The elastic transition in s-triazine (C3N3H3) from a trigonal (R3c) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.

Motivated by the recent calculation of Guiliani and Overhauser in which a criterion for the preferred orientation of the wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$ of a charge density wave (CDW) in the alkali metals was derived, we have carried out a neutron-diffraction search for CDW's in potassium metal. Although the search was not exhaustive of all the possibilities, we have been unable to detect any satellite peaks at (or near) the predicted positions in reciprocal space. However, we have found small diffraction peaks along certain symmetry axes at other positions in reciprocal space. They are strongly temperature dependent, resembling the Debye-Waller effect in potassium metal, but they do not satisfy the symmetry requirements of a CDW modulation of the body-centered-cubic crystal structure.

The order parameter of the second order magnetic phase transition in Ho at 131 K has been measured in a magnetic field by neutron diffraction. It was expected that the applied field would change the number of components (n) of the order parameter from four to two. Such a change would be visible in the order parameter exponent β. However, in a field of 40 kOe, β is found to be 0.38 ± 0.02, in agreement with the zero field measurement and with an n = 4 model and in contradiction to an n = 2 model.

The molecular crystal s-triazine, C3N3H3, is known to undergo a transition from a trigonal to a monoclinic structure when cooled below Tc=200K. Inelastic neutron scattering measurements have been carried out on a single crystal of s-triazine at various temperatures. A pronounced softening of transverse modes governed by the elastic constant c44 is observed in the vicinity of Tc. The phonon response remains underdamped at energies down to at least 0.07 meV and no 'central peak' is observed. The transition seems to be slightly first order ('quasi-continuous') in accordance with recent observations, while it has been reported to be of second order at higher pressures.

Orientational ordering of nitrogen-molecule overlayers on graphite has been investigated by neutron diffraction. The ordering transition is found to occur at 30 K in both the registered and dense-solid phases despite a 5% difference in the nearest-neighbor distance.

The magnetic properties of Cu${(\mathrm{N}{\mathrm{O}}_{3})}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2.5 ${\mathrm{H}}_{2}$O are dominated by the presence of antiferromagnetically coupled pairs of Cu ions with a singlet ground state separated by about 5.2 K from an excited triplet. Neutron-diffraction studies have been performed on a crystal of Cu${(\mathrm{N}{\mathrm{O}}_{3})}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2.5 ${\mathrm{D}}_{2}$O between 0.08 and 0.25 K in fields of up to 60 kOe applied along the monoclinic $b$ axis. At 0.125 K and between 28 and 44 kOe, weak magnetic scattering appears at some of the nuclear peak positions. The magnetic structure consists of alternating antiferromagnetic chains approximately in the $a\ensuremath{-}c$ plane which are antiferromagnetically coupled to neighboring chains in the same plane. The antiferromagnetic axis is perpendicular to the field direction, lying roughly halfway between the $a$ and $c$ axes, and the moment attains a maximum of about ${0.45}_{\mathrm{\ensuremath{\mu}}\mathrm{B}}$ per Cu atom at 38 kOe, falling off sharply close to the upper and lower critical fields. Magnetic scattering also appears at a different set of nuclear peak positions above 28 kOe, reaching a saturation value at about 46 kOe. This results from an induced ferromagnetic component along the $b$ axis with a saturation moment of roughly ${0.9}_{\mathrm{\ensuremath{\mu}}\mathrm{B}}$ per Cu atom. The results are in accordance with numerous previous studies, and in particular allow a choice between two antiferromagnetic structures proposed by Diederix et al. from proton resonance measurements.

The phonon dispersion relations in high-density crystals of fcc $^{4}\mathrm{He}$ have been measured along high-symmetry directions by the neutron-inelastic-scattering technique. A recent study of the lattice dynamics of fcc $^{4}\mathrm{He}$ by Eckert et al. has been extended to cover the fcc phase diagram at pressures below 5 kbar. Molar volumes of 9.03, 9.43, and 9.97 ${\mathrm{cm}}^{3}$/mole have been studied in the temperature range from near the melting curve to near the fcc-hcp transition line. The phonon dispersion relations are in good agreement with a first-order self-consistent phonon theory calculation by Goldman. The observed phonon-group line shapes at large energy and momentum transfers show evidence for multiphonon scattering in agreement with calculations by Glyde. Eckert et al. reported extremely large anharmonic isochoric temperature shifts of the phonon energies. The present work studied the shifts as a function of molar volume and temperature. Mode-Gr\"uneisen-parameter dispersion curves have been measured using the present data and earlier measurements at lower density in the fcc phase by Traylor et al. Macroscopic Gr\"uneisen parameters have been calculated from the phonon density of states obtained from the data.

The neutron-inelastic-scattering technique was used to measure the phonon dispersion relations in two high-density crystals of hcp $^{4}\\mathrm{He}$ with molar volumes of 11.61 and 9.41 ${\\mathrm{cm}}^{3}$/mol. These densities are on the order of twice that of $^{4}\\mathrm{He}$ at 30 bars. The crystals were grown from the melt in the fcc phase, and cooled across the fcc-hcp phase transition. The observed phonon spectra show anharmonic effects much less prominent than those observed in earlier measurements at 21 ${\\mathrm{cm}}^{3}$/mol. In particular, multiphonon interference effects were not found to be very pronounced, while multiphonon scattering appeared to influence phonon line shapes strongly for large wave-vector and energy transfers. Elastic constants d\\ifmmode \\dot{e}\\else \\.{e}\\fi{}termined from the initial slopes of the dispersion curves were found to be in good agreement with those calculated by Goldman using the first-order self-consistent-phonon theory. The dependence on volume of the phonon energies is discussed with reference to the earlier studies at 21.1 and 16.0 ${\\mathrm{cm}}^{3}$/mol. No significant dispersion of mode Gr\\uneisen parameters was found unlike the case of fcc He. Gr\\uneisen parameters were found to depend on volume approximately in a manner given by Ahlers for the thermodynamic Gr\\uneisen parameter.

Development of new ternary superconductors has led to materials which also show a strong tendency toward magnetic order when one of the constituent elements is a rare earth. Powder neutron diffraction data on superconducting (Ts∼6 K) samples of ErMo6Se8 taken in the temperature range 0.05–2.0 K show that magnetic Bragg peaks develop at TM=1.1 K, in agreement with specific heat studies. However, it is not possible to index these new reflections using a simple antiferromagnetic unit cell based on the ErMo6Se8 lattice. It is also not possible to index the magnetic reflections based on a single modulation vector, or with a modulation vector along a high symmetry direction including higher order harmonics. Thus either the long range magnetic order corresponds to a more complicated magnetic structure in the ErMo6Se8 lattice, or at least some of the peaks develop in impurity phases. These results are compared with the recent neutron data on the reentrant superconductors HoMo6Se8 and ErRh4B4, in which the development of ferromagnetic order is clearly shown to be responsible for quenching the superconductivity.

The neutron-inelastic-scattering technique was used to measure the phonon dispersion relations in a high-density crystal of fcc He at 38 K. The crystal was grown at a pressure of 4.93 kbar and a temperature of 38.5 K in a high-pressure sample holder. Its lattice parameter was determined to be 3.915 \ifmmode\pm\else\textpm\fi{} 0.002 \AA{}, equivalent to a molar volume of 9.03 ${\mathrm{cm}}^{3}$/mol. The measured dispersion curves were found to be in good agreement with a recent calculation by Goldman using the first-order self-consistent phonon theory without short-range correlation functions. The strong anharmonic effects observed in earlier measurements on the crystals of 21 ${\mathrm{cm}}^{3}$/mol were found to be much less prominent in this He crystal. The magnitude of the multiphonon interference effects on the one-phonon intensities is shown to be less than half of that observed in the low-density crystals. Thermodynamic analysis of the data yielded ${\ensuremath{\Theta}}_{D}^{M}=154$ K which indicates that the ratio of mean amplitude of vibration to the nearest-neighbor distance is 8.6%, as opposed to nearly 30% for the lowest-density He crystals. The dependence of the phonon energies on volume is discussed with reference to the earlier work of Traylor et al. on an fcc crystal at 11.7 ${\mathrm{cm}}^{3}$/mol. Limited measurements were also made at 22 K to determine the temperature dependence of the phonon energies. Unusually large isochoric temperature shifts of as much as 15% for some phonons close to the zone center were found over the range of 22-38 K.

Using neutron scattering techniques, we show that the destruction of superconductivity in Er${\mathrm{Rh}}_{4}$${\mathrm{B}}_{4}$ at ${T}_{c2}=1.0$ K is accompanied by the development of long-range ferromagnetic ordering of the Er sublattice. The observed magnetic Bragg intensities indicate that the Er ion has a moment of 5.6${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ which is oriented in the tetragonal basal plane. The magnetic transiton appears second order, but shows anomalously strong precursor scattering.

The temperature dependence of the order parameter associated with the magnetic phase transition in Ho near 131 K has been measured by neutron diffraction. The results show a power law behavior for the order parameter with an exponent of β = 0.39 (+0.04, −0.03) for the reduced temperature range 0.007 ⩽ ϵ ⩽ 0.3. This value is in agreement with recent predictions for an n = 4 vector model studied in the renormalization group technique.

Neutron inelastic scattering has been used to study the longitudinal and one of the transverse acoustic phonons, propagating along the [001]∗ direction in triclinic K1.75[Pt(CN)4]·1.5H2O. This material appears to be a quasi-one-dimensional conductor, with a commensurate distortion. We observe a reasonably well defined Kohn anomaly, which shows little temperature dependence between 80 and 300 K.

The phonon dispersion relations in a high-density Ne crystal have been measured in the [100], [110], and [111] symmetry directions at 8 K using a triple-axis neutron spectrometer. The crystal was grown at a pressure of 6 kbar and at 82 K in a new high-pressure sample holder. Its lattice parameter was determined to be 4.209 \ifmmode\pm\else\textpm\fi{} 0.002 \AA{}, equivalent to a molar volume which is 84% of that of an equilibrium density crystal at $T=0$ K. A Born-von K\'arm\'an force-constant analysis indicated that the interatomic forces in Ne remain predominantly central in nature at this density. The data are well represented in the harmonic approximation by a Lennard-Jones potential with $\ensuremath{\epsilon}=50.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ erg and $\ensuremath{\sigma}=2.818$ \AA{}. The force constants from the most general model were used to derive the zero-sound elastic constants. Corrected for pressure, the analysis gives ${c}_{11}=547\ifmmode\pm\else\textpm\fi{}8$, ${c}_{12}=278\ifmmode\pm\else\textpm\fi{}8$, and ${c}_{44}=289\ifmmode\pm\else\textpm\fi{}4$ (${10}^{8}$ dyn ${\mathrm{cm}}^{\ensuremath{-}2}$). A new analysis of the results of three previous studies of phonon dispersion relations in Ne crystals of approximately equilibrium density was used in conjunction with the present results to derive a dispersion of quasiharmonic mode Gr\"uneisen parameters. These were appropriately summed to obtain the macroscopic Gr\"uneisen parameter. Quasiharmonic calculations were shown to overestimate the compression dependence of the phonon energies in Ne. Further thermodynamic calculations also demonstrated the relative decreased importance of zero-point motion in the high-density crystal.