A density-functional theory investigation of the structural and spin properties of (PO2)4(WO3)8 model bronzes

Plane-wave density-functional theory calculations of P4W8O32 mono-phosphate tungsten bronzes with pentagonal tunnels demonstrate that (1) these materials possess a ground (singlet) state structure of orthorombic symmetry, as observed by X-ray diffraction crystallography at room temperature; (2) open-shell states lie very close in energy to the ground state and are significantly populated at room temperature; and (3) these low-lying states are likely to be implicated in the formation of a superstructure due to the tilting of WO6 octahedra (Peierls-like structural transformations) upon temperature decrease. The latter finding is consistent with the existence of a low-temperature incommensurate structure, as evidenced by the satellite peaks observed in X-ray diffraction patterns.