Abstract
The Green's function method with the symmetry decoupling scheme has been used in the present paper to investigate the antiferroelectric phase transition in squaric acid (H2C4O4) crystal. It is shown that the phase transition is of first order. The transition temperature Tc and the order parameter Sc at transition point are obtained. The dispersion relation, internal energy and transition entropy are derived respectively. Theoretical results are in agreement with those of recent experiments.