Oxide pyrochlores — A review

The group theoretical methods by which the symmetries of normal modes in crystals may be determined are outlined, and a series of tables are presented to facilitate rapid determination of the selection rules for vibrational transitions. Emphasis is placed on the method of nuclear site group analysis in which the number of infrared and Raman active modes of each symmetry may be obtained without detailed analysis of the symmetry elements in the crystallographic unit cell or the construction of tables. By using the tables presented here for most cases identification of the crystallographic space group is sufficient information to allow determination of the vibrational mode selection rules by inspection. Several examples are included in which crystals are analyzed by each of the methods.

Normal mode determination in crystals

Surface, interfacial and molecular aspects of polymer bioadhesion on soft tissues

Monolithic TiO 2 gels can be reproducibly obtained when the hydrolysis of titanium alkoxides is performed in the presence of acetic acid. This carboxilic acid does not act only as an acid catalyst, but also as a ligand and changes the alkoxide precursor at a molecular level therefore modifying the whole hydrolysis condensation process. Infra-red experiments show that bidentate acetates replace OR groups and are directly bounded to the titanium. Both, chelating and bridging acetates, are observed, leading to Ti(OR) x (Ac) y . oligomers. Hydrolysis of this new molecular precursor removes first (OR) groups and bridging acetates. Chelating acetates are still observed in the gel. They can only be removed upon heating above 200 °C.

Hydrolysis of titanium alkoxides: modification of the molecular precursor by acetic acid

Bismuth compounds and preparations with biological or medicinal relevance.

Infrared absorption study of metal oxides in the low frequency region (700-240 cm−1)

With the recent growth in interest in the ir and Raman spectra of crystals, it has become very important to know which vibrational modes are optically active Hornig, Winston and Halford, and Bhagavantam and Venkatarayudu pioneered in developing methods for this However, heretofore the determination has been a laborious procedure fraught with difficulty and with many points of indecision Among the latter is the choice of the primitive cell and the correct site symmetry of each atom What is needed is a short, straightforward, foolproof method We propose here an alternate procedure for obtaining the activity of the vibrations from the correlation tables which comes close to meeting these goals The calculation is reduced to but a few minutes' work The method will be explained in detail by use of numerous examples

Infrared and Raman Selection Rules for Lattice Vibrations: The Correlation Method:

The infrared lattice spectra from 700 to 50 cm−1 of eight cubic rare earth oxides are reported. It is evident from these data that the crystal structure determines the frequency position, and clearly shows the effect of the lanthanide contraction in this series of compounds.

Infrared Lattice Spectra of Cubic Rare Earth Oxides in the Region 700 to 50 cm −1

The infrared lattice spectra of a large number of rare-earth garnets have been recorded. Of the seventeen predicted lattice modes, we observed 15 for the aluminum series, 12 for the gallium, and 10 for the iron. A set of three bands in the high-frequency region of each spectrum has been assigned to the motion of the oxygen anions bonded to the rare-earth cation. The lowest-frequency bands in each spectrum have been assigned to the translational motions of the rare-earth ion. All of the frequencies were assigned to external motions of the crystal lattice.

Infrared Lattice Spectra of Rare-Earth Aluminum, Gallium, and Iron Garnets

Infrared absorption spectra are shown for monoclinic ZrO2 and for cubic stabilized ZrO2. Nine bands are reported in monoclinic ZrO2 in the region 800 to 200 cm−l, whereas only one broad band is observed in cubic ZrO2 over the same frequency range.

Neil T. McDevitt

Papers

Infrared absorption study of metal oxides in the low frequency region (700-240 cm−1)

Infrared and Raman Selection Rules for Lattice Vibrations: The Correlation Method:

Infrared Lattice Spectra of Cubic Rare Earth Oxides in the Region 700 to 50 cm −1

Infrared Lattice Spectra of Rare-Earth Aluminum, Gallium, and Iron Garnets

Infrared Absorption Spectroscopy in Zirconia Research