Fluorite Raman

A small but artistically interesting use of fluorspar is ia the productioa of vases, cups, and other ornamental objects popularly known as Blue John, after the Blue John Mine, Derbyshire, U.K. Optical quaUty fluorite, sometimes from natural crystals, but more often artificially grown, is important ia use as iafrared transmission wiadows and leases (70) and optical components of high energy laser systems (see Infrared and RAMAN spectroscopy Lasers) (71). 

Raman spectroscopy [24,25] Six Raman-active modes of Aig + 3 Eg + 2 Big symmetry are observed for tetragonal Z1O2 (space group P42/nmc), while for the cubic fluorite structure (space group Fm3m) only one p2g mode centred at around 490 cm is observed for c-Zr02 [22,24,25,32]. An example of the variation of the Raman patterns with composition is reported in Fig.6.4. 

We prepared by sol-gel some thermally stable Zro.io(Cei-xPrx)o.9o02 mixed oxides (x between 0 and 0.75) with a fluorite-type structure. This structure was confirmed by the presence, in the Raman spectrum, of a single band ca. 460 cm characteristic of the M-0 vibration in the fluorite-type structure. Moreover, the band position and the shoulder at 570 cm indicate the presence of oxygen vacancies probably associated with praseodymium cations. Consequently, high OSCs appears to be the result of the presence of both cerium and praseodymium atoms. Thus, addition of praseodymium atoms into zirconia-ceria oxides appears to be very promising for the design of new automotive catalysts. 

For the materials with fluorite structure, the only Raman active vibrational mode of T2g symmetry is observable. This mode arises from symmetric shift of the anions relatively to cations. Therefore the ions shifts owing to the thermal effects or impurities insertion influence Raman spectra. This is important for oxide materials and for Zr02iY203 in particular, where oxygen vacancies play an important role in ionic conductivity. 

As above mentioned, usually the points of phase transitions are indicated in situ by a change of the XRD pattern or of IR or Raman spectra. Usually, the high-pressure phases do not survive after the pressure is relaxed and revert to the initial phase, but occasionally are stabilized in an intermediate phase. Thus, SnOz at 25 GPa and 1,000 °C has the structure of fluorite, and in the unloaded state the a-PbOz structure [85] which is denser than the initial, mtile-type structure. Dioxides under high pressures undergo the following transformations of structural types ... 

The most intense feature in the Raman microscopy spectrum of the ash specimen is at 475 cm Actinide oxides are known to crystallize at high temperature with a fluorite (CaFs) structure and space group Fm3m(Ol). This structure is predicted to possess a simple vibrational structure with one IR active phonon of T symmetry (302 cm for PUO2) and one Raman active phonon of T2g symmetry (478 cm for PUO2) at = 0 [88]. Therefore, the 475-cm Raman band can be assigned to the T2 mode of PUO2. 

As has been described, the catalytic decomposition of NgO on Rh/ceria catalysts follows a redox mechanism where not only rhodium but also ceria is progressively reduced and oxidized by NgO. Temperature-programmed reduction by Hg (Hg-TPR) is a useful technique for correlating catalytic decomposition with the reducibility of the ceria material. The oxidation processes involved in NgO decomposition have also been studied by in situ Raman spectroscopy and by X-ray photoelectron spectroscopy (XPS) measurements performed after in situ treatments under different conditions. Figure 4.4 shows details of the band, attributed to the fluorite structure of ceria, measured by in situ Raman spectroscopy performed with Ar... 

---