On the differences between X-ray and neutron thermal vibration parameters

Abstract

For crystal structures analyzed by both X-ray and neutron diffraction, the anisotropic mean-square displacement parameters of the non-H atoms are sometimes found to differ significantly. The differences can usually be adjusted by either: (1) an isotropic factor q, defined by UijX = qUijN, to correct for a temperature difference between the two experiments; (2) anisotropic factors qij, defined by UijX = qijUijN, to correct for a temperature difference and different anisotropic diffraction effects of absorption, extinction, thermal diffuse scattering, multiple reflection, or systematic measuring errors in the two experiments; (3) anisotropic diffraction correction terms delta Uij, defined by UijX = UijN + delta Uij; (4) the sum of an isotropic temperature correction and anisotropic diffraction corrections, defined by UijX = qUijN + delta Uij. Correction parameters q, qij and delta Uij are easily calculated by linear least-squares fit, and the corrections from (3) or (4) seem to be the most reliable. Corrections calculated from X-ray and neutron Uij's of the non-H atoms of a crystal can be useful for adjusting the neutron Uij's of the H atoms for adoption, along with the neutron coordinates of the H atoms, as fixed parameters in an X-ray analysis of the electron density distribution.

Experimental Electron Density and Neutron Diffraction Studies on the Polymorphs of Sulfathiazole

“…31 The method of Madsen 32a (SHADE2 program 32b ) is known to provide an excellent approximation to H-atom adp’s. 33 For those forms of 1 for which neutron diffraction data were available, models using H atom adp’s from the SHADE2 calculated or the scaled 34 experimental neutron values were carefully compared, and little difference was found. In the final refinements for forms II–IV, the scaled experimental neutron adp’s were used with fixed contributions, while for form I, the SHADE determined parameters were used in the same way.…”

Growth and characterization of high quality LuVO3 single crystals

“…Final unit-cell parameters were obtained by the Bruker GLOBAL least-squares orientation matrix refinement procedure based on the positions of all measured reflections and are reported in Table 1. The semi-empirical absorption correction of Blessing [7], based on the determination of transmission factors for equivalent reflections, was applied using the Bruker program SADABS [8]. The values of equivalent reflections were averaged and the resulting discrepancy factors are reported in Table 1.…”

Experimental electron density study of 1,2,4-triazole at 15 K. A comparison with ab initio-calculations

“…\ > 0.8 j lead to anisotropic displacement parameters (ADP) slightly different from the neutron values. The differences can be accounted for by an anisotropic discrepancy tensor (At/n = 0.00115, At/22 = 0.00064 and Ac/33 = 0.00030) [16]. This correction was applied to the ADP's of the hydrogen atoms and the values obtained were fixed during the entire refinement.…”