Search

The i.r. absorption and Raman scattering spectra of some aluminium and gallium garnets have been studied. They show a stronger force field in the aluminates than in the gallates and the influence of the ionic radius of the Ln 3+ cation on the frequencies of the aluminium—oxygen or gallium—oxygen network. The study of the system Er 3 Ga 5 O 12 -Er 3 Al 5 O 12 permitted us to propose an assignment of the different ranges of frequencies and to point out that the tetrahedra vibrations are higher than the octahedra ones. The i.r. spectrum of the perovskite NdGaO 3 is compared to the garnet Nd 3 GasO 12 one.

The i.r. and Raman spectra of antimonates M II Sb 2 O 6 (M II = Pb, Ca, Sr, Ba, Cd) belonging to the ‘PbSb 2 O 6 type’ structure are studied. A vibrational assignment is proposed and the normal coordinate calculations permitted us to calculate the force field and to characterize the chemical bonds in these compounds.

Unpolarized Raman spectra of the compounds M 3 UO 2 F 5 ( M = K, Rb, Cs, NH 4 ) were recorded in the region 1000-100 cm −1 at room and liquid nitrogen temperatures. Polarized Raman spectra of single crystals K 3 UO 2 F 5 , Rb 3 UO 3 F 5 were measured at room temperature and permitted a complete assignment of the A 1 ′, E 2 ′ and E 1 ′ fundamental modes of the UO 2 F 5 3− ion of the D,7, symmetry group. Factor group analysis was applied to the interpretation of the spectra. Site and correlation splitting due to the internal modes of the UO 2 F 5 3− ion are discussed.

A systematic study has been made of the influence of two factors on the synthesis of titanium dioxide gels: (i) the acidity of the reaction medium and (ii) the presence of a solvent. This study demonstrates that, in an aqueous acid medium, the development of textural properties is related to the concentration of free hydrogen ions. The presence in an acid reaction medium of a solvent, such as butanol or benzene, of low dielectric constant, inhibits the development of the structural properties by lowering the ionic dissociation of the acid. Our results provide an explanation for the apparently divergent measurements in the literature.

A comparison is made between the force fields calculated for the MIINbV2O6 and MIITaV2O6 columbite series and the MIITaV2O6 and MIISbV2O6 trirutile series. The trirutile structure is shown to be particularly more rigid than the columbite structure. For this reason, the trirutile structure exists only for divalent cations of small size. The stability of the two structures and their relative modifications are then compared. From a comparison of the vibrational spectra of the rutile FeSbO4 with those of the trirutile MSb2O6 compounds, a short-range order in the rutile structure is proposed.

The normal coordinate analysis of PbNb 2 O 6 and PbTa 2 O 6 compounds of “1 × 2 block” structural type has been carried out using a crown model unit corresponding to the pseudo-cell unit of the crystal structure. The results permitted the assignment of the observed i.r. and Raman frequencies and the force fields obtained have been compared to the “1 × 2 block” homologues, i.e. BaNb 2 O 6 , CaTa 2 O 6 , BaTa 2 O 6 (II), SrNb 2 O 6 whose force fields have been previously determined.

The vibrational assignment of compounds A2IIIB2IVO7 (A = La, Nd; B = Zr, Hf) belonging to the pyrochlore structure is proposed. A normal coordinate analysis permitted us to calculate their force field which is discussed.

The Raman spectra of partially deuterated ethylenediamine (5 and 95% on NH 2 groups) have been recorded between − 46 and − 196°C in the region of the NH and ND stretching vibrations. The vibrational frequencies of the isotopically diluted NH and ND bonds enabled us to demonstrate the importance of the hydrogen bonds in the cohesion of the crystals and during the allotropic transformation, and also to show the influence of the asymmetric effect on the form of the normal modes of vibration of the NH 2 groups.

Infrared (4000-100 cm −1 ) and Raman (1000-100 cm −1 ) spectra of the complexes M 4 (UO 2 ) 2 F 8 -2H 2 O (M = Rb, Cs) were reported. Factor group analysis was used to classify the normal vibration modes of the binuclear ion (UO 2 ) 2 F 4− 8 and to interpret the spectra. Polarized Raman spectra of Rb 4 (UO 2 ) 2 F 8 -2H 2 O made it possible to assign the Raman fundamental vibrations of the anion (UO 2 ) 2 F 4− 8 . The infrared fundamental vibrations of this anion was made by comparison with those of the mononuclear UO 2 F 3− 5 anion. The result of the normal coordinate analysis of the (UO 2 ) 2 F 4− 8 ion was used to support the preceding assignment and to analyze the effect of the association of a double fluorine bridge with two UO 2 F 5 groups on the spectra and on the G.V.F.F. constants, compared to those of the mononuclear ion UO 2 F 3− 5 .

The vibrational study of the three families of the niobates and tantalates of “1 × 2 block”-type structure shows that some vibrational frequencies are characteristic of this structure. The relation between the calculated force constants and the structural characteristics permitted us to distinguish the different types of Nb O and Ta O bonds from each other, and to establish a relation between the polarizability of the divalent cation and the mode of linkage of the double octahedra in the layer planes. Finally, the comparison of these results with those obtained for other niobates and tantalates of various structures permitted us to determine the relative stability of all these compounds.

Resume Infrared and Raman spectra of the niobates of columbite structure M Nb2O6 (M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd) were recorded in the region 1100-50 cm−1. All of them are similar at the shorter wavelengths (> 400 cm−1) but depend on the cation at lower frequencies. By comparison of i.r. and Raman spectra, the main vibration frequencies have been assigned.