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Canada's pre-eminent neutron source since 1957 has been the NRU reactor at Chalk River. It is unlikely to operate beyond the year 2005. In 1994, AECL prepared the case and concept for a new research reactor, the Irradiation Research Facility (IRF), to replace NRU. The IRF was developed with the dual purpose of meeting the needs of both R&D programs to support existing and advanced CANDU® designs and also of condensed matter science and materials research using extracted neutron beams. In November 1995, AECL began a pre-project engineering programme to develop the design of the facility and to begin the safety analysis and “up-front” licensing process. The dual-purpose concept continues to be pursued and the design modified, to achieve maximum performance in the most cost-effective manner. The planned neutron-beam facilities, which include a cold source and a guide hall, will greatly enhance Canada's programs of neutron-beam research and applications. The current status of the IRF design and of efforts to secure funding for the neutron-beam components will be presented. more...

By measuring and calculating oblique-incidence transmittance spectra of a negative-index photonic metamaterial layer, the authors infer the in-plane dispersion relation of the magnetization wave. From the geometry and the dispersion shape, the authors conclude that coupling is predominantly via magnetic dipoles for wave propagation along the magnetic-dipole direction. These magnetization waves are the classical analog of magnon excitations of quantum-mechanical spins. more...

Inelastic neutron scattering experiments in tetragonal KNb${\mathrm{O}}_{3}$ show a high anisotropy in some regions of the phonon dispersion relation and the existence of strong one-dimensional intercell correlations between atomic motions along the [100] and [010] directions, in agreement with x-ray diffuse-scattering measurements in the same phase. The transverse phonon modes propagating normally to these directions of strong correlations, that is, with polarization vectors parallel to them, are relatively soft, not only near the zone center but all the way to the zone boundary. Comparison of these results with the Raman, neutron, and x-ray data obtained in the other phases suggests a description of the ferroelectric phase transition mechanism in KNb${\mathrm{O}}_{3}$ in terms of soft modes and dynamic correlations in each phase. more...

The power of neutron scattering methods to probe the dynamical behaviour of atoms and molecules in solids is illustrated by several examples taken from the now enormous literature on phonons, related excitations and phonon interactions in perfect and not so perfect crystals. The seminal role played by the soft mode concept is stressed. Although references are made to early investigations in some areas, the emphasis is on the last decade of research. Nearly all of this research has been carried out at steady state reactor sources with triple axis crystal spectrometers. more...

The coherent, elastic neutron scattering from polycrystalline samples of SF6 and CBr4 in their plastic phases has been measured. The powder line component of the neutron distributions has been analysed in terms of Debye-Waller factors to give information about the orientational disorder in the solids. An extensive diffuse scattering component is also observed in CBr4. The scattering calculated for a simple model of the molecular disorder in CBr4 agrees reasonably well with this observed diffuse scattering, but improvements in the model are clearly necessary. more...

The contributions to the dynamic form factor S(Q, omega ) arising from interference between its one-phonon and multiphonon parts have been observed in the scattering of neutrons from potassium. Interference makes two qualitatively distinct contributions to S(Q, omega ). One is approximately proportional to the one-phonon part, S1(Q, omega ), and contributes scattering intensity largely within the one-phonon peak region. The other is asymmetric around the one-phonon frequency and for narrowly peaked phonon groups contributes intensity of opposite sign in the multiphonon region directly on each side of the one-phonon peak. Both contributions, and their dependence on wavevector Q and frequency omega , can be described by an anharmonic theory (Ambegaokar et al. (1965)) which includes interference between the one- and two-phonon parts of S(Q, omega ) via the cubic anharmonic term. more...

The technique of inelastic neutron scattering has been used to determine translational and librational lattice modes in a single crystal of solid nitrogen in the cubic $\ensuremath{\alpha}$-phase at 15 K. The measurements were concentrated at the high-symmetry points $\ensuremath{\Gamma}$, $R$, and $M$, where the degeneracies ease the task of symmetry assignments to the observed modes. The assignment was made on the correlation of the observed structure-factor variation with force-model predictions. The calculations were based on a general potential function which includes Lennard-Jones or six-exponential-type interactions as well as electric quadrupole forces. Good agreement was found between both the observed frequencies and the observed intensities and their calculated counterparts. The temperature dependence of selected modes has been established and it is compared to a recent self-consistent phonon calculation. more...

The wave vector and temperature dependences of transverse acoustic modes propagating along the [001] and polarized along the [100] orthorhombic axis in KNbO3 have been studied by means of coherent inelastic neutron scattering methods. The results are consistent with previous elastic constant and neutron scattering measurements. The mechanism of the orthorhombic-tetragonal phase transition is discussed in the light of all these data. more...

In contrast to the three finite-frequency peaks in the dynamical structure factor S(ate) predicted in a recent computer-simulation study of aluminum at 800 K, the line shape observed by means of neutron scattering is smooth and exhibits a single narrow peak at each wave vector. The observed line shape is, however, in agreement with approximate analytical calculations based on self-consistent-phonon theory, using the same interatomic force model as employed in the computer simulation. These comparisons suggest that exact computer simulation on a finite lattice can produce artificial peaks or oscillations not characteristic of the real solid. We find that these artificial peaks are associated with long-time noise in the computer simulation of S(Q,t); their efFect on S(Q,co) can be minimized by use of a damping function. The study of anharmonic effects in solids, that is, the ways in which the lattice vibrations deviate from harmonic excitations having sharply defined, temperature-independent frequencies, is a subject of continuing interest. These effects are important in solids at elevated temperatures and in materials which exhibit large zero-point oscillations. Up to about 1968, as described in a review by Cowley, work in this field was primarily concerned with the measurement and theoretical calculation of the shift and width of phonon frequencies as a function of temperature. The studies of aluminum' and potassium' are examples of thi. s work. Subsequently the effect of pressure on phonon modes, for example in Rb, ' has been studied, and several investigations have been made of a very characteristic anharmonic effect on the onephonon scattering of both x rays and neutrons — the so-called one-phonon-two-phonon interference effect, ' which has been reviewed by Glyde. ' Once again we may cite as examples both aluminum" and potassium, ' in addition to the pioneering work on KBr, '" and also the studies of the 'He sol1ls 12 more...

The neutron diffraction pattern of liquid D20 has been investigated at pressures up to 9·1 kbar at 25°C and 15·6 kbar at 85°C using an aluminium alloy pressure vessel of novel design. The corrected neutron counts were scaled by a constant factor so that our low-pressure measurements agreed with the best absolute measurements of the structure factor at low pressure. The structure factors were Fourier transformed to give the neutron-weighted pair distribution functions G(r). The principal conclusions drawn from the results are as follows. (1) The amplitudes of the peaks in the radial distribution function greatly increase under pressure, showing that there is an increase in the correlations of molecular positions and orientations. A graph of 4πr 2 G(r) against pressure shows no evidence that the first-neighbour coordination number increases by much more than the density. The number of near neighbours at 15·6 kbar and 85°C appears to be about 6·4. The volume of melting of ice VII near the triple point of 81·... more...

The nature of the spin correlations in Zn1−cMncTe alloys for several values of c has been studied by neutron powder diffraction as a function of temperature T between 2 and 300 K. Results have been obtained for c=0.376, 0.565, 0.594, and 0.68. The correlations are weak at low c and high T, but intensify progressively as c increases and/or T decreases. For c as high as 0.68, the experiments do not indicate any phase transition to a fully ordered magnetic structure, but the appearance of broad, intense magnetic diffuse scattering peaks at particular positions suggests a strong tendency towards antiferromagnetic ordering of type III. The results are analyzed in terms of a theory of the magnetic scattering from magnetic ions surrounded by near‐neighbor shells of correlated spins. Comparison is also made with the correlations expected from an ordered type III structure modified by various exponential functions of a correlation length κ(c). Application of a magnetic field up to 2 Tesla to the c=0.594 specimen p... more...

The frequencies of normal modes belonging to the Sigma 4 and G4 branches in potassium at 4.3K have been measured by means of coherent inelastic neutron scattering experiments. No evidence was found in the Sigma 4 branch of the anomaly predicted in a recent theoretical calculation based on a rather sophisticated pseudopotential model. It is likely that this anomaly arises from a particular mathematical problem in calculating the Sigma 4 normal mode frequencies, and that when this problem is avoided, the model provides an excellent description of the observed frequencies. The parameters of a widely used phenomenological model have been re-evaluated in the light of all the phonon frequency data now available for potassium. more...

The spin correlations between Mn ions in Zn1−cMncTe alloys have been measured as a function of temperature and concentration. The spin correlations follow the same sign sequence as for a type III antiferromagnet out to seven shells of neighbours. Complete long range magnetic order was not observed for any c or T. more...

The frequencies of particular intermolecular modes of vibration and libration in single crystals of solid carbon disulphide have been measured by coherent inelastic neutron and Raman scattering techniques 148K. Measurements were made at the special points in the Brillouin zone Gamma , S, R, Z, A, Q and Y and, including earlier infrared results, 37 of the 42 distinct frequencies at these points have been identified. Six of the ten dispersion curves along Lambda (001) have been measured and three dispersion curves have been outlined along Delta (100). Values of the elastic constants C11, C33, C44and C55have been determined from the acoustic branches. The present results are compared with calculations based on two different models for the interatomic potentials. Both models calculate the general level of the intermolecular frequencies to be too high and the bandwidth of the frequencies to be too large. Neither model can be considered an adequate description of the present results. more...

Collective excitations in the orientationally disordered phase of SF6 have been studied by inelastic neutron scattering and molecular dynamics simulation techniques. Experimental measurements to observe acoustic modes were made along the high symmetry directions at temperatures of 100 K and 200 K. The excitations observed showed little evidence of discrete peaks but were all broad and overdamped. They showed little temperature dependence. The dynamical structure factors S(Q, ω) calculated from the simulation are in qualitative agreement with the observed spectra but quantitatively show discrepancies. For smaller wave vectors than those studied experimentally the calculations show the existence of well-defined, long wavelength acoustic phonons. The wave vector at which the transition occurs between propagating and overdamped excitations was found to be temperature dependent. The results are interpreted in terms of the concept of orientational frustration. Some difficulties in the application of molecular d... more...

Lattice dynamics of La 11 B 6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B 6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB 6 , the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB 6 , no softening of C 12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds. more...

The dispersion curves for phonons propagating along the trigonal axis of the layer-structure emiconductor Bi2Te3 at 77 K have been determined by coherent inelastic neutron scattering techniques. Analysis of these results in terms of a simple interplanar force model shows a) that the forces between adjacent planes of Te atoms are comparable to those between nearest neighbour planes of Bi and Te and b) that foreces between next-nearest-neighbour planes have to be considered. In addition, a limited number of phonons were measured at 300 K and some low-frequency modes propagating along the bisectrix within the planes were determind. The results are compared, where possible, with previous Raman scattering, infrared reflection, and elastic constant measurements. more...

The circumferential distributions of axial, hoop, and radial residual strains in a bent INCOLOY-800* tube have been measured by neutron diffraction. High residual axial tensile strains are observed just above the neutral axis with corresponding compressive strains just below the neutral axis. There are marked and systematic differences in the amount of strain in grains with a [001] direction aligned along the axis of the tube and those with a [111] direction aligned along the tube axis. Model calculations show that these differences originate in the different yield points and elastic moduli for different crystallographic directions. more...

An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF6) and carbon tetrabromide (CBr4) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3m) for SF6 and face-centred cubic (space group Fm3m) for CBr4. The bond-orientation distribution function, f(Ω), has maxima in the directions for SF6 and in the directions for CBr4. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The tr... more...

An attempt to observe inelastic neutron scattering from short; wavelength charge fluctuation modes in molten potassium bromide is described. Measurements were made using a triple axis crystal spectrometer with fixed scattered neutron energy (19 THz, or 78.6 meV) and scans were recorded at scattering angles between 2 degrees and 7 degrees . An analysis of the results indicates weak shoulders on each side of the elastic peak, corresponding to energy transfers about 4 THz. A calculation of the expected scattering, based on the computer simulation results of Hansen and McDonald (1975) is described, and the neutron scattering results are discussed in the light of this calculation. more...

The first two authors have measured the dispersion curves for phonons whose wave vector lies along the symmetry lines $\ensuremath{\Sigma}$ and $Z$ in the Brillouin zone. The measurements were made by the method of inelastic neutron scattering. With the aid of these new results the latter two authors have determined the shell-model parameter $\ensuremath{\delta}$ which could not be determined from the previously measured dispersion curves along the lines $\ensuremath{\Delta}$ and $\ensuremath{\Lambda}$. A figure is given which summarizes all the measured dispersion curves of diamond and gives the most recently calculated shell-model fit to the data. Because of the large size of this parameter $\ensuremath{\delta}$, it was necessary to recompute some of the thermodynamic and optical properties of diamond previously reported by the last two authors elsewhere. Thermodynamic quantities such as the Debye temperature were very little affected, but some of the critical-point frequencies used in the analysis of the two-phonon infrared absorption spectrum had to be altered. A corrected table of critical points is given. more...

The frequency-wave-vector dispersion relation, $\ensuremath{\nu}(\mathrm{q})$, for the normal vibrations of a nickel single crystal at 296\ifmmode^\circ\else\textdegree\fi{}K has been measured for the [$00\ensuremath{\zeta}$], [$0\ensuremath{\zeta}\ensuremath{\zeta}$], [$\ensuremath{\zeta}\ensuremath{\zeta}\ensuremath{\zeta}$], and [$0\ensuremath{\zeta}1$] symmetric directions using inelastic neutron scattering. The results can be described in terms of the Born-von K\'arm\'an theory of lattice dynamics with interactions out to fourth-nearest neighbors. The shapes of the dispersion curves are very similar to those of copper, the normal mode frequencies in nickel being about 1.24 times the corresponding frequencies in copper. The fourth-neighbor model was used to calculate the frequency distribution function $g(\ensuremath{\nu})$ and related thermodynamic properties. more...

The usefulness of group theoretical methods in the study of the normal modes of vibration in crystals is illustrated by applying them to the case of solid carbon dioxide. Emphasis is placed on a detailed explanation of how to obtain the irreducible representations and basis vectors for selected normal mode wave vectors. These examples have been chosen specifically to show how to treat (i) zone boundary points in non-symmorphic space groups and (ii) various effects of time-reversal symmetry. Compatibility relations and the gradients of dispersion curves at the zone boundaries are also discussed. The group theoretical analyses are presented in a straightforward manner suitable for practical application, without detailed proofs or justifications. more...

The crystal dynamics of sodium nitrite has been studied by coherent neutron inelastic scattering techniques. The normal modes of vibration propagating along the three principal symmetry axes in the crystal at 296°K have been measured. The results have been used to find the parameters of rigid ion models for NaNO 2 in which the NO 2 ion was treated as a group. Although none of the models are entirely satisfactory they do give a fair description of the results. The temperature dependence of the frequencies of some of the normal modes was measured and none were found to be very temperature dependent near the ferroelectric transition. Ferroelectric critical scattering was observed which was quasi-elastic showing that the ferroelectric fluctuations change slowly compared with the frequencies of almost all the normal modes of vibration. A model is deduced for the ferroelectric fluctuations incorporating the motion of the atoms but it is found that the motion of the atoms in these fluctuations cannot be describe... more...

The spin-wave dispersion relations have been measured in antiferromagnetic uranium dioxide by inelastic neutron-scattering techniques. The triple-axis crystal spectrometer at the C5 facility of NRU was used throughout in its constant-Q mode of operation. The dispersion relations were obtained for spin waves propagating along the main symmetry directions at 9\ifmmode^\circ\else\textdegree\fi{}K, and less complete measurements were made at higher temperatures both above and below the N\'eel temperature. The theory of spin waves in U${\mathrm{O}}_{2}$ is developed and various models are used in attempts to deduce the exchange and anisotropy parameters from the experimental results. None of the models are completely satisfactory, because of the difficulties arising from the multidomain character of the specimen, and from the strong interaction between the magnons and the phonons. A theory of this interaction is also developed which gives quite reasonable agreement with experiment. more...

The normal modes of vibration of the ordered alloy of copper and zinc, $\ensuremath{\beta}$ brass, have been extensively studied at 296\ifmmode^\circ\else\textdegree\fi{}K by means of the coherent one-phonon scattering of slow neutrons from single-crystal specimens. The frequencies of normal modes propagating along the high-symmetry directions [$00\ensuremath{\zeta}$], [$\ensuremath{\zeta}\ensuremath{\zeta}0$], [$\ensuremath{\zeta}\ensuremath{\zeta}\ensuremath{\zeta}$], and [$\frac{1}{2}\frac{1}{2}\ensuremath{\zeta}$] have been measured by means of a triple-axis crystal spectrometer for two specimens mounted in different orientations. These results may be satisfactorily described in terms of a restricted Born-von K\'arm\'an model ($4E$) with interatomic forces extending out to fourth-nearest-neighbor atoms, although Fourier analysis of sums of squares of certain normal-mode frequencies (taken in pairs) indicates the probable existence of nonzero forces extending at least to seventh neighbors. A feature of interatomic-force models for $\ensuremath{\beta}$ brass is the large difference between the second-nearest-neighbor Cu-Cu and Zn-Zn forces. The experimental evidence does not, however, permit an unambiguous determination of which force is to be identified as Cu-Cu and which as Zn-Zn. A fairly successful attempt has been made to interpret the results in terms of more physically realistic models, in which the effective long-range forces are represented by an oscillatory potential arising from the mild singularity in the dielectric function at the Fermi level. The precise form of the coefficient of the oscillatory potential has been chosen on an empirical basis. Model 4E has been used as an interpolation formula for computing the frequency distribution function and its moments, and also the heat capacity of $\ensuremath{\beta}$ brass. A selection of normal modes has been studied at several temperatures above 296\ifmmode^\circ\else\textdegree\fi{}K, particularly in the vicinity of the order-disorder phase transition at about 727\ifmmode^\circ\else\textdegree\fi{}K. The over-all structure of the dispersion curves appears to be substantially unchanged in the disordered phase, although certain "splittings" observed at 296\ifmmode^\circ\else\textdegree\fi{}K become blurred into apparently continuous bands of frequencies at elevated temperatures. In all cases, the frequencies decrease and energy widths increase as the temperature increases. Two particular longitudinal-optic modes display a sharp increase in energy width at the transition temperature, in contrast to the generally smooth behavior of the other modes. No satisfactory explanation of these effects has yet been found. more...

A simple method is described for growing massive single crystals of the alloy series SnTe-GeTe, suitable for inelastic neutron studies. A table of the time-temperature cycle is provided. The conventional techniques to establish the single crystal character of opaque solids were found to be useless. A new technique (neutron photograph of a Bragg reflection) is described, which gives reliable data about the single crystal character of the solids formed. Single crystal lattice parameters were obtained by neutron diffraction and are in good agreement with the values obtained from X-ray powder diffraction studies. more...

Neutron inelastic scattering methods are used to demonstrate the close connection between the antiferroelectric phase transition at 110°K in SrTiO 3 and the temperature dependence of the triply degenerate Γ 25 mode of vibration of wave vector q = (2π/ a ) (0.5, 0.5, 0.5). The results underline the importance of such methods for studying antiferroelectric transitions as well as the ferroelectric transitions previously investigated. more...

Measurements have been made of phonon frequencies and widths in β-brass at 296°K, 499°K, 714°K and 751°K, using neutron spectrometry. These properties vary smoothly with temperature and show no large discontinuities near the order-disorder phase transition. A possible correlation between the phonon lifetimes and the degree of short range order is discussed. more...

Recent neutron diffraction data are used to investigate triplet correlations in liquid argon by means of a method suggested by Egelstaff et al . A careful analysis is made of the errors involved and it is found that the superposition approximation generally overestimates the magnitude of triplet correlations and that the deviations are largest for nearest-neighbour triplet configurations. more...

The spin correlations between Mn ions in disordered MncZn1−cTe alloys (c = 0.68, 0.594, and 0.376) have been measured as a function of temperature and concentration by neutron diffraction. The scattering peaks principally around the wave vector corresponding to the (1,1/2,0) reflection, which is the ordering wave vector of the type III antiferromagnetic structure. The magnetic intensity was analyzed in terms of the correlations between a central spin and successive shells of neighbors and these were found to follow the sign sequence for the type III antiferromagnet but with progressively smaller amplitudes. At no temperature or concentration was long range order observed. However, the width of the (1,1/2,0) peak at low temperatures is consistent with a power law and the critical concentration for magnetic ordering is just outside the present concentration range. more...

Measurements have been carried out on selected phonon modes in sodium at temperatures down to 36 °K. No phonon instabilities have been found to be associated with the martensitic transformation in this metal. more...

In an orientationally disordered solid the equilibrium positions of the molecular centers of mass lie on an ordered three‐dimensional lattice, but the molecular orientations are disordered. The effects of this disorder can be observed in the scattered neutron distributions in several ways. The intensity of the Bragg reflections, particularly those with high Miller indices, is greatly reduced. Intense, diffuse scattering appears which may be distributed throughout reciprocal space. Well‐defined librational phonons are difficult to observe and the translational phonons are often very broad. A brief outline of the theory of neutron scattering from orientationally disordered solids is given and the information which may be derived from neutron scattering is discussed with reference to results on CBr4, SF6 and β‐N2. more...

Calculations based on experimental phonon frequency distributions show that significant equal-time displacement correlations are present in niobium and molybdenum lattices.

The triple axis crystal spectrometer at Chalk River has been used to observe coherent slow neutron scattering from a single crystal of pure gallium arsenide at 296°K. The frequencies of normal modes of vibration propagating in the [ ζ 00], [ ζζζ ], and [0 ζζ ] crystal directions have been determined with a precision of between 1 and 2·5 per cent. A limited number of normal modes have also been studied at 95 and 184°K. Considerable difficulty was experienced in obtaining well resolved neutron peaks corresponding to the two non-degenerate optic modes for very small wave-vector, particularly at 296 °K. However, from a comparison of results obtained under various experimental conditions at several different points in reciprocal space, frequencies (units 10 12 c/s) for these modes (at 296°K) have been assigned: T 8·02 ± 0·08 and L 8·55 ± 0·2. Other specific normal modes, with their measured frequencies are (a) (1,0,0): TO 7·56 ± 0·08, TA 2·36 ± 0·015, LO 7·22 ± 0·15, LA 6·80 ± 0·06; (b) (0·5,0·5, 0·5): TO 7·84 ± 0·12, TA 1·86 ± 0·02, LO 7·15 ± 0·07, LA 6·26 ± 0·10; (c) (0, 0·65, 0·65): optic 8·08 ± 0·13, 7·54 ± 0·12 and 6·57 ± 0·11, acoustic 5·58 ± 0·08, 3·42 ± 0·06 and 2·36 ± 0·04. These results are generally slightly lower than the corresponding frequencies for germanium. An analysis in terms of various modifications of the dipole approximation model has been carried out. A feature of this analysis is that the charge on the gallium atom appears to be very small, about ± 0·04 e. The frequency distribution function has been derived from one of the force models. more...