Synthesis and characterization of Na_0.3RhO₂·0. 6H₂O - A semiconductor with a weak ferromagnetic component

We have prepared the oxyhydrate Na_0.3RhO₂·0. 6H₂O by extracting Na⁺ cations from NaRhO₂ and intercalating water molecules using an aqueous solution of Na₂S ₂O₈. Rietveld refinement, thermogravimetric analysis (TGA), and energy-dispersive X-ray analysis (EDX) reveal that a non-stoichiometric Na_0.3(H₂O)_0.6 network separates layers of edge-sharing RhO₆ octahedra containing Rh ³⁺(4d⁶, S=0) and Rh⁴⁺ (4d⁵, S=1/2). The resistivities of NaRhO₂ and Na_0.3RhO ₂·0.6H₂O (T<300) reveal insulating and semi-conducting behavior with activation gaps of 134 and 7.8 meV, respectively. Both Na_0.3RhO₂·0.6H₂O and NaRhO ₂ show paramagnetism at room temperature, however, the sodium-deficient sample exhibits simultaneously a weak but experimentally reproducible ferromagnetic component. Both samples exhibit a temperature-independent Pauli paramagnetism, for NaRhO₂ at T>50 K and for Na_0.3RhO₂·0.6H₂O at T>25 K. The relative magnitudes of the temperature-independent magnetic susceptibilities, that of the oxide sample being half that of the oxyhydrate, is consistent with a higher density of thermally accessible electron states at the Fermi level in the hydrated sample. At low temperatures the magnetic moments rise sharply, providing evidence of localized and weakly-ordered electronic spins with effective moment per formula unit values of 2.0×10^-1 μ_B for NaRhO₂ and 0.8×10^-1 μ_B for Na_0.3RhO₂·0.6H₂O.