Fluorite study

J. S. Elsholtz described emission of bluish-white light when fluorspar was heated. Also described by J. G. Wallerius, 1750 the name fluorescence was coined in 1852 by G. G. Stokes 1768 First chemical study of fluorite undertaken by A. S. Marggraf... 

The fluorite phase is found to be extremely high in energy (it falls outside the energy range of Figure 1). Its equilibrium volume at P=0 would be 27.648 A /mol, and calculated equation of state gives B=287 GPa and B"=4.18. These values make fluorite structure the least compressible of all titanium dioxide polymorphs studied here, but still leaves the observation of a phase with B>500 GPa unexplained. ... 

The main problem related to the use of pseudopotentials in studies of solids under pressure is to make sure that the overlap of ionic cores does not increase significantly when interatomic distances decrease. The present study is certainly not affected by this potential pitfall since Ti-O distances typically change by no more than 0.1 A over the pressure range investigated. However, theoretical studies of fluorite and related phases at pressures of around 100 GPa should be performed with added caution. 

During the course of exploratory experimentation involved in the preparation of 8-242pU203, some limited oxygen potential measurements over Pu02-X fluorite phase were made at 1750 and 2050 K. The transpiration method was used for this study because, for a given temperature, the composition of the condensed phase can be fixed by appropriate choice of oxygen potential (via H2/... 

This theoretical result is completely substantiated by experiment. Goldschmidt,31 from a study of crystal structure data, observed that the radius ratio is large for fluorite type crystals, and small for those of the rutile type, and concluded as an empirical rule that this ratio is the determining factor in the choice between these structures. Using Wasastjerna s radii he decided on 0.67 as the transition ratio. He also stated that this can be explained as due to anion contact for a radius ratio smaller than about 0.74. With our radii we are able to show an even more satisfactory verification of the theoretical limit. In Table XVII are given values of the radius ratio for a large number of compounds. It is seen that the max-... 

Adularia is abundant in Au-Ag deposits, where it is commonly found with Au-Ag minerals only rarely does it occur in Pb-Zn and Cu deposits. Albite is very rare and is reported only from the Nebazawa Au-Ag deposits. Barite is a common gangue constituent in Pb-Zn-Mn deposits, especially those in the southwestern part of Hokkaido and the northern part of Honshu, where it is usually a late-stage mineral coexisting with carbonate and quartz but rarely with sulfide minerals. Other rare gangue minerals include fluorite, apatite, gypsum, bementite, rutile, and sphene, but they have not been studied. 

R. M. Mahbubar, Y. Michihiro, K. Nakamura, and T. Kanashiro, LDA Studies on Polarizabilities and Shielding Factors of Ions in Fluorite Structure Crystals, Sol. St. Ionics, 148,227 (2002). 

As a first case study, we borrow from the modeling work of Rowan (1991), who considered the origin of fluorite (CaF2) veins in the Albigeois district of the southwest Massif Central, France. Production and reserves for the district as a whole total about 7 million metric tons, making it comparable to the more famous deposits of southern Illinois and western Kentucky, USA. 

Amy Berger helped me write Chapter 10 (Surface Complexation), and Chapter 31 (Acid Drainage) is derived in part from her work. Edward Warren and Richard Worden of British Petroleum s Sunbury lab contributed data for calculating scaling in North Sea oil fields, Richard Wendlandt first modeled the effects of alkali floods on clastic reservoirs, and Kenneth Sorbie helped write Chapter 30 (Petroleum Reservoirs). I borrowed from Elisabeth Rowan s study of the genesis of fluorite ores at the Albigeois district, Wendy Harrison s study of the Gippsland basin, and a number of other published studies, as referenced in the text. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

This ore is heavily weathered ore, with more than 30% of the bastnaesite contained in the -7 (iim fraction. The major host minerals present in this ore are barite and fluorite. Table 24.6 shows the chemical analyses of the ore used in various research studies. 

W. Liu, and M. Flytzani-Stephanopoulos, Transition metal-promoted oxidation catalysis by fluorite oxides A study of CO oxidation over Cu—Ce02, Chem. Eng. J. 64, 283—294 (1996). 

The most well-studied and useful materials to date are those with fluorite-related structures, especially ones based on ZrOj, ThOj, CeOj and Bi203 (Steele, 1989). To achieve high oxide ion conductivity in ZrOj, CeOj and ThOj, aliovalent dopants are required that lead to creation of oxide vacancies. Fig. 2.2, scheme 4. The dopants are usually alkaline earth or trivalent rare earth oxides. 

XRD measurements showed that the compound has a cubic lattice with the lattice parameter a = 0.6443 nm and metal atom arrangement of the fluorite type (space group Fm3m). Theoretical studies of its X-ray absorption spectra were conducted soon after its discovery by Orgaz and Gupta [35]. 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

The fluorite in our study consisted of 40 samples from different environments. Concentrations of luminescence impurities in several samples are given in Table 4.6. By using laser-induced time-resolved spectroscopy we were able to detect and ascribe the following emission centers Eu +, Ce ", Gd +, Sm +, Dy3+, Eu +, Pr +, Er +, Tm +, Ho +, Nd +, Mn + and the M-center (Figs. 4.10-4.12). 

The structure of pyrochlore is considered to be an anion deficient derivative of the fluorite structure type. Ca atoms are in eight-fold coordination, while Nb atoms are in six-fold coordination. Steady-state luminescence spectra of pyrochlore revealed emission of REE, such as trivalent Dy and Nd (Gorobets and Rogojine 2001). The natural pyrochlore in our study consisted of four... 

The lines of Sm + connected with several types of centers are well studied in fluorite by steady-state luminescence spectroscopy (Tarashchan 1978 Krasilschikova et al. 1986). In time-resolved spectra it is mostly prominent after long delay times and is mainly characterized by the Hnes at 562,595 and 651 nm (Fig. 4.10d). 

Characteristic bands of Mn + well studied by steady-state luminescent spectroscopy (Tarashchan 1978 Gorobets and Rogojine 2001) have been found in time-resolved luminescence spectra of calcite (Fig. 4.14a), fluorite (Fig. 4.10d), datolite (Fig. 4.16d), wollastonite (two bands at 555 and 603 nm Fig. 4.42a,c), and spodumen (Fig. 4.61a). 

---