Crystals body-centered tetragonal

Tin exists in two ahotropic 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 stmcture and may be formed when very high purity tin is exposed to temperatures well below zero. The ahotropic transformation is retarded if the tin contains smah 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 a-tin particles accelerates the transformation. 

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.

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).

Continuing with our survey of the seven crystal systems, we see that the tetragonal crystal system is similar to the cubic system in that all the interaxial angles are 90°. However, the cell height, characterized by the lattice parameter, c, is not equal to the base, which is square (a = b). There are two types of tetragonal space lattices simple tetragonal, with atoms only at the comers of the unit cell, and body-centered tetragonal, with an additional atom at the center of the unit cell. 

The compound Cs2 [Pt(CN)4 ] (FHF)0.38 forms body-centered tetragonal crystals that have a distinctive reddish-gold metallic luster. The cell constants deter-... 

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. 

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. 

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. 

Body-centered tetragonal crystals, sublimes at 120°. d 5,55. Very sensitive to moisture, discolors quickly in moist air. Violent reaction with water forming BiFa and ozone. Reids with liqnid petrolatum above 50. ... 

These facts suggested that the oxide MoO was essential in bringing about the ring-closing reaction, and the special features of its crystal lattice were considered. This is of the rutile type (body-centered tetragonal system) and is shown in Fig. 3, together with a plan of the 110-plane in Fig. 4. Here the shortest distance between molybdenum atoms is 2.79A., the next... 

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