Domaines d'existence, distorsions structurales et modes de vibration des ions conducteurs dans les reseaux hotes de type nasicon

Abstract

The NASICON structure domain of stability has been investigated in the ZrO 2-P 2O 5-M 2O-SiO 2 (M = Li, Na, K, Ag) system (Hf, Ti and partial Ta substitutions of Zr ions have also been studied) using X-Ray diffraction and vibrational spectroscopy. Metastability phenomena have been pointed out for some compositions. The different compounds have been synthetized in the form of powder using a sol-gel process and in the form of single crystals using various techniques. IR and Raman spectra are interpreted by using a neso-silicate (phosphate) structural description: a framework of XO 4 tetrahedra (X = P, Si) surrounded by octahedral cavities filled with M and M″ cations M″ = Sc, Ti, Zr, Hf). As different kinds of structural distortions are observed as a function of composition, distortions also occur against temperature. Local disorder and long range correlations are simultaneously observed : for some compositions e.g. Na 5Zr(PO 4) 3, a NASICON single crystal can be described as a “glass” with respect to the tetrahedra sublattice and as quasi a perfect crystal with respect to the cations one. Assignment of internal modes (tetrahedra modes), of M″ cations modes and of conducting cations (M) modes and of tetrahedra external modes is proposed on the basis of chemical substitution shift. Phase transitions are pointed out for the two kinds of sublattice (tetrahedra, cations). The influency of the “history” of samples is evidenced by DTA and vibrational spectroscopy. Correlations between local disorder, phase transitions and variations of conductivity activation energy are discussed.