Structural phase transitions in BaMo/sub 6/S/sub 8/: Evidence for an incommensurate phase

The structure of BaMo/sub 6/S/sub 8/ has been studied over the temperature range 19 K to 573 K by time-of-flight neutron powder diffraction. Below 175 K the data can be suitably refined in a triclinic, P1, cell with volume equal to the rhombohedral, R3, cell common to most Chevrel-phase structures. At temperatures immediately above 175 K, the rhombohedral, R3, Bragg peaks are broadened by satellite reflections which appear to be identical to those recently observed at low temperature in PbMo/sub 6/S/sub 8/ and SnMo/sub 6/S/sub 8/. An abrupt change in the sign of the temperature dependence of the hexagonal c axis (par. deltac/par. deltaT) signals the transition to an undistorted rhombohedral, R3, structure at temperatures above about 350 K. An extended Landau theory determines both continuous and discontinuous transitions from R3 induced by a single order parameter. Analysis of the order parameters inducing commensurate transitions imposes symmetry restrictions on the atomic displacements in the lower symmetry phases. The assumption of an R3 commensurate phase is not consistent with the bond lengths obtained for the distortions to the P1 (or P1) phase for any of the possible cells preserving order parameters. Thus the phase immediately above 175 K cannot be amore » commensurate R3 structure. This is consistent with experimental evidence. 25 refs., 11 figs., 8 tabs.« less