In Brillouin scattering the wave-vector transfer Q of the scattering process is equal to the wave-vector q of collective excitations. In the case of neutrons the kinematical limitation restricts the range of momentum and energy transfer. Investigations on liquid Pb, Rb and Cs are reviewed. Molecular liquids CO and D2O have been studied also. Water has been investigated by Brillouin scattering with X-rays with very high energy resolution. The latter technique has the advantage that there is no kinematical limitation. Two different groups have interpreted the experimental results on water in different ways. After 40 years of Brillouin scattering, this technique remains very fruitful for the study of liquids and amorphous materials, both with neutrons and with X-rays.

In traditional data analysis a model function, convoluted with the resolution, is fitted to the measured data. In case that integrated intensities of signals are of main interest, one can use an approach which does not require a model function for the signal nor detailed knowledge of the resolution. For inverse TOF technique, this approach consists of two steps: (i) Normalisation of the measured spectrum with the help of a monitor, with 1/k sensitivity, which is positioned in front of the sample. This means at the same time a conversion of the data from time of flight to energy transfer. (ii) A Jacobian [I. Waller, P.O. Fröman, Ark. Phys. 4 (1952) 183] transforms data collected at constant scattering angle into data as if measured at constant momentum transfer Q. This Jacobian works correctly for signals which have a constant width at different Q along the trajectory of constant scattering angle. The approach has been tested on spectra of Compton scattering with neutrons, having epithermal energies, obtained on the inverse TOF spectrometer VESUVIO/ISIS. In this case the width of the signal is increasing proportional to Q and in consequence the application of the Jacobian leads to integrated intensities slightly too high. The resulting integrated intensities agree very well with results derived in the traditional way. Thus this completely different approach confirms the observation that signals from recoil by H-atoms at large momentum transfers are weaker than expected.

. The temperature evolution of the structure of NaNO 2 nanocomposite ferroelectric material in a porous glass with 7 nm pores was studied by neutron diffraction in temperature region from room temperature up to the melting, i.e. in the ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with the structure of the bulk, but above the ferroelectric phase transition (and up to ≈ 513 K) a volume premelted state is formed, manifesting itself in a growth of amplitudes of ion thermal vibrations, a steep increase of elementary cell volume and “softening” of lattice. For the first time the temperature dependence of order parameter η for confined sodium nitrite is determined. η (T) follows a power law with T C =425.6± 2.1 K and β = 0.31± 0.04, which is essentially different from that for bulk NaNO 2 . Our obtained data are in a good agreement with the results of earlier dielectric and neutron diffraction measurements.

The temperature evolution of the crystal structure of KD2PO4 (DKDP) embedded in a porous glass with 7 nm pore diameter has been studied by neutron diffraction in temperature interval 90 K-308 K. It is shown that confined DKDP forms interconnected clusters with characteristic size of 18(0.5) nm. The structure of this nanocomposite material is found to be monoclinic and corresponds to the space group P2(1). The lattice parameters and characteristic cluster size are temperature independent and do not reveal any peculiarities in the vicinity of expected ferroelectric phase transition of 223 K.

Preliminary results of lattice-dynamical shell-model calculations for the relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) in the virtual-ion approximation are presented. The model parameters are fitted to Raman scattering and low-frequency inelastic neutron scattering data, and higher-frequency dispersion curves are predicted.

A detailed lattice-dynamic investigation on the relaxor ferroelectric PMN was started to search for "additional" modes (dispersion curves). But the results are complex with 4 branches observed below 6.5 THz, a broad unresolved continuum between 6.5 and 12 THz, a well defined excitation at 13.4 THz with longitudinal and transverse character and further weak signals above 16 THz. Possible effects caused by disorder are discussed.

Results of study of the spatial distribution of polarization by scattering (X-ray and neutron) and Piezoresponse Force Microscopy techniques are presented for several relaxors both cubic (PMN and PMNPT solid solutions) and uniaxial (SBN). It is demonstrated that in all cases except pure PMN cooled in zero electric field polar nanodomains are formed. In case of PMN cooled in the applied field nanodomain state is preserved even after subsequent zero field heating to above freezing temperature. AFM measurements of PMNPT10 are used to determine the quantitative characteristics of these nanodomains.

Results of a study of low-energy excitations and critical scattering in the field-induced ergodic ferroelectric phase of the relaxor ferroelectric PbMg1/3Nb2/3O3 are presented. It is demonstrated that a transition to the ferroelectric phase does not result in the appearance of a clearly defined soft mode. The observed quasielastic scattering was found to be strongly anisotropic. The q-dependence of the scattering intensity is described in terms of a 'fractal' law. For the first time we have observed a strong difference between the topology of polar microregions at room temperature in a 'virgin' crystal and in a crystal cooled in an electric field and then reheated.

It is known that finite-size effect results in the drastic changes of the phase transition (PT) features, and these anomalies become especially significant if the characteristic size is comparable with correlation length of the order parameter critical fluctuation. The porous matrices give the unique possibility to study confined materials and their properties as a function of size and topology of pores. Recent dielectric measurements of some ferroelectric materials confined in different porous matrices [1, 2] have shown unexpected growth of dielectric constant ε above the temperature Tc of ferroelectric PT for all materials and all matrices, but microscopic origin of observed anomalies was incomprehensible. To clarify the situation we have attempted to study the temperature evolution of structure of highdeuterated KD2PO4 (DKDP) embedded into porous glass with pore sizes 70 ± 3Å at temperatures 90 K < T < 310 K (below and above Tc) by neutron diffraction. The data treatment of obtained diffraction patterns shows that embedded DKDP forms a system of interconnected clusters with average size essentially exceeds the pore sizes. The structure of embedded DKDP does not change in investigated temperature region and corresponds to the space group P21. The lattice parameters a, b and c are practically temperature independent (Fig.1) and do not demonstrate any peculiarities in the vicinity of expected PT at 250 K. It is known that DKDP crystals may exist in two polymorths due to isotopic

Overbending of the longitudinal-optical-phonon branch in diamond has been evidenced along all three principal directions by a joint inelastic neutron and x-ray experiment. The observed overbending of 1.5, 0.5, and 0.2 meV ~along the D, L, and S directions, respectively! confirms previous ab initio lattice-dynamics calculations, thus providing experimental proof for the explanation of the anomalous peak in the two-phonon Raman spectrum.

The crystal structure of a solid solution of 3.5 at.% deuterium in α-Mn was determined by neutron diffraction and the spectrum of optical deuterium vibrations at 5 K was studied by inelastic neutron scattering (INS). The results give conclusive evidence of the tunnelling origin of the strong peaks at 6.4 and 1.6 meV observed earlier at temperatures up to 100 K in the INS spectra of hydrogen and deuterium solutions in α-Mn, respectively.

Hydrides of iron and iron-based alloys are thermodynamically stable only at hydrogen pressures in the gigapascal range and rapidly lose hydrogen under ambient conditions. At low temperatures, however, these hydrides can be retained in a metastable state at atmospheric pressure after being cooled under high pressure to liquid nitrogen temperature. This review will discuss the current state of studies on phase transformations in the Fe–H and related systems and also on the composition, crystal structure and physical properties of the hydrides, both under high hydrogen pressures and in the 'quenched' metastable state at ambient pressure. The studies at ambient pressure include magnetization measurements, x-ray and neutron diffraction, Mössbauer spectroscopy and inelastic neutron scattering. In the sections on Mössbauer and structural investigations of hydrides of Fe–Cr and Ni–Fe alloys new experimental results will be presented.

INTRODUCTIONCe cours est le second d'une série de trois.L'approche théorique de la diffusion magnétique et les techniques expérimentales ont été développées par Bjôrn Fâk.Mon exposé commence par une brève introduction sur la résolution expérimentale en insistant sur les spectromètres trois axes (TAS), Ensuite, deux exemples de détermination des courbes de dispersion des magnons sont présentés : La chaîne ferromagnétique unidimensionnelle (1-D) CsNiF 3 et le grenat antiferromagnétique 3-D Ca 3 Fe 2 (Ge0 4 )3.Après une approche générale des systèmes ferro-et antiferromagnetiques, de nouveaux développements pour certains systèmes antiferromagnétiques sont expliqués.Les Spinons, excitations dans des systèmes S=l/2 ont été prédits et observés.Les résultats expérimentaux sur CuGe03 obtenus en temps de vol (TOF) avec le spectromètre MARI installé sur la source puisée ISIS montre que le TOF est particulièrement utile pour les systèmes 1-D.A la fin, les excitations dans les composés de type CsFeCl3 présentant un état singulet fondamental sont discutées.Ces excitations sont appelées excitons car elles ont un moment net nul dans l'état fondamental. LA RÉSOLUTION EN DIFFUSION DE NEUTRONS INÉLASTIQUELa relation entre la section efficace doublement différentielle et la fonction de structure S(Q,

The vibrational spectrum of fcc γ-MnH0.41 synthesized under high pressure of gaseous hydrogen was studied by inelastic neutron scattering at 2 K in the range of energy transfers from 25 to 400 meV. The fundamental band of optical hydrogen vibrations consists of a peak at 111 meV with a broad shoulder towards higher energies, which extends up to about 140 meV. At higher energy transfers, the spectrum originates from multiphonon neutron scattering and exhibits approximately harmonic behaviour. The results are compared with the available data for other metal hydrides.

Inelastic neutron and X-ray scattering are used to investigate high-frequency acoustic modes in glasses, in particular glassy selenium and densified silica. Strong scattering takes over above a crossover frequency where the excitations loose their plane-wave character.

Vibrational spectra of α-MnH0.07 and α-MnD0.05 were studied by inelastic neutron scattering at temperatures from 1.7 to 200K over a wide range of energy and momentum transfers. Together with the high-energy bands of the optical vibrations, pronounced peaks at 6.3 and 1.6meV were observed in the spectra of the samples loaded with H and D, respectively. The study of the temperature, momentum-transfer and isotope dependence of the spectra demonstrated the tunnelling origin of these peaks.

The spin-wave spectrum of the antiferromagnet with the garnet structure has been studied by inelastic neutron scattering on the triple-axis spectrometers IN12 and IN14 at ILL. Magnon dispersion curves were measured along and directions up to energies of 0.3 THz. The spin-wave symmetry analysis, which reduced the number of independent exchange parameters to six, was performed to ensure correct description of the spin-wave branches. As determined by the fitting, five of the parameters are statistically significant and have been found to be , and . This quite unusual sequence of exchange parameters clearly demonstrates the importance of the superexchange chain geometry.

Cobalt hydrides and deuterides with the hcp metal lattice and H(D)-to-metal atomic ratios 0.18≤x≤0.5 were prepared under high pressures of hydrogen or deuterium, respectively, and studied by neutron diffraction in a metastable state at 120 K and ambient pressure. A profile analysis of the spectra showed that in all samples the hydrogen and deuterium atoms occupy octahedral interstitial sites. In the samples with x≤0.26, the hydrogen and deuterium atoms are randomly distributed over these sites. In the samples with x≥0.34, they form layered superstructures, occupying every third octahedral base layer at x=0.34 and every second layer at x=0.38 and 0.5.

Results of inelastic neutron scattering experiments between 300 K and 900 K on the relaxor ferroelectric PbMg 1/3 Nb 2/3 O 3 are presented. Within a mode-coupling analysis the data are consistent with the observation of a strongly damped quasi-optic excitation. It is demonstrated that below T d ≈ 650 K a dynamical crossover takes place manifesting itself by a narrow central peak. This crossover is accompanied by the appearance of strong damping of the transverse acoustic phonons, with the damping constant proportional to q 4 . Different physical models of the crossover are discussed.

The splitting due to dipolar interactions was investigated for the hexagonal [110] direction (y-direction). The Sx and Sy fluctuations were exclusively visible in the non-spin-flip and the spin-flip channels, respectively. The splitting creates minima in the dispersion curves away from the commensurate K-point (13130). In CsFeCl3, which does not exhibit long-range order, the excitations are temperature independent at low temperature. In RbFeCl3 a soft mode behaviour is observed and the new minima in the dispersion curves correspond to the observed incommensurate structures. In the commensurate phase of RbFeCl3 below 1.95K an acoustic and an optic mode were observed.