Cubic transformation

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Vibrations of the crystal components may destabilise a lattice structure above a particular temperature, but this does not necessarily result in melting if an alternative phase is stable. Normally the form stable at the higher temperature, generated by a reversible recrystallisation, is less ordered. An example is the orthorhombic to cubic transformation of the alkali metal perchlorates [7] which is associated with increased rotational freedom of the perchlorate anion. Studies of phase transformations are of value in advancing understanding of solid state decompositions. 

Fig. 4 Schematic representation of lamellar— hexagonal phase transformation (a through d) and the hexagonal— cubic transformation (e and f). The shaded circles around the surfactant aggregates represent the inorganic species (generally metal alkoxides or other metal-oxo species).

Melting points and the tetragonal-cubic transformations of the rare earth dicarbides... 

Tetragonal to cubic transformation temperature of rare earth dicarbides (°C). 

The average atom model also affords an examination ofthe effect ofCa concentration on the structure of silicate perovskite. Given that CaSi03 is cubic, we expect (Mgi Ca )Si03 to become cubic for some value of x, even for a static lattice, and to lower the temperature of the predicted temperature induced orthorhombic to tetragonal/cubic transformation. Figure 11 shows the b/a and c/a lattice ratios as functions of. At the low concentrations that are compatible with our miscibility estimates (figure 10), Ca is unlikely to have a substantial effect on the structure of the static lattice. However, the effect of Ca on the critical temperature remains unknown. A full assessment of calcium s effect on the structure of silicate... 

ZnSnAs2, order-disorder transitions take place at a few degrees below the melting point, leading to a tetragonal-cubic transformation. 

The Cm203 (white) displays three crystal modifications, namely the A-, B- and C-type lanthanide structures as shown in table 23 (Eller and Pennemann 1986). A small uptake of oxygen can cause the oxide to acquire a tan to light brown appearance. The C-type (bcc) structure is the low-temperature form, which converts to the B-type (monoclinic) structure above 800°C, which in turn changes to the A-type (hexagonal) structure above 1600°C (Baybarz and Haire 1976). It is the C-type structure that is readily oxidized to higher oxides the monoclinic form is very resistent to oxidation and the monoclinic to cubic transformation via temperature treatment is very diflicult ( irreversible transformation). The B to A and the A to B transformations occur more readily with temperature. Self-irradiation (especially noticeable with the more readily available, shorter-lived Cm-244 isotope) converts the C-form of the sesquioxide to the A-form (Wallmann 1964, Noe et al. 1970). 

Occasionally, solid-solid transformations will be dependent on the specific composition. An example is the tetragonal to cubic transformation in solid solutions of perovskites where Sr or Pb have been substituted for Ba in BaTi03. The transition temperature measured by DTA or DSC wiU be indicative of the extent of substitution. 

Fig. 46. - Practical cases of the course of equilibrium background, Xa,. and actual kinetic degree of traiisformalion, A (dashed), for the variant processes taking place during the tetragonal-cubic transformation of manganese spinels of the composition MnxFe3-s04 for the various levels of manganese content, see the inserted values,. t. in the region from 2.3 to 3.0.

The temperature dependence of nano-sized Ce ri 02 was studied by in situ synchrotron X-ray powder dififractometry. Due to peak broadening for the nano-sized Ce Zr. Og and weak X-ray scattering by oxygen, the reported tetragonal-cubic transformation temperature might be incorrect. 

These are the fluctuations observed in a structure, when a phase transition is approached, and which can be considered as precursors of the transformation. In general their local symmetry is closely related to that of the stucture to come. For instance, in the case of the solid/Iquid transition for which this term was proposed first , they are local perturbations of the crystalline order when the transition to the liquid is approached. Similar phenomena have indeed been observed in the case of the lame liar/cubic transformation in Cj2E0g/H20. They manifest themselves, on the X-ray pattern of the... 

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