Fluorite impurity

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

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). 

Cubic fluorite-structure (Fm3m) zirconia-based solid solution, (Zr,ACT,REE)02 x, exhibi ts significant compositional flexibility to incorporate high concentrations of Pu, neutron absorbers, and impurities contained in Pu-bearing wastes (Gong et al. 1999). The phase has excellent radiation stability. No amorphization was observed under ion irradiation at room temperature to a dose corresponding to 200 dpa, and at 20 K to a dose of 25 dpa. Irradiation with I+ and Sr+ up to 300 dpa produced defect clusters in Y-stabilized zirconia, but did not cause amorphization. Amorphization... 

Direct observation of point defects in metals has been possible by field ion microscopy. Impurity point defects may be usefully investigated by electron microscopy in combination with electron diffraction and electron spectroscopy. Direct observation of the dopant environment in fluorites has been attempted by Catlow et al. (1984) by employing EXAFS in conjunction with computer simulation. 

The mineral fluorite, CaF2, in Figure 13-5 has a cubic crystal structure and often cleaves to form nearly perfect octahedra (eight-sided solids with equilateral triangular faces). Depending on impurities, the mineral takes on a variety of colors and may fluoresce when irradiated with an ultraviolet lamp. 

In carrier flotation, small-sized (several pm diameter) particles become attached to the surfaces of larger particles (perhaps 50 pm diameter, the carrier particles) [630]. The carrier particles attach to the air bubbles and the combined aggregates of small desired particles, carrier particles, and air bubbles float to form the froth. An example is the use of limestone particles as carriers in the flotation removal of fine iron and titanium oxide mineral impurities from kaolinite clays [630]. The use of a fatty acid collector makes the impurity oxide particles hydrophobic these then aggregate on the carrier particles. In a sense, the opposite of carrier flotation is slime coating, in which the flotation of coarse particles is decreased or prevented by coating their surfaces with fine hydrophilic particles (slimes). An example is the slime coating of fine fluorite particles onto galena particles [630],... 

The triboluminescence of minerals has been studied visually (see the footnotes to Table I) but only a few minerals have been examined spectroscopically. There are a few clear examples of noncentric crystals, such as quartz, whose emission is lightning, sometimes with black body radiation. Most of the triboluminescent minerals appear to have activity and color which is dependent on impurities, as is the case for kunzite, fluorite, sphalerite and probably the alkali halides. Table I attempts to distinguish between fracto-luminescence and deformation luminescence, but the distinctions are not clear cut. A detailed analysis of the structural features of triboluminescent and nontriboluminescent minerals may make it possible to draw conclusions about the nature and concentration of trace impurities that are not obvious from the color or geological site of the crystals. Triboluminescence could be used as an additional method for characterizing minerals in the field, using only the standard rock hammer, with the sensitive human eye as a detector. 

For ceramic use, the most important mineral containing fluorine is fluorite (CaF2) which occurs in fluorspar. Natural deposits have a purity of 90-98% with silica as the principal impurity. Fluorite mineral powders have angular surfaces which result from cleavage and conchoidal fracture of the mineral [14]. Fluorspar is used in many forms of optical glass of low index of refraction and in enamels. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

Big off-centre motions 1 A) have been reported from EPR measurements performed on IT (Ni+, Cu + or Ag +) [47, 65-67,120,121] and (Cr +j [212-214] impurities in some fluorite type crystals (Table 2). Experimental results collected in Table 2 again indicated that off-centre instabilities are not due to size effects. In fact, Ni moves off-centre in the three Cap2, Srp2 and SrCl2 lattices [68, 120,121], while the smaller isoelectronic Cu ion remains on-centre in Cap2, the fluorine ligands suffering an orthorhombic T2g t2g + eg) IT distortion [47,48,68]. 

A relatively great number of papers have been reported for on Mn + and Fe " " centres d ions with ground state) on fluorite crystals. With the possible exception of Fe + in Bap2 [151], the two d impurities remain on-centre. Nevertheless, Roelfsema and den Hartog [215, 216] have investigated the effect of an applied... 

Table 2 Summary of experimental information and DFT calculations on the on-centre/off-centre character for several impurities in fluorite type lattices [19,41,48,122]. The ground state of cations in hexahedral geometry is shown. When available the experimental or DFT-calculated value of the equilibrium coordinate, Zq, is also given in parenthesis. Experimental values ofa/4 (a is the lattice parameter) are also given under each host lattice. All distances are given in pm units ...

Off-Centre Impurities in Fluorite-Type Crystals Microscopic Origin... 

Barium sulfate, BaS04, occurs in the mineral barite (a). Calcium fluoride, Cap2, occurs in the mineral fluorite (b). Both are clear, colorless crystals. Minerals are often discolored by impurities. 

Pure titania has the rutile structure and therefore has limited solubility in YSZ. The observed linear decrease in lattice parameter with increasing titania concentration in these solid solutions suggests that titanium cations enter the lattice substitutionally for zirconium. Concordant with the data from XRD measurements [29,30,123] the cubic fluorite structure is retained upon addition of 12-20 mol% titania, above which a second phase appears, claimed to be ZrTi04 [123]. The spread in data of the solubility limit produced by different authors may be due to slight differences in, e.g., yttria concentration, sample processing, sintering temperature and impurity content in the cited studies. Microstructural investigations based on SEM and TEM indicated that precipitates of the second phase actually may appear already at lower titania contents [123,125]. 

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