With high-resolution Guinier diffractometry the low-temperature stability of the intercalated lithium vanadium oxide bronzes epsilon - LixV2O5, with 0.32 less than or equal to x less than or equal to 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 Gamma(3)(+) irreducible representation of space group Pmmn. The symmetry is orthorhombic P2(1)/m2(1)/m2/n above T-c, but monoclinic P2(1)/m11 below T-c. The transition temperatures depend strongly on the stoichiometry of the intercalated lithium ions. The critical exponent beta, and, hence, the character of the phase transition do not change significantly with the lithium concentration. However, for x less than or equal to 0.52 corresponding phase transitions could not be detected, supporting the idea that the accepted stability field of epsilon - LixV2O5 (0.32 less than or equal to x less than or equal to 0.80) should be subdivided into two fields, epsilon- and epsilon'-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.
