Transformation cubic-monoclinic

I. Cryolite—563.7 °C endotherm, transforms from monocline to cubic. 

Figure 7.44 Several potential ceramic failure mechanisms of Y-PSZ thermal barrier coatings. (A) Spalling due to stresses introduced by thermal cycling (B) Destabilization ofzirconia due to cubic-monoclinic phase transformation (C) Erosion and...

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... 

Colorless monoclinic or hexagonal crystals transforms to cubic form at 500°C refractive index 1.465 density 2.221 g/cm sublimes at 845°C soluble in water, solubility decreases with an increase in temperature (26.1 and 23.2 g at 0 and 100°C, respectively) insoluble in absolute ethanol and acetone. 

White, heavy, amorphous powder or monoclinic crystals refractive index 2.13 density 5.68 g/cm Mohs hardness 6.5 transforms to tetragonal structure above 1,100°C and cubic form above 1,900°C melts at 2,710°C and vaporizes at about 4,300°C insoluble in water soluble in hydrofluoric acid and hot sulfuric, nitric and hydrochloric acids. 

The linear log-log plots of reaction rate (in terms of oxygen consumption) versus time show for many alloys a discontinuity, or increase of reactivity. It appears that this transition is associated with the phase transformation in the protective film of Zr dioxide. The initial film formed on Zr is the cubic polymorph of Zr dioxide. After a period of oxidation this transforms to the tetragonal, and finally to the monoclinic (stable) form of Zr dioxide. When certain alloying constituents... 

Zirconia exists in three solid phases-cubic, tetragonal and monoclinic. It undergoes the following transformations... 

The transformations above room temperature (monoclinic-orthorhombic, orthorhombic-tetragonal I) are more easily understood. One notes the expected decrease in transition temperature with increasing sodium content, as is found in many alloy phase diagrams. Attempts to observe transitions to a cubic phase at higher temperatures failed, owing to sublimation of samples of very low sodium content (x < 0.02) or decomposition at higher sodium concentrations. 

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