Neutron-diffraction study of Tb(OH)3 - a case of severe extinction

Neutron-diffraction experiments on a single crystal of Tb(OH)3 have indicated severe extinction effects. For some Bragg reflections the observed neutron cross section is attenuated by a factor of ten owing to extinction. A least-squares refinement (including extinction) of the structure factors deduced from integrated intensities measured at room temperature gives structural parameters in good agreement with a previous determination, and new information about the position and thermal vibrations of the hydrogen atom. The final residual of 0.036 indicates that extinction is well accounted for by the Zachariasen formulae, and the value of g, the extinction parameter, is (6 ± 1) × 104. The polarized-neutron technique has been used to measure the ratio between the magnetic and nuclear structure factors at 2.6°K, at which temperature Tb(OH)3 is ferromagnetic (9 μB/Tb atom). The effect of extinction is particularly dramatic in this experiment, but again the results may be explained with the simple Zachariasen correction and the same extinction parameter as used for the unpolarized data. The corrected magnetic scattering amplitudes deduced from the polarized-neutron experiment allow the magnetic form factor of the terbium atom to be found, and compared to that measured in terbium metal; this aspect of the investigation is discussed in a separate article.