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  • 1
    Digitale Medien
    Digitale Medien
    Thermodynamics of sodium feldspar II: Experimental results and numerical calculations (1985)
    Springer
    Physics and chemistry of minerals 12 (1985), S. 99-107 
    Details
    ISSN: 1432-2021
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Geologie und Paläontologie , Physik
    Notizen: Abstract The free energy, entropy and enthalpy of sodium feldspar in thermal equilibrium and in metastable states are derived from investigations of the heat capacities of albite, analbite, ordered and disordered Or31. The lattice strains of all stable and metastable states are calculated from the two-order parameter theory published in the preceding paper. This approach also allows one to distinguish between the influence of Al, Si order and of the displacive lattice distortion on the thermodynamic behaviour of albite. The thermal cross-over between high albite and low albite is found to be a smooth function of temperature.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01046834
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  • 2
    Digitale Medien
    Digitale Medien
    Phase transition(s) in titanite CaTiSiO5: An infrared spectroscopic, dielectric response and heat capacity study (1995)
    Springer
    Physics and chemistry of minerals 22 (1995), S. 41-49 
    Details
    ISSN: 1432-2021
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Geologie und Paläontologie , Physik
    Notizen: Abstract The structural phase transition in titanite near 500 K (averaged symmetries A2/a→P2 1/a) and a second anomaly around 900 K have been studied using infrared spectroscopy on single crystals aqnd powder samples, measurements of the dielectric properties and the specific heat. The same synthetic single crystal was used in all experiments. The phase transition near 500 K is associated with a break in the temperature evolution of phonon frequencies and absorption intensities. Some phonon signals decrease rapidly under further heating and their extrapolated intensities disappear at ca. 850 K. The most dominant temperature effect relates to Ti-O phonons with amplitudes along the crystallographic a axis. These phonons show large LO-To splitting and continue to soften under heating even at temperatures above the transitions point (ca. 500 K). The softening of these modes correlates directly with the increase of the real part of the dielectric constant with a well-pronounced anomaly at 500 K. The dielectric losses also increse with increasing temperature. Measurements under strong field do not show antiferroelectricity. The transition at 500 K generates a small but sharp λ-anomaly in the excess specific heat. A second, weaker anomaly was found near 850 K. The results are discussed in terms of thermodynamic models.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00202679
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  • 3
    Digitale Medien
    Digitale Medien
    Domain wall formation in minerals: I. theory of twin boundary shapes in Na-feldspar (1985)
    Springer
    Physics and chemistry of minerals 12 (1985), S. 132-140 
    Details
    ISSN: 1432-2021
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Geologie und Paläontologie , Physik
    Notizen: Abstract Domains of triclinic Na-feldspar which are a direct consequence of structural strain are investigated theoretically. The following conclusions are arrived at. Only two types of twin walls are created by spontaneous strain, namely the Albite- and Pericline twin walls. If intersections occur, rounded corners are predicted. Any third domain wall close to such an intersection must bend into an S-shape. Narrow domains between two twin walls tend to become needle-shaped close to the intersection with a further, perpendicular, twin wall. The combination of the elastic stiffness coefficients c 44 c 66-c 46 2 is expected to become small at temperatures near to the transition temperature. In the same temperature interval elastic fluctuations occur; their amplitudes and propagation directions are given.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00308205
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  • 4
    Digitale Medien
    Digitale Medien
    Order-parameter saturation and low-temperature extension of Landau theory (1991)
    Springer
    The European physical journal 82 (1991), S. 399-404 
    Details
    ISSN: 1434-6036
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The temperature evolution of the structural order parameter in displacive phase transitions with high transition temperatures (e.g. quartz, As2O5, LaAlO3, CaCO3, NaNO3, Pb3(PO4)2 etc.) follows a Landau-type behaviour over large temperature intevals. Below a saturation temperatureT s , however, the order parameter tends to become temperature independent. A quantitative analysis shows the correlationk B T s =ħΩ/4, where Ω is the frequency of the relevant local excitation atT s . A general Landau-type expression for the Gibbs energy is given which includes the effects of order-parameter saturation.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01357186
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