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 reflect ions and first-order satellites. The superspace group is P(I41/amd)s1s 1.BAR Final agreement factors are 0.0193, 0.0186 and 0.045 for all, ma in and satellite reflections, respectively. It is shown that the effec t of phasons on the atomic Debye-Waller factors can be quantified by t wo additional structural parameters: the modulus beta(Br)11,2 and the phase chi(Br)11,2 of a second harmonic that spatially modulates the te mperature factors of Br atoms. Results are in good agreement, within t he resolution of the experimental data, with the theoretically expecte d 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 angs trom, wavevector q = 0.32c.