1,721,032 research outputs found
High resolution neutron powder diffraction investigation of the low temperature crystal structure of molecular iodine (I2)
No full textHigh resolution neutron time-of-flight powder diffraction has been used to determine the crystal structure of solid iodine as a function of temperature. The results show the structure is orthorhombic, space group Cmca, and is isomorphous with the solid halogens, bromine and chlorine. The observed structural trends are discussed
Phonon softening in tantalum below 4.2 K
No full textThe phonon lifetime of longitudinal and transverse phonons along the [100] direction in tantalum has been measured at 1.2, 4.2 and 6 K and room temperature by neutron inelastic scattering. An anomalous softening of acoustic phonons with energies between 6 and 12 meV is observed when the temperature is lowered from 4.2 to 1.2 K. Such a softening is an indication that a lattice instability can be present at low temperatures in otherwise stable transition metals
Dispersion relations of acoustic phonons in pyrope garnet: Relationship between vibrational properties and elastic constants
No full textCoherent inelastic neutron scattering measurements have been performed on a natural garnet pyrope from the Dora Maira Massif, Italian Western Alps, [(Mg0.92Fe0.05Ca0.03)(3)Al2Si3O12, space group , Z = 8, a = 11.459 Angstrom] to determine the acoustic phonon-dispersion relations at room temperature. Phonon measurements have not previously been made on natural garnets, although the dispersion curves form a basis for developing and testing physically correct interatomic potentials for use in lattice dynamical models. Acoustic phonons propagating in the high-symmetry [100] and [110] directions were observed using the PRISMA spectrometer installed at the ISIS Neutron Spallation Source, U.K. The inelastic scans were performed around the (660), (080), (860), (10,6,0), and (0,10,0) Bragg points: Two transverse acoustic and one longitudinal acoustic phonon branches were measured. The observed acoustic phonon frequencies are in good agreement with those calculated on the basis of a semiempirical interatomic potential model. As a check, the elastic constants obtained from the slopes of the acoustic phonon branches are comparable with previous measurements obtained by Brillouin spectroscopy, although their large estimated errors reflect the complexity of the experimental measurements of phonon-dispersion curves on pyrope garnets
High resolution neutron powder diffraction studies of the low temperature crystal structure of molecular iodine (I2)
No full textThe crystal structure of iodine has been investigated using high resolution neutron time-of-flight powder diffraction in the temperature range 5 to 150 K. The results confirm the structure is orthorhombic, space group Cmca, and is isomorphous with the solid halogens bromine and chlorine. The accurate structural parameters obtained in this investigation have been modelled assuming a simple Einstein functionality to compare the refined anisotropic temperature factors with the directly determined molecular dynamics and predict minimum energy (0 K) lattice parameter values. These values are expected to be directly applicable in the development and verification of anisotropic atom-atom pair potentials which will successfully reproduce the observed condensed phase crystalline structure
Inelastic neutron scattering from pyrope powder: experimental data and theoretical calculations
No full textTime-of-flight (TOF) coherent inelastic neutron scattering measurements have been carried out at 14 and 30 K, on a powder sample of natural pyrope garnet [(Mg0.92Fe0.05Ca0.03)(3)Al(S)i(3)O(12), space group Ia(3) over bar d, Z = 8, a = 11.455(1) Angstrom] from the Dora Maira Massif, Italian Western Alps, in order (i) to investigate its vibrational properties and (ii) to determine the contribution from component atoms to the one-phonon density of states, for clarifying the sources of some thermodynamic anomalies. An interatomic potential function has been modeled by simultaneous fitting to the structural, elastic and vibrational observables of the mineral. The developed potential has been employed to interpret the experimental inelastic neutron scattering patterns. The calculations have been performed within the framework of the lattice dynamics
Time-of-flight Neutron-diffraction Investigation of Temperature Factors In the Zn Blende Semiconductor Inp
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Competing Interactions in Hexagonal PrxNd1-xCo5 Pseudobinary Intermetallic Compounds
No full textMeasurements of the cone magnetic structure in PrxNd1−xCo5 compounds show that the spin reorientation transition temperature from easy plane to easy cone and the low temperature value of the angle between the easy magnetization direction and the c-axis both decrease with decreasing Nd concentration. The observed behaviour of the magnetic anisotropy implies a particularly complex competition between a non-axial Pr and planar Nd anisotropy as well as the large Co axial anisotropy
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