The crystal structure of α-monoclinic selenium

Abstract

The CoAl distances determined by Mrs Douglas vary somewhat about the average value discussed above. The variations might be attributed either to differences in bond numbers or to differences in the amount of d character of the cobalt orbitals. Their distribution in the crystal (especially the 180 ° angle between the strongest bonds) indicates that the second rather than ¢he first effect is mainly involved. The smallest observed distance, 2.375 A., would result from a 2/3 bond with the cobalt bond orbital having 60% d character. The possibility of unequal distribution of d character among the bond orbitals permits a variety of behavior of the transition elements that is not shown by elements such as aluminum.Additional experimental evidence about the electronic structure proposed here for Co~Al9 could be obtained by study of the magnetic properties of the substances. The above theory requires that the magnetic moment of cobalt in Co~h_l 9 be greater than in the elementary substance, whereas the theory of Raynor & Waldron requires that it be less. The crystal structure of a-monoclinie selenium has been determined by the application of the Harker-Kasper phase inequalities and two-dimensional Fourier syntheses. The a-selenium molecule is an eight-membered, puckered ring similar to the rhombie sulfur molecule. It conforms, within the limit of experimental error, to the non-crystallographic point-group symmetry ~2m; the covalent bond distance is 2.34 _+ 0.02 A., and the covalent bond angle is 105.3 _+ 2-3 °. Certain abnormally short packing distances are observed which are discussed in terms of the metallic character believed to be present in the intermolecular bonding of hexagonal selenium.

Formation and phase transformation of selenium nanowire arrays in anodic porous alumina templates

“…Their diameters were about 50 nm, similar to that of the nanochannels. Se nanowires deposited at room temperature consisted of large grains, in which different phases (monoclinic a-Se [9] and b-Se [10]) and defects have been observed. However, Se nanowires deposited at 40 1C were structurally uniform, and showed better morphology.…”

“…Moreover, the high-resolution TEM (HRTEM) study (see Fig. 3 [9,10] and trigonal (0.237 nm) [12] Se. The volume occupied by single Se atom in the cF12 structure (0.0158 nm 3 ) is much smaller than that in the trigonal structure (0.0271 nm 3 ) [12], implying that this fcc phase should exist under a high-pressure synthesis condition.…”

“…The Se-Se bond length of the helical chains is 0.237 nm. Both a-Se (a ¼ 0:905 nm; b ¼ 0:907 nm; c ¼ 1:161 nm; b ¼ 90:51) [9] and b-Se (a ¼ 1:285 nm; b ¼ 0:807 nm; c ¼ 0:931 nm; b ¼ 93:81) [10] consist of 4 eight-membered rings in their unit cells (Se-Se bond length 0.232 nm). The various Se monoclinic structures are actually due to different modifications of the eight-membered ring structure.…”

Nachweis von Se6, Se7 und Se8 in Lösungen yon Selen mittels Hochdruckflüssigkeitschromatographie / Detection, of Se6, Se7 and Se8 in Selenium Solutions by High-Pressure Liquid Chromatography

“…Während vom elementaren Schwefel über 20 verschiedene Ringmoleküle Sn (n > 6) nachgewiesen [1] und 8 davon in reiner Form isoliert worden sind [2], kennt man beim elementaren Selen in reiner Form bisher nur den achtgliedrigen Ring, der in verschiedenen Gittern kristallisiert (a-Ses [3], ß-Se» [4], y-Se8 [5]). Die wahrscheinlich ringförmigen Moleküle Sen (n = 5-10) wurden bisher nur massenspektroskopisch im gesättigten und ungesättigten Selendampf bei Temperaturen von 160-500 °C nachgewiesen [6][7][8]; daß Selenatome aber auch viergliedrige Ringe bilden können, zeigt die Existenz des planaren Kations Se4 2+ [9].…”

Derivation of van der Waals radii from known crystal structures

“…This results from its existence in three crystalline forms whose structures have been determined. These are ~-monoclinic: Burbank (1951); fl-monoclinic: Marsh, Pauling & McCullough (1953);and hexagonal: Straumanis (1940). The thermal constants needed to establish approximate 0 °K structures are available for the ~ (Newton & Colby, 1951), and hexagonal (Straumanis, 1940) phases.…”