We present a study of the thermodynamic behavior and the transitions between smectic phases of the PBnA (phenyl-4 benzylidene 4＇-alkylanyline) liquid crystal family, Four different compounds of the homologous series PBnA were synthesized. Temperatures of the transitions and changes in enthalpy between the phases present in the polymorphism of these compounds were determined by differential scanning calorimetry. X-ray diffraction patterns of the samples aligned by magnetic field were obtained for all regions of stability of the mesophases. Temperature dependence of the layer spacing in the tilted hexatic phase together with its discontinuity at the transition to the smectic A phase was derived from the X-ray experiment. It is shown that the slope of this dependence changed its sign with increase of the alkyl chain length. Asymmetry in the small angle Bragg reflections in the smectic phases was observed,