The neutron diffraction anomalous dispersion technique and its application to vitreous Sm2O3·4P2O5

The first successful complete neutron diffraction anomalous dispersion experiment has been performed to investigate the role of the Sm3+ ions in the structure of vitreous Sm2O3·4P2O5. The ideal form of this technique, which employs the wavelength dependence of the real and imaginary parts of the neutron scattering length close to an absorption resonance, is used and involves measurements at two pairs of wavelengths: the real part of the scattering length is varied, keeping the imaginary part constant, and then the imaginary part is varied, keeping the real part constant. If A denotes the element with the isotope (149Sm) having the absorption resonance and X any other element present in the sample, the first measurement can be used to extract the A−A+A−X or A−A+X−X contribution to the real space correlation function, T(r), and the second yields the A−A component correlation function. For the present glass, the Sm−Sm+Sm−X contribution (X=P or O) reveals that the Sm3+ ions have an average co-ordination number, nSm(O), of 7, with a mean SmO bond length of 2.375±0.005 Å, while the anomalous difference correlation function indicates that the Sm3+ ions are ∼4.6 Å apart. The paper concludes with a discussion of the relative merits of the various neutron diffraction techniques for isolating individual or subsets of real-space component correlation functions.