Electron density and properties of rock-salt type crystals

Abstract

DREAM (Blessing, 1986) and the recently developed multipole refinement and analyses package XD (KOiitzantszk.-y et aL, 1994, we will present charge density analyses results on Creatine Monohydrate using both the Seed-Skewness method and the elliptical peak-mask method.An interactive version of this abstract can be obtain from the URL: http://www.cf.ac.uk!uwcc/chemy/bracke/iucr/iucr96.html Blessing, R. H., Crystallographic Rev., 1. 3-58 (1986). Bolotovsky, R., White, M.A., Darovsky, A. & Coppens. P., J. Appl. CJ}•st., 28, 86-95(1995). Koritsanszky, T., Hansen, N., Howard, S., Su, Z. & Mallinson. P., Berlin/ Glasgow, first release: May 1994.Kabsch, W. J., J. Appl. Cryst., 21, 916-924(1988a); J. Appl. Clyst., 21.67-71(1988b Antiferromagnetic order in MnF2 induces a dipolar distortion of the fluorine inner electron shelL The core-deformation generates a substantial electric field at the nucleus, which is not compensated by peripheral lattice contributions, thus giving rise to an apparent Coulomb force on the nucleus. The force exerted on any nucleus should vanish in the stable equilibrium configuration. A local source of an opposing electric field could consist in a small skewness of the fluorine nuclear vibrational distribution which should persist even in the limit of zero temperature. This model rests upon the assumption that the electron deformation does not rigidly follow the nuclear motion.Pulsed single-crystal neutron diffraction (T = 15 K) at the spallation source IPNS has been used to test this hypothesis. Data have been collected up to very high diffraction vectors, (sin81A)max = 2.75 A-1. The harmonic mean-square displacement parameters are in excellent agreement with previous results from both gamma-ray and neutron diffraction. Statistically significant third-order coefficients of a Gram-Char-lier expansion could be extracted from the experiment. The shape of the antisymmetric part of the nuclear distr·ibu-tion function substantiates a subtle balance between the mean thermal electric fields due to the electronic and the nuclear charge density distribution. The sense of the skewness around the equilibrium position is opposite to the one found previously for the par·amagnetic state. It is to be noted that data from nuclear scattering alone can provide valuable indications concerning a local redistribution of electron density.Experiment thus confirms the conclusion that the core polarization in antiferromagnetic MnF 2 is dynamically stabilized. A theoretical description of the detailed physical mechanism is lacking at present. The X-ray diffraction studies of the electron density (ED) of the crystals with rock-salt structure, traditional test crystals in solid state computational methods, should provide the basis for direct comparison of the theoretical and experimental data. However, the severe extinction and sometimes ign01ing TDS distort the diffraction intensities and the problem of description of highly ionic electronic states exists in theoretical calculations for these crystals. Therefore the relia...