The structure of the σ-phase Co₂Mo₃

Abstract

The a-phase structure of Co2Mo s has been confirmed by single-crystM measurements. The structure has been refined and the interatomic distances compared with those in CoTMo 6 and three other a-phases. The distribution of atoms in the available sites has been determined: cobalt occupies the 12-fold, molybdenum the 15-fold, and a mixture of Co and Mo the 14-fold co-ordinated positions.

The ordering of the σ phases Cr2Ru and Cr2Os

“…Two slightly different values occur, as expected, when the .74 parameter deviates from the ideal value of 0.25. The 12 sites form rows GIEI2G of a planar net where the distances between any pair of atoms, particularly 12--/2, are noticeably short compared to the sums of atomic radii of the 'compound' atoms at those sites; this has also been observed in o'-MoaCo 2 (Forsyth & Alte da Veiga, 1963). The abnormally short J-J distance, suggesting strong electronic bonding (Nevitt, 1962), has been associated with bond angles of 180 ° (Forsyth & Alte da Veiga, 1962); it is difficult, however, to explain this directional character of the bonding between identical atoms of a transition metal occupying equivalent positions in the structure.…”

“…4 indicate the sensitivity of the R factor to small variations of the individual site-occupancy parameters: as expected, for each atomic site, the shape of the corresponding curve depends on the number of equivalent positions in the unit cell. The pronounced U shape of the curve for the J site is particularly striking, reflecting a higher degree of sensitivity to small occupancy changes at these sites; this effect was noticed throughout the two-dimensional refinements of CrERU, Cr2Os and MoaCo 2 (Forsyth & Alte da Veiga, 1963).…”

“…A detailed analysis of Table 2 of their paper, however, shows that the model presented does not lead to conclusive results, as far as the prediction of ordering is concerned. This is particularly evident in the case of the a phase Mo3Co 2 which is known to be ordered as a result of precise measurements (Forsyth & Alte da Veiga, 1963); it happens that the difference in the atomic radii of molybdenum and cobalt is one of the largest among the known binary a phases; therefore, a significant difference should be expected between the values of a and c when these are calculated on the assumption of an ordered structure or of a disordered one. Contrarily, a has the same calculated value, 9.297A, when both an ordered and a disordered structure are assumed, whilst the corresponding values for c are 4.815 and 4.847 A, respectively.…”

“…The arrangement and positional parameters obtained from the study of the [100] projection, together with z 4 = 0.25 as for a-Mo3Co2 (Forsyth & Alte da Veiga, 1963), were postulated as a trial structure. The corresponding R factor was 0.11.…”

Atomic ordering in binary Al5-type phases

“…C.N.12 C.N.15 C.N.14 C.N.12 C.N.14 (atomic%B)* C.N.12 C.N.15 C.N.14 C.N.12 C.N.14 V-Ni Kasper & Waterstrat (1956), Wilson & Spooner (1963), Forsyth & d'Alte da Veiga (1963), Wilson (1963, Spooner & Wilson (1964), Algie & Hall (1966). extent on the relative positions of the interchanging elements in the periodic table.…”