Structural evolution of high energy density V<sup>3+</sup>/V<sup>4+</sup> mixed valent Na<sub>3</sub>V<sub>2</sub>O<sub>2x</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3−2x</sub> (<i>x</i> = 0.8) sodium vanadium fluorophosphate using <i>in situ</i> synchrotron X-ray powder diffraction
Journal of materials chemistry. A