Polymorphism of heptalithium nitridovanadate(V) Li7[VN4]

The system Li-V-N was studied by means of X-ray and neutron powder diffraction, thermal and chemical analyses, and XAS spectroscopy at the vanadium K-edge, Three polymorphs of Li-7[VN4] have been established from X-ray and neutron powder diffraction (gamma-Li-7[VN4], space group P (4) over bar 3n, No. 218, a = 960.90(4) pm, V = 887.23(6) x 10(6) pm(3), Z = 8; beta-Li-7[VN4], space group Pa (3) over bar, No. 205, a = 959.48(3) pm, V = 883.31(5) x 10(6) pm(3), Z = 8; alpha-Li-7[VN4], P4(2)/nmc, No. 137, a = 675.90(2) pm, c = 488.34(2) pm, V = 223.09(1) x 10(6) pm(3), Z = 2). Crystallographic and phase relations are discussed. All three modifications are diamagnetic, indicating vanadium in the oxidation state +5. The V-K XAS spectra support the oxidation state assignment, the non-centrosymmetric coordination (tetrahedral), and the nearly identical second coordination sphere of vanadium, made up from Li in all three phases. The 3d-related features of the spectra display strongly localized properties. The phase transitions appear to be reconstructive; no direct group-subgroup symmetry relations of the crystal structures exist. The formation of solid solutions between Li2O and beta-Li-7[VN4] with the general formula Li-175{(V0.25(1- x)Li0.25x)(N1-xOx)} with 0 less than or equal to x less than or equal to 1 leads to increasing substitution of V by U. At the approximate composition with x = 0.125 the Li-V-disorder seems to be complete: X-ray diffraction lines give rise to only one-half of the original unit cell dimensions, a' = 476,47(3) pm approximate to (1/2)a(beta-Li-7[VN4]).