Fluorite , structure

There are many compounds of various chemical classes that crystallize in this structural type fluorides of alkaline-earth elements, lead and cadmium high-temperature modifications of zirconium and hafnium oxides, solid solutions Mi xRxF2+x (M = Ca, Sr, Ba, Pb, Cd R = RE elements) [7,21, 31 and others] Bai ,.Bi ,.(0,F)2+6 [32], fluorite-Uke modifications of MOF (M = RE elements, Bi) and Mi xTex(0,F)2+ oxyfluoride phases [33,34], solid solutions in the BiOF-YOF system [24] etc. 

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Values of M depend upon the structure varying between 1-763 (CsCI) and 2-519 (fluorite structure). 

The structural relationships in Bi203 are more complex. At room temperature the stable fonn is monoclinic o -Bi203 which has a polymeric layer structure featuring distorted, 5-coordinate Bi in pseudo-octahedral iBiOs units. Above 717°C this transforms to the cubic -form which has a defect fluorite structure (Cap2, p. 118) with randomly distributed oxygen vacancies, i.e. [Bi203D]. The )3-form and several oxygen-rich forms (in which some of the vacant sites are filled... 

Dioxides are known for all the actinides as far as Cf. They have the fee fluorite structure (p. 118) in which each metal atom has CN = 8 the most common preparative method is ignition of the appropriate oxalate or hydroxide in air. Exceptions are Cm02 and Cf02, which require O2 rather than air, and Pa02 and UO2, which are obtained by reduction of higher oxides. 

The room temperature transformation of the columbite phase to baddeleyite commences at 13-17 GPa 6, with transition pressure increasing linearly with temperature Direct transition from rutile to baddeleyite phase at room temperature and 12 GPa has also been reported 7. The baddeleyite phase undergoes further transition to an as yet undefined high-symmetry structure at 70-80 GPa. The most likely candidate for the high-pressure phase is fluorite, which is consistent with the general pattern of increasing Ti coordination number from 6 in rutile, to 7 in baddeleyite (a distorted fluorite structure), and to 8 in fluorite. 

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. ... 

High pressure polymorphs are naturally characterized by wider bands with a smaller gap between the upper and lower VBs. The upper VB width in columbite, baddeleyite and fluorite structures is 5.37 eV, 6.22 eV and 7.44 eV, respectively, while the lower VB width is 2.32 eV, 3.30 eV and A.60 eV, respectively. This trend is due to the increasing overlap between the 2s-states of oxygen under compression. 

The compounds of the MMe205F type, where Me = Nb or Ta M = Rb, Cs, Tl, crystallize in cubic symmetry and correspond to a pyrochlore-type structure [235-237]. This structure can be obtained from a fluorite structure by replacing half of the calcium-containing cubic polyhedrons with oxyfluoride octahedrons. 

Other binary compounds include MAs3 (M = Rh, Ir), which has the skutterudite (CoAs3) structure [33] containing As4 rectangular units and octahedrally coordinated M. The corresponding antimonides are similar. M2P (M = Rh, Ir) has the anti-fluorite structure while MP3 has the CoAs3 structure. In another compound of this stoichiometry, IrSi3, 9-coordination exists for iridium. 

The Fluorite Structure.—In Table XI are given the observed interatomic distances in crystals with the fluorite structure. There is good... 

We have accordingly shown that for values of the ratio of the crystal radius of the cation to that of the anion greater than 0.65 the fluorite structure is stable for values less than 0.65 the rutile structure is stable. 

Goldschmidt predicted from his empirical rule that calcium chloride would not have the fluorite structure, and he states that on investigation he has actually found it not to crystallize in the cubic system. Our theoretical deduction of the transition radius ratio allows us to predict that of the halides of magnesium, calcium, strontium and barium only calcium fluoride, strontium fluoride and chloride, and barium fluoride, chloride,... 

Many complex ions, such as NH4+, N(CH3)4+, PtCle", Cr(H20)3+++, etc., are roughly spherical in shape, so that they may be treated as a first approximation as spherical. Crystal radii can then be derived for them from measured inter-atomic distances although, in general, on account of the lack of complete spherical symmetry radii obtained for a given ion from crystals with different structures may show some variation. Moreover, our treatment of the relative stabilities of different structures may also be applied to complex ion crystals thus the compounds K2SnCle, Ni(NH3)3Cl2 and [N(CH3)4]2PtCl3, for example, have the fluorite structure, with the monatomic ions replaced by complex ions and, as shown in Table XVII, their radius ratios fulfil the fluorite requirement. Doubtless in many cases, however, the crystal structure is determined by the shapes of the complex ions. 

The theoretical result is derived that ionic compounds MXS will crystallize with the fluorite structure if the radius ratio Rm/Rx is greater than 0.65, and with the rutile (or anatase) structure if it is less. This result is experimentally substantiated. 

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

This compound has the cubic fluorite structure with one octahedral interstice per Ce atom. Therefore, a =1, and S = 2 for CeH2. We can therefore write ... 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

Figure 1 Computer-generated schematic drawn to scale showing the fluorite structure of pure Ce02 (A), and the vacancies present in the doped Ce0 9W0 iOy reservoirs when fully charged (B) and after releasing 30% of its oxygen atoms (C).

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

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). 

FIGURE 7.5 The calcium fluoride structure (also known as the fluorite structure). 

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