PHASONS MODULATE THE ATOMIC DEBYE-WALLER FACTORS IN INCOMMENSURATE STRUCTURES - EXPERIMENTAL-EVIDENCE IN THBR4 AT 55-K

The incommensurate displacive structure of beta-ThBr4 at 55 K has been determined from a neutron diffraction data set including main reflections and first-order satellites. The superspace group is P(I41/amd)s1s1.BAR Final agreement factors are 0.0193, 0.0186 and 0.045 for all, main and satellite reflections, respectively. It is shown that the effect of phasons on the atomic Debye-Waller factors can be quantified by two additional structural parameters: the modulus beta(Br)11,2 and the phase chi(Br)11,2 of a second harmonic that spatially modulates the temperature factors of Br atoms. Results are in good agreement, within the resolution of the experimental data, with the theoretically expected value for chi(Br)11,2. CryStal data for the average structure: M(r) = 551.65, tetragonal, I4(1)/amd, a 8.919 (1), c = 7.902 (1) angstrom, V = 628.6 (2) angstrom3, Z = 4, D(x) = 5.82 Mg m-3, lambda = 0.84 angstrom, wavevector q = 0.32c*.