ROLE OF POLAR PHONONS IN THE CHEMICAL BOUND AT STRUCTURAL PHASE TRANSITIONS CHARACTERIZED BY REPETITIVE FOURIER SPECTROSCOPY

Abstract.- The temperature dependence of dynamic effective charges carried by Ti and Nb ions which are the movable atoms involved in the ferroelectric phase transition of BaTi03 and KNbO, respectively, are reported, as deduced from TO-LO splittings of polar latgice modes, including soft modes. Restllts below the semiconducting-metal phase transition of N ~ O ~ are also discussed. There is an aspect of certain structural phase transitions with displacements of atoms, but which are not properly displacive, which has been little exploited up to now. When a structural transition occurs, certain cation-anion distances are sig-nificantly changed. Such changes of bond lengths affect not only the bond strength k and, therefore, the frequency of the oscillator we = (k/p)'/', but also the amount of the fractional charge which is supplied by the cation and localized on the anion. But things can be more complicated in compounds like oxides because oxygen becomes unstable, i.e. its polarisability increases rapidly with the increase of the radius of a spherical repartition of positive charges. ' The knowledge of instantaneous dipo-le momentscreated by the motion of positive against negative ions, by infrared reflec-tivity spectroscopy, gives information not only on polar optical modes and their splitting into TO and LO components, but also on the effective charge Ze. We will use-4