VANADIUM PENTOXIDE AND VANADIUM-OXIDE BRONZES STRUCTURAL CHEMISTRY OF SINGLE (S) AND DOUBLE (D) LAYER MXV2O5 PHASES

Investigation of the details of the V2O5 crystal structure shows the vanadium coordination to be purely square pyramidal SP, the single [V2O5] layer S being built up by endless double strings of VO5 SP associated by corner sharing. This unit allows to describe all the VOB's structures exhibiting S layers, i.e., α-MxV2O5 (M = all elements), ε-, δ-, γ-LixV2O5, α′-NaxV2O5, Cu0.64V2O5. The structural mechanisms which govern the phase transitions ε → δ, δ → γ have been described, as well as the transformations by crystallographic shears, slips, and VO5 SP reversal of ε (or α) into the 3-D networks of β or β′-MxV2O5 phases. The complete LixV2O5 system versus f(T, x) is described and the evolution of the cell parameters for ε and γ phases is accounted for by introducing the puckering angle μ of the S layers. The demonstration of the slipping of the S layers quantified by shift Δ, expressed as a fraction or unit of the height of an octahedron, has made it possible to propose various original stackings for these layers; some ideal structures are described. A mechanism explaining the M intercalation between S layers shows that for large cations new layers are formed after shrinkage of two S layers giving double [V2O5] layers D of the D4 and D4M types built up by endless quadruple strings and DZ type obtained from cross-linked double strings of VO6 octahedra sharing edges and corners. This assumption is supported by the structure of the new VOB, τ-Na0.70V2O5, which is an intergrowth of the D4 and DZ layer types. Using these units in combination with Δ slips has permitted a unifying description is given for all known structures: δ-AgxV2O5, δ-SrxV2O5, δ-K0.5V2O5, ν-Ca0.60V2O5, ν-K0,62V2O5, ν-Rb0.40V2O5, ν-(NH4)0.50V2O5, ε-CuxV2O5, ρ{variant}-K0.50V2O5, ..., and many other possibilities are suggested. © 1992.