Zachariasen

In addition to the Zachariasen and radius ratio rules, for oxides the electronegativity of the predominant cation should be between 1.7 and 2.1 (7). If the cation electronegativity is too high, the compound tends to form molecules or discrete polyatomic ions rather than a connected network. For example, CrO satisfies the radius ratio rule, but the highly electronegative Cr ions promote the formation of discrete dichromate(VI) ions, Cr202 , in the presence of other oxides. 

Figure 7.10. (a) Zachariasen s two-dimensional model of an AiOj glass, after Zachariasen (1932). (b) Two-dimensional representation of a sodium silicate glass,... 

Although a number of solid compounds of plutonium were synthesized by ultramicrochemical techniques during our first year and a half at the Metallurgical Laboratory, it was not until November, 1943 that a positive identification of a crystal structure was made. W. H. Zachariasen joined the project in the fall of 1943 and very soon began to make definitive identifi-... 

Data for which no reference is given are from the Slrukturbericht of P. P. Ewald and C. Hermann. 6 R. W. G. Wyckoff, Z. Krisl., 75,529 (1930). W. H. Zachariasen, ibid., 71, 501, 517 (1929). d The very small paramagnetic susceptibility of pyrite requires the presence of electron-pair bonds, eliminating an ionic structure Fe++S2. Angles are calculated for FeS2, for which the parameters have been most accurately determined. The parameter value (correct value = 0.371) and interatomic distances for molybdenite are incorrectly given in the Slrukturbericht. 

It was concluded by Zachariasen [Norsk geol. Tidsskrift, 8, 189 (1925) Z. physik. Chem., 119, 201 (1926)] from the intensities of reflection of x-rays that beryllium oxide does not contain Be++ and O" ions. However, it has since been shown by Claassen [ibid., 124, 139 (1926)] and Zachariasen himself [Z. Physik, 40, 637 (1926)] that if the electron distribution of the ions is taken into account, the x-ray measurements are compatible with an ionic structure. 

The tetragonal crystal WSi2 (Zachariasen, 1927) may serve as another example. Each silicon atom forms five bonds with surrounding silicon atoms at 2-633 A and five bonds with tungsten atoms at the same distance. Each tungsten atom forms ten bonds with silicon atoms at 2-633 A and four bonds with tungsten atoms at... 

Bixbyite, found only in Utah, about 35 miles southwest of Simpson, is described by Penfield and Foote2) as forming shiny black cubic crystals with a trace of octahedral cleavage. The composition assigned it by them was Fe++Mn+40A, with a little isomorphous replacement of Fe++ by Mg++ and Mn++ and of Mn+i by Ti+i. It was shown by Zachariasen that the X-ray data exclude this formulation, and indicate instead that the mineral is a solid solution of Mn20A and Fe20A. We shall reach a similar conclusion. 

This choice of space group is further substantiated by Zachariasen s data for the other substances as well as bixbyite. His reproduced Laue photographs of Tl .03 and of bixbyite show no spots due to 071, 017, 091, 019, 0.11.1 or 0.1.11, although planes of these forms were in positions favorable to reflection, while the powder data show that 031, 013, 073, and 037 gave no reflections for any of the sesquioxides studied. Zachariasen s rejection of ZJ arose from his... 

Dr. Zachariasen has kindly informed us that he now agrees with our choice of the space group T7. 

Zachariasen s arrangement of the metal atoms approximates the first of our two (that with the positive parameter value), and would be identical with it if his parameters were taken to be 0.030 and 0.530 rather than 0.021 and 0.5 2. 

If one-fourth of the fluorine ions are removed and the others are replaced by oxygen ions, calcium being replaced by (Mn, Fe), a structure is obtained which approximates that of bixbyite, which differs from it only in small displacements of the ions. This similarity is shown by the fact that the highly distorted octahedra have corners which are nearly at six of the eight corners of a cube, the six being chosen differently for the 8e and the 24 e octahedra, as is seen from Fig. 4 and 5. This analogy was, indeed, pointed out by Zachariasen for his incorrect structure. As a matter of fact the "ideal structure, with u = 0 and... 

As mentioned by Zachariasen, Goldschmidt1) found that the range of radius-ratio values leading to stability of the C-modification is about... 

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