Preparation, Structure Evolution, and Metal?Insulator Transition of Na_xRhO₂ Crystals (0.25 ≤ x ≤ 1)

The triangular lattice Na_xRhO₂ contains a 4d Rh element with large spin-orbit coupling, and the electron-electron correlation effect is expected to have some novel physical properties. Here we report NaRhO₂ crystal growth by Na₂CO₃ vapor growth and a series of Na_xRhO₂ (0.25 ≤ x ≤ 1) crystals prepared using the chemical desodiation method. Na_xRhO₂ reveals a layer structure with the space group R3-m, and the lattice parameter a evolves from 3.09 to 3.03 Å and c from 15.54 to 15.62 Å when x decreases from 1.0 to 0.2. Decreasing potassium concentration leads to a contraction of the RhO₆ octahedral layers, which may be attributed to a higher covalency of Rh-O bonds. More important, the metal-insulator transition in Na_xRhO₂ was observed in resistivity along the ab plane. The conducting mechanism of Na_xRhO₂ is strongly dependent on x. Two-dimensional variable range hopping (VRH) mechanisms (0.67 ≤ x ≤ 1) and metallic behaviors (0.42 and 0.47) are observed in temperature-dependent resistivity. The origin of this metal-insulator transition was discussed on the basis of the Ioffe-Regel criterion. Our work demonstrates the strong correlation between sodium concentration and physical properties of Na_xRhO₂.