ER2RHSI3 AND R2COGA3 (R-EQUIVALENT-TO-Y, TB, DY, HO, ER, TM, YB) WITH LU2COGA3 TYPE-STRUCTURE - NEW MEMBERS OF THE ALB2 STRUCTURE FAMILY

The structure of Er2RhSi3 was redetermined by X-ray single-crystal diffraction (A(Mo Kalpha) = 0.71073 angstrom, mu = 42.392 mm-1, F(000)=892, T=293 K, R=0.037, wR=0.030 for 244 contributing unique reflections). It is shown that this silicide has a hexagonal structure of the LuCoGa3 type, hP24, (194) P63/mmc-khfb, a = 8.1130(7), c = 7.7556(9) angstrom, V=442.09(9) angstrom3, Z=4, M(r)=521.68, D(x)=7.838 mg mm-3. The structure of isotypic Er2Co1.4Gal2.6 (hP24, (194) P63/mmc-khfb, a = 8.607(3), c = 6.898(3) angstrom, V= 442.5(4) angstrom3, Z = 4, M(r) = 598.30, D(x) = 8.980 Mg MM-3) was refined by X-ray powder diffraction (lambda(Fe Kalpha) = 1.93735 angstrom, F(000) = 1017.6, T= 293 K, R = 0.092 for 61 reflections). The cell parameters of R2Co1+xGa3-x phases (R=Y, Tb, Dy, Ho, Er, Tm, Yb; x= +/-0.4) with the same structure type were obtained from X-ray powder diagrams. The Lu2CoGa3 structure is a distorted substitution variant of the AlB2 type, where the trigonal Lu6 prisms centred by cobalt atoms share triangular faces in infinite columns. The cobalt atoms are displaced away from the prism centres like the mercury atoms in the orthorhombic KHg2 structure type. The distortions in Er2RhSi3 are of a lesser magnitude than those observed in Lu2CoGa3. A progressive substitution of gallium by cobalt along the RGa2-RCoGa cross-sections of the R-Co-Ga systems (R=Y, Th, Dy, Ho, Er, Tm, Yb, Lu) leads to a step by step deformation of the trigonal prisms. The main features of other deformation and substitution derivatives of the AlB2 type are discussed.