Polarized rst-order Raman scattering studies at 300 K were carried out in L -asparagine monohydrate crystal in order to obtain the general assignment of its phonon spectra. A careful analysis of the vibrational spectra shows that the assignment of the fundamental vibrational modes can be done on the basis of amino, carboxilic and water groups vibrations, and correlations with previous data reported for other amino acids. However, some vibrations were correctly assigned from the study of its temperature -dependence behavior performed in the range 10 -300 K. The assignment proposed con rm the zwitterionic structure of L-asparagine monohydrate crystal.

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The pressure dependence of the room-temperature Raman spectrum of LiNbOS and LiTaO, in the range 1 bar-80 kbar is reported. The studies were confined to the transverse optical modes, since these are the modes which could be involved in the ferroelectric-paraelectric phase transition occurring in both materials at high temperatures. All mode frequencies increase linearly as the pressure increases and no distinctive behaviour is observed in the lowest frequency A, (TO) modes, identified by some workers as soft modes.The niobate and tantalate of lithium, LiX03, where X is Nb or Ta, are ferroelectrics with Curie temperatures of ca 1470 and ca 890 K, respectively (the properties of these and other ferroelectric materials are extensively reviewed in Ref. 3).1-3In the room-temperature ferroelectric phase they have a trigonal crystal structure belonging to the space group R3c (&), with two formula units per primitive cell. The high-temperature paraelectric phase has a structure belonging to a centrosymmetric space group, generally believed to be R f c ( D : d ) . On the basis of the temperature dependence of the Raman spectra of these materials, Johnston and Kaminow4 proposed that the ferroelectric-paraelectric phase transition is of the displacive type. They reported the existence of a soft mode, the lowest frequency TO mode of A , symmetry in the ferroelectric phase, the frequency of which tends to zero as the transition temperature, T,, is approached from below.This interpretation was brought into doubt by later results from Raman' and inelastic neutron scattering: which favour an order-disorder mechanism for the transition. In particular, Penna et aZ. ' and Chowdhury et aL6 found no evidence of softening (i.e. o+O as T + T,) in any of the zone-centre optical modes. Displacive transitions driven by a soft phonon are expressed by a quasi-harmonic ionic potential in which phonons are used as basis states and anharmonic phonon-phonon interaction renormalizes the self-energy of these p h o n o n~.~ Within this picture, the structural phase transition is caused by lattice anharmonicities. The type of order-disorder transition favoured by Chowdhury et aZ.6 and Penna et al? would be best treated with a pseudo-spin formalism. In this case there might still be a tunnelling mode which softens as the transition temperature is approached.Lines7 proposed a mechanism intermediate between both extremes outlined above, but which involves softmode behaviour in the lowest frequency Al mode (in the ferroelectric phase) far from T,. Close to T,, fourthand sixth-order anharmonicities would introduce side minima in the free energy, allowing for two alternative equilibrium positions.

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In this paper, we study the effect of Ni(II) doping on the structure of L-histidine hydrochloride monohydrate crystals using x-ray diffraction and Raman spectroscopy. X-ray powder diffraction shows no significant change in the unit cell parameters of the doped single crystal, whereas x-ray multiple diffraction using synchrotron radiation indicates that the Ni ions are located in interstitial positions in the crystal lattice. The temperature-dependent Raman spectra reveal a structural phase transition in the 10–300 K temperature range. The proposed mechanism of this phase transition supports the suggestion that the Ni ions occupy interstitial positions.