Correlations between crystallographic data and physical properties at the 240K and 80K phase transitions in stoichiometric BaVS3

The two phase transitions (at 240K and 80K) which occur in stoichiometric BaVS 3 as a function of temperature have been investigated by very accurate powder X-ray diffraction data. These data indicate that the 240K transition, where the crystal symmetry is lowered from hexagonal to orthorhombic, is a displacive transition of second-order type. The thermal variations of a 0 and b 0 / square root 3 show that between 240K and 80K the orthorhombic distortion increases progressively and does not reach saturation even at 80K. The electric-magnetic 80K transition is accompanied by an additional discontinuity of the lattice parameters. This discontinuity is only observed in the basal a 0 b 0 plane, whereas the c 0 parameter follows a normal Debye-expansion law. The data strongly indicate that the 80K transition is of first-order type and that the lattice is still orthorhombic below the transition.