Neutron needs large crystals

The former way is not useful when X-ray diffraction is used, because the difference between the scattering curves of Si and A1 is too small. If neutron diffraction is used, the neutron diffraction curves differ by as much as 25 %, so that the Si/Al ratio can be satisfactorily refined. Unfortunately, large crystals are needed at present (nearly 1mm in volume), therefore this method can only be applied to a restricted number of zeolites. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

Up to now, neutron inelastic scattering has not played a major role in the study of the actinides and there are good reasons for this a large amount of material is needed (0.5 cm crystals) so that studies started only recently and are confined to uranium systems. Moreover, the complexity of the results obtained has made interpretation of the experiments difficult. 

In cryogenic detectors, a simultaneous measurement of both ionization and thermal energy allows the discrimination of nuclear recoils from electrons produced in radioactive decays or otherwise. This discrimination, however, cannot tell if the nuclear recoil was caused by a WIMP or an ambient neutron. The detector, most often a germanium or silicon crystal, needs to be cooled at liquid helium temperature so that its low heat capacity converts a small deposited energy into a large temperature increase. Only relatively small crystals can be currently used in these cryogenic detectors, with relatively low detection rates. 

The production of further comprehensive compendia of X-ray and neutron diffraction results has been precluded by the steep rise in the number of published crystal structures, as illustrated by Figure A.l. Printed compilations have been effectively superseded by computerized databases. In particular, the Cambridge Structural Database (CSD) now (October 1992) contains bibliographic, chemical, and numerical results for over 100,000 organo-carbon crystal structures. This machine-readable file fulfils the function of a comprehensive structure-by-structure compendium of molecular geometries. However, the amount of data now held in CSD is so large that there is also a need for concise, printed tabulations of average molecular dimensions. 

---