The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

K -edge absorption spectra of selected vanadium compounds

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

Excitations of the $2p$ subshell in the $3d$ transition metals and their oxides have been studied by inelastic scattering of 75-keV electrons. The ${L}_{23}$ "white lines" which arise from dipole transitions to unoccupied $d$ states have been investigated in terms of their threshold energies, widths, and intensity ratios. Shifts in the ${L}_{3}$ threshold energy between the metal and oxide are different from the chemical shifts measured by x-ray photoemission spectroscopy and this suggests the importance of relaxation effects. Single-particle calculations for the ${L}_{3}$ spectra are also discussed. Measured ${L}_{3}$ linewidths are generally larger than those predicted by suitably broadened theory. A variation from the statistical ${L}_{3}$-to-${L}_{2}$ white-line intensity ratio of 2:1 has been observed across the $3d$ transition row, with values ranging between 0.8:1 for Ti to 5:1 for FeO. This behavior appears to be associated with the white lines since Cu with a filled $3d$ band exhibits the statistical results. It is suggested that the anomalous ratios may be explained by a breakdown of $j\ensuremath{-}j$ coupling caused by an exchange mechanism. Finally, the extended x-ray absorption fine-structure-type structure extending several hundred eV above the white lines is analyzed for Cr to provide the radial distribution function.

Study of the L 23 edges in the 3 d transition metals and their oxides by electron-energy-loss spectroscopy with comparisons to theory

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

The titanium LII, III x‐ray emission and absorption spectra (λ∼27.5 A) from pure metal, oxides, nitride, carbide, and boride have been investigated using a plane‐crystal vacuum spectrometer with electron‐beam excitation and flow‐proportional counter detection. Emission spectra were studied over a wide range of accelerating voltages and takeoff angles, showing that satellite emission and self‐absorption effects can significantly distort the band shapes and energy positions of intensity maxima. A replica of the LII, III absorption spectrum can be constructed solely from emission spectra affiicted with widely different amounts of self‐absorption. The LII, III emission spectra from the oxides, nitride, and carbide exhibit an important crossover transition from the 2p level of the anion to the LII and LIII levels of titanium. Results indicate formation of a 3d band in titanium compounds which is only partially filled, giving rise to metallic conduction. X‐ray data is compared to density of states calculations ...

Band Structure and the Titanium LII, III X‐Ray Emission and Absorption Spectra from Pure Metal, Oxides, Nitride, Carbide, and Boride

Wavelength and intensities are reported for the Al K series using primary excitation. Included in the tabulation are the diagram lines α1α2 and β1, and the nondiagram lines α′, α3, α4, α5, α6, β′, β″, and β″′. Spectra are shown and line positions and intensities are detailed using both aluminum metal and aluminum oxide as the target materials. Significant differences are seen between metal and oxide spectra, especially in wavelength and shape of the Kβ1 band, and large changes are noted in the intensity of some satellite lines. The oxygen K band at 23.60 A from an anodized film is shown. The band is only slightly asymmetrical and no satellites are detected on the band contour. The application of x‐ray spectroscopy to the study of elements in anodic oxide pores is discussed with reference to quantitative changes and coordination determination.

Diagram and Nondiagram Lines in K Spectra of Aluminum and Oxygen from Metallic and Anodized Aluminum

The Mα and Mβ emission spectra and the MIV and MV absorption spectra have been studied for the rare earth elements. It is conclusively shown that the complicated multiplet structure observed in the emission spectra is not real emission structure but is, instead, produced by sample self‐absorption. This is demonstrated by observing the emission spectra over wide variations in take‐off angle and bombarding‐electron energies and finally by comparing the detailed structure of both the emission and absorption spectra. The MIV and MV absorption structure completely overlaps the Mα and Mβ emission lines which are each found to have but one intensity maximum when obtained under conditions of minimum self‐absorption. Some of these spectra have never been shown previously, while others have been studied in detail by several investigators. Points of agreement and disagreement with previous work are mentioned and the wavelengths of the emission lines and absorption edges are listed for all of the lanthanides. It is c...

Self‐Absorption Effects in the Soft X‐Ray Mα and Mβ Emission Spectra of the Rare Earth Elements

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.

William L. Baun

Papers

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

Band Structure and the Titanium LII, III X‐Ray Emission and Absorption Spectra from Pure Metal, Oxides, Nitride, Carbide, and Boride

Diagram and Nondiagram Lines in K Spectra of Aluminum and Oxygen from Metallic and Anodized Aluminum

Self‐Absorption Effects in the Soft X‐Ray Mα and Mβ Emission Spectra of the Rare Earth Elements

Infrared Absorption Spectroscopy in Zirconia Research