F. Romain

Double perovskites with oxygen structural vacancies: Raman spectra, conductivity and water uptake

Abstract The perovskite-related phases Sr 6 Ta 2 O 11 , Sr 5.92 Ta 2.08 O 11.12 (V o s ) 0.88 , Sr 5.46 Ta 2.54 O 11.81 (V o s ) 0.19 , Ba 6 Nb 2 O 11 (V o s ) 1 , Ba 4 Ca 2 Nb 2 O 11 , where V o s stands for ‘oxygen structural vacancies’, have been studied by thermogravimetry, AC-conductivity and IR and Raman spectroscopy. A correlation between water uptake and nominal V o s concentration was established, but the nature of the alkaline earth metal atom influences the oxygen sublattice: Ba 6 Nb 2 O 11 (V o s ) 1 has zero water uptake. The relationship between short-range disorder and water uptake is clear: only compounds having a strong oxygen and cation lattice disorder exhibit maximal water uptake: Sr 6 Ta 2 O 11 and Ba 4 Ca 2 Nb 2 O 11 . The protonic species change during cooling–heating.

Raman study of the high-temperature phase transition in CsH2PO4

Abstract The Raman spectra of the high temperature superionic phase of CsH 2 PO 4 were investigated in the 4000-20 cm −1 range. There was no evidence for an irreversible phase transition at 422 K and the only reversible high temperature transition was observed near 504 K. The high temperature phase corresponds well to CsH 2 PO 4 and not to CsH 2 P 2 O 7 and its Raman spectra are characteristic of a plastic phase, implying high dynamical disorder of H 2 PO − 4 ions but with a relatively ordered Cs + sublattice.

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

Abstract Infrared and Raman spectra on Na 3 H(SO 4 ) 2 , K 3 H(SO 4 ) 2 and (NH 4 ) 3 H(SO 4 ) 2 crystals have been investigated at 300 and 100 K in the 4000 to 30 cm −1 region. An assignment of bands in terms of OH group frequencies and more or less distorted tetrahedra of ammonium and sulphate ions is given. The crystallographic and spectroscopic symmetry and/or dissymetry of OH⋯O hydrogen bonds linking sulphate ions into dimers is discussed using OH group frequencies and the splitting of the v 1 (SO 4 ) Raman bands as criteria. In the particular case of (NH 4 ) 3 H(SO 4 ) 1 compound containing several solid phases it can be shown that the room temperature phase (II) is strongly disordered, principally because of an orientational disorder of ammonium ions, and that a progressive ordering takes place with temperature lowering.

Phase transitions and crystal dynamics in the cubic perovskite CH3NH3PbCl3

Abstract Raman spectra of polycrystalline samples of CH3NH3PbCl3 were studied at temperature ranging from 80 K to 300 K. An assignment of most of the observed bands is proposed. The first order phase transitions previously detected at 172 and 179 K were characterized: III←(172 K )→II←(179 K )→I (P222 1 ) (P4/mmm) (Pm3m) The spectral evolution and the mechanism proposed is in agreement with the theoretical analysis of normal vibrations and with previous X-ray diffraction studies and dielectric measurements. The I ↔ II transition involves mainly the conformational and orientational motions of the organic cation, whereas the antiferroelectric transition at 172 K is connected with a complex mechanism involving the freezing of the cation motions coupled with the distorsion of the PbCl6 octahedra. A systematic measurement of the CH3NH3+ torsional vibration (at 483 cm−1) was performed and the activation energy Ea = 10.1 kJ mol−1 was extracted from the half width vs temperature curve.

Etude par spectroscopie (IR et Raman) de l'éthylène diammonium bis dihydrogénomonophosphate NH3(CH2)2NH3(H2PO4)2

Abstract Infrared and Raman Spectra of single crystal and polycrystalline samples of NH 3 (CH 2 ) 2 NH 3 (H 2 PO 4 ) 2 and of its deuterated polycrystalline derivative have been recorded at 298 K. All the observed bands have been assigned using group theory. The IR and Raman spectra are consistent with the presence of strong OH⋯O hydrogen bonds.

Etude par spectroscopic Raman et infrarouge des verres de pseudo-spin Rb0,65(NH4)0,35H2PO4

Abstract Raman and infrared spectra of RbH2PO2 and NH4H2PO4 crystals and of the solid solution Rb0.65(NH4)0.35H2PO4 have been studied at room and liquid helium temperatures. The paraelectric→ ferroelectric transition of RbH2PO4 is characterized by the increased number of bands and to some extent by band narrowing. The paraelectric→antiferroelectric transition of NH4H2PO4 has a much more profound effect on the Raman and infrared spectra, mainly because of the dynamic orientational disorder of ammonium ions in the paraelectric phase. The solid solution Rb0.65(NH4)0.35H2PO4 spectra at 20 K containing a frustrated OH·O bonded system in the competing ferroelectric and antiferroelectric interactions show that the hydrogen bonds are not much modified with respect to those of pure crystals as far as OH-stretching frequencies (and thus distances) and band shapes are concerned. The bands associated with the vibrations of ammonium ions also remain broad at low temperature indicating a frozen-in NH4+ orientational disorder in the solid solution. This is probably the factor determining the overall characteristics of the spectra.

Raman scattering in tellurium-metal oxyde glasses

Abstract Structural studies were undertaken of glasses in the (1−x)TeO 2 − xZnO system. The glass domain (0,09 x > 0,40) was verified and variations observed in the Raman spectra were correlated with changes induced on the TeO 2 polyhedra upon glass formation with the addition of the metal oxyde. Spectra were recorded as these glasses were heated through T C and recrystallized. The spectral profiles observed in the recrystallized systems closely resemble those of the broad bands in the initial glasses. The sharp bands in the final spectra, characteristic of a more ordered phase, are interpreted (by correlation with neutron diffraction data) on the basis of chains of TeO 3 TeO 4 polyhedra interlaced with chains of ZnO 6 groups.

Crystal structure, DSC and Raman studies in CH3C6H4NH(CH3)2·SbCl4

Abstract The salt para methyl phenyl dimethyl ammonium tetrachloroantimonate (III) crystallizes in the monoclinic system with space group Cc. The unit cell dimensions are: a=13.780(1) A, b=14.943(2) A, c=8.192(1) A, β=113.39(1)°, with Z=4. The structure consists of ammonium cations and polynuclear anions in which distorted SbCl5 square pyramids sharing a common Cl atom are held together in infinite chains parallel to the c axis. These chains are themselves interconnected by means of the NH⋯Cl hydrogen bonds. Differential scanning calorimetry study was carried out. The Raman of polycrystalline samples have been recorded at different temperatures between 77 and 300 K. A low-temperature phase transition at 230 K of order-disorder type was found.

LO-TO splittings, effective charges and interactions in electro-optic meta-nitroaniline crystal as studied by polarized IR reflection and transmission spectra

Abstract The polarized IR reflection spectra of the meta-nitroaniline ( m -NA) single crystal along the a, b and c crystallographic axes as well as the b and c polarized transmission spectra have been measured in the 100–400 cm −1 region. The LO-TO splitting values have been calculated from the reflection spectra by fitting them with the four parameter dielectric function. The dipole moment derivatives, relevant to dynamic effective charges, of the vibrations have also been calculated and used to check the applicability of the oriented gas model (OGM) to reflection spectra. The discrepancies from the OGM have been discussed in terms of vibronic couplings, weak hydrogen bondings (HB) and intramolecular charge transfer.

Polymorphisme Cristallin des Sept Premiers Termes de la Serie des 4,4′-Di-n-alcoxyazoxybenzenes—I—Enthalpimetrie Differentielle—Spectroscopie Raman (Modes de Reseau)

Abstract A crystalline polymorphism has been detected in the seven first members of the homologous series 4,4′-di-n-alkoxyazoxybenzenes (n = 1 to 7) by D.S.C and Raman spectroscopy. In the case of the compounds n = 1,3,4,5, the crystalline polymorphism is characterized in the high temperature phase by a statistical disorder of the azoxy group, the molecules being pseudo-centrosymmetrical. Besides, for n = 3 and 5, con-trarily to n = 1 and 4, an important increase of the molecular volume due to conformational changes appears at the transition. As for the members n = 2, 6 and 7, the crystalline systems of the two solids are practically similar.