Monoclinic 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... 

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). 

An important oxide with the fluorite structure is Zr02. At room temperature, zirconia has a monoclinic structure in which zirconium is seven-coordinate. This transforms to a... 

The nonstoichiometric monohydrides formed by V, Nb and Ta have structures that are determined by T and the H content . Order-disorder transitions involving the H atoms lead to structural complexity in the ordered phases the bcc metal lattice distorts to tetragonal, orthorhombic or monoclinic. In addition, V and Nb, but not Ta, at >0.1 MPa Hj form dihydrides that have the fluorite structure. Hence, the phase diagrams of these elements with H are complex. 

XRD spectra of UO3, 0363 and H2-reduced UO3 are presented in Figure 1, H2-reduction of UO3 at 800 K for 10 hours resulted in transformation of the monoclinic phase of P UO3 into the orthorhombic fluorite structure of UO2, although some orthorhombic a U30g is also present. Similar results were observed from H2-reduction of U30g [7], with complete transformation of a U30g to UO2, however. 

Pure zirconia has a distorted fluorite (monoclinic) structure at room temperature, which transforms to a tetragonal structure at above 1200 C and finally to a cubic form at >2300°C. The cubic fluorite form has a crystal structure as shown in fig. la. The exact transformation temperature and behavior are probably very sensitive to any impurity present and also influenced by hysteresis. If the Zr is partially replaced by a divalent or trivalent cation with relatively large ionic radius, the fluorite structure can be stabilized at lower temperatures. This stabilized zirconia is often metastable at room temperature and does not decompose to the thermodynamically stable phases. 

Bi203 is monoclinic at room temperature, and exhibits electronic conductivity. On heating to 1050 K, Bi203 undergoes a phase change to a cubic fluorite structure (6-Bi203), which... 

The high temperature electrolytes are mostly oxides of composition MO based upon the fluorite, structure. The best investigated is "calcia stabilized zirconia (CSZ) which consists of a solid solution of 12-15% CaO in ZrO. The addition of calcia transforms ZrO from the monoclinic to tne cubic (fluorite) structure and also introduces anion vacancies for charge compensation. Conduction is by 0 ion diffusion through anion vacancies and ZrO -CaO has a resistivity of 30 ohm-cm at 950 C. Trivalent cations may also be used to stabilise ZrO with resistivities at 950 C of 12 ohm-cm for ZrO -Y 0 and - 6 ohm-cm for ZrO -Yb O or ZrO -Sc O (Figure l). Staoilized zirconia is of interest as an electrolyte for fuel cells, but no battery applications have been proposed and the temperature of conduction is too high to be of real interest. 

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