Hexagonal-cubic transformation

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).

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... 

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. 

Silvery gray lustrous metal or bluish black amorphous powder close-packed hexagonal lattice transforms to a body-centered cubic structure at 865°C density 6.506 g/cm melts at about 1,852°C vaporizes at 4,377°C elec-... 

Thallium is a sUver-gray, soft, heavy, and ductile metal having three forms. The normal close-packed hexagonal lattice transforms to a body-centered cubic stmcture above 230 °C and a face-centered cubic form is stable at high pressmes. The triple point is at 110°C and 30 kbar. Thallium vapor is essentially monatomic, but on heating to 2000 °C, the vapor emits a visible band due to TI2. Some properties are listed in Table 1. ... 

With Th carbides, two carbohydride phases, ThjCHj and ThjCH, are obtained by heating the carbide in Hj gas at 0.1 MPa and 850°C. Pressure-composition isotherms show that these are definite phases, not solid solutions. The ThjCHj is hexagonal and the ThjCH monoclinic (probably distorted hexagonal). These are stable compounds extrapolation of the pressure-composition-temperature data to RT indicates dissociation pressures of ca. 10 and 10 Pa, respectively. However, the hexagonal ThjCHj transforms to a cubic phase at 380°C. 

It has been shown (Barrett 1956. Hull Rosenberg 1959) that sodium partially transforms at low temperatures from the normal body-centered cubic structure to close packed hexagonal. The transformation is of the martensitic type and is promoted by cold-working at the low temperatures. Ina.smuch as none of the calorimetric measurements on sodium were accompanied by crystallographic analysis, the tabulated data below 100 K are to some degree ambiguous. 

Hydration of CA leads to the formation of two hexagonal hydrates CAHj and C2AHg. CAHjo is formed at lower temperatures, not exceeding 20 °C the ratio of C2AHg increases with temperature. At temperature above 30 °C both hexagonal hydrates transform to the only stable cubic CjAHg phase. 

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). 

Rutkowski, B., Malzbender, J., Steinbrech, R.W., Beck, T., and Bouwmeester, H.J.M. (2011) Influence of thermal history on the cubic-to-hexagonal phase transformation and creep behaviour of... 

At 31OC, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865C it again transforms into a body-centered cubic structure. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235oG, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure. 

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

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