SYNCHROTRON POWDER DIFFRACTION STUDY OF THE LOW-TEMPERATURE LATTICE DISTORTION OF PBMO6S8

Synchrotron and Guinier x-ray powder diffraction data as a function of temperature on a superconducting PbMo6S8 sample (T-c=14.7 K) showed e vidence for a subtle structural phase transition at T-l=140 K, leading from a rhombohedral high-temperature to a triclinic low-temperature m odification. The transition is incomplete and presents a hysteresis of about 110 K. The triclinic cell parameters obtained at 130 K, a=6.534 (9) Angstrom, b=6.532(9) Angstrom, c=6.529(9) Angstrom, alpha=89.27(8) degrees, beta=89.12(8)degrees, gamma=88.97(8)degrees, show that the di stortion affects mainly the angles (delta alpha/alpha(r)=0.3%) and les s the edges (delta alpha/alpha(r)<0.1%). This result contrasts with pr evious neutron diffraction work [Phys. Rev. 35, 5365 (1987)] where the distortion was found to be much greater and to affect both the angles and the edges (delta alpha/alpha(r),=1.2%, delta alpha/alpha(r),=1.2% ), and to be of the same order of magnitude than in the MMo(6)S(8) (M= Sr,Ba,Eu) analogues (M=Ba: delta alpha/alpha(r),=1.2%,delta alpha/alph a(r),=3.5%). Guinier data on an oxygen containing sample having a lowe r superconducting transition temperature (T-c=10.5 K) showed a smaller distortion that started at lower temperature (T-l=100 K).