Crystal Chemistry of a New Mineral-like Phosphate Na6.9Ni2+0.9V3+4.3Al0.8(PO4)8(H2O)2 in the Series of α-CrPO4 Derivatives

A novel mineral-like phosphate hydrate Na6.9Ni2+0.9V3+4.3Al0.8(PO4)8(H2O)2 (Z = 2) was obtained under high-temperature hydrothermal conditions by modeling the chemistry of geothermal brines in natural geological solutions. The compound, characterized by scanning electron microscopy and microprobe analysis, possesses an orthorhombic symmetry with the Cccm space group; the unit cell parameters are a = 6.4082(8), b = 19.6813(19), c = 10.5035(11) Å. Here we report its crystal structure studied by low-temperature single crystal X-ray diffraction and discussed as derived from the α-CrPO4 archetype, known for a large range of compounds with promising properties. Three-dimensional continuous migration pathways for Na+ within the structure were found and confirmed by a bond valence energy landscape analysis. The migration barriers turned out to be ~0.44 eV along the a and b directions and ~0.42 eV along the c axis. These values suggest that the compound may be a potential electrode material for sodium-ion batteries.