Neutron scattering under pressure

Caution For safety procedures see Appendix 2. Neutron scattering under high pressure is a very powerful but comparatively new method in condensed-matter studies. It can only be performed at a limited number of neutron facilities, such as the reactors in Grenoble (ILL), Orsay (LLB), or Dubna (Russia), at the spallation sources in the UK (ISIS, Rutherford Appleton Laboratory, Didcot) or the USA (LANSCE, Los Alamos). Potential users of these facilities are encouraged to submit requests for beamtime. Requests should only be made after gathering information on the experimental facilities available and after careful examination of the published results from individual sources. 

By far the largest part of the experimental results presented in this work, has been obtained with the aid of optical studies. Only in exceptional cases other methods, like electron-spin resonance or neutron scattering, have been employed to get information about energy levels under pressure. Obviously, these methods must be used in the case of non-transparent materials, where optical methods are not suitable. 

Now, we may evaluate the surface reconstruction generated by the suppression of a few kilobars of stress along the d axis, using the deformation coefficients under hydrostatic pressure. The authors of Ref. 136 have determined, in neutron-scattering experiments, the variation of the crystallographic parameters of anthracene ... 

Probably the most notable work on the structure in liquid water based upon experimental data has been that of Soper and co-workers [6,8,10,30,46,55]. He has considered water under both ambient and high temperature and pressure conditions. He has employed both the spherical harmonic reconstruction technique [8,46] and empirical potential structure refinement [6,10] to extract estimates for the pair distribution function for water from site-site radial distribution functions. Both approaches must deal with the fact that the three g p(r) available from neutron scattering experiments provide an incomplete set of information for determining the six-dimensional pair distribution function. Noise in the experimental data introduces further complications, particularly in the former technique. Nonetheless, Soper has been able to extract the principal features in the pair (spatial) distribution function. Of most significance here is the fact that his findings are in qualitative agreement with those discussed above. 

Because structural phase transitions are often ferroelastic or coelastic in character it is essential to have a well-defined stress applied to the crystal at high pressures. In effect, this means that a hydrostatic pressure medium must be used to enclose the crystal. A 4 1 mixture by volume of methanol ethanol remains hydrostatic to just over 10 GPa (Eggert et al. 1992) and is convenient and suitable for many studies. If the sample dissolves in alcohols, then a mixture of pentane and iso-pentane which remains hydrostatic to 6 GPa (Nomura et al. 1982), or a solidified gas such as N2, He, or Ar can be employed. Water appears to remain hydrostatic to about 2.5 GPa at room temperature, just above the phase transition from ice-VI to ice-VII (Angel, unpublished data). The solid pressure media such as NaCl or KCl favoured by spectroscopists are very non-hydrostatic even at pressures below 1 GPa and have been shown to displace phase transitions by at least several kbar (e g. Sowerby and Ross 1996). Similarly, the fluorinert material used in many neutron diffraction experiments because of its low neutron scattering power becomes significantly non-hydrostatic at -1.3 GPa. Decker et al. (1979) showed that the ferroelastic phase transition that occurs at 1.8 GPa in lead phosphate under hydrostatic conditions is not observed up to 3.6 GPa when fluorinert was used as the pressure medium. At pressures in excess of the hydrostatic limit of the solidified gas and fluid... 

A.I. Kolesnikov, V.E. Antonov, A.M. Balgurov, S.M. Bennington M. Prager (1994). J. Phys. Condens. Matter, 6, 9001-9008. Neutron scattering studies of the structure and dynamics of the PdCuH ordered phase produced under a high hydrogen pressure. 

The systematics of the lattice constants of the RE monochalcogenides, see Fig. 11, indicate that the Tm ion changes its valence in going from the metallic sulfide (essentially 3+) to the semiconductor-like telluride (essentially 2+). Magnetization measurements of TmSe (48) revealed a complicated behavior, and inelastic neutron scattering studies were unable to detect the structure of the crystal field levels (49). This was unexpected because Tm is known to be a favorable element for neutron scattering, as confirmed in various studies of TmSb (50). It was therefore proposed by Wohlleben and Coles (4) that TmSe, like TmTe under pressure, is a homogeneous mixed valence system. 

VE1 Vennemann, N., Lechner, M.D., and Oberthtir, R.C., Thermodynamics and conformation of polyojgrethylene in aqueous solution under high pressure. 1. Small-angle neutron scattering and densitometric measurements at roomten rature. Polymer, 28,1738, 1987. 

Inelastic neutron scattering from both y- and a-Ce under pressure was investigated by Rainford et al. (1977). Their results indicate that the s-f exchange interaction, J, in y-Ce increases with increasing pressure as predicted by Coqblin. Also they find no experimental evidence for a magnetic contribution to the inelastic neutron scattering intensity of a-Ce. They conclude that if there is some residual 4f electronic character in a-Ce its dynamical response is too weak or diffuse to be observed in their experiment. 

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