Quaternary Derivatives of the Structure Type BaHg11

One known and nine new quaternary aluminides in the Al-rich region of R–T–Ag–Al (R = Y, Ce, Pr, Nd, Sm, and Gd, T = V, Nb, Ta, Cr, and Mo) systems were synthesized by arc melting and their crystal structures were studied by X-ray powder diffraction. The crystal structures of the compounds R3T(Ag,Al)20Al12 (R = Ce, Pr, Nd, and Sm, T = V, Nb, Ta, Cr, and Mo) and R3TAgx(Ag,Al)20Al12 (R = Y and Gd, T = Ta) belong to new structure types: Ce3Ta (Ag,Al)20Al12 (Pearson symbol сP36, space group Pm-3m, a = 8.6675(2) Å) and Gd3TaAgx(Ag,Al)20Al12 (сP4912.86, Pm-3m, a = 8.6158(5) Å), respectively. Within the rows of isotypic compounds the unit-cell parameter increases with increasing metallic radius of the rare-earth or d-element atom. The structure type Ce3Ta (Ag,Al)20Al12 (Ce3TaAg7.97Al24.03) is a partly ordered quaternary substitution derivative of the structure type BaHg11, whereas Gd3TaAgx(Ag,Al)20Al12 (Gd3TaAg8.89Al23.25) is a distorted fill-in quaternary substitution variant of the same prototype. The latter can also be described as a derivative of the structure type Y3TaNi6+xAl26.

Study of Faulting in Laves Phase TiCr2 Alloys

The Laves phases TiCr2 have shown polytypism dependent on temperature. TiCr2 is 2H (C14, MgZn2 structure) above 1100°C (C15, MgCu2 structure) at lower temperatures with 4, 6, and 12 layer variants occurring as intermediate structures. Of particular interest are the phase transformations which presumably proceed by a shear mechanism.Laves phase structures and their stacking variants can be described in terms of the stacking sequence of six similar compound layers. These layers form topologically close packed structures of AB2 composition in which the metallic radius of the A atom is approximately 1.22 times larger than that of the B atom. Fig. 1 shows the atomic arrangement within a single compound layer. Similar layers are then stacked as in close packed structures. In TiCr2 the most common transition structures seem to be the 4H, 6H1, and 6H2 with the latter being less prominent. It is possible to form all these structures by the glide of arrays of Shockley type partial dislocation dipoles which are highly mobile at practical transformation temperatures. Models for each of the transformations have been proposed but have not been confirmed by direct observation. The partial dislocation dipoles may arise from the dissociation of perfect dipoles, emission from grain boundaries (or interfaces), or by nucleation from perfect crystal.

The adsorption of carbon monoxide on evaporated metal films

Calorimetric measurements at room temperature have been made of the heats of adsorption of carbon dioxide on evaporated films of iron, cobalt, nickel, titanium, molybdenum, tungsten, tantalum and niobium. These new data, considered along with similar data obtained previously for oxygen and carbon monoxide, provide strong evidence for dissociative adsorption of carbon dioxide into CO and O, except possibly on iron, cobalt and nickel. The saturation coverages for the adsorption of carbon dioxide on all the metals are distinguished by their smallness. Despite this, the adsorptive capacity for hydrogen of surfaces saturated with carbon dioxide is very small, except for nickel and to a lesser degree for iron. These observations have also been accounted for in terms of dissociative adsorption of carbon dioxide and the concentrations of krypton atoms in the monolayer deduced previously. For nickel and cobalt (and to some extent iron), adsorption of carbon dioxide in an undissociated form is a distinct possibility, but a clear conclusion cannot be reached. The linear dependence of heat of adsorption on metallic radius is discussed.