Abstract
The kinetics of monodomain order-disorder processes in monoclinic (I2/c) BaAl2Ge2O8-feldspar have been investigated by X-ray powder diffraction, Hard Mode IR Spectroscopy, and TEM darkfield imaging on quenched samples. Compared to the behaviour predicted by the TDGL approximation ordering kinetics observed at low temperatures slow down significantly when equilibrium is approached. Such a delay is not observed in disordering experiments starting from essentially ordered cation distributions. The deviation from TDGL behaviour is interpreted in terms of partial order parameter conservation in a non-uniformly ordered phase. Modifications to the uniform TDGL rate equation are tested against the available data. An activation energy of 352 ± 28 kJ/mol is obtained for Al,Ge ordering. The mixing coefficient ξc 2/ξ2, which describes the degree of order parameter conservation, is obtained as a function of temperature. While this coefficient vanishes in the vicinity of the transformation temperature T tr, it saturates towards a level of ξc 2/ξ2≈0.4 for T→0. ξ c 2/ξ2 determines the kinetic stability of ordered clusters quenched from T≈T tr.
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Received: 21 April 1999 / Revised, accepted: 19 July 1999
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Malcherek, T., Kroll, H. & Salje, E. Al,Ge cation ordering in BaAl2Ge2O8-feldspar: monodomain ordering kinetics. Phys Chem Min 27, 203–212 (2000). https://doi.org/10.1007/s002690050008
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DOI: https://doi.org/10.1007/s002690050008