heavily investigated rare-earth A2Ir2O7 iridate series under extreme conditions is presented. From Pr2Ir2O7 to Lu2Ir2O7, the series is sufficiently covered by iridates with A = Pr, Sm, Dy, Ho, Er, Tm, Yb, and Lu; general trends and systematics within the series, including the understudied heavy-rare-earth members, are dependably followed. Temperature- and pressure-dependent synchrotron X-ray powder diffraction experiments reveal robustness of the pyrochlore structure throughout the series, down to 4 K and up to 20 GPa. The thermal expansivity of the pyrochlore lattice is determined, all falling in the Debye temperature range of θD = 360–420 K. The pressure compressibility shows a systematic increase of the bulk modulus with the rare-earth atomic number from K = 180–210 GPa. Combining the results of thermal measurements (Debye temperature) and pressure measurements (bulk modulus) enables us to determine the Grüneisen parameter of selected members and compare it to previous studies. Temperature and pressure evolution of the fractional coordinate of oxygen at 48f Wyckoff position, the sole free fractional coordinate in the crystal structure, is investigated and discussed regarding the antiferromagnetic ordering of the Ir magnetic moments. In addition to results on A2Ir2O7 iridates, the temperature and pressure evolution of the crystal structure of an IrO2 minority phase is followed. The tetragonal rutile-type structure is stable down to the lowest temperature. However, an application of pressure of approximately 15 GPa induces a structural transition: The tetragonal structure is orthorhombically distorted. The orthorhombic structure is still not fully stabilised at 20 GPa, and further distortion of the lattice (or subsequent structural transformations) is expected with increasing external pressure.