Cerium under high pressure

CeCujSij as obtained from the spectra, exhibited in fig. 33 (cf. also fig. 16). The linewidths are pressure (volume) dependent as expected from the relationship of atomic volume and spectral density (section 7.3.). W4. exhibits a larger pressure derivative than IF3. Plotting W3, W4 vs the occupation number v of the tetravalent state, we find a correlation of the linewidth differences W4 — W3 and the fractional valence. The derivative 0(1 4— lF3)/0v = 8.6 1 eV. This number seems to be a universal number in Ce systems. It has been found in elemental cerium under high pressure and Ce (Pdi, tT,t)3 (T = Ag,Rh,Y) also. From the Lm spectra in 2 jZ mixed valent materials (Pr,Sm,Eu) one extracts 2.6eV (cf. e.g. Rohler et al. (1983a), Liibcke (1985)). At the time there is no sound interpretation of these empirical relationships between valence and Lm linewidths. 

The first breakthrough in the large-scale synthesis of ammonia resulted from the development of the Haber process in 1913 in which ammonia was produced by the direct combination of two elements, nitrogen and hydrogen, in the presence of a catalyst (iron oxide with small quantities of cerium and chromium) at a relatively high temperature (550°C) and under a pressure of about 2940 psi (20.3 MPa). 

The hydrothermal crystallization is the most popular technique in preparing ceria-based nanoparticles. Precipitation from aqueous solutions under elevated temperature and high pressure are involved in the process. Usually the hydrothermal crystallization is carried out as follows. An excess amount of precipitates is added to the cerium salt solutions. The precipitated gels are sealed in Teflon-lined autoclaves and hydrothermally treated at 423-573 K for several hours. The autoclaves are quenched and the crystalline powder products are washed and dried. Using the hydrothermal crystallization method, a number of ceria-based nanoparticles have been prepared as summarized in Table 3.2. The particle size clearly depends on the reaction temperature and the starting materials used. It is shown that by heating at low temperature and by using tetravalent cerium salt solutions smaller particles can be obtained. 

Several investigators have reported the co-existence of two of the cerium phases, such as y and a, or /3 and a, or a and o, under conditions of pressure and temperature where only one phase would be expected from the known phase relationships and hysteresis effects (see section 2.4). The lack of truly hydrostatic pressure in the high pressure X-ray device probably accounts for most of the problem, but slow reaction kinetics may also contribute. 

Recently, Jayaraman et al. (1976) have discovered a volume collapse in cerium phosphide under a pressure of 100 kbar resembling the yja transition of pure Ce. They suggest that CeP may therefore be a superconductor at high pressure. The arrow in fig. 10.21 shows the reduced Ce-Ce spacing at their maximum pressure of 200 kbar. It is still appreciably larger than the apparent critical distance of — 3.3 A for the appearance of superconductivity. In this light, it does not seem... 

Effect of pressure on phase relationships Jayaraman et al. (1966) in a study of the effects of high pressure and temperature on binary cerium-rare earth alloys, tested three cerium-yttrium alloys that, under normal conditions, had the hep the Sm-type and the dhcp structures. Pressure-induced phase transformations in the sequence hep - Sm type dhcp fee were observed in several other intra rare earth alloy systems tested but in the Ce-Y... 

Demetalation of ferralactone complexes with Cerium(IV) Ammonium Nitrate produces p-lactones (at low temperature) and/or 8-lactones (at high temperature, under a high pressure of carbon monoxide). 

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

High polishing speeds are essential in todays economy, and the latest equipment employs much higher spindle speeds and pressures than those used just a few years ago. Cerium oxide is ideal under these more modern conditions, A spherical lens that required 8 minutes to polish 15 years ago is now polished in less than one minute, A toric (cylinder) lens that previously took 15 minutes to polish, now requires 4-1/2 minutes. 

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