The influence of the lithium concentration on the low-temperature phase transition in phases LixV2O5 with shcherbinaite-structure

With high-resolution Guinier diffractometry the low-temperature stability of the intercalated lithium vanadium oxide bronzes ε - Li _x V₂O₅, with 0.32 ≤ x ≤ 0.80, was studied. In the course of these investigations a line of thermally induced structural phase transitions was detected in the x-T field. The reversible continuous phase transitions can be described in terms of a classical Landau-type theory. They are proper (potentially) ferroelastic with an one-dimensional order parameter transforming as the Γ⁺ ₃ irreducible representation of space group Pmmn. The symmetry is orthorhombic P2₁/m2₁/m2/n above T_c , but monoclinic P2₁ /m11 below T_c .

The transition temperatures depend strongly on the stoichiometry of the intercalated lithium ions. The critical exponent β, and, hence, the character of the phase transition do not change significantly with the lithium concentration. However, for x ≤ 0.52 corresponding phase transitions could not be detected, supporting the idea that the accepted stability field of ε - Li _x V₂O₅(0.32 ≤ x ≤ 0.80) should be subdivided into two fields, ε- and ε'-phase, according to whether the phase transition occurs, or not. The influence of the reduction rate of the intercalated phases on the critical parameters will be discussed in terms of a mean-field theory.