Body-centered tetragonal crystal structure

At atmospheric pressure, pure solid tin adopts two structures or allotropes, depending on temperature. At room temperature white metallic tin is stable but, at temperatures below 13°C, white tin undergoes a phase transformation into gray tin. White tin (also known as / -tin) adopts a body-centered tetragonal crystal structure (Fig. 8.5.1). Allotropic gray tin (a-tin) crystallizes in a cubic diamond... 

Figure 8.5.1 A body-centered tetragonal crystal structure adopted by white tin. crystal structure (Fig. 8.5.2).

Another common metal that experiences an al-lotropic change is tin. White (or p) tin, having a body-centered tetragonal crystal structure at room temperature, transforms, at 13.2°C (55.8°F), to gray... 

Martensite—this has platelike or needle-like grains of an iron-carbon solid solution that has a body-centered tetragonal crystal structure. Martensite is produced by rapidly quenching austenite to a sufficiently low temperature so as to prevent carbon diffusion and the formation of pearlite and/or bainite. 

FCC), and if the material is cooled slowly, the metal transforms back to body-centered cubic. If, however, the steel is rapidly cooled (quenched), the material transforms to the body-centered tetragonal crystal structure. This structure, termed martensite, is very strong and hard, and, at low temperature, it is metastable with respect to the equilibrium body-centered cubic structure. The transformation rate from the metastable body-centered tetragonal structure to the stable body-centered cubic crystal structure is very slow at room temperature and poses no problems in use. All of these structures exhibit metallic bonding with positive temperature coefficients of resistivity. 

FIG. 6 Schematic depiction of the body-centered tetragonal crystal structure. 

Of historic interest is the question of the existence of a high-temperature form of tin with a transformation temperature at 161 °C. This form of tin was said to have a rhombic crystal structure and called gamma or brittle tin. It has been demonstrated that this form of tin does not exist. It has been established that tin exists as the two polymorphic forms, gray tin (alpha) with a diamond cubic crystal structure up to 13.2°C, which transforms to white tin (beta) with a body-centered tetragonal crystal structure up to the melting temperature (232°C). 

URANIUM. [CAS 7440-61-1], Chemical element symbol. U, at. no. 92, at. wt, 238,03, periodic table group (Actinides), mp 1,131 to i. 33°C, bp 3,818°C, density 18.9 g/cm3 (20UC). Uranium metal is found in three allotropic forms (1) alpha phase, stable below 668°C, orthorhombic (2) beta phase, existing between 668 and 774°C. tetragonal and (3) gamma phase, above 774°C, body-centered cubic crystal structure. The gamma phase behaves most nearly that of a true metal. The alpha phase has several nonmetallic features in its crystallography. The beta phase is brittle. See also Chemical Elements. 

K+, Rb+ and Cs+ form salts with Cgg which contain monomeric fulleride ions at all temperatures. The structure of these salts is I4/mmm body centered tetragonal (bet) structure at all temperatures [4,59,60], except CS4C60 at room temperature and below, which is Immm orthorhombic (bco) [61]. According to our present knowledge, the fulleride ions in these phases are not rotating [4,60]. Thus the effect of the crystal field must be taken into account. 

Fig. 63. (a) The hexagonal AlBj type crystal structure of the RGaj compounds. Spheres with and without pattern show the Ga atoms and the R atoms, respectively (b) Brillouin zone of the body-centered tetragonal crystal lattice for da > -Jl. 

Tao R and Xiao D. 2002. Three-dimensional dielectric photonic crystals of body-centered-tetragonal lattice structure. Applied Physics Letters 80 4702-4704. 

Fig. 2. Structures for the solid (a) fee Cco, (b) fee MCco, (c) fee M2C60 (d) fee MsCeo, (e) hypothetical bee Ceo, (0 bet M4C60, and two structures for MeCeo (g) bee MeCeo for (M= K, Rb, Cs), and (h) fee MeCeo which is appropriate for M = Na, using the notation of Ref [42]. The notation fee, bee, and bet refer, respectively, to face centered cubic, body centered cubic, and body centered tetragonal structures. The large spheres denote Ceo molecules and the small spheres denote alkali metal ions. For fee M3C60, which has four Ceo molecules per cubic unit cell, the M atoms can either be on octahedral or tetrahedral symmetry sites. Undoped solid Ceo also exhibits the fee crystal structure, but in this case all tetrahedral and octahedral sites are unoccupied. For (g) bcc MeCeo all the M atoms are on distorted tetrahedral sites. For (f) bet M4Ceo, the dopant is also found on distorted tetrahedral sites. For (c) pertaining to small alkali metal ions such as Na, only the tetrahedral sites are occupied. For (h) we see that four Na ions can occupy an octahedral site of this fee lattice.

Tin exists in two allotropic forms white tin (p) and gray tin (a). White tin, the form which is most familiar, crystallizes in the body-centered tetragonal system. Gray tin has a diamond cubic structure and may be formed when very high purity tin is exposed to temperatures well below zero. The allotropic transformation is retarded if the tin contains small amounts of bismuth, antimony, or lead. The spontaneous appearance of gray tin is a rare occurrence because the initiation of transformation requires, in some cases, years of exposure at —40°C. Inoculation with Ot-tin particles accelerates the... 

X-Ray powder diffraction data of a large number of NF4+ salts are described by various workers. The data (Table I) indicate these salts have a tetragonal lattice. Single crystals of 98.9% pure (NF4)2NiF6 showed that the compound has a body-centered tetragonal cell, space group /4/m (37). The salt is made up of octahedral NiF62- ions and tetrahedral NF4+ cations and has the antifluorite structure. The interatomic N-F distance in the NF4+ tetrahedron is 130-140 pm and the F---F distance is 220 pm. 

Additional doping results in the formation of the body centered tetragonal (b.c.t.) structure of AjCeo and the b.c.c. phase A Ceo (Fig. 11) in which the distinction between the tetrahedral and octahedral sites disappears, and for each fullerene molecule there exists six tetrahedral positions of interstitials. According to [76] the change of crystal packing fixjm frc.c. to b.c.c. is not accompanied by a significant displacranent of Ceo molecules. For equal numbers of fullerene molecules, the volume of the b.c.c. phase is only 9% larger than the volume of the f.c.c. phase. 

---