Closest packing in dense oxides: The structure of a polymorph of Co _{3}(AsO _{4}) _{2}
Abstract
Crystals of Co _{3}(AsO _{4}) _{2} were grown from the melt of a mixture of Co _{2}As _{2}O _{7} and As _{2}O _{5}. The crystals are isostructural with Mg _{3}(AsO _{4}) _{2} and are tetragonal with a = 6.858(2), c = 18.872(5) Å, Z = 6, and space group I overline42d . A total of 1048 independent reflections were measured by diffractometer and used in the fullmatrix refinement to a final R value of 0.069. The structure contains two distinct AsO _{4} groups. Two of the cobalt ions are octahedrally coordinated and a third occupies a overline4 site with four short and four long CoO distances. The crystal structure of Co _{3} (AsO _{4}) _{2} is not based on the continuous threedimensional closest packing of oxygen atoms. Nevertheless the number of oxygen atoms per cubic centimeter is 5.4 × 10 ^{22}, which falls in the range of values for hexagonal and cubic closest packed structures. A better measure of the degree to which closest packing is achieved by a structure is suggested. It is based on an analysis of the polyhedra of oxygen atoms which surround each of the oxygen atoms in a structure and their relation to the polyhedra in ideally closest packed structures. In order to facilitate the analysis, polytopes of 11 and 12vertex polyhedra were studied. A new decahexahedral 11vertex polyhedron was found.
 Publication:

Journal of Solid State Chemistry France
 Pub Date:
 March 1980
 DOI:
 10.1016/00224596(80)902650
 Bibcode:
 1980JSSCh..32...29G