Crystal structure high temperature

Varghese, 0. K. Paulose, M. Gong, D. Grimes, C. A. Dickey, E. C. (2003) Crystallization and high temperature structural stability of titanium oxide nanotube arrays. J Materials Research 18 156-165... 

Phase transitions are not only characterized by atomic or molecular structural changes - they can also be characterized by significant modifications in the microstmcture and domains and at a much larger size scale. One notable example has been recently reported by Glazer et al. [110] using linear birefringence measurements in LiTaOs and LiTa cNbi c03 crystals at high temperature. 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

It is noteworthy that the corresponding silver compounds do not exist in this structure. AgLuS2 (468), which we expect to be non-metallic, crystallizes in a disordered NaCl structure (high-temperature modification ). The structure of AgYS2, on the other hand, is a monoclinic, strongly distorted but ordered version of the rocksalt type (468). 

This conclusion might be anticipated on the basis of the crystal structures A solid of orthorhombic structure (UFe) is expected to be more soluble in another solid of orthorhombic structure (low temperature form of MoFe) than in a solid of cubic structure (high temperature form of MoFe). 

The potential window necessary to induce nanotube formation was 10-25 V which could be maintained for several hours (Table 1). The as made nanotubes present an amorphous structure which crystallizes at high temperature (>280 °C) to give anatase and rutile. The mtile phase becomes dominating at temperatures higher than 620 °C. 

The most intense feature in the Raman microscopy spectrum of the ash specimen is at 475 cm Actinide oxides are known to crystallize at high temperature with a fluorite (CaFs) structure and space group Fm3m(Ol). This structure is predicted to possess a simple vibrational structure with one IR active phonon of T symmetry (302 cm for PUO2) and one Raman active phonon of T2g symmetry (478 cm for PUO2) at = 0 [88]. Therefore, the 475-cm Raman band can be assigned to the T2 mode of PUO2. 

The relation between n and the supermolecular structure that is formed is of interest. The low molecular weight polyethylene fractions that have n values of 4 form a uiuque type of superstructure. They can he represented by either rods or a rod-like assembly of the lamellar crystallites.(5) For molecular weights 7800 and 11 500, crystallized at high temperatures, 129 °C and 130°C, n is also equal to 4 and similar superstructures are observed. In this molecular weight range the high crystallization temperatures are borderline between the different type superstructures that are formed by hnear polyethylene.(5) As the crystallization temperature... 

Figure 3.4 Grain growth in a UO2 fuel rod during high-temperature service, showing the three zones of the original grain structure (I), the equi-axed central zone (II), and the mainly single crystal zone (III) surrounding the central void...

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