56SHORT COMMUNICATIONS 57-162 rnm. having been determinated by calibration with sodium chloride. The dimensions of the unit cell were found to be: ao=7-37±0.02, b0=4.60±0.02, c0=14-34±0.03/~; fl = 108 ° 59' +_ 8'.These data give the axial ratios: a: b : c = 1.602:1 : 3.118. The observed density of 1.42 g.cm.-a agrees well with the density of 1.417 g.cm. -3 calculated on the basis of two formula units per unit cell.When the diffraction spots in the Weissenberg and precession photographs were indexed the following interferences were observed: hkl in all orders; hO1 when l = 2n; 0/c0 when /c = 2n. These data are consistent with the space group C~t~-P21/c. Structural investigations on 5-oxiphenazine and 5-10-dioxiphenazine are in progress.
Graphs like those in Figs. 4 and 5 allow optimum dimensions to be chosen for the target, from the standpoint of the transmission factor in diffraction studies. They also allow the value of the transmission factor to be found for non-tabulated a and/~ values, by an interpolation method. Fig. 6 shows a plot of the transmission factor versus scattering angle for a=O.4 and for various values of the parameter ft. The smallness of its variation with the scattering angle makes it easy to find, by interpolation, the values of the transmission factor for other scattering angles. We wish to express our gratitude to G. Al~moreanu, D. B~descu, M. Suveie~, for assistance given in tabulating the transmission factor. An X-ray diffraction study has been made of the ordering of atoms in the following binary a phases : NbOs, NbIr, NbRe, MoOs, MoIr and CrRe. Ordering of atoms among the different atomic sites has been shown to exist in all cases. From these results and others it is deduced that the size of the constituent atoms is a major factor in governing the filling of A, B, and D sites but in addition some valency electron factor governs the filling of C and E sites.
A model is presented for the determination of the unit-cell size for ordered and disordered binary o" phases from a geometry assuming spherical atoms. The basis of the calculations is the assumption of an average radius of the atoms occupying each of the five crystallographic sites in the unit cell. The model is then used to predict the changes in lattice parameters produced by fast-neutron irradiation and by the addition of a third element in some ternary a phases. The calculated parameters are compared with experimental results.
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