New precious metal containing normal spinels: LiRhRu_1-xIr_xO₄, LiFeIr_1-xRu_xO₄, and LiCoIr_1-xRu_xO₄

Large spin-orbit-coupled cations in geometrically frustrated crystal structures have the most suitable setting for exploring novel exotic states of matter. Spinel oxides (AM₂O₄) are well-known examples of geometrically frustrated systems. In this study, we report for the first time the synthesis of compositions LiRhRu_1-xIr_xO₄ (x = 0–0.5), LiFeIr_1-xRu_xO₄ (x = 0–0.5), and LiCoIr_1-xRu_xO₄ (x = 0–0.3) containing precious metal cations on edge-sharing octahedral M-sites, and systematically investigate their magnetic and electrical properties. ⁵⁷Fe Mössbauer spectroscopy revealed that iron is trivalent in all LiFeIr_1-xRu_xO₄ solid solutions. Magnetic measurements indicate deviations from theoretical spin-only magnetic moment values, indicating the influence of spin-orbit coupling owing to the presence of 4d and 5d block elements. The LiFeIr_1-xRu_xO₄ series shows spin-glass-like freezing behavior with T_g ≈ 20 K, and a small frustration index (f ≈ 1-2), indicating that the frustration originates from site disorder. LiRhRu_1-xIr_xO₄ and LiCoIr_1-xRu_xO₄ exhibit strongly geometrically frustrated magnetism. Electrical resistivity measurements as a function of temperature indicate that all phases are semiconducting. Seebeck coefficient measurements show that LiRhRu_1-xIr_xO₄ and LiFeIr_1-xRu_xO₄ are p-type semiconductors with holes as the major charge carriers. A sign reversal of the Seebeck coefficient indicates both holes and electrons as carriers for LiCoIr_1-xRu_xO₄ (x = 0–0.2), but only holes as major carriers for x = 0.3. The Seebeck coefficient and power factor increase drastically in the LiRhRu_1-xIr_xO₄ solid solution with Ir substitution, reaching a maximum of ≈ +125 μV/K and ≈2.3×10^-6 W/mK² at ∼650 K for x = 0.5.