Isotopes in Neutron Diffraction - Detailed Structural Analysis at the Metal-Insulator Transition in SmNiO 3
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Isotopes in Neutron Diffraction - Detailed Structural Analysis at the Metal-Insulator Transition in SmNiO3 Mark T. Weller, Paul F. Henry and C.C. Wilson1 Department of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. 1 ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX, UK. ABSTRACT The use of isotopically enriched materials on high intensity neutron powder diffractometers allows the derivation of much higher quality structural information than has hitherto been possible. The technique can be applied to materials such as ferroelectrics, superconductors and oxides exhibiting colossal magneto-resistance, where small structural changes associated with phase transitions need to be characterised. Three isotopically pure samples of samarium nickelate have been studied around the metal-insulator transition at 403 K. Simultaneous multi-histogram refinements permit extraction of very high quality structural information that shows smooth variations in bond lengths and angles with all nickel oxygen distances decreasing as the electrons localise. INTRODUCTION Neutron diffraction is a key technique in the characterisation of solids, often providing information unobtainable with any other method. Its importance has been amply demonstrated in recent years with investigations of high temperature superconductors [1], fullerenes and fullerides [2], compounds displaying colossal magneto-resistance [3], ferroelectrics [4], zero and negative coefficient of thermal expansion materials [5], zeolites [6] and molecular and organometallic solids [7]. Despite this widespread use, the method still has limitations deriving from the fundamental neutron scattering properties of many systems, coupled with (until now) relatively weak neutron count-rates, leading to the requirement for large sample sizes. With the advent of very high count-rate neutron diffraction instrumentation, such as D20 at the ILL, GEM at ISIS and the planned instruments at the SNS at Oakridge, incorporating large detector areas, sample sizes can be reduced dramatically whilst maintaining good data statistics. The work reported here is the first to address another aspect of powder neutron diffraction structural work that is possible with small sample sizes, namely the use of otherwise prohibitively expensive isotopes. By using isotopically pure materials with strongly contrasting scattering powers [8] for the elements present and multiple data set analysis techniques a new generation of powder neutron diffraction experiments, producing much higher quality structural information and structural data on systems, has become possible. The potential of isotopes in neutron diffraction can be seen by consideration of their fundamental properties. For over half the elements in the periodic table there exists, at reasonable cost (
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