Solving Crystal Structures from Powder Neutron Data

7 Solving Crystal Structures from Powder Neutron Data 

The neutron-atom interaction involves also the interaction between the magnetic moment associated with the spin of the neutron and the magnetic moment of the atom (this last generated by the presence of unpaired electrons). This allows the investigation of magnetic structures, whose magnetic scattering does decline with sin 0/2. 

The interaction of neutrons with matter is weaker than for X-rays and electrons. Therefore higher neutron fluxes are needed to measure appreciable scattered intensities. 

Coherent scattering, giving rise to Bragg scattering, defines the structure factor  

We have underlined that positivity and atomicity of the electron density are basic conditions for the validity of traditional DM. What then is the effect of the possible violation of the positivity criterion on DM procedures It has been assessed that positivity is not an essential ingredient of DM. In particular the triplet phase invariants can again be evaluated via a von Mises distribution, but now the value of N has to be replaced by Aeq, where  

---

Within the last few years high resolution data collected either from synchrotron or neutron sources has allowed new structures to be solved from powder samples. Powder data from these new sources are superior to that collected from a conventional x-ray source both in resolution and peak to background ratio these improvements in data quality have the capability of making the determination of new molecular sieve topologies easier but not necessarily routine. Synchrotron radiation has possible future capabilities of being used to collect data from crystals that are only a few microns in size. At that time most newly synthesized molecular sieve materials will be solved by single crystal techniques instead of powder techniques. At the present time an acceptable structure refinement can be obtained from a crystal with a... 

Because of the progress in structure solution from powders and in neutron diffraction, more than 300 crystal structures have already been fully determined, most of them during the past two decades. Here, crystal structures are only considered as fully characterized if all atomic positions have been determined and refined to an acceptable precision. The hydride fluoride database (HFD) provides a comprehensive, critical compilation of crystal structure data of metal hydrides. The power of the method described of combining X-ray and neutron powder diffraction may be illustrated by the example of MgeCoaDn, whose rather complex crystal structure, which contains 63 free positional parameters, was solved ab initio from high-resolution synchrotron and neutron powder diffraction data. Because of the main problems discussed in this section, the accuracy of metal hydride structural parameters is generally lower than for other inorganic compounds. 

---