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A General Approach to First Order Phase Transitions and the Anomalous Behavior of Coexisting Phases in the Magnetic Case
Author(s) -
Gama Sergio,
de Campos Ariana,
Coelho Adelino A.,
Alves Cleber S.,
Ren Yang,
Garcia Flávio,
Brown Dennis E.,
da Silva Luzeli M.,
Carvalho A. Magnus G.,
Gandra Flávio C. G.,
dos Santos Adenilson O.,
Cardoso Lisandro P.,
von Ranke Pedro J.
Publication year - 2009
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.200801185
Subject(s) - phase transition , classification of discontinuities , condensed matter physics , critical point (mathematics) , materials science , thermodynamics , first order , transition point , magnetic field , statistical physics , work (physics) , plane (geometry) , physics , quantum mechanics , mathematics , mathematical analysis , geometry
Abstract First order phase transitions for materials with exotic properties are usually believed to happen at fixed values of the intensive parameters (such as pressure, temperature, etc.) characterizing their properties. It is also considered that the extensive properties of the phases (such as entropy, volume, etc.) have discontinuities at the transition point, but that for each phase the intensive parameters remain constant during the transition. These features are a hallmark for systems described by two thermodynamic degrees of freedom. In this work it is shown that first order phase transitions must be understood in the broader framework of thermodynamic systems described by three or more degrees of freedom. This means that the transitions occur along intervals of the intensive parameters, that the properties of the phases coexisting during the transition may show peculiar behaviors characteristic of each system, and that a generalized Clausius–Clapeyron equation must be obeyed. These features for the magnetic case are confirmed, and it is shown that experimental calorimetric data agree well with the magnetic Clausius–Clapeyron equation for MnAs. An estimate for the point in the temperature‐field plane where the first order magnetic transition turns to a second order one is obtained (the critical parameters) for MnAs and Gd 5 Ge 2 Si 2 compounds. Anomalous behavior of the volumes of the coexisting phases during the magnetic first order transition is measured, and it is shown that the anomalies for the individual phases are hidden in the behavior of the global properties as the volume.