Structural Transition with a Sharp Change in the Electrical Resistivity and Spin-Orbit Mott Insulating State in a...

We report the successful synthesis, crystal structure, and electrical properties of Sr₃Re₂O₉, which contains Re⁶⁺ with the 5d¹ configuration. This compound is isostructural with Ba₃Re₂O₉ and shows a first-order structural phase transition at ∼370 K. The low-temperature (LT) phase crystallizes in a hettotype structure of Ba₃Re₂O₉, which is different from that of the LT phase of Sr₃W₂O₉, suggesting that the electronic state of Re⁶⁺ plays an important role in determining the crystal structure of the LT phase. The structural transition is accompanied by a sharp change in the electrical resistivity. This is likely a metal-insulator transition, as suggested by the electronic band calculation and magnetic susceptibility. In the LT phase, the ReO₆ octahedra are rotated in a pseudo-a⁰a⁰a⁺ manner in Glazer notation, which corresponds to C-type orbital ordering. Paramagnetic dipole moments were confirmed to exist in the LT phase by muon spin rotation and relaxation measurements. However, the dipole moments shrink greatly because of the strong spin-orbit coupling in the Re ions. Thus, the electronic state of the LT phase corresponds to a Mott insulating state with strong spin-orbit interactions at the Re sites.