X-rays, diffuse scattering

Any perturbation from ideal space-group symmetry in a crystal will give rise to diffuse scattering. The X-ray diffuse scattering intensity at some point (hkl) in reciprocal space can be written as... 

Figure 3 Calculated X-ray diffuse scattering patterns from (a) a full molecular dynamics trajectory of orthorhombic hen egg white lysozyme and (b) a trajectory obtained by fitting to the full trajectory rigid-body side chains and segments of the backbone. A full description is given in Ref. 13.

A dynamic transition in the internal motions of proteins is seen with increasing temperamre [22]. The basic elements of this transition are reproduced by MD simulation [23]. As the temperature is increased, a transition from harmonic to anharmonic motion is seen, evidenced by a rapid increase in the atomic mean-square displacements. Comparison of simulation with quasielastic neutron scattering experiment has led to an interpretation of the dynamics involved in terms of rigid-body motions of the side chain atoms, in a way analogous to that shown above for the X-ray diffuse scattering [24]. 

It is simplest to consider these factors as they are reflected in the entropy of the solution, because it is easy to subtract from the measured entropy of solution the configurational contribution. For the latter, one may use the ideal entropy of mixing, — In, since the correction arising from usual deviation of a solution (not a superlattice) from randomness is usually less than — 0.1 cal/deg-g atom. (In special cases, where the degree of short-range order is known from x-ray diffuse scattering, one may adequately calculate this correction from quasi-chemical theory.) Consequently, the excess entropy of solution, AS6, is a convenient measure of the sum of the nonconfigurational factors in the solution. 

X-Ray Diffuse Scattering from Self-Oi anized Mesoscopic Semiconductor Structures... 

In the case of structural studies with x-rays, sample size is rarely a serious problem since single crystals as small as about 0.001 mm3 (sealed x-ray tube) or 10-9 mm3 (synchrotron radiation) will do. In the case of x-ray diffuse scattering measurements with the assembly described in Section VI.D, the crystal must be 2 mm long at least but can be relatively thin. With neutrons, however, sample size is more critical. Indeed, for structure measurements, 0.1 mm3 is a minimum, with the highest neutron fluxes available nowadays, and more usual sizes are in the range of several cubic millimeters to several tens of cubic millimeters. For polarized neutrons or inelastic scattering studies, much larger samples are necessary. 

Because the amplitude of the lattice distortion in the Peierls insulator is very small, the X-ray scattering associated with it is weak. Furthermore the one-dimensional nature of this distortion gives rise to diffuse Bragg planes instead of the usual well-defined Bragg reflections. These two facts have led to the development of a special diffuse X-ray photographic technique often referred to as the monochromatic Laue technique or XDS (for X-ray diffuse scattering) (64). 

Renaud G, Belakhovsky M, Lefebvre S, Bessiere M (1995) Correlated chemical and positional fluctuations in the gold-nickel system-a synchrotron X-ray diffuse-scattering study. J Phys III 5 1391-1405... 

The bis(oxalato)platinate(II) complexes based on [Pt(C204)2] , forms another family of linear chain systems, and the partially oxidised complex series, Cx[Pt(C204)2]-n(H20) (C = K, Rb, Mg " ", Co, Ni, Zn ), was reported.According to elemental analysis and X-ray diffuse scattering measurements, the formal oxidation state of platinum in this series is estimated in the range of -1-2.19 to -f2.38. The average Pt-Pt distance is 2.84-2.89 A and the conductivity at room temperature is 10 -10 S cm , depending on the associated cation. 

Chemical analyses and theoretical values are reported in Table I.8 The calculated platinum oxidation state, based on chemical analysis and crystallographic analysis, is +2.25. An X-ray diffuse scattering analysis also indicates that the Pt oxidation state is 2.25(2).ls... 

Neutron and X-Ray Diffuse Scattering Study of Tetrathiofulvalene Tetracyanoquino-dimethane (TTF-TCNQ) 445... 

Some results of a X-ray diffuse scattering investigation on the charge transfer compounds TSeF-TCNQ and HMTSeF-TCNQ are presented. In the metallic state the results show the formation of a 1-D distortion or a Kohn anomaly in the phonon spectrum. For TSeF-TCNQ this is observaibelow T 238 K and corresponds to a periodicity of (3.15+ 0.05) Ibl along the chain axis. For HMTSeF-TCNQ the distortion was detected even at room temperature and has a period of (2-7+ 0,1) Icl In both materials the distortion has both transverse components. In the insulating state of TSeF-TCNQ below 29 K the 1-D distortion on each stack are correlated to font a 3-D super-lattice. 

To summarise, it seems clear that at low T the conductivity of HMTSF TCNQ arises from small pockets of electrons with a degeneracy temperature of about 2CK and there is mounting evidence that these are an intrinsic property of the material. The proposed band structure accounts for many of the experimental observations but a number of problems remain. One interesting experiment is X-ray diffuse scattering at low T. It is known that there is a longitudinally polarised "2kp" soft phonon down to 100, if this condenses to... 

NEUTRON AND X-RAY DIFFUSE SCATTERING STUDY OF TETRATHIOFULVALENE TETRACYANOQUINODIMETHANE (TTF - TCNQ)... 

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