Abstract
We consider mixed crystals of the form(MY) x (MX) 1−x , whereY is an active component which drives a structural phase transition, while the componentX has no active internal degree of freedom. We describe this system by a compressible Ising model, including the dilution of the spinsY and the elastic strain fields caused by the componentX. We derive a Landau expansion of the free energy for this system, within molecular field theory. The coefficients of this expansion depend on temperature, pressure, spin concentrationx and the strain fields. This simple model exhibits a rich phase diagram. At sufficiently high concentrationsx, the phase transition is first order. Decreasingx, the transition passes through a tricritical point and eventually becomes second order. For lowx or high strain fields, no transition occurs.
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Theuns, T., Michel, K.H. Free energy and structural phase transitions in mixed crystals: A microscopic derivation. Z. Physik B - Condensed Matter 86, 125–131 (1992). https://doi.org/10.1007/BF01323556
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DOI: https://doi.org/10.1007/BF01323556