Rare‐Earth‐Rich Magnesium Compounds RE₂₃Rh₇Mg₄ (RE = La, Ce, Pr, Nd, Sm, Gd) with Tetrahedral Mg₄ Clusters

Abstract

The rare earth‐rich compounds RE23Rh7Mg4 (RE = La, Ce, Pr, Nd, Sm, Gd) were prepared by induction‐melting the elements in sealed tantalum tubes. The new compounds were characterized by X‐ray powder diffraction. They crystallize with the hexagonal Pr23Ir7Mg4 type structure, space group P63mc. The structures of La23Rh7Mg4 (a = 1019.1(1), c = 2303.7(4) pm, wR2 = 0.0827, 1979 F2 values, 69 variables), Nd23Rh7Mg4 (a = 995.4(2), c = 2242.3(5) pm, wR2 = 0.0592, 2555 F2 values, 74 variables) and Gd23Rh6.86(5)Mg4 (a = 980.5(2), c = 2205.9(5) pm, wR2 = 0.0390, 2083 F2 values, 71 variables) were refined from single crystal X‐ray diffractometer data. The three crystallographically different rhodium atoms have trigonal prismatic rare earth coordination with short RE–Rh distances (283–300 pm in Nd23Rh7Mg4). The prisms are condensed via common edges, leading to a rigid three‐dimensional network in which isolated Mg4 tetrahedra (312–317 pm Mg–Mg in Nd23Rh7Mg4) are embedded. Temperature dependent magnetic susceptibility data of Ce23Rh7Mg4 indicate Curie‐Weiss behavior with an experimental magnetic moment of 2.52(1) μB/Ce atom, indicative for stable trivalent cerium. Antiferromagnetic ordering is evident at 2.9 K.

Intermediate-valent Ce23Ru7Mg4 and RE23Ru7Mg4 (RE = La, Pr, Nd) with Pr23Ir7Mg4-type Structure

“…Careful evaluation of the data sets then showed that the systematic extinctions were compatible with space group P6 3 mc, in agreement with the Pr 23 Ir 7 Mg 4 type [10]. The atomic parameters of isotypic La 23 Rh 7 Mg 4 [12] were taken as starting values, and the three structures were refined using SHELXL-97 [23] (full-matrix least-squares on F 2 ) with anisotropic atomic displacement parameters for the rare earth and transition metal atoms and isotropic displacement parameters for the light magnesium atoms. Since the structure refinement of the prototype Pr 23 Ir 7 Mg 4 [10] revealed small defects on the Ir2 site, also the occupancy parameters of the RE 23 Ru 7 Mg 4 crystals were refined in separate series of leastsquares cycles.…”

“…F43m) and hexagonal for the Pr 23 Ir 7 Mg 4 type (space group P6 3 mc). Cavities within these networks are filled by tetrahedral Mg 4 clusters, a rare structural motif in magnesium chemistry [12]. First investigations revealed interesting properties for these rare earth-rich materials.…”

Ternary Rare-Earth-Metal Nickel Indides RE23Ni7In4 (RE = Gd, Tb, Dy) with Yb23Cu7Mg4-Type Structure

“…An assignment of the Pr 23 Ir 7 Mg 4 -type structure would seem to be evident given its overwhelming pervasiveness compared to the Yb 23 Cu 7 Mg 4 -type structure (Table ). − Or is it?…”

“…According to literature data, all atomic positions are fully occupied in the structure of Yb 23 Cu 7 Mg 4 as well as in most Pr 23 Ir 7 Mg 4 -type compounds. Some deficient occupancies of transition-metal sites were reported for the structure of Pr 23 Ir 7 Mg 4 itself (Pr 23 Ir 6.81 Mg 4 ), Gd 23 Rh 6.86 Mg 4 , and La 23 Ru 6.87 Cd 4 . The largest homogeneity regions were found for Ce 23 Ru 7+ x Al 4– x (0 < x < 2.97) and Ce 23+ y Ru 7+ x Ga 4– x – y (0 < x < 3.00, 0 < y < 0.97) .…”

Rare Earth-rich Cadmium Compounds RE23T7Cd4 (T = Co, Ni, Ru, Rh, Ir, Pt)

“…The three structures were refined using SHELXL-97 [11] (full-matrix least-squares on F 2 ) with anisotropic atomic displacement parameters for all atoms. Since the structures of Pr 23 Ir 6.81 Mg 4 [5] and Gd 23 Rh 6.86 Mg 4 [7] showed small defects on the T 2 sites, the occupancy parameters for all three crystals were refined in separate series of least-squares cycles. …”