MgCuj type

The ABj intermetallics discussed here have either a cubic MgCuj-type or a hexagonal MgZn -type structure where A has a metallic radius larger than that of B, ideally by a factor of 1.225. The radius ratios of A to B vary from ca. 1.05 to 1.68, but the radii contract or expand on compound formation to approach the ideal value of 1.225. 

Binary and Ternary Chromides. A compound of formula HfCrj has been identified in the V-Hf-Cr system. At 1470 K HfCrj has the MgCuj-type structure which transforms to a phase with a MgNij-type structure. At temperatures > 1570 K only the A-phase with a MgZuj-type structure exists. 

Fig, 13.3. Compound formation in the R-Fe, R-Ru, R-Os systems. Notes 1) TbFe2 also crystallizes with a rhombohedral modification derived from the MgCu2 type. 2) (Pr, Nd, Yb)Fe2 synthesized under pressure, crystallize with the MgCuj type structure. 3) (Nd, SmIRuj and (La, CelOsj under pressure transform to the MgZn2 structure type. 4) The following compounds have been claimed to exist but no structural details are available (Sc, Y, La, Ce)jRu and (Y, La, Ce)30s. 

Crystallographic data for RXj compounds with the cubic MgCuj type structure... 

LaOs2, CeOsj, and PrOs2 of the MgCuj type transformed to the hexagonal MgZn2 type at temperatures around 1270 K and pressures ranging from about 1 to about 7 GPa (Cannon et al. 1973). 

Only a MgCuj-type derivative is known. The MgSnCurtype (GIKT, 52), shown in the right hand part of fig. 17 is found with 8320 RNi4Au compounds, where R = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc (Dw, 75). However, since no compounds are known where the M element is from the boron or silicon group these compounds shall not be discussed further. 

No ternary compounds were observed, mutual solid solubihties of binary phases were negligible. The binary Y-Al compounds YjAl (PbClj-type), YjAlj (ZrjAla-type), YAl (CrB-type), YAlj (MgCuj-type), YAI3 (NijSn-type) are in agreement with the compilation of landelli and Palenzona (1979). 

The ternary Sc-Ni phases observed are in accordance with literature data ScNis (CaCuj-type), ScjNi, (Ce2Ni7-type), ScNiz (MgCuj-type), ScNi (CsCI-type) and ScjNi (TijNi-type). For a discussion of nickel and scandium borides, see Y-Ni-B and Sc-Co-B, respectively. 

---