Dynamics of cubic-tetragonal phase transition in KNbO<Subscript>3</Subscript> perovskite

The low-energy part of the vibration spectrum of KNbO₃ was studied by cold neutron inelastic scattering in the cubic phase. In addition to acoustic phonons, we observe strong diffuse scattering, which consists of two components. The first one is quasistatic and has a temperature-independent intensity. The second component appears as quasielastic scattering in the neutron spectrum, indicating a dynamic origin. From analysis of the inelastic data, we conclude that the quasielastic component and the acoustic phonon are mutually coupled. The susceptibility associated with the quasielastic component grows as the temperature approaches T _C .     

Phonon- and libron-excitation with ortho-para conversions by neutron scattering on solid hydrogen

The cross section for incoherent neutron scattering on solid ortho hydrogen in the ordered phase is investigated for processes where ortho-para conversions take place in combination with the excitation of phonons and librons. The phonon part of the cross section is calculated and compared with the experimental results of Stein, Stiller and Stockmeyer [2]. The coupling of the librons to the ortho-para conversion is discussed. We show that librons give a contribution to the cross section in the vicinity of both the ortho → para and the para → ortho conversion line.     

Lattice dynamics of cubic PbTiO3 by inelastic neutron scattering

High-temperature dispersion relations of the phonon modes in a cubic PbTiO₃ single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T _c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed.     

Theory of spinconversion in XH3-systems

In a simple model of (mechanically and magnetically) isolated methyl groups with mutualintramolecular dipolar coupling between the nuclear magnetic moments of the three protons the conversion rate (i.e. for the symmetry changing transitionsE ^a,b A) is investigated. It turns out, that within the model of a fixed rotational axis only half of theE-symmetric rotor states can convert intoA-symmetric ground states.The usual coupling to the phonon bath of thermal lattice excitations is considered in lowest order perturbation theory, and used to describe the temperature dependence of shift and broadening of tunnelling-and librational lines by inelastic neutron scattering (INS). For the two possible types of this phonon coupling,breathing andshaking, a different temperature dependence of the conversion rate is obtained. The temperature dependence of the INS-spectra and of the conversion rate are predicted to be mutually interdependent.Only in case of a non zero phonon coupling of breathing type a non vanishing conversion rate at zero temperature is found; this rate is estimated to be proportional to _t ³ .     

First measurement of a phonon dispersion curve by inelastic X-ray scattering

Inelastic scattering of 13.8 keV X-rays with very high energy resolution of E=55 meV was used to measure the phonon dispersion curves for theLA andLO modes in the [00] direction in Be. The results agree with inelastic neutron scattering data known from the literature. The X-ray scattering intensities of the phonon excitations for different momentum transfers are in very good agreement with the prediction from the scattering law.     

Magnetic relaxation spectra of EuNi2P2

We report on inelastic neutron scattering experiments on EuNi₂P₂ prepared from¹⁵³Eu-isotopes. The sample was checked by Mössbauer spectroscopy; a satellite phase of about 4% was found. The neutron scattering experiment was performed with thermal neutrons. AboveT=50 K a strong quasielastic magnetic line was detected with a nearly temperature independent width of about 6 meV. At low temperatures the spectrum changes to an inelastic character with a distinctQ-dependence in the intensity. For comparison static susceptibility and magnetization measurements were performed down to 250 mK.     

LaAg x In1−x

The phonon dispersion of LaAg _x In_1–x (x=1, 0.89, 0.8) has been studied by inelastic neutron scattering in the cubic high temperature phase. A soft mode behaviour was observed at theM-point. The doubling and the cubic to tetragonal deformation of the elementary cell was observed through the phase transition. The measurements of the elastic constants were extended to 450 K and their magnetic field dependence was investigated.This work was supported by the BMFT and the Sonderforschungsbereich 65 Darmstadt/Frankfurt     

Origin of negative thermal expansion in cubic ZrW2O8 revealed by high pressure inelastic neutron scattering

Isotropic negative thermal expansion has been reported in cubic ZrW2O8 over a wide range of temperatures (0-1050 K). Here we report the direct experimental determination of the Grüneisen parameters of phonon modes as a function of their energy, averaged over the whole Brillouin zone, by means of high pressure inelastic neutron scattering measurements. We observe a pronounced softening of the phonon spectrum at P = 1.7 kbar compared to that at ambient pressure by about 0.1-0.2 meV for phonons of energy below 8 meV. This unusual phonon softening on compression, corresponding to large negative Grüneisen parameters, is able to account for the observed large negative thermal expansion.     

Structural disorder and phase transitions in ZrO2-Y2O3 system

The phase transitions in the ZrO₂-YzO₃ system have been investigated as a function of temperature using Raman scattering. The cubic phase was found to be structurally disordered and the spectrum has been compared with a one phonon density of states derived from a rigid ion model. The frequencies and symmetries of the modes were obtained for the cubic, tetragonal and monoclinic phases and comparisons have been made between the various temperatures and compositions.     

Density effect in d - d catalyzed fusion with ortho-and para-enriched deuterium

The ortho-para dependence of d-d fusion neutrons time spectra has been investigated using different target densities at similar temperatures. Even at the similar temperatures, the ortho-para effect on the fusion neutron yields in the liquid phase showed the opposite tendency from that in the gas phase. The result evidenced significant density dependence on resonant ddμ formation.     

α-quartz as a substrate in thermal phonon radiation

Using the mechanical strong anisotropic-quartz, measurements of radiation temperature as a function of phonon radiation power were performed for gold and copper radiators evaporated on theX- orZ-crystal face, respectively. The comparison with theoretical model calculations of phonon transmission across the interface yields agreement with the isotropic/anisotropic acoustic mismatch model. A strong increase in radiation temperature in comparison to linear emission models is observed at temperatures above about 40 K. We suppose that this effect is due to phonon back-scattering which leads to a back-heating of the metal film.Supported by Bundesministerium für Forschung und Technologie     

Ortho-para conversion rate in solid hydrogen under pressure

We report new measurements of the ortho-para conversion rate in solid H₂ at 4.2 K for densities ?₀ ? ? ? 1.7?₀, where ?₀ is the density at zero pressure. The conversion rate shows evidence of two maxima, at ～ 1.3?₀ and at ～ 1.5?₀ respectively. A qualitative discussion of this behavior is presented in terms of the theory of phonon creation during the conversion, and this discussion shows the consistency between the observed maxima and the shape of the phonon spectrum.     

Soft rotary mode and structural phase transitions in K2ReCl6

The triple-axis neutron scattering technique has been used to observe the soft-phonon modes associated with the second-order structural phase transitions in the cubic antifluorite K₂ReCl₆. The transverse (ReCl₆)^?2 rotary mode has been found to be soft all along the [ζ00] direction, with the structural distortions being triggered by a condensation first at the zone center and then at the [100] zone boundary. In addition to the soft phonon modes there is a temperature dependent central component to the spectrum along the [ζ00] direction.     

Lattice dynamics and hydrostatic-pressure-induced phase transitions in ScF3

New phase transitions induced by hydrostatic pressure in a cubic (under standard conditions) ScF₃ crystal are discovered by the methods of polarization microscopy and Raman scattering. The space groups $R\bar 3c$ for Z=2 and Pnma for Z=4 are proposed for the high-pressure phases. A nonempirical computation of the lattice dynamics of the crystal is carried out. It is shown that, under normal pressure, the cubic phase is stable down to T=0 K, while the application of a hydrostatic pressure gives rise to a phonon branch in the vibrational spectrum (between points R and M of the Brillouin zone) with negative values of squares of frequencies. The condensation of soft mode R₅ at the boundary point of the Brillouin zone leads to rhombohedral distortion of the cubic structure with the unit cell volume doubling. The calculated frequencies at q=0 of the ScF₃ lattice in the distorted rhombohedral phase are real-valued; the number and position of frequencies active in Raman scattering are in accord with the experimental values.     

Dissipation of ripplon flow at the surface of superfluid <Superscript>4</Superscript>He

Within the framework of quantum hydrodynamics of a superfluid helium surface, the momentum relaxation rate caused by the annihilation of two ripplons with phonon creation, inelastic phonon scattering with ripplon annihilation, and in the case of helium films one-particle ripplon scattering from the surface-level inhomogeneities introduced by the substrate roughness (new relaxation mechanism) was obtained for a ripplon gas at T?0.25 K. The contribution from the inelastic phonon scattering is negligible at these temperatures. For a film at T≤0.15 K, one-particle scattering dominates, leading to a temperature dependence of the form K∝T^5/3 for the convective thermal conductance.At higher temperatures, phonon creation with annihilation of two ripplons is the dominant mechanism, giving K∝T^?3. These results are in quantitative agreement with the available experimental data.     

Contours of constant phonon energies in thebc-plane of sapphire (Al2O3) by means of inelastic neutron scattering

By means of inelastic neutron scattering the contoursv=const. have been determined for the transverse acoustic phonon branch which is polarized in the mirror plane (bc-plane). The results show that in all directions effects of dispersion can be neglected forq0.5q _max.     

Investigation of the closed porosity of functional ceramic materials by spin-echo small-angle neutron scattering

The closed porous structure in ceramic materials is investigated by spin-echo small-angle neutron scattering. A series of ceramic samples of oxygen–ion conductors based on bismuth molybdate with the general formula Bi_12.8 X _0.2Mo₅O_{34 ± δ} (X = Mg, Ba, Ca, Sr) is obtained by powder sintering for 6?45 h at a temperature close to the melting point. The samples are characterized by scanning electron microscopy and X-ray fluorescence analysis. It is found that they had a stoichiometric chemical composition, are singlephase, and contain clean pores between crystal grains. The pore size is determined by spin-echo small-angle neutron scattering and ranges from 2.2 to 3.5 μm. It is demonstrated that longer sintering times correspond to larger pores (the increase in their average diameter is as large as 30%). It is found that the studied materials lack a fractal pore structure.     

Martensitic transformation and soft modes in KAlF4

Abstract

The structural phase transition observed at ?13°C in KAlF₄ is shown to be martensitic. The results of an investigation of the phonon spectrum by inelastic neutron scattering are reported. It is shown that the transition is preceded by the softening of a flat phonon branch. A model is proposed to explain how such a softening is related to the martensitic transition.     

Mode dependent scattering of phonons by domain walls in ferroelectric KDP

Thermal conductivity and ballistic phonon imaging measurements in KH₂PO₄ (KDP) at low temperature (T<3K) indicate that scattering from domain walls has a large effect on phonon transport. kDP has a ferroelectric phase transition from tetragonal to orthorhombic structure atT _c =122 K. BelowT _c domains of opposite electric polarization and crystal orientation form unless the sample is colled in an electric field. Thermal conductivity measured along the [100] (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. We attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domainwall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission.