High-Pressure Synthesis, Crystal Structure, and Metal–Semiconductor Transitions in the Tl2Ru2O7−δPyrochlore

Thallium ruthenium oxides, Tl₂Ru₂O_7−δ, with the pyrochlore structure were synthesized under a pressure of 1–5 GPa and 1173 K and characterized by resistivity, magnetization, and TOF neutron-diffraction measurements. The oxygen vacancy,δ, varied with the synthesis conditions and significantly affected their electrical properties. The pyrochlores synthesized at high pressure and atmospheric pressure are classified into four groups which depend on their oxygen nonstoichiometry. (i) Nonstoichiometric Tl₂Ru₂O_6.71shows a metallic conductivity with almost temperature-independent magnetization. (ii) Stoichiometric Tl₂Ru₂O₇synthesized under high oxygen pressure using KClO₄shows a metallic–semiconducting transition at 120 K with magnetization anomalies at 120 and 40 K. (iii) Slightly nonstoichiometric Tl₂Ru₂O_6.96shows spin-glass-like behavior around 40 K accompanying a resistivity increase at the transition. (iv) Tl₂Ru₂O₇synthesized at 773 K and atmospheric pressure is semiconducting with magnetization anomalies at 120 and 40 K. The change from the metallic to semiconducting state is discussed from the viewpoint of structure changes.