Cobalt Centered Trigonal RE6 Prisms and Mg4 Clusters as Basic Structural Units in RE4CoMg (RE = Y, La, Pr, Nd, Sm, Gd–Tm)

Abstract

AbstractThe new rare earth metal rich intermetallic compounds RE4CoMg (RE = Y, La, Pr, Nd, Sm, Gd–Tm) were prepared via melting of the elements in sealed tantalum tubes in a water‐cooled sample chamber of a high‐frequency furnace. The compounds were investigated by X‐ray diffraction of powders and single crystals: Gd4RhIn type, $F{\bar 4}{\rm 3}m$, a = 1428.38(9) pm, wR2 = 0.0638, 680 F2 values, 20 variables for La4CoMg, a = 1399.5(2) pm, wR2 = 0.0584, 589 F2 values, 20 variables for Pr4CoMg, a = 1390.2(3) pm, wR2 = 0.0513, 634 F2 values, 20 variables for Nd3.90CoMg1.10, a = 1381.0(3) pm, wR2 = 0.0730, 618 F2 values, 22 variables for Sm3.92Co0.93Mg1.08, a = 1373.1(4) pm, wR2 = 0.0586, 611 F2 values, 20 variables for Gd3.92CoMg1.08, a = 1362.1(3) pm, wR2 = 0.0576, 590 F2 values, 20 variables for Tb3.77CoMg1.23, a = 1344.8(2) pm, wR2 = 0.0683, 511 F2 values, 20 variables for Dy3.27CoMg1.73, and a = 1343.3(2) pm, wR2 = 0.0560, 542 F2 values, 20 variables for Er3.72CoMg1.28. The cobalt atoms have trigonal prismatic rare earth coordination. Condensation of the CoRE6 prisms leads to a three‐dimensional network which leaves larger voids that are filled by regular Mg4 tetrahedra at a Mg–Mg distance of 316 pm in La4CoMg. The magnesium atoms have twelve nearest neighbors (3 Mg + 9 RE) in icosahedral coordination. In the structures with Nd, Sm, Gd, Tb, Dy, and Er, the RE1 positions which are not involved in the trigonal prismatic network reveal some RE1/Mg mixing and the Sm3.92Co0.93Mg1.08 structure shows small cobalt defects. Considering La4CoMg as representative of all studied systems an analysis of the chemical bonding within density functional theory closely reproduces the crystal chemistry scheme and shows the role played by the valence states of the different constituents in the electronic band structure. Strong bonding interactions were observed between the lanthanum and cobalt atoms within the trigonal prismatic network.