Abstract
Heat capacity of (CH3NH3)2FeCl4 (MAFeC) and (C2H5NH3)2FeCl4 (EAFeC) at constant pressure have been measured by an AC calorimetry. Three phase transitions were detected on MAFeC and four phase transitions on EAFeC. The structural phase transitions of both compounds at highest temperature region are described by the renormalization group method for 3-d XY model (d=3, n=2). In the II–III phase transition of EAFeC, the logarithmic divergence due to the dipole-dipole interaction is found. The crossover scheme of the 2-d system from XY to Ising was proposed for the magnetic phase transitions of both compounds at the lowest temperature region.
