Tetragonal system

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. 

Zircon belongs to the tetragonal system and is a positive uniaxial. The typical form shows the ill and the 110 planes. The two orientations selected for luminescence polarization study were the (110) plane, parallel to the basal section and the [100] row. In such cases the axis perpendicular to the (110) plane will be called X. The orientation notation is made according to the so-called Porto notation (Porto et al. 1956). The Xi(ZX2)Xi orientation means that the laser light entered parallel to the Xi axis of the crystal and is polarized in the Z direction, while the emission is collected along the Xi axis with X2 polarization. By polarization spectroscopy with a high spectral resolution (less then 0.1 nm) six lines are observed for the Dq- Fi transition of the Eu-II center instead of the maximum three allowed for an unique site (Fig. 5.12). In Z(XX)Z geometry which corresponds to observation of a-polarized luminescence we... 

Orthorhombic crystals are similar to both tetragonal and cubic crystals because their coordinate axes are still orthogonal, but now all the lattice parameters are unequal. There are four types of orthorhombic space lattices simple orthorhombic, face-centered orthorhombic, body-centered orthorhombic, and a type we have not yet encountered, base-centered orthorhombic. The first three types are similar to those we have seen for the cubic and tetragonal systems. The base-centered orthorhombic space lattice has a lattice point (atom) at each comer, as well as a lattice point only on the top and bottom faces (called basal faces). All four orthorhombic space lattices are shown in Figure 1.20. 

The nitrate, [Ir(NH3)e](N03)3, crystallises in large quadratic plates belonging to the tetragonal system which are isomorphous with hex-ammino-eobaltic nitrate. It is soluble in water. 

Fig. 36. Tetragonal system. (See also Fig. 28.) a. Unit cell type. b. Phloroglucinol diethyl ether. Class 4jm. c. Wuifenite, PbMoQ4. Class 4. d. Anatase, TiOs. Class 4/mmm. e. Zircon, ZrSi04. Class 4/wmm.

Tellurium dioxide is known in two different crystalline forms. Crystals of the tetragonal system,2 but almost regular (a c — 1 1-1076), of density 5-66, are obtainable from the solution in nitric acid, while the molten dioxide when slowly cooled deposits rhombic needles 3 (a b c=0 4566 1 0-4693) of density 5-93 and identical with the rarely occurring mineral tellurite. 

Rutile and anatase crystallize in the tetragonal system, brookite in the rhombic system. The melting point of TiOz is ca. 1800 °C. Above 1000 °C, the oxygen partial... 

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