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  • 1
    Electronic Resource
    Electronic Resource
    Ab initio total energy study of brucite, diaspore and hypothetical hydrous wadsleyite (1995)
    Springer
    Physics and chemistry of minerals 22 (1995), S. 461-467 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Ab initio total energy calculations based on the local density approximation (LDA) and the generalised gradient approximation (GGA) of density functional theory have been performed for brucite, Mg(OH)2, diaspore, AlOOH and hypothetical hydrous wadsleyite, Mg7Si4O14(OH)2. The use of a general gradient approximation (GGA) is essential to obtain a good agreement (≈ 1%) of the calculated lattice parameters to diffraction data. The calculated fractional coordinates of brucite and diaspore are in good agreement (≈ 1.5%) with experimental data. The angle of the non-linear hydrogen bond in diaspore is reproduced well, and the calculated Raman active OH stretching frequency in brucite is in very good agreement with spectroscopic data. There are no significant differences between the calculated fractional coordinates and the second derivative of the energy when GGA is used instead of standard LDA. It is concluded that the description of the static and the dynamic behavior of the OH groups in these hydroxides is very good. It is therefore inferred that the parameter free model is predictive and it has been used to evaluate a hypothetical structure of hydrous wadsleyite. The model reproduces the unusual Si-O bond length of 1.7 Å, observed in β-Mg2SiO4. It predicts an O-H distance of 0.97 Å, which is significantly shorter than the distance obtained from earlier model calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00200324
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  • 2
    Electronic Resource
    Electronic Resource
    phonon dispersion and martensitic phase transition in ordered alloys (1999)
    Springer
    The European physical journal 11 (1999), S. 75-81 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 63.20.Dj Phonon states and bands, normal modes, and phonon dispersion - 81.30.Kf Martensitic transformations - 61.12.-q Neutron diffraction and scattering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: In a previous neutron scattering study, we had observed that the TA phonon softening in L12-ordered ferromagnetic Fe72Pt28 Invar is pronounced at the zone boundary M-point and leads to an antiferrodistortive phase transition at low temperatures. Here, we report on similar neutron scattering investigations on two ordered crystals with higher Fe content to investigate the relation between the TA phonon softening and the martensitic transformation, which occurs in Fe-rich ordered Fe-Pt. We find that the TA phonon softening, especially at the M-point zone boundary, does not depend on the composition of the investigated crystals. In Fe74.5Pt25.5, however, the antiferrodistortive phase transition temperature is enhanced due to tetragonal strain preceding the martensitic transition. In Fe77Pt23 a precursor driven premartensitic phase transition is not observed. The structure of the martensite is, however, influenced by the soft mode lattice instability of the austenite. This would explain the origin of structural details found previously for Fe3Pt thermoelastic martensite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050918
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  • 3
    Electronic Resource
    Electronic Resource
    New slow and short range magnetic correlations in superconducting (1998)
    Springer
    The European physical journal 3 (1998), S. 163-167 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 74.25.Ha Magnetic properties - 74.72.Dn La-based cuprates - 78.70.Nx Neutron inelastic scattering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Inelastic neutron scattering measurements have been performed on high quality single crystals of in order to study the spin dynamics of this compound. In addition to the well-established incommensurate magnetic response, we show the existence of a new set of low energy excitations present in the whole superconducting region of the phase diagram. This new feature of the dynamical cross section is characterized, below about 10 K, by very short range ( lattice spacing) antiferromagnetic correlations and by a low energy scale of meV. At higher temperatures these fluctuations become nearly Q-independent. Different possible origins of these new spin correlations are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050297
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  • 4
    Electronic Resource
    Electronic Resource
    Soft modes and phonon interactions in studied by neutron scattering (1998)
    Springer
    The European physical journal 5 (1998), S. 169-178 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 64.70.Kb Solid-solid transitions - 61.12.-q Neutron scattering - 63.20.Dj Phonon states and bands, normal modes, and phonon dispersion - 63.20.Kr Phonon-electron and phonon-phonon interactions - 61.44.Fw Incommensurate crystals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn2P2S6 is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a*-c* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (Px), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn2P2Se6, a strong coupling between the TO(Px) and TA(uxz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(Px) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(Px) dispersion in the a*-b* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(uyx) and LA(uxx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050431
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  • 5
    Electronic Resource
    Electronic Resource
    Anharmonic dynamical behaviour in bcc zirconium (1998)
    Springer
    The European physical journal 3 (1998), S. 447-454 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 63.20.Ry Anharmonic lattice modes - 63.20.Dj Phonon states and bands, normal modes, and phonon dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We present inelastic neutron scattering measurements of the low energy and strongly damped phonons in the high temperature bcc phase of zirconium. These phonons were investigated at different scattering vectors but equivalent phonon wave vectors in different Brillouin zones or along different but equivalent paths in the same Brillouin zone. Neither the observed differences in intensity nor in line shapes can be explained by the coherent one-phonon scattering law . This leads to an apparent violation of the fundamental symmetry of lattice dynamics. Taking into account the strong anharmonicity of these phonons, interferences between one- and multi-phonon scattering are held responsible for these effects. Measurements in different scattering planes reveal that due to the symmetry of the bcc lattice, these effects can only be observed in certain directions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050334
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