P. Tarte

Infra-red spectra of inorganic aluminates and characteristic vibrational frequencies of AlO4 tetrahedra and AlO6 octahedra

Abstract Some aspects of the interpretation of IR spectra of inorganic solids are discussed, with emphasis on the factors influencing the vibrational frequencies of cation-oxygen co-ordinated groups, namely the value of the co-ordination number, “isolated” or “condensed” state of the co-ordinated groups and vibrational interactions with neighbouring groups. These considerations are applied to the study of AlO stretching frequencies in a series of aluminates. Characteristic frequency ranges are as follows: “Condensed” AlO 4 tetrahedra 900-700 cm −1 , “Isolated” AlO 4 tetrahedra 800-650 cm −1 , “Condensed” AlO 6 octahedra 680-500 cm −1 , “Isolated” AlO 6 octahedra 530-400 cm −1 . Several cases of mixed vibrations (AlO + LiO) are found in the particular case of lithium aluminates from abnormal or erratic 6 Li 7 Li isotopic shifts.

Infrared studies of spinels—III: The normal II–III spinels

Abstract In accordance with theoretical expectations, 4 bands are generally observed in the i.r. spectrum of a number of cubic normal II–III spinels. The systematic investigation of pure compounds, solid solutions and isotopic species leads to a realistic and consistent set of assignments: the 2 high-frequency bands v 1 and v 2 essentially depend (in shape and position) on the chemical nature of the octahedral, trivalent cation and thus are essentially related to vibrations of the lattice of octahedral groups. The 2 low-frequency bands v 3 and v 4 must be assigned to complex vibrations involving the simultaneous participation of both cations, tetrahedral and octahedral. This complex origin is definitely demonstrated by the study of isotopic species. These assignments, which are in accordance with all the experimental data collected on normal II–III spinels, cannot be extended to other types of spinels (inverse II–III, normal II–IV,⋯)

Spectres d'absorption infrarouge de borates de terres rares

Abstract The infra-red spectra of either trivalent cations or rare-earth borates related to the calcite, the aragonite and the vaterite structures have been studied in the 1500-250 cm −1 region. The infra-red pattern given by the rare-earth borates of the YBO 3 type cannot be reconciled with a three-fold co-ordination, but instead points to a four-fold co-ordination of boron, a fact in contradiction with the alleged vaterite structure of these borates. The low-frequency spectrum is strongly structure-dependent, and is clearly influenced by the co-ordination number of the trivalent cation. The strong band observed in this region may be assigned to a cation—oxygen vibration, or possibly (for the calcite or aragonite-type borates) to a libration of the borate anion. Some significant discrepancies between our results and some data recently published in the literature are found to be related to spurious effects, such as the granulometry of the sample and possibly orientation effects.

Vibrational studies of olivine-type compounds—II Orthophosphates, -arsenates and -vanadates AIBIIXVO4

The i.r. and Raman spectra of ABIIXO4 phosphates, arsenates and vanadates of olivine structure have been investigated, and their interpretation is discussed on both theoretical and experimental bases. Assignments have been deduced from the systematic investigation of pure compounds, solid solutions and isotopic species. For all the compounds investigated, the stretching frequencies of the tetrahedral XO4 anion may be considered as internal modes, but the corresponding bending frequencies can be considered as “internal” only for the phosphates and for the sodium compounds. For the arsenates and vanadates where A is lithium, the 6Li7Li isotopic shifts reveal important interactions between the Li translations and the AsO4 or VO4 bending vibrations. It has also been possible to evidence a number of vibrational interactions in the case of the external vibrations. Finally, we have further investigated to what extent the internal stretching vibrations of the complex anion are modified by the chemical nature of the bivalent cation BII and an original correlation with the second ionization potential of the BII cation is proposed.

Infrared studies of spinels—II: The experimental bases for solving the assignment problem

Abstract The problem of interpreting the i.r. spectrum of spinels is discussed. Since the four i.r. active fundamentals belong to the same representation T1u vibrational interactions must be expected and any assignment of the observed frequencies to localized vibrations is necessarily an approximation. The validity of this approximation is discussed for different types (weak or strong) of vibrational interactions, and emphasis is put on the identification of the cations which are involved in either “localized” or “complex” vibrations. These assignment problems may be solved by appropriate experimental methods, namely the systematic investigation of the vibrational behaviour of: (i) homogeneous families of pure compounds; (ii) isotopic species, including isotopes of medium-weight cations; (iii) solid solutions.

Infrared studies of spinels—I: A critical discussion of the actual interpretations

Abstract As a general introduction to an extensive investigation of the infrared spectrum of spinels, the authors present a critical discussion of the data and interpretations actually published in the literature. It is shown that the generally accepted interpretation relies on an oversimplified vibrational scheme, and also on too fragmentary experimental data. Moreover, this interpretation may be questioned on both theoretical and experimental bases. It is thus suggested that the assignment problem is more complicated than previously expected and should be reinvestigated on a systematic experimental basis.

Infrared studies of spinels—IV: Normal spinels with a high-valency tetrahedral cation

Abstract The investigation of normal II-IV germanates, I-II-V vanadates and normal I-VI molybdate and tungstate spinels show that the high-frequency band of their i.r. spectrum must be assigned to a vibration of the tetrahedral group. A comparison is made with the assignment previously proposed for the normal II-III spinels, and it is suggested that the high-frequency absorption band of a spinel must be assigned to a vibration between the oxygen and the highest-valency cation, irrespective of the co-ordination of this cation. The medium- and low-frequency bands are often related to complex vibrations, as evidenced from the study of isotopic species. The Raman spectrum of the molybdates has also been investigated and the fundamentals have been assigned to the different symmetry species with the help of 92Mo-100Mo isotopic shifts. The i.r. spectrum of the I-II-V vanadates exhibits several abnormal features: the stretching vibration of the V04 tetrahedron is represented by a broad, complex absorption, and the low-frequency band is missing.

Etude infra-rouge des orthosilicates et des orthogermanates—II: Structures du type olivine et monticellite

Abstract The method of isomorphic substitution is applied to, and gives a fairly detailed assignment of, the infra-red spectra of silicates and germanates of the olivine type. Two regions are easily distinguished in the spectrum: the first (1000–1450 cm −1 ) is characterized by a fairly uniform pattern, highly specific of the SiO 4 (or GeO 4 ) tetrahedra involved in the olivine structure ; these bands are assigned to the v 1 , v 3 and v 4 modes of SiO 4 or GeO 4 tetrahedra. The second region (450–1280 cm −1 ) is more or less influenced by the nature of the cation X and may be assigned, in part to vibrations of XO 6 octahedra, in part to the v 2 mode of SiO 4 tetrahedra; this last mode has been identified by the study of a complex solid solution (Mg, Ni, Co, Mn) 2 SiO 4 . The deformation of the tetrahedral sites is proved by the splitting of the v 3 mode in dilute solid solutions of the type X 2 (Si, Ge)O 4 . A non-linear, but fairly regular relationship exists between the SiO 4 frequencies and the ionic radius of the cation X. The influence of isomorphic substitution on band contour is discussed for solid solutions (X, Y) 2 SiO 4 and X 2 (Si, Ge)O 4 .

Vibrational studies of molybdates, tungstates and related compounds—I: New infrared data and assignments for the scheelite-type compounds XIIMoO4 and XIIWO4

Abstract New experimental data lead to revised assignments for the i.r. frequencies of molybdates and tungstates of the scheelite family. The investigation of isotopic species reveals non-negligible interactions between the internal bending modes and the external translational modes, and allows an easy discrimination between translational and rotational modes. As a consequence, the low-frequency band of these compounds must be assigned to a rotation, and not to a translation as previously assumed.

Vibrational studies of molybdates, tungstates and related compounds—II: New Raman data and assignments for the scheelite-type compounds

Abstract New experimental data on pure compounds, isotopic species and solid solutions lead to more detailed and, in some cases, to revised assignments for the Raman spectrum of scheelite-type molybdates and tungstates. The 92Mo-100Mo isotopic shifts allow a more reliable assignment of the 2 types of internal bending modes, namely v4 and v2; it is found that v4 ⪢ v2; this is the reverse of the i.r. active modes, for which isotopic data show that v2 ⪢ v4. Thus, such assignments cannot be made by comparison. Likewise, the investigation of isotopic species and solid solutions leads to an easy discrimination between the rotational and the translational modes. Moreover, it appears that, for the lighter cations Ca and Sr at least, the 2 lowest-lying translational modes are essentially related to Mo-Mo (or W-W) motions, whereas the 2 highest-lying frequencies are related to a cation-cation (Ca-Ca or Sr-Sr) translation. This separation of the translations has only a practical significance, since it is not imposed by symmetry considerations; it is no longer really significant for the compounds of the heavy cations Ba and Pb. Some aspects of the relations between the internal frequencies and the nature of the cations are also discussed.