Abstract
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 LiRhRu1-xIrxO4 (x = 0–0.5), LiFeIr1-xRuxO4 (x = 0–0.5), and LiCoIr1-xRuxO4 (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 LiFeIr1-xRuxO4 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 LiFeIr1-xRuxO4 series shows spin-glass-like freezing behavior with Tg ≈ 20 K, and a small frustration index (f ≈ 1-2), indicating that the frustration originates from site disorder. LiRhRu1-xIrxO4 and LiCoIr1-xRuxO4 exhibit strongly geometrically frustrated magnetism. Electrical resistivity measurements as a function of temperature indicate that all phases are semiconducting. Seebeck coefficient measurements show that LiRhRu1-xIrxO4 and LiFeIr1-xRuxO4 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 LiCoIr1-xRuxO4 (x = 0–0.2), but only holes as major carriers for x = 0.3. The Seebeck coefficient and power factor increase drastically in the LiRhRu1-xIrxO4 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.
