BRILLOUIN-SCATTERING STUDIES ON POLYTYPES OF STEARIC ACID-B FORM

Almost all the elements of stiffness tensors of two polytypes (the single-layered Mon and the double-layered Orth II) of stearic acid B form were determined by Brillouin spectroscopy. The magnitude of stiffness elements differed more or less between two polytypes. In particular, the element c33, which expresses tensile modulus in the lamellar stacking direction of Mon was measured to be twice as great compared with that of Orth II. The stiffness tensors of Mon and Orth II polytypes were calculated by using the intra- and intermolecular forces and crystallographic data. The calculated values were in good agreement with the observed ones, and the large difference between the c33 values for the two polytypes was interpreted to arise from the difference in molecular displacement induced by the tensile stress.