diffraction intensity in plane is of the order of |% of the incident beam 0 Strong vicinal out-of-plane peaks recorded in scans using a larger detector acceptance angle (1.4°) indicate that the total diffractive scattering may be a substantial fraction of I 0 as expected for an estimated surface Debye temperature of 360 K" 5 There is some indication that a hard-wall potential with a weak attractive well is appropriate for this scattering system 0 The different structural models which have been proposed for the Si(lll) 7x7 reconstruction 7 " 9 are expected to generate quite different intensity sequences versus 9 r for the range of incident conditions we reporto Consequently, we think that the data we present here are sufficient to give each model a critical test, x R a E. Schlier and H. E. Farnsworth, J. Chem. Phys. The a-a' phase transition of H in Nb resembles a gas-liquid transition of a real gas. 1 ' 2 In this transition the lattice retains its original bcc symmetry, and the phases are only distinguishable by their different lattice expansions. This transition and similar transitions in PdH (Ref. 3) and PdAgH (Ref. 4) are still under discussion,, 5,6They have often been treated as textbook examples of a lattice gas , 7 • 8 and the interaction energy, responsible for the phase transition, is introduced as a suitable parameter proportional to the measured critical temperature T c . In contrast to the lattice-gas model the real lattice is deformable and the interstitially dissolved protons set up long-range distortion fields. 9 " 11 The a-a' 3£, 917 (19593£, 917 ( ) j J 0 . Technol. 15, 423 (1978. 4 M. J. Cardillo and G. E. Becker, Phys. Rev. Lett. 40, 1148Lett. 40, (1978.Assuming 0 D surface =j(9 D bulk , with 0 D bulk =543 K [B. W. Batterman and D. R 0 Chipman, Phys. Rev. 127, 690 (1962)1, we estimate a reduction in the total diffraction intensities to I/IQ^ 10-20%, for a range of the He/Si attractive well depth of 10-20 meV. 6 E. Grimmelmann and J. C. Tully, private communication. 7 J.transition has therefore been attributed to an elastic interaction via this lattice distortion induced by the protons. 12 ' 13 The model of elastic interaction leads in turn to the concept of macroscopic elastic modes 14 ' 15 caused by internal stress produced by the inhomogeneous hydrogen density variation in a coherent lattice. At T c only a few modes become unstable which depend upon the sample geometry through the fulfillment of the elastic boundary condition. Therefore, the study of the shape dependence of the phase transition is a crucial test of the validity of the elastic interaction as the main contribution to the attractive part of the energy. Nb samples (single crystals, wires, and foils)The coherent OL-GL' phase transiton of H in Nb has been studied in several sample shapes. The samples were loaded in situ with the critical hydrogen concentration of 0.31 H/Nb above T C =171°C and slowly cooled. Below T c a coherent macroscopic spinodal decomposition takes place. By x-ray scattering and examination of the relaxed c...
This review gives first a short introduction to the theoretical relations between experimentally observed diffuse scattering results and the desired information about defects. The situation is discussed under which diffuse scattered X‐ray intensity from defects can be distinguished from other diffuse intensities (thermal diffuse scattering, Compton scattering). A typical experimental set‐up is described to show what the requirements are in experimental resolution and how intensity distribution in reciprocal space can be measured most conveniently. A few typical experiments are discussed to demonstrate the physical potential of this method for studying impurities in metals and radiation‐induced defects and defect clusters in ionic crystals. Special emphasis is given to the comparison of the experimental results with theoretical predictions; the intensity close to the Bragg peaks (Huang scattering) falls off as 1/g2, g being the distance from a reciprocal‐lattice point G. The scattering intensity goes as G2. The intensity distribution in reciprocal space gives typical isointensity curves from which the symmetry of the double force tensor and its components can be deduced. Together with the measured shift of a Bragg peak, measurement of absolute scattering intensity gives a unique way of determining the defect concentration. Further away from the Bragg peaks (asymptotic scattering) the intensity is proportional to 1/g4 and shows characteristic oscillations.
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