Synthesis and Physical Characterization of Pyrochlore oxides

Abstract

5d Transition metal oxides with Ir(4+) as the transition metal ion has been of great interest due to its comparable on-site Coulomb interaction and spin-orbit coupling, which together result in a rich array of physical properties. The iridates of the pyrochlore structure are a promising candidate for realizing the Weyl semimetals. They change their physical properties from that of an antiferromagnetic insulator for smaller or heavier rare earth (i.e., A = Gd, Tb, Dy, Ho, Er, and Yb, including Y) to an exotic, nonmagnetic metal for Pr2Ir2O7. The intermediate members corresponding to A = Nd, Sm, and Eu show a thermally induced metal-insulator transition. Literature reveals that stuffing in Eu2Ir2O7 show a strong correlation between the physical properties and structure. This project started with the idea of understanding the effect of stoichiometry on the physical properties ofSm2Ir2O7, which is lacking for the pyrochlore iridates. Till now, all the pyrochlore iridate sample reported in the literature are synthesized using the conventional ceramic route, which takes more than 600 hrs of sintering to stabilize the pyrochlore phase along with significant IrO2 losses. Through this project 6 samples of lanthanide pyrochlore iridates (Ln= Pr, Sm, Gd, Er and Yb) were synthesized using a wet-chemical technique through a single sintering. The wet-chemical method is shown to be an efficient way of obtaining single-phase pyrochlore iridate samples with sintering duration as low as 12 hrs and negligible IrO2 losses. As the final stoichiometry can affect the physical properties, comparison of results obtained from the solid-state reaction technique and the wet-chemical route were done using resistivity and magnetic susceptibility measurements. A metal to insulator transition was seen for Sm2Ir2O7 and Gd2Ir2O7 concomitant with AFM ordering in the insulating regime. The specific heat data were analyzed, using an equation considering schottky anomaly, electronic and phonon contributions.