Structure-property relationships in pyrochlores: low-temperature structures of Tl2Ru2O7-δ (δ=0.00 and 0.05)

Abstract

Structural, electrical and magnetic properties are investigated for ruthenium pyrochlores. The structure of the thallium ruthenium pyrochlore, Tl ₂ Ru ₂ O _7–δ , has been determined for δ=0.00 and 0.05 by neutron diffraction measurements at low temperatures. Stoichiometric Tl ₂ Ru ₂ O _7.00 synthesized under a high oxygen pressure of 3 GPa using KClO ₄ shows a first-order metallic-semiconducting transition at 120 K. The high-temperature metallic phase has a cubic pyrochlore structure with a three dimensional network of both ruthenium fand thallium. The low-temperature semiconducting phase has thallium sites with slightly short and long Tl-O distances, and the two Tl sites are ordered one dimensionally in the orthorhombic cell. Charge disproportionation of the Tl ions was suggested by valence bond sum calculations. By contrast, non-stoichiometric Tl ₂ Ru ₂ O _6.95 showed a spin-glass-like or canted-spin behavior around 40 K with resistivity increase also observed at this temperature. No significant structure changes were observed at the transition. Metallic-semiconducting transitions are discussed on the basis of their bond distance and angle changes.