Rh. Blessing, ON THE DIFFERENCES BETWEEN X-RAY AND NEUTRON THERMAL VIBRATION PARAMETERS, Acta crystallographica. Section B, Structural science, 51, 1995, pp. 816-823
For crystal structures analyzed by both X-ray and neutron diffraction,
the anisotropic mean-square displacement parameters of the non-H atom
s are sometimes found to differ significantly. The differences can usu
ally 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 t
wo experiments; (2) anisotropic factors q(ij), defined by U-X(ij) = q(
ij)U(N)(ij), to correct for a temperature difference and different ani
sotropic diffraction effects of absorption, extinction, thermal diffus
e scattering, multiple reflection, or systematic measuring errors in t
he 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 is
otropic 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 f
it, 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 atom
s 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 dis
tribution.