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 Li7[VN4] have been established from X-ray and neutron powder diffraction (gamma-Li7[VN4], space group P43n, No. 218, a = 960.90(4) pm, V= 887.23(6) x 10^6 pm^3, Z = 8; beta-Li7[VN4], space group Pa3, No. 205, a = 959.48(3) pm, V = 883.31(5) x 10^6 pm^3, Z = 8; alpha-Li7[VN4], P42/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-Li7[VN4] with the general formula Li1.75{(Vo.25(1-x)Lio.25x)(N1-xO),)} with 0 < x < 1 leads to increasing substitution of V by Li. 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, aʹ = 476.47(3) pm ~ (1/2)a(betaLi7[VN4]).