Characterisation of Pb2Rh2O7 and Y2Rh2O7: an unusual case of pyrochlore stabilisation under high pressure, high temperature synthesis conditions

Abstract

Two novel oxides with Pb₂Rh₂O₇ and Y₂Rh₂O₇ compositions were synthesised using high pressure, high temperature techniques at 19 GPa and 8 GPa, respectively. Structurally, both compounds were determined to crystallise in the cubic pyrochlore structure, space group Fdm, with no observed oxygen vacancies. Both oxides have effectively identical Rh–O bond lengths of 1.987 Å and a bond-valence sum (BVS) of 4.2 that confirm a Rh⁴⁺ oxidation state. Physical property measurements for Pb₂Rh₂O₇ are consistent with a metallic ground state. This is similar to other Pb₂M₂O₇ oxides where M = Ru, Ir, and Os. Y₂Rh₂O₇ represents an unusual case of the lower density (6.356 g cm⁻³) pyrochlore structure being stabilised under high pressure conditions, while the analogous, higher density (7.031 g cm⁻³) perovskite YRhO₃ is stabilised by synthesis under ambient pressure conditions. The Rh⁴⁺ state results in a S = ½ magnetic ground state. Magnetisation measurements suggest strong AFM coupling in Y₂Rh₂O₇. However, long range AFM order is not observed down to 2 K presumably due to the geometric frustration of the pyrochlore lattice. Specific heat and resistivity measurements indicate a large electronic contribution to the heat capacity. The Wilson ratio of 4.78(11) is well above 2, indicating nearness to magnetism and the likely presence of Rh moments in the background of the conduction electrons. Catalytic activity indicated a greater correlation with other Rh pyrochlores as opposed to dependence on the Rh oxidation state.