ON THE DIFFERENCES BETWEEN X-RAY AND NEUTRON THERMAL VIBRATION PARAMETERS

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 U-X(ij) = qU(N)(ij), to correct for a temperature difference between the two experiments; (2) anisotropic factors q(ij), defined by U-X(ij) = q(ij)U(N)(ij), 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 U-ij, defined by U-X(ij) = U-N(ij) + Delta U-ij; (4) the sum of an isotropic temperature correction and anisotropic diffraction corrections, defined by U-X(ij) = qU(N)(ij) + Delta U-ij. Correction parameters q, q(ij) and Delta U-ij 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 U-ij's of the non-H atoms of a crystal can be useful for adjusting the neutron U-ij'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.