A. N. Christensen

Elastic diffuse neutron scattering study of the defect structure of TiC0.76 and NbC0.73

Abstract The elastic diffuse neutron scattering of the non-stoichiometric carbides TiC0.76 and NbC0.73 has been measured at room temperature, in the (110) reciprocal lattice plane, for 0.2⩽q = 4π sin O/λ⩽4 A -1 . The Cowley-Waren short-range order parameters were obtained for the eight first shells of neighbours of the metalloid f.c.c. sublattice. The degree of short-range ordering was found much larger in NbC0.76. Strong displacements of the metal atoms towards their first neighbour carbons occur in both cases.

Order-disorder transition and structure of the ordered vacancy compound Nb6C5: powder neutron diffraction studies

High-resolution neutron diffraction studies of NbC0.83 powders are described. In a first set of experiments the order-disorder transition was studied by measuring the integrated Bragg intensities. A first-order transition is observed around 1025+or-5 degrees C on heating, which shows a hysteresis. In a second set of experiments, a profile refinement method was used to get information on the crystal structure of a well ordered Nb6C5 sample. The V6C5-type trigonal superstructure previously proposed was confirmed (rather than the V6C5 monoclinic form). A shift of the niobium atoms ( approximately=0.05 AA) away from their first-nearest-neighbour vacant site, towards a first-nearest-neighbour occupied site (trans to the former) is found.

Phonon anomalies in transition metal nitrides: δ-NbN

Abstract The phonon dispersion curves in δ-NbN have been measured at room temperature in the high symmetry directions Δ, Σ, and ʌ by coherent inelastic neutron scattering. Anomalies in the dispersion of the acoustic branches have been detected which are quite different from those which have already been reported for superconducting transition metal compounds with 9 valence electrons. In the 10 valence electron compound δ-NbN the soft mode region has moved from the middle of the Σ-direction to the zone boundary. The experimental results are well described by a double shell model which has also been used to calculate the density of states. Using the formalism of Varma and Weber the measured dispersion curves have been reproduced. This shows that the change in the anomalies when going from 9 to 10 valence electrons, is due to a shift in Fermi energy in an essentially rigid band scheme.

122122122) versus (1 12 0) type ordering of nitrogen vacancies in TiNx

Abstract A single crystal of TiN 0.82 was studied by elastic neutron diffuse scattering at 300 K. The diffuse intensity is maximum at the ( 1 2 1 2 1 2 ) type reciprocal lattice points, whereas TiN 0.50−0.61 shows long-range ordering of nitrogen vacancies with superlattice reflexions of the type ( 1 1 2 0 ). This suggests that the effective first and second neighbour interatomic potentials on the nitrogen sublattice are positive with V 1 ≅ 2 V 2 . In TiN 0.82 a contraction of the Ti-N average first neighbour distance of ≅0.01 A was found.

Short-range-order determination in quenched NbC0.73 by elastic diffuse neutron scattering

The elastic diffuse neutron scattering of an NbC0.73 single-crystal has been measured at room temperature on two different spectrometers, the four-circle D10 at the HFR–ILL (Grenoble), and the two-axis G44 with time-of-flight analysis at the Laboratoire Leon Brillouin (Saclay). The data were treated either by Fourier transformations or by least-squares fit. The short-range-order parameters and the interatomic static displacements obtained by the various experiments and/or data treatment are in good agreement: carbon vacancies tend to be third neighbours in the f.c.c. metalloid sublattice and metal atoms relax away from the vacancy first neighbours by ~0.05 A. It is shown that the treatment of diffuse scattering data limited to two reciprocal planes [(001) and (1{\bar 1}0)] gives satisfactory results.

Lattice dynamics of the A15 compound Mo 3 Si

The phonon dispersion curves of Nb/sub 3/Sb have been studied by inelastic neutron scattering. All the branches have been determined in the main symmetry directions and the results are analyzed with Born--von Karman models of different complexity. A model with general forces up to the eighth-nearest neighbors gives excellent agreement with experiment. However, a rather good description of the dispersion curves is obtained with a model including axially symmetric forces up to the third-nearest neighbors, which indicates that these interactions are dominant. Temperature effects were found to be very small. Additional experiments were performed on polycrystalline samples to determine directly the phonon density of states. Our results obtained on Nb/sub 3/Sb are compared with the available phonon data on Nb/sub 3/Sn.

Neutron Scattering Studies of the Defect Structures in TiC1−x, and NbC1−x

Single crystals of TiC1−x. and NbC1−x. were studied by elastic neutron diffuse scattering at room temperature in the (110) reciprocal lattice plane; the spectra of TiC0.76, TiC0.79and NbC0.73 were analysed by the Sparks and Borie method, which allowed to determine the first Cowley-Warren. short-range order coefficients and a shortening (0.03 A) of the average first neighbor metal-carbon distances. The order-disorder transformation in TiC1–x (0.52 ⩽ 1–x ⩽ 0.67) was studied by high temperature powder neutron diffraction; the transition temperatures (≃760– 790°C) and critical coefficients β were determined. The results are discussed in terms of interatomic pair potentials.

The electronic structure of ZrN0.93(110) studied by angle-resolved photoemission

An angle-resolved photoemission study of a ZrN0.93 (110) single crystal is reported. Off-normal spectra recorded using unpolarized Ne I and He I resonance radiation are presented and the determined experimental peak dispersion are compraed with possible direct transitions calculated for ZrN1.0. The results show that dominant emission peaks at these photon energies probably are one-dimensional density of states features, i.e., peaks originating from band extrema. A peak observed at about 2eV below the Fermi energy is interpreted as originating from occupied vacancy states.