Baddeleyite structure

Monoclinic zirconia (baddeleyite structure) stable below 1197°C, tetragonal zirconia (rutile structure) stable between 1197 and 2300°C, cubic zirconia (fluorine structure) stable above 2300°C or at lower temperature if stabilized by addition of magnesia, calcia or yttria. Maximum service temperature 2400°C. Zirconia starts to act as an oxygen anion conductor at 1200°C. Highly... 

Figure 4.9 A clinographic view of the baddeleyite structure showing the 7-coordinated oxygen polyhedra situated about each zirconium cation. The oxygen atoms are bonded to either 3 or 4 zirconium atoms. The ideal density is 5.83 g/cc. 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

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. 

Preliminary results for baddeleyite phase indicate that it is very close in energy to columbite but at P=0 its volume is 8% smaller than that of the columbite structure. The predicted structure for this monoclinic phase at 0 GPa is a=4.7901 A, b=4.9151 A,... 

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. 

Baddeleyite has a monocHnic structure with space group Plljc. The Zr + ion has seven-fold coordination, while the idealized ZrOz polyhedron is close to tetrahedral orientation, where one angle in the structure is different significantly from the tetrahedral value. Natural baddeleyite is a raw material for zirconium. In industry ZrOz, named usually zirconia, is important in areas such as surface chemistry, where its activity as a red ox material and its acid-based functions are important. As a ceramic material, zirconia can resist very high temperatures and its stabihzed form, yttrium-stabihzed zirconiiun, shows remarkable mechanical properties. 

Cubic stabilized zirconia (CSZ) Pure zirconia (Zr02) is either chemically extracted and purified from the mineral zircon (ZrSi04) or purified from baddeleyite. It occurs as three crystalline polymorphs with monoclinic, tetragonal and cubic structures. The monoclinic form is stable up to 1170°C... 

Structures Related to Both the Baddeleyite and Yttrium Trifluoride Structure Types 135... 

Finally, it may also be pointed out that the structure of baddeleyite itself may be described in a similar manner very shghtly distorted (100) unit layers of fluorite-type (ZrO layers of edge-shared OZt4 tetrahedra) alternate with commensurate 3 4 3 4 layers of anions. Small rotations of the triangles in the latter convert 3 4 3 4 to 3 (the anion-only layers in Y 0 iF 4.2) similarly, small rotations of the squares convert 3 4 3 4 to 4 (the anion-only layers in the fluorite type) . ... 

J.D. McCullough and K.N. Trueblood, The crystal structure of baddeleyite (monoclinic Zr02), Acta Crystallogr. 12, 507-511 (1959)... 

Zirconium oxide, or zirconia, occurs as the mineral baddeleyite, but zirconium oxide is obtained commercially mainly via its recovery from zircon. Zircon is treated with molten sodium hydroxide to dissolve the silica. Zirconia is used as a ceramic, but it must be doped with about 10 percent CaO or Y2O3 to stabilize it in its face-centered cubic form. Zirconia is monoclinic, meaning that it has one oblique intersection of crystallographic axes, but it undergoes a phase change at about 1,100°C (2,012°F), its crystal structure becoming tetragonal, and above 2,300°C (4,172°F) it becomes cubic. To... 

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