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The article of record as published may be found at https://doi.org/10.1016/0956-7151(94)90171-6 Using critical voltage electron diffraction, Fox has recently determined the lowest seven X-ray structure factors of [gamma]-TiAl (L1[sub 0] structure). The authors present here a comparison of these accurately measured (0.15%) structure factors with first-principles local density calculations, finding an agreement within 0.7% and an r.m.s. error of 0.013 e/atom. While such measurements are limited to the first few structure factors [rho](G) (where G is the crystal momentum), theory is able to obtain [rho](G) for arbitrarily high G. If they construct charge density deformation maps by Fourier summations up to the lowest measured G, the calculated and experimental density deformation maps agree very closely. However, if they include in the theoretical density deformation map high G values (outside the range accessible to experiment), qualitatively different bonding patterns appear, in particular between Ti atoms. Systematic study of the total, valence, and deformation charge densities as well as comparison with result for NiAl in the hypothetical L1[sub 0] structure elucidate the bonding patterns in these transition metal aluminides. AC36-83CH10093

Structure factors and site-site distribution functions for models of liquid carbon disulphide (CS2) and acetonitrile (CH3CN) are obtained by using the site-site Ornstein-Zernike (SSOZ) integral equation with the Percus-Yevick (PY) and the hypernetted chain (HNC) closures. The calculated structure factors are found to be in good agreement with the neutron and X-ray diffraction data as well as with the simulation data. The site charges have a significant effect on the distribution functions but not on the structure factors of both the systems. There is very good qualitative agreement between the calculated distribution functions and the results from computer simulations. Distinctive shoulders found in the simulation results for the first peaks of the C-N and CH3-CH3 distribution functions are enhanced in the calculations using the integral equations.

Neutron diffraction measurements on liquid KTl were performed using Tl-205 isotope substitution. The measurement gives evidence for the occurrence of Tl-clusters with an average mutual distance of 0.96 nm. The structure in liquid KTl is compared with the crystal structures of KTl and K8Tl11, where resp. Tl-6 and Tl-11 clusters occur. To extend the search for analogies in the structure of liquid and solid alloys, we also measured liquid K8Tl11, K8Tl10Zn, K10Tl10Pd, and K18Tl20Au3, they all exhibit interesting crystal structures with large clusters (Tl-11, Tl10Zn, Tl10Pd and Tl9Au2). Indeed, some similarities are found between the structures in these liquid alloys and their solid counterparts.

Les systèmes désordonnés amorphes ont commencé à susciter l'intérêt des Physiciens et des Chimistes bien avant la découverte de leurs propriétés de commutation d'un état semiconducteur à un état métallique. La caractéristique essentielle de ces matériaux est bien évidemment la non-périodicité à longue distance pour des systèmes à l'état solide. D'un point de vue phénoménologique, un amorphe était défini comme un corps ne présentant pas de spectre de diffraction fin. Nous tenterons de définir de façon plus rigoureuse cet état amorphe puis, dans le cadre des propriétés structurales, dont la connaissance est essentielle pour une meilleure compréhension de ces systèmes, nous rappellerons comment est définie la fonction de distribution de paire g(r). Nous montrerons quelles sont les informations que cette fonction permet d'obtenir pour la connaissance de l'ordre local, et quelles sont les limites de son utilisation en fonction des approximations utilisées. Nous examinerons ensuite brièvement les différentes méthodes d'investigation dont nous disposons pour obtenir cette fonction de corrélation de paire. Nous insisterons alors plus particulièrement sur le cas des neutrons thermiques en montrant la démarche suivie pour obtenir à partir de l'information expérimentale (intensité diffusée) la fonction de corrélation de paire g(r) que nous utilisons comme paramètre d'étude de l'ordre à courte distance.

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Particles occupying sites of a random lattice present density fluctuations at all length scales. It has been proposed that increasing interparticle interactions reduces long range density fluctuations, deviating from random behaviour. This leads to power laws in the structure factor and the number variance that can be used to characterize deviations from randomness which eventually lead to disordered hyperuniformity. It is not yet fully clear how to link density fluctuations with interactions in a disordered hyperuniform system. Interactions between superconducting vortices are very sensitive to vortex pinning, to the crystal structure of the superconductor and to the value of the magnetic field. This creates lattices with different degrees of disorder. Here we study disordered vortex lattices in several superconducting compounds (Co-doped NbSe$_2$, LiFeAs and CaKFe$_4$As$_4$) and in two amorphous W-based thin films, one with strong nanostructured pinning (W-film-1) and another one with weak or nearly absent pinning (W-film-2). We calculate for each case the structure factor and number variance and compare to calculations on an interacting set of partially pinned particles. We find that random density fluctuations appear when pinning overcomes interactions and show that the suppression of density fluctuations is indeed correlated to the presence of interactions. Furthermore, we find that we can describe all studied vortex lattices within a single framework consisting of a continous deviation from hyperuniformity towards random distributions when increasing the strength of pinning with respect to the intervortex interaction.