Vladimir Ivanovski

Vibrational spectra of hexaaqua complexes.: IX. Reflection infrared spectra, optical constants and longitudinal optical phonon–transversal optical phonon splitting in some alums

Abstract The Fourier transform infrared (FT-IR) specular reflection spectra from 35 alums with a general formula M I M III (RO 4 ) 2 ·12H 2 O were measured. The application of the Kramers–Kronig transformation on the reflection spectra gives the optical constants n and k which are then used to calculate the absorption spectrum and the dielectric parameters ( e r ′ and e r ″). From the latter two parameters, the conductivity ( σ ) and resistivity ( ρ ) functions are further calculated, the maxima of which give the frequencies of the transversal optical (TO) and longitudinal optical (LO) phonons, respectively. The LO–TO splitting for various modes varies between 0 and 60 cm −1 , depending on the oscillator strength. Reliable spectra are obtained, for the first time, for some highly efflorescent alums such as KFe(SO 4 ) 2 ·12H 2 O, RbIn(SO 4 ) 2 ·12H 2 O and NaCr(SO 4 ) 2 ·12H 2 O.

A reflectance model function for the ν3(SO42−) modes in gypsum—comparison with the specular reflectance IR spectrum

Abstract The IR polarized reflection spectra of gypsum CaSO 4 ·2H 2 O were recorded at near normal incidence (≈10°) and at 16° incidence angle. n -polarized ν 3 (SO 4 2− ) modes of B u symmetry type were obtained upon reflection from the (010) crystal plane. Band triplet or singlet was observed in the B u spectrum, depending on the direction of polarization, instead of doublet or singlet, respectively, according to expectations based on group theoretical considerations. Explanations for the appearance of the triplet were offered in terms of dispersion of the permittivity function, when two modes have close frequencies and rather large LO–TO splitting, but do not interact (the oscillating dipoles of the modes are along different principal dielectric axes). A model reflection function was constructed that gave statistically satisfactory results.

On the origin of the splittings of the (SO42−) modes in the specular reflectance IR spectra of gypsum

Abstract The IR polarized spectra of gypsum CaSO 4 ·2H 2 O were recorded at incidence angles of approximately 10 and 16 degrees. Band singlet or doublet was observed for the higher frequency ν 3 (SO 4 2− ) mode of B u symmetry type, depending on polarization ( n or p ). A doublet was observed for the lower frequency ν 3 (SO 4 2− ) mode of B u symmetry type too, irrespectively of the type of polarization. In order to give an explanation for the doublets origin, a model permittivity function was constructed. Quite good agreement exists between the reflectance based on the model permittivity function and the experimentally measured one for the high-frequency doublet. The origin of the lower frequency doublet could not be explained in this way, but may be speculated to result from an Evans type interaction between a combination of a water libration and ν 2 (SO 4 2− ), with the lower frequency ν 3 (SO 4 2− ) mode.

Vibrational spectra of hexaaqua complexes. X. Raman and IR studies of the sulfate group disorder in α-alums

Abstract Earlier X-ray diffraction and Raman spectroscopic studies indicated that the sulfate groups in some α-alums are orientationally disordered. The current studies of the Raman and IR spectra of various alums give evidence that the disorder is indeed present in a number of sulfate α-alums. This is best manifested through the appearance of surplus bands in the spectral regions of the symmetric (Raman) and antisymmetric (IR) stretching vibrations of the sulfate anion. It was reconfirmed that the extent of disorder is cation-sensitive, the smaller the univalent cation, the larger the disorder. The spectroscopic results are in an excellent agreement with the crystallographic ones. A sulfate group disorder was detected in sulfate-doped selenate alums as well but no positive spectroscopic evidence was found for a selenate group disorder, again in agreement with the crystallographic results for pure selenate alums.

Dispersion analysis of arbitrarily cut orthorhombic crystals

We developed a measurement and evaluation scheme to perform dispersion analysis on arbitrarily cut orthorhombic crystals based on the schemes developed for triclinic and uniaxial crystals. As byproduct of dispersion analysis the orientations of the crystal axes are found. In contrast to the spectra of arbitrarily cut uniaxial crystals, where the fit routine has to separate two independent principal spectra, the spectra of arbitrarily cut orthorhombic crystals are a combination of three independent spectra and the evaluation scheme gets more complex. Dispersion analysis is exemplary performed on two different crystals, which show different spectral features and different levels of difficulties to evaluate. Neodymium gallate (NdGaO3) has broad overlapping reflections bands while topaz (Al2SiO4 [F, OH]2) has a quite high total number of infrared active bands.

Dispersion analysis with inverse dielectric function modelling.

We investigate how dispersion analysis can profit from the use of a Lorentz-type description of the inverse dielectric function. In particular at higher angles of incidence, reflectance spectra using p-polarized light are dominated by bands from modes that have their transition moments perpendicular to the surface. Accordingly, the spectra increasingly resemble inverse dielectric functions. A corresponding description can therefore eliminate the complex dependencies of the dispersion parameters, allow their determination and facilitate a more accurate description of the optical properties of single crystals.

IR-ATR investigation of surface anisotropy in silicate glasses

Several samples of flat soda-lime silicate glass were investigated by the Infrared Attenuated Total Reflection (IR-ATR) spectroscopy technique. The specular reflectance spectra together with the results of the performed dispersion analysis and the generated reflectance spectra, using Fresnel equations, suggest that the samples are isotropic. In contrast, spectra recorded by the ATR technique suggest an anisotropic structure on the surface of the specimen different from that in the bulk. This is established through differences in the s- and p-polarized IR-ATR spectra, which cannot be simply transformed into one another employing Fresnel formula for an isotropic case. It appears that this thin film having a structure different from the bulk is larger than the ATR effective penetration depth of the evanescent ray for each incidence angle above the critical one. The investigation suggests C2 symmetry of the SiO4 unit.

Generalized dispersion analysis of arbitrarily cut monoclinic crystals

Dispersion analysis is applicable to arbitrarily cut monoclinic crystals of unknown orientation in order to find the symmetry axis. By this it is possible to differentiate between the transition moments oriented parallel and normal to the b-axis and to determine the dielectric tensor functions of those two principal directions. Dispersion analysis of arbitrarily cut monoclinic crystals is based on an extension of the evaluation scheme developed for arbitrarily cut orthorhombic crystals. We present dispersion analysis of monoclinic crystals exemplarily on spodumene (LiAl(SiO3)2) and yttrium orthosilicate (Y2SiO5).

A direct method of quantification of maximal chemisorption of 3-aminopropylsilyl groups on silica gel using DRIFT spectroscopy

3-Aminopropylsilyl (APS) modified silica gel plays an important role as a precursor for further modifications, where APS acts as a spacer or bridging molecule. A monolayer of APS which is most suitable for this purpose was obtained in anhydrous conditions. The properties of the APS-modified silica gel depend on the amount of molecules chemisorbed on the surface. A direct quantitative method using Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy was proposed. The obtained results were further supported by elemental analysis. The conclusion was that the proposed methodology can be used for the quantification of APS groups chemisorbed on silica gel when the grafting chemical reaction was mainly irreversible.

Vibrational spectra and dispersion analysis of K2Ni(SeO4)2·6H2O Tutton salt single crystal doped with K2Ni(SO4)2·6H2O

Dispersion analysis of the polarized IR reflectance spectra of K2Ni(SeO4)2·6H2O doped with K2Ni(SO4)2·6H2O has been performed. Vibrational parameters like oscillator strength, attenuation constant and frequency of the transversal phonons for the modes of Au symmetry type plus the orientation of the transition dipole moments for the modes of Bu symmetry type in the ac crystal plane have been obtained. The spectra-structure correlation of the H2O stretching vibrations show that bands appearing in the spectra for polarization of the external radiation oriented along the b axis are mainly due to the H2O stretching vibrations of one of the three crystallographically distinct sets of water molecules. The orientation of the transition dipoles of stretching vibrations of the selenate ion differ from the characteristic spectra of the sulfate analog in that no mutually perpendicular transition dipoles are found in the ac crystal plane. Water librational bands masked with the bands of the ν4(SO4(2-)) mode in the sulfate analog have now been unveiled and assigned. The ratio between the oscillator strength and the attenuation constant appears to be a helpful tool in the assignment of the sulfate stretching vibrations and water librations. The vibrational and orientational characteristics of the ν4(SeO4(2-)) modes were obtained. Тhe ν3(SO4(2-)) frequency region of the isomorphously isolated SO4(2-) ion in the K2Ni(SeO4)2·6H2O matrix was investigated in some detail. Contrary to the expected three, four bands can be identified. Three of them were assigned to ν3(SO4(2-)) based on the orientation of the transition dipole moments. On the basis of the IR, but also Raman spectra of the pure and mixed crystals, a discussion of the influence of the potential field and the hydrogen bonds with the change in the volume of the unit cell is given.