Theory of coupled phase transitions: Phase separation and abnormal variation of order parameter

A simplified Ginzburg-Landau theory is developed to study coupling of a fir st-order gas-liquid phase transition (FOPT) with a second-order orientation al phase transition (SOPT). We show analytically that, due to the coupling between the two phase transitions, the SOPT may exhibit a FOPT-like phase s eparation in which a partially ordered liquid phase is separated from a dis ordered gaseous phase. This inhomogeneous phase separation leads to an abno rmal variation of the order parameter of the SOPT, including a distinct mod e of growth and an apparent mean-field critical exponent qualitatively diff erent from the normal one. (C) 2000 American Institute of Physics. [S0021-9 606(00)51734-6].