Se monoclinic

Monoclinic selenium is metastable and its enthalpy of formation and entropy are needed as auxiliary data for some evaluations. They were derived from a thermodynamic cycle involving the enthalpy and entropy of fusion at the melting point 413 K, the selected data for trigonal and liquid selenium, and the heat capacity of monoclinic selenium. The selected enthalpy of fusion is that in [81GAU/SHU]  

The Gibbs energy of formation is calculated from the selected enthalpy of formation and entropy to be  

The major contribution to the error limits is the uncertainty in the molar mass of Se(g) due to the natural variation in the isotopic composition of selenium. The selected value of the heat capacity was calculated from the same data as the entropy yielding  

This value is combined with the selected enthalpy of formation of Sc2(g) to give the selected enthalpy of formation of Se(g)  

---

AlSCl has an orthorhombic structure, with the lattice constants a - 8.09, b = 10.52, c = 3.86 A, andZ = 4. It is probably isotypic with SbSCl and BiSCl, crystallizing in a layer type of lattice (157) (see Section XII,C,5). The selenide halides are monoclinic, with the probable space-group P2,/m. The lattice constants are given in Table XVII. The constancy of the b parameters for all three compounds suggests the general presence of an Al-Se chain extending in that direction (266). 

Figure 34 Solid-state structure of grey Se. (see ref. 96) The structure ofpolymeric S (see refs. 100 and 101) and grey Te (see ref. 95) is similar (Se and Te are trigonal space group P3j21, that of S is monoclinic and slightly distorted). E-E distances 206.6pm (S), 237.5pm (Te). (a) A view that shows the orientation of the chains with respect to the np2-np2 lone pair repulsion. The E-E-E-E torsion angles are 85.3° (S), 100.6° (Se), 100.7° (Te) (b, c) views showing the helical arrangements of the chains. The E-E-E bond angles are 106.0° (S), 103.1° (Se) and 103.1° (Te)...

Cu zone is abundant (up to 40%) and dominantly monoclinic, with lesser hexagonal forms also present suggesting incomplete retrograde equilibration. Analyses of pyrrhotite by EPMA revealed elevated contents of Co and Se accounting for their enrichment in the basal sulfide facies. 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

We have discussed structures of O2, S6, and Ss- Now we consider Se, Te, and Po. Six crystalline forms of selenium have been reported ot-Se (stable under normal conditions), a-monoclinic, and (3-monoclinic Se, and three cubic forms deposited as thin films by vacuum evaporation. Metallic a-Se is trigonal, but also described as a distorted simple cubic structure, 3PO, similar to the structure of Te with more distortion for Se. There are infinitive helices parallel to the c axis. The space groups of a-Se are Dg, P3i21 and Dg, P3221 for the other enantiomorph (see Section 10.1.3). 

Both a- and (3-monoclinic Se are deep red and revert to a-Se on heating. These forms contain Seg "crown rings packed in a complicated arrangement. Cubic (3-Se was reported to be simple cubic, with one atom per unit cell. Cubic a-Se, obtained by heating cubic (3-Se, was reported to be face-centered cubic (ccp or 3P) with four atoms in the unit cell. A third cubic Se was reported to have the diamond structure. These cubic structures have not been confirmed. 

Red monoclinic selenium exists in three forms, each containing Ses rings with the crown conformation of Sg (Fig. 16.4.1). Vitreous black selenium, the ordinary commercial form of the element, comprises an extremely complex and irregular structure of large polymeric rings. 

The crown-shaped molecule Se8 has long been known to crystallize in two monoclinic lattices termed a- and jS-Se8, respectively (see Table II). Crystals of various sizes have been obtained from CS2 solutions prepared by dissolution of either red amorphous Se (22) or of vitreous selenium in CS2 and subsequent cooling or evaporation of the solvent (3a,b, 4a, 5). Red amorphous Se is readily soluble in CS2, but since both vitreous and red amorphous Se do not contain more than small amounts of Se8, the dissolution must be accompanied by a chemical interconversion. It has, however, been reported (26) that the dissolution of red amorphous Se in CS2 requires illumination with photons of energies in excess of 2.3 eV, ambient room light levels being sufficient. 

The spontaneous but slow crystallization of monoclonic selenium (Se8) from red amorphous Se, prepared by quenching of selenium vapor (1000°C) in liquid nitrogen, depends on the storage temperature of the samples. Below 303 K, red amorphous Se(vap) is completely transformed into the monoclinic phase, whereas above 303 K transformation into the metallic phase takes place. (The relative Se8 content of the material can be determined by DSC.) In contrast, chemically prepared red amorphous Se(redn) is transformed only into the metallic phase, even below 303 K (21). 

---