High-temperature single-crystal neutron diffraction study of natural chondrodite

The H-atom environment in a Tilly Foster chondrodite was analyzed using single-crystal neutron diffraction data collected at 500, 700 and 900 K and previously published low temperature data collected at 10, 100 and 300 K on the same crystal Mg_4.64Fe_0.28Mn_0.014Ti_0.023(Si_1.01O₄)₂F_1.16(OH)_0.84 (Friedrich et al. in Am. Mineral 86, 981–989, 2001).

The full mean square displacement matrix S of the O–H pair was determined from the temperature dependence of the anisotropic displacement parameters, enabling a proper correction of the O–H bond for thermal vibration without assumptions about the correlation of O and H movements. The results show that the perpendicular O–H motions in chondrodite are intermediate between the riding and the independent motion models. The corrected O–H bond lengths do not change with temperature whereas the corrected H…F distances show an increase of 0.02 Å with temperature, as do the Mg–O distances. This result shows that spectroscopic observations on the strength of the covalent O–H bond cannot be interpreted unambiguously in terms of a corresponding behaviour of the associated H…O/F hydrogen bond.

Reference: Kunz M., Lager G.A., Bürgi H.B., Fernández-Díaz M.T. Phys. chem. miner. 33, 17-27 (2006)

Molecular Motion in Crystalline Naphthalene: Analysis of Multi-Temperature X-Ray and Neutron Diffraction Data

Single crystals of h₈-naphthalene have been examined by both X-ray and neutron diffraction over a range of temperatures from 5 to 295 K (figure 1). The aim of this case study was to measure the anisotropic displacement parameters (ADPs) of carbons and hydrogens and to interpret them using the model of thermal motion proposed by Bürgi and Capelli (Acta Cryst. 2000, A56, 403). The traditional rigid-body analysis expresses the low frequency motions in terms of molecular translations and librations only, whereas the Bürgi-Capelli treatment also includes the high-frequency internal modes. We show that a considerable improvement occurs by representing the internal modes by a single second-rank tensor and that a further improvement follows by including a Grüneisen parameter to account for volume thermal expansion. By applying the treatment to multi-temperature diffraction data, there is a considerable reduction in the ratio of number of adjustable parameters/number of independent observations.

Atomic resolution neutron holography (principles and realization)

Atomic resolution neutron holography constitutes a novel technique to obtain structural information. It is based on the recording of the interference of neutron waves coherently scattered by atoms located on a crystal lattice with a suitable reference wave. This process can be accomplished by two complementary schemes. In the frame of the first approach, a point-like source of spherical neutron waves is required inside a single crystal. Such a source can be realized owing to the extremely large value of the incoherent neutron scattering cross section of the proton. Hydrogen atoms imbedded in a sample which is placed in a monochromatic beam of slow neutrons will emit spherical neutron waves as a result of an incoherent scattering process. The interference between the undisturbed wave field and that part of the wave which is scattered by neighbouring atoms can be recorded, thereby producing a hologram. The second approach utilizes a source of plane neutron waves outside the sample. The interference between the undisturbed and the scattered parts of the neutron wave field is recorded by point-like detectors, i.e. strongly neutron-absorbing nuclei, which are placed inside the crystal lattice that is to be imaged (figure 2). The experimental feasibility of these two techniques is demonstrated.