epsilon-LixV2O5 bronzes (0.33<=x<=0.64) a joint study by X-ray powder diffraction and Li-6, Li-7 MAS NMR

A kind of miscibility gap was detected by XRD and NMR experiments in the homogeneity range of the epsilon phase of LixV2O5 bronzes for lithium yields around x=0.5. Lattice parameter determinations show a break in the evolution of the a parameter versus the lithium content suggesting the existence of two varieties of the epsilon phase, i.e. epsilon(1) and epsilon(2) Li-6 and Li-7 magic-angle-spinning (MAS) NMR experiments confirm such event. The evolution of the chemical shift of the isotropic band allows the separation of the epsilon-LixV2O5 phase region into three parts: two monophasic domains epsilon(1) with the 0.33 < x < 0.47 and epsilon(2) with the 0.53 < x < 0.64 ranges and a biphasic one with 0.47 < x < 0.53 range. Both monophasic domains correspond to the homogeneous variation of the a lattice parameter observed by XRD experiments. Calculations show that the lithium surrounding remains essentially unchanged in both epsilon 1 and epsilon 2 phases. For low lithium contents (x < 0.47), an explanation of the structural evolution could be provided by an ordering of the lithium ions every second site in the [010] direction involving a doubling of the b lattice parameter and a space group P2(1)/m. The second monophasic domain (x > 0.53) could correspond to a phase similar to alpha'-NaxV2O5, i.e., an ordering of the V4+ atoms which infers a loss of symmetry driving to P2(1)mn space group. The biphasic domain (x approximate to 0.5) may be attributed to a mixture of the preceding phases.