RAMAN-SPECTROSCOPIC INVESTIGATIONS OF THE STRUCTURE AND PHASE-TRANSITIONS OF LIQUID-CRYSTALLINE LEAD(II) ALKANOATES

Lead(II) alkanoates with even chain lengths from octanoate to octadeca noate have been investigated by Raman spectroscopy. In the low frequen cy region, transverse and longitudinal acoustical modes (TAM, LAM) hav e been assigned. It was shown that LAM-1 is the vibration of the doubl e chain with the node of the vibration in the Pb2+ layer. A fully exte nded conformation of the chains in the low temperature phase was confi rmed. The frequencies and intensities of the LAMs as compared with tho se of the alkanes and the fatty acids led to an estimate of the force constant and polarizability of the Pb2+-COO- bond relative to the C-C bond. The defects at the chain ends were investigated in the rho(CH3) and nu(CC) region. For the intermediate (CM) phase, both the Pb2+ laye r distance reduction and the chain length independent enthalpy contrib utions can be attributed mainly to defects at the chain ends.