Raman spectra of anatase have been observed in natural and synthetic crystals. Both crystals show the same spectral features. The Raman band occurring at 516 cm−1 at room temperature is split into two peaks centred at 519 cm−1 and 513 cm−1 at low temperature (73 K). The six Raman active fundamentals predicted by group theory are all observed and assigned. The spectra are analyzed by a simple model considering only short-range forces and the calculated vibrational frequencies are in good agreement with the observed Raman frequencies.

Raman spectrum of anatase, TiO2

Electrical and optical spectroscopic studies of TiO2 anatase thin filmsdeposited by sputtering show that the metastable phase anatase differs in electronic properties from the well‐known, stable phase rutile. Resistivity and Hall‐effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self‐trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.

Electrical and optical properties of TiO2 anatase thin films

▪ Abstract The synthesis of the two currently used superhard materials, diamond and cubic boron nitride, is briefly described with indications of the factors influencing the quality of the crystals obtained. The physics of hardness is discussed and the importance of covalent bonding and fixed atomic positions in the crystal structure, which determine high hardness values, is outlined. The materials investigated to date are described and new potentially superhard materials are presented. No material that is thermodynamically stable under ambient conditions and composed of light (small) atoms will have a hardness greater than that of diamond. Materials with hardness values similar to that of cubic boron nitride (cBN) can be obtained. However, increasing the capabilities of the high-pressure devices could lead to the production of better quality cBN compacts without binders.

Synthesis and Design of Superhard Materials

Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder

Phase transformation of TiO2 from anatase to rutile is studied by UV Raman spectroscopy excited by 325 and 244 nm lasers, visible Raman spectroscopy excited by 532 nm laser, X-ray diffraction (XRD), and transmission electron microscopy (TEM). UV Raman spectroscopy is found to be more sensitive to the surface region of TiO2 than visible Raman spectroscopy and XRD because TiO2 strongly absorbs UV light. The anatase phase is detected by UV Raman spectroscopy for the sample calcined at higher temperatures than when it is detected by visible Raman spectroscopy and XRD. The inconsistency in the results from the above three techniques suggests that the anatase phase of TiO2 at the surface region can remain at relatively higher calcination temperatures than that in the bulk during the phase transformation. The TEM results show that small particles agglomerate into big particles when the TiO2 sample is calcined at elevated temperatures and the agglomeration of the TiO2 particles is along with the phase transformat...

UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk

which is obtained from potassium tetratitanate (K2Ti409), has been studied by examining the titration curve in relation to the structural change of the material. Structural characterization has also been performed for layered hydrous titanium dioxide and its potassium ion exchanged phases. The material has almost the same sheet structure of [Ti40$In as that of potassium tetratitanate, in which hydronium ions and hydroxylated protons are situated as exchangeable ones. The ion exchange proceeds stepwise through four stages (0-25, 25-50, 50-70, and 70-98% conversion), indicating that potassium ions replace the exchangeable protons one by one. During the process four solid phases appeared with different interlayer distances and cation/water contents: one-fourth-exchanged phase, &, SH1,5Ti409.0.6H20; half-exchanged phase, KHTi409.0.5H20; approximately three-fourths-exchanged phase, K1.4H,,6Ti409.1 .2H20 and fully exchanged phase, K2Ti409.2.2H20. The study of the unit cell dimensions of these phases indicates that the ion exchange takes place accompanied by a contraction/expansion of interlayer distances and a shift of adjacent sheets in relation to each other along the c axis. A lattice type change occurred during the process, which is considered to be due to the arrangement of potassium ions and/or water molecules incorporated in the interlayer space. A structural model is proposed for the fully exchanged phase in which interlayered potassium ions and water molecules occupy the sites arranged' in a double row and shifted above and below the levels at y = 0 and Models for the other ion-exchanged phases are also proposed to interpret the ion-exchange mechanism from a structural aspect.

Layered hydrous titanium dioxide: potassium ion exchange and structural characterization

Graphite BC[sub 2]N has been compressed with Co metal at a pressure of 5.5 GPa and temperatures of 1400-1600[degrees]C. The principal resulting products were crystals (average dimension 3 [mu]m) with cubiclike facets. The powder X-ray diffraction pattern revealed two kinds of cubic phase, in approximately equal amounts, which were identified as diamond and cBN on the basis of their lattice parameters. Microelemental analyses on individual crystal fragments by K-edge electron energy-loss spectroscopy confirmed this disproportionating crystallization scheme: half of the grains were composed of carbon-only signals of which gave fine-structure characteristic of sp[sup 3] bonding and the other half gave spectra characteristic of sp[sup 3] boron and nitrogen. The crystallization of cBN as well as diamond in the catalytic solvent of pure Co metal is observed here for the first time and is of relevance to the mechanism of the accepted catalytic action of cobalt on the hexagonal/cubic transformation. 26 refs., 4 figs., 2 tabs.

Simultaneous crystallization of diamond and cubic boron nitride from the graphite relative boron carbide nitride (BC2N) under high pressure/high temperature conditions

Protonated pentatitanate: preparation, characterizations and cation intercalation

ALTHOUGH recent work has indicated that more than one structural type of tin sulphide (SnS2) exists1, the magnitude of polytypism in this substance has not been studied until now We have recently grown crystals of tin sulphide and have studied Weissenberg X-ray diffraction photographs of ninety specimens Here we report the several polytypes which have been observed to date and indicate something about the frequency of the various structures

Yoshinori Fujiki

Papers

Raman spectrum of anatase, TiO2

Layered hydrous titanium dioxide: potassium ion exchange and structural characterization

Simultaneous crystallization of diamond and cubic boron nitride from the graphite relative boron carbide nitride (BC2N) under high pressure/high temperature conditions

Protonated pentatitanate: preparation, characterizations and cation intercalation

Structural polytypism of SnS 2