Monoclinic phase

Oxides such as CaO, MgO, and Y2O2 are added to Zr02 to stabili2e the tetragonal phase at temperatures below the tetragonal to monoclinic phase-transition temperature. Without stabili2et, the phase transition occurs spontaneously at temperatures below 850—1000°C, and no fracture toughness enhancement occurs (25). 

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,... 

W0x-Zr02 samples with surface densities above 5-6 W-atoms nm contain only the metastable tetragonal phase (Figure 4), but pure Zr02 contains only the thermodynamically stable monoclinic phase. This apparent saturation coverage is higher than... 

As an example of a structure derived from the union of different polyhedra, the structural motif resulting from the connection of two trigonal prisms (around a Co atom), and a distorted rectangular prism (around an A1 atom) has been identified by Richter et al. (2005) in the monoclinic phase Co4A17+vSi2 v this motif is shown in Fig.4.37. 

According to Cameron and Papike (1982), pyroxenes contain Cr " and TF" in rocks equilibrated at low fo (lunar specimens, meteorites). However, spectroscopic evidence is ambiguous and insufficient for a safe attribution (Rossman, 1982). Some authors (Bocchio et ah, 1979 Ghose et al., 1986 Griffin and Mot-tana, 1982) report the presence of Mn " in Ml sites in clinopyroxene. Davoli (1987) reexamined this hypothesis, proposing precise structural criteria to detect the presence of Mn " in the monoclinic phase (the ratio Mn /Mn may be a potential /02 barometer). 

Figure 10. Chain formation in the monoclinic phase of diphenylguanidine (left) and in the orthorhombic phase of diphenylguanidine (right). 

In the case of anthracene, the stable monoclinic phase transforms under stress to a triclinic phase in which molecules are favourably oriented for dimerization to occur. Although the triclinic phase has not been isolated as a pure phase, its structure has been established using low-temperature electron microscopy and atom-atom potential calculations (Jones Thomas, 1979). In l,8-dichloro-9-methyl anthracene, isolated dislocations with (201) [010] translation bring the molecules to the required geometry (Fig. 8.17) to facilitate photodimerization. 1, 5-dichloroanthracene is an interesting case. Instead of the expected 100% head-to-head dimers, photoreaction yields 80%... 

Fig. 17.2. Results of polarized Raman spectroscopic analyses on the surface of yttria-stabilized zirconia in the presence of an indentation print (a) optical micrograph of the indentation print, (b) map of transformed monoclinic fraction, (c) map of 6-axis orientation of the monoclinic phase, and (d) map of the angle of c-axis orientation of the residual tetragonal phase. In both (c) and (d), the represented angle gives the inclination of the respective axes with respect to the sampling plane...

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