CE3RH2GE2 AND ISOTYPES WITH THE ORTHORHOMBIC LA3NI2GA2 TYPE

Ce3Rh2Ge2, M(r) = 771.35, orthorhombic, La3Ni2Ga, type, oP28, (57) Pbcm - e2d2c, a = 5.7001 (9), b = 8.099 (1), c = 13.461 (3) angstrom, V = 621.4 (1) angstrom 3, Z = 4, D(x) = 8.244 Mg m3, lambda(Mo K-alpha) = 0.71073 angstrom, mu - 35.492 mm-1, F(000) = 1312, T = 293 K, wR = 0.022 for 725 contributing unique reflections. Isotypes are found for R3Rh2Ge2 with R = Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. The structure is characterized by infinite columns of trigonal-prism blocks along the c axis, each block being rotated with respect to its two neighbours by 180-degrees around the column axis and shifted by 1/2c. Every block consists of three rectangular-face-linked trigonal prisms which are centred by Rh or Ge atoms. These columns are linked to neighbouring columns in a way similar to the linkage of the trigonal-prism columns in the FeB structure. Ignoring the difference in the site occupation of the prism centres the La3Ni2Ga2 type can be considered a slab-stacking variant of the Y3Co3Ga type. The geometrical relation between these types corresponds to that already described for the FeB and CrB structure types, allowing the use of the same transformation equations for the cell parameters.