Vanadium clustering/declustering in P2-Na1/2VO2 layered oxide
Résumé
The new layered phase P2-Na1/2VO2 has been synthesized by sodium electrochemical deintercalation. Its structure has been studied by high resolution powder diffraction, pair distribution function analysis, and nuclear magnetic resonance spectroscopy between 300 and 350 K. An increase of 2 orders of magnitude in its electronic conductivity has been observed at approximately 322 K, and a structural transition has been found to occur simultaneously. The arrangement of sodium ordering in P2-Na1/2VO2, which maximizes sodium-sodium distances to lower electrostatic repulsions between alkali ions, is found to be unchanged across this transition. At room temperature, high resolution powder diffraction and pair distribution function analysis reveal the triangular lattice formed by vanadium ions to be distorted by the formation of pseudotrimers clusters with vanadium-vanadium distances as short as 2.581 Å. Above the transition, the pseudotrimers disappear and the triangular vanadium lattice becomes more regular with a mean vanadium-vanadium distance of 2.88 Å. At 350 K, the increase in P2-Na1/2VO2 electronic conductivity is due to enhanced charge transport resulting from the declustering of vanadium ions. These results highlight how sodium ordering between the MO2 layers and the electronic transport within the MO2 layers are intimately correlated in NaxMO2-type sodium-layered oxides.