Structural Transition with a Sharp Change in the Electrical Resistivity and Spin-Orbit Mott Insulating State in a Rhenium Oxide, Sr3Re2O9.

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.

Quantifying Diffusion through Interfaces of Lithium-Ion Battery Active Materials.

Detailed understanding of charge diffusion processes in a lithium-ion battery is crucial to enable its systematic improvement. Experimental investigation of diffusion at the interface between active particles and the electrolyte is challenging but warrants investigation as it can introduce resistances that, for example, limit the charge and discharge rates. Here, we show an approach to study diffusion at interfaces using muon spin spectroscopy. By performing measurements on LiFePO4 platelets with different sizes, we determine how diffusion through the LiFePO4 (010) interface differs from that in the center of the particle (i.e., bulk diffusion). We perform ab initio calculations to aid the understanding of the results and show the relevance of our interfacial diffusion measurement to electrochemical performance through cyclic voltammetry measurements. These results indicate that surface engineering can be used to improve the performance of lithium-ion batteries.