Transformation cubic-orthorhombic

The crystal structure change associated with the solid-state transformation of pure MoFe, i.e., body-centered cubic above the transformation to orthorhombic below it, is analogous to the cubic-to-orthorhombic transformation reported (20) for the 5d hexafluorides, WFe, ReFe, OsFe, IrFe, and PtFe. Another investigation has shown that the solid-state transformation in MoFe involves the crystal structure change from body-centered cubic to orthorhombic see Ref. 14). 

The matrices of symmetrical transformations for all the types (P, F, 1) of cubic lattices may also be obtained from the matrices of symmetrical transformations for orthorhombic lattices if one sets ni = ri2 = ns = n. 

Fig. 11.21 shows the results of TG and DTA measurements on mixtures of AP particles and catalysts. The endothermic peak observed at 513 K is caused by the crystal structure transformation of AP from orthorhombic to cubic. A two-stage exothermic decomposition occurs in the range 573-720 K. The decomposition of the AP is seen to be drastically accelerated by the addition of catocene. The exothermic peak accompanied by mass loss occurs before the AP crystal transformation. Although the AP is sensitized by the addition of carborane, no effect is seen on the AP decomposition. The results indicate that carborane acts as a fuel component in the gas phase but does not catalyze the decomposition of AP. Thus, the critical friction energy is lowered due to the increased reaction rate in the gas phase. The results imply that the initiation of ignition by friction is caused by the ignition of the gaseous products of the AP pyrolants.PI... 

Silvery-white metal exhibits three crystalline modifications an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm when heated at 280°C the beta form converts to a body-centered cubic crystaUine gamma modification at 577°C, having a density 18.0 g/cm . 

Above its Curie point (approximately 130 °C) the unit cell is cubic with the ions arranged as in Fig. 2.39. Below the Curie point the structure is slightly distorted to the tetragonal form with a dipole moment along the c direction. Other transformations occur at temperatures close to 0°C and — 80 °C below 0 °C the unit cell is orthorhombic with the polar axis parallel to a face diagonal and below —80 °C it is rhombohedral with the polar axis along a body diagonal. 

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. 

Orthorhombic black phosphorus, the most thermodynamically stable and least reactive form, is obtained by heating white phosphorus under pressure. It has a graphitic appearance and consists of double layers as shown in Fig. 10-2(a). When subjected to pressures above 12 kbar, the orthorhombic form transforms successively to the rhombohedral and cubic forms, Fig. 10-2(b) and (c). 

Deviations from the ideal structure with orthorhombic, rhombohedral, tetragonal, monoclinic, and triclinic symmetry are known, althongh the latter three are scarce and poorly characterized. The cubic cell of LaAlOs, LaNiOs, or LaCoOs often has a small deformation from rhombohedral symmetry. The distorted stracture usually exists at room temperature, but transforms to a cubic structure at high temperature. 

The effects of firing conditions on samples whose La Mn ratio is fixed at 1 is illustrated by the following data reported by Hauback and coworkers.If the sample is fired in Ar at 1000 °C, it has the composition LaMnOs.oo and a structure that is an orthorhombic distortion of the cubic perovskite. If the sample is heated in air at 750 °C, a sample with a rhombohedral perovskite structure results that becomes ferromagnetic at low temperatures before transforming at 9 K to the ORTH 1 variant of the GdFeOs structure. On the other hand if the firing temperature is lowered to 900 °C, 5 increases to 0.09 and a phase with a rhombohedral modification of perovskite is formed. If the La/Mn ratio is lowered to 0.88, firing at 750 °C results in a rhombohedral phase with S = -0.08 that... 

The Tb halides are similar to the sdver hahdes in that they are sensitive to light. The yellow compound TII has a curious orthorhombic layer structure, which is transformed to a red metastable cubic form (CsCl type) at 4.7kbar or 175 °C, becoming a metallic conductor at about 160kbar. If a small quantity of a Tb halide is added to an aqueous solution of an alkali halide, a blue Luminescence is emitted furthermore, TlCl doped KCl behaves as a thalhum alkali halide phosphor. In both cases, TlCb is believed to be the active species. ... 

---