Polarized Raman spectroscopy has been used to obtain the room-temperature phonon spectra of the series of rare earth orthovanadate single crystals: SmVO4, HoVO4, YbVO4, and LuVO4. The observed Raman frequencies follow the overall mode distribution expected for REVO4 compounds with the tetragonal zircon structure. The variation of the mode frequency with atomic number across the lanthanide orthovanadate series was investigated, and the trend exhibited by the internal modes was explained by considering the force constants of VO4 tetrahedron.

Raman scattering is used to investigate the room temperature phonons of tetragonal (I4∕m) Sr2CoWO6 and monoclinic (P21∕n) Ca2CoWO6, Sr2CoTeO6, and Ca2CoTeO6 double perovskites. Both tetragonal and monoclinic structures result from small distortions of the Fm3¯m cubic lattice. Therefore, phonons were assigned according to correlations with those observed in the prototype A2B′B″O6 Fm3¯m cubic structure and the internal vibrational modes of the oxygen octahedra. The observed shifts of some vibrations in the A2CoBO6 compounds upon changing either A or B elements are tentatively explained.

SrAlF 5 has been considered as one of the rare fluoride ferroelectric crystals, belonging to the polar I 4 group. However, recent x-ray diffraction data suggest that the correct structure is centrosymmetric (I 4 1 /a). Since ferroelectricity is forbidden in this structure, the existence of an inversion centre was investigated by vibrational spectroscopy. Thus, measurements on SrAlF 5 single crystals have been made by polarized Raman scattering and infrared reflectance spectroscopy. The results, discussed on the basis of the factor group analysis of the proposed structures and other members of the ABF 5 family, favour a centrosymmetric structure.

Dielectric constant and ionic conductivity measurements have been performed in SrAlF 5 single crystals, along the [100] and [001] directions, as functions of frequency (10 Hz-10 MHz) and temperature (300-800 K). The real part of the ac conductivity showed the 'universal dynamic response of dielectrics', evidenced by a power-law dependency on the frequency. A first-order phase transition was observed around 715 K, with a considerable thermal hysteresis in the real part of the dielectric constant and in the electrical conductivity. The ionic conductivity varies by about four orders of magnitude in the temperature interval studied and shows various regions with Arrhenius behaviour with activation energies ranging from 0.2 to 1.40 eV.

The aim of this study was to test the hypothesis that salinity can affect indirectly the nitrate acquisition by a negative modulation triggered by glutamine accumulation. Cowpea plants were exposed to a mild NaCl concentration (50 mM) in order to restrict growth and N-demand. After 21 d, pretreated plants and control plants were supplied with 0, 5 and 10 mM of Ca(NO 3 ) 2 for 3 d in absence of NaCl. Salt pretreated plants showed a great limitation in acquisition of NO 3 ⎯, indicated by decline in the nitrate uptake rate, NO 3 ⎯ accumulation, nitrate reductase activity and protein content. The restriction of NO 3 ⎯ utilization was positively associated with increased glutamine synthetase activity and glutamine accumulation, especially in roots.