Thermodynamics of the superconducting phase transformation in high T c ceramics with magnetoelectric effects
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A second order phase transformation in a dielectric ceramic superconductor is shown to relate jumps in physical properties to the ratio of state variables, i.e., to the phase boundary, near the critical point. The major jumps in specific heat, compressibility, permittivity, permeability, and spinodal composition curvature, between the normal and superconducting phases, are found from the continuity of the entropy, volume, electromagnetic fields (with zero fields), and chemical equilibrium. The thermal expansion, pyroelectric effects, and the piezoelectromagnetic effects are important differences between ceramic and metallic superconductors. The most important conclusions from experimental measurements are that the lattice plays a minor role in the superconductor transformation while the magnetoelectric jump may be related to the jump in permeability, i.e., the Meissner effect, and the jump in permittivity. The oxygen miscibility gap which controls order/disorder transformations thermodynamically mandate that only metastable compositions are obtained in the metal oxide, so absolute stability of the system may never be achieved. An explicit criterion to suggest other superconductor systems is given from magnetoelectric materials, i.e., some of the pervoskites.
I. INTRODUCTION The discovery of high temperature ceramic superconductors12 has led to an intense search for mechanistic understanding of the superconductivity and for immediate optimization of material properties for device applications. The defect perovskites with high Tc behavior are very complex materials even without structural twins.3 The Y,Ba2Cu3O7_s superconductor is ferroelectric,4 antiferromagnetic,5'6 magnetoelectric, and on the oxygen composition axis has a miscibility gap.7"9 The material in the superconducting phase becomes diamagnetic due to the Meissner effect and unable to support an internal electric field. Physical properties change across phase boundaries. Thermodynamics provides some information on the changes in two phase systems. The Clausius-Clapeyron equation is the classic example for first order phase changes. Second order phase changes are generally considered to apply to type II superconductors and are independent of any mechanism which describes the superconductivity. Proposed mechanisms in high Tc materials must satisfy thermodynamic constraints. Thermodynamics also provides interrelationships between physical properties. The thermodynamics of phase transformations have been applied to metallic superconductors. Testardi10 assumed the form of the equation of state between the normal and superconducting phases using the free energy and a stress dependent transition temperature. The superconducting phase changes in V3Si and V3Ge do not seem to be in good agreement with the pressure dependence of the critical J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
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temperature, especially for V3Ge." The high Tc materials have already received preliminary checks for the pressure dependence o
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