We have developed a computer program to calculate the thermal diffuse scattering (TDS) intensity distribution for single-crystal specimens in a diffractometer with no energy analysis. We assumed that the phonon frequencies are approximated by those of elastic waves and that the elastic constants, density and lattice parameters of the system under study are known. The results of the calculations were compared to experimental data obtain for single crystals of Si, diamond and NiAl at the wide-angle neutron diffractometer (WAND) at the HFIR at Oak Ridge National Laboratory. Excellent agreement was found between the calculations and the experimental observations.

The crystal structures, phonon behavior, and pre-martensitic behavior of Li, Na, K, Rb, and Cs as a function of temperature and pressure (Li, Na, Rb, and Cs) are reviewed. Li has been shown to partially transform to the 9R (Sm-type) structure in the vicinity of 75 K and then partially to fcc on annealing. Its [Tc] increases with applied pressure. Na, on the other hand, partially transforms to a mixture of 9R and nearly hexagonal R-polytypes, which approach hcp on annealing. Its [Tc] is rapidly suppressed by application of modest pressures. No martensitic or pre-martensitic phenomenon, or phonon anomaly, has been observed in K down to 5 K. Also, a search for CDWs in the vicinity of (110) has been inconclusive. Similar negative results have been obtained for Rb.

an incommensurate phase (IP) between a (0~) and !3 (0~) phases (K.Gouhara, Y. H. Li and

Neutron diffraction measurements on highly oriented pyrolytic graphite intercalated with SbCl5 have been carried out and indicate that these intercalation compounds have practical application as long-wavelength neutron monochromators. This is due, in part, to their large d spacings and, in part, to their long-range c-axis stacking fidelity and chemical stability. The stage 2 compound is particularly suitable because it has intense first-order scattering accompanied by very weak second-order scattering. These intensity considerations are not as favorable for the stage 4 compound but applications at longer wavelengths are possible. At a scattering angle of 60° (2θ) neutrons of wavelengths 12.8 and 19.5 Å can be obtained from the stage 2 and stage 4 compounds, respectively. Structure-factor calculations for the 001 peaks are presented and are found to be in reasonable agreement with the diffraction data.

The effects of uniaxial stress applied during the aging time for precipitation growth on the formation of G.P. zones in an Al−1.8% Cu alloy have been studied by means of neutron scattering. When a a compression is applied along the [100] direction the precipitates grow preferentially on the (100) planes. Making use of this result, the effect of the formation of G.P. zones on the resonance anomaly in the [ζ00]T phonon branch previously observed in this alloy has also been studied with respect to the relative direction between the phonon wave vector q and the plane of the precipitate. The experimental result indicates that the precipitates perpendicular to q are more effective for the formation of the resonance anomaly than those parallel to q.

The acoustic phonons in cubic Na${\mathrm{O}}_{2}$ have been studied above and below the orderdisorder transition by inelastic neutron scattering. Above the transition the ${\mathrm{O}}_{2}^{\ensuremath{-}}$ ions are disordered and undergoing rapid reorientation about the body diagonals. This reorientation couples with and affects certain acoustic modes in the crystal. The results are described in terms of a phonon-pseudospin coupling theory.

New phonon-dispersion curves as determined by inelastic neutron scattering experiments are presented for the hcp metals, Ti, Co, and Tc, along with their theoretical analysis in terms of the phenomenological charge-fluctuation model. The model was also used to reanalyze the phonon data for some other hcp metals, Sc, Y, Zr, and Hf. For all the metals other than Sc and Y it was found that the data could be successfully analyzed only with a dipolar-charge-fluctuation model in contrast to Nb which required only monopolar terms. For Sc and Y the dispersion curves were satisfactorily analyzed in terms of a third-neighbor Born --- von K\'arm\'an model without any charge-fluctuation terms. The softening of the longitudinal [001] mode near $\ensuremath{\Gamma}$ observed in some of the metals appears to be closely related to an incipient instability in the electronic system toward the formation of a dipolar charge-density wave. The temperature dependence of the large anomaly in Tc could be reproduced by extremely small changes in one of the parameters of the dipolar-charge-fluctuation model.

The discovery of a charge-density wave (CDW) in $\ensuremath{\alpha}$-U at \ensuremath{\sim}43 K is reported. The mode corresponds to the condensation of a longitudinal-optic phonon near $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=[\frac{1}{2}, 0, 0]$ in which the atoms are displaced by \ensuremath{\sim}0.006 \AA{}A from their special positions. Large precursor effects are seen in the phonon spectra. The wave vector of the CDW is related to that of the fundamental lattice in a rather complex way, which is discussed in terms of discommensurations between the CDW (${\ensuremath{\alpha}}^{\ensuremath{'}}$ phase) and the primary $\ensuremath{\alpha}$ phase.

The knowledge of the decay heat of nuclear fuel is necessary for performing the reactor safety analysis, determining the heating load in fuel pools, shielding requirements on fuel discharge and transport routes when irradiated reactor fuel is transferred from the reactor, via some intermediate storage location, to the final disposal or the chemical reprocessing plant. In this study, analysis of the decay heat parameters of the Miniature Neutron Source Reactor (MNSR) including radioactivity, decay heat and the isotopic mass variation with time since reactor shutdown for the potential Low Enriched Uranium (LEU) (UO2-Zircaloy and U3Si-Al) and the standard Highly Enriched Uranium (HEU) (HEU-Al4) cores has been performed using the ORIGEN-2 code. For this purpose, a new one-group cross-section data base of the ORlGEN-2 computer code for the MNSR with LEU and HEU fuels has been developed using the MCNP-4C code. The variation of fission products, actinides and daughters and activation products with post shutdown time for the standard core and the potential LEU cores have been considered in the analysis of the decay heat power resources. It was found that, all the three types of MNSR fuels show close agreement in the total decay heat, which is mainly due to the fission products. This behavior continued for about 1.0E05 days. After this time, the fission products decay heat became comparable with the corresponding actinides decay heat in which the standard HEU UAl4-Al fuel showed the smallest decay heat values while the potential LEU-UO2 fuel had the highest decay heat followed by the LEU-U3Si fuel. The time variation of the total radioactivity followed the same behavior as the corresponding total decay heat. Finally, the relative decay heat as a function of reactor shutdown time has been also computed by various analytical functions and the results compared with the ORIGEN-2 code for the three types of fuels.

The relationship of phonon anomalies and lattice instabilities to high Tc superconductivity is explored. Inelastic neutron scattering studies have revealed an intimate and dramatic relation between certain phonons and lattice transformations, real or incipient, for moderate to high Tc superconductors, which are minimal or nonexistent in very low Tc or nonsuperconductors. Experimentalevidence is presented for a wide variety of superconducting materials and possible theoretical explanations for their behaviour are discussed.

We show that doping of hole charge carriers leads to formation of electric dipolar clusters in cuprates. They are created by many-body interactions between the dopant ion outside and holes inside the CuO planes. Because of the two-fold degeneracy holes in the CuO plane cluster into four-particles resonance valence bond plaquettes bound with dopant ions. Such dipoles may order into charge-density waves (CDW) or stripes or form a disordered state depending on doping and temperature. The lowest energy of the ordered system corresponds to a local anti-ferroelectric ordering. The mobility of individual disordered dipoles is very low at low temperatures and they prefer first to bind into dipole-dipole pairs. Electromagnetic radiation interacts strongly with electric dipoles and when the sample is subjected to it the mobility changes significantly. This leads to a fractal growth of dipolar clusters. The existence of electric dipoles and CDW induce two phase transitions with increasing temperature, melting of the ordered state and disappearance of the dipolar state. Ferroelectricity at low doping is a natural consequence of such dipole moments. We develop a theory based on two-level systems and dipole-dipole interaction to explain the behavior of the polarization as a function of temperature and electric field.

The many predictions of structure transitions and soft modes in ..cap alpha..-U were verified by elastic and inelastic neutron scattering. Such verification was made possible by earlier work that revealed the pronounced anomaly in the ..sigma../sub 4/-LO branch, for the identification of the eigenvector associated with the anomalous mode led to the regions in reciprocal space where one would expect to observe the superlattice reflections. The discovery of the unusual shift in the superlattice reflections toward the origin is somewhat puzzling; perhaps the shift is due to discommensurations in terms of wide stacking solitons. 5 figures, 1 table. (RWR)

The room-temperature lattice dynamics of $\ensuremath{\alpha}$-uranium has been investigated. The phonon dispersion curves along the orthorhombic high-symmetry directions have been measured using inelastic neutron scattering techniques. The dispersion curves in the $[0\ensuremath{\zeta}0]$ and $[00\ensuremath{\zeta}]$ directions are not very unusual. However, a depression and pronounced dips are observed in the $[\ensuremath{\zeta}00]$ direction. The slopes of the long-wavelength acoustic branches are in agreement with the slopes predicted from the elastic constants. A four-neighbor Born-von K\'arm\'an general-tensor model, a 12-neighbor axially symmetric model, a simple shell model, and a simple Heine-Abarenkov pseudopotential model are unable to satisfactorily reproduce some of the $[\ensuremath{\zeta}00]$ branches. However, a modified form of the shell model which includes forces to six neighbors is found to reproduce most of the branches well, including those in the $[\ensuremath{\zeta}00]$ direction. A preliminary low-temperature experiment has revealed extra elastic scattering as well as significant phonon softening along the $[\ensuremath{\zeta}00]$ direction.

The phonon dispersion relations of the substitutional disordered alloys ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}$ ($x=0.05, 0.30, 0.50, 0.75, \mathrm{and} 0.95$) were studied by neutron inelastic scattering. The resonance mode for the sample with $x=0.05$ is well described by the mass-defect coherent-potential-approximation (MCPA) theory. Experimental results deviate from the MCPA calculations for the samples with $x\ensuremath{\ge}0.30$ and the difference is most conspicuous for the maximum phonon frequency which decrease faster with increasing Pt concentration than the theory predicts. The energy splitting of the phonon dispersion curves at the resonancemode frequency has a branch dependence in contrast to the MCPA prediction; that of the $L$ branch always takes place at a higher frequency than that for the $T$ branch.

A lattice dynamics study of α‐U at room temperature revealed a pronounced dip in the Σ4 Lo mode at a wavevector ζ=0.475. Additional studies at low temperatures showed this mode to soften considerably. Below 60 K elastic peaks appear near (h+1/2, k,l), increasing in intensity to a maximum at 10.4 K, the lowest temperatured studied. The current tentative interpretation is the xoexistence of two phases of uranium, the α phase, and a similar, but slightly distorted, α1 phase with a sinusoidal modulation in the x‐direction.

The phonon dispersion relations of disordered alloys Ni1-xPtx were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with x ≧ 0.30, the energy splitting of the phonon dispersion curve for the L-branch takes place at a higher frequency than that for the T-branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.

Inelastic neutron scattering measurements have been carried out on 1T-TiSe2 at various temperatures in order to obtain information about the dynamics of the phase transition associated with the charge density waves. A large damping of a transverse phonon at the L-point has been observed near the transition temperature. A simple lattice dynamical model was used to study the effect of the Ti-Se bonding on the transition.

We present theoretical calculations and experimental measurements of the phonon linewidth for Nb. The calculations employ the rigid muffin-tin approximation and a realistic band structure. The measurements were done at low temperature to minimize the contributions to the linewidth from phonon-phonon interactions. The calculations predict and experiments confirm strong electron-phonon coupling for the longitudinal phonon modes in the [110] direction.

Phonon dispersion curves of single crystal iodine at 77 K have been measured by one-phonon coherent inelastic neutron scattering techniques. The data are analysed in terms of two Buckingham-six intermolecular potentials; one to represent the shortest intermolecular interaction (3.5 Å) and the other to represent the more distant interactions. Moderate agreement is obtained between the observed and calculated frequencies, but it also oappears necessary to treat the second-nearest-neighbor interaction (3.97 Å) separately from the van der Waals interactions (distances ⩾ 4.2 Å).