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The crystal chemical features of the new series of compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr6Zn23Si prototype (cF120, space group Fm3m, Z = 4), a filled variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare earth-magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and makes possible the formation of the ternary R6Mg23Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the Bärnighausen formalism within group theory. [ABSTRACT FROM AUTHOR]

A series of four homologous silicides have been discovered during systematic explorations in the central part of the La-Ni-Si system at 1000 °C. All compounds La 12.5 Ni 28.0 Si 18.3 ( n = 3; a = 28.8686(8), c = 4.0737(2) Å, Z = 3), La 22.1 Ni 39.0 Si 27.8 ( n = 4; a = 20.9340(6), c = 4.1245(2) Å, Z = 1), La 32.9 Ni 49.8 Si 39.3 ( n = 5; a = 24.946(1), c = 4.1471(5) Å, Z = 1), and La 44.8 Ni 66.1 Si 53.4 ( n = 6; a = 28.995(5), c = 4.158(1) Å, Z = 1) crystallize in the hexagonal space group P 6 3 / m and can be generalized according to La n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)- z with n = 3-6. Their crystal structures are based on AlB 2 -type building blocks, fused La-centered Ni 6 Si 6 hexagonal prisms, yielding larger oligomeric equilateral domains with the edge size equal to n . The domains extend along the c axis and show checkered ordering of the cationic and anionic parts, while all their atoms are located on mirror planes. La n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)- z can be considered as a mirror series to the La-rich La ( n +1)( n +2) Ni n ( n -1)+2 Si n ( n +1) , where an exchange of the formal cationic and anionic sites, i.e., La and Si, occurs. The La-Ni-Si system is the first system where two such analogous series have been observed.

We report two new rare-earth (R) ternary intermetallic compounds-Ho 2 Ni 0.8 T 1.2 with T = Si and Ge-that correspond to the R 5 Ni 2 T 3 phase earlier reported to form in Dy-Ni-T and Ho-Ni-T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr 2 Ni 1- x P-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho 2 Ni 0.8 T 1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R 2 Ni 0.8 T 1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr 2 Ni 1- x P structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at T C = 38 K for Ho 2 Ni 0.8 Si 1.2 and T C = 37 K for Ho 2 Ni 0.8 Ge 1.2 , which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below T C with both ferromagnetic and antiferromagnetic components and magnetic propagation vector κ 1 = [0, 0, 0]. Below T N ≅ 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector κ 2 = [κ x , 0, 0] appears to coexist with the first magnetic structure.

We investigated the U-Ni-B and Nb-Ni-B systems to search for possible new heavy fermion compounds and superconducting materials. The formation, crystal chemistry, and physical properties of U 2 Ni 21 B 6 and Nb 3- y Ni 20+ y B 6 [ternary derivatives of the cubic Cr 23 C 6 -type ( cF 116, Fm 3̅ m )] have been studied; the formation of the hypothetical "U 3 Ni 20 B 6 " and "Nb 2 Ni 21 B 6 " has been disproved. U 2 Ni 21 B 6 [ a = 10.6701(2) Å] crystallizes in the ordered W 2 Cr 21 C 6 -type, whereas Nb 3- y Ni 20+ y B 6 [ a = 10.5842(1) Å] adopts the Mg 3 Ni 20 B 6 -type. Ni in U 2 Ni 21 B 6 can be substituted by U, leading to the solid solution U 2+ x Ni 21- x B 6 (0 ≤ x ≤ 0.3); oppositely, Nb in Nb 3 Ni 20 B 6 is partially replaced by Ni, forming the solution Nb 3- y Ni 20+ y B 6 (0 ≤ y ≤ 0.5), none of them reaching the limit corresponding to the hypothetically ordered "U 3 Ni 20 B 6 " and "Nb 2 Ni 21 B 6 ". These results prompted us to investigate quaternary compounds U 2- z Nb z Ni 21 B 6 and U δ Nb 3-δ Ni 20 B 6 : strong competition in the occupancy of the 4 a and 8 c sites by U, Nb, and Ni atoms has been observed, with the 4 a site occupied by U/Ni atoms only and the 8 c site filled by U/Nb atoms only. U 2 Ni 21 B 6 , U 2.3 Ni 20.7 B 6 , and Nb 3 Ni 20 B 6 are Pauli paramagnets. Interestingly, Nb 2.5 Ni 20.5 B 6 shows ferromagnetism with T C ≈ 11 K; the Curie-Weiss fit gives an effective magnetic moment of 2.78 μ B /Ni, suggesting that all Ni atoms in the formula unit contribute to the total magnetic moment. The M ( H ) data at 2 K further corroborate the ferromagnetic behavior with a saturation moment of 10 μ B /fu (≈0.49 μ B /Ni). The magnetic moment of Ni at the 4 a site induces a moment in all of the Ni atoms of the whole unit cell (32 f and 48 h sites), with all atoms ordering ferromagnetically at 11 K. Density functional theory (DFT) shows that the formation of U 2 Ni 21 B 6 and Nb 3 Ni 20 B 6 is energetically preferred. The various electronic states generating ferromagnetism on Nb 2.5 Ni 20.5 B 6 and Pauli paramagnetism on U 2 Ni 21 B 6 and Nb 3 Ni 20 B 6 have been identified.

A new series of intermetallic compounds R 3 Au 9 Pn (R = Y, Gd-Tm; Pn = Sb, Bi) has been discovered during the explorations of the Au-rich parts of rare-earth-containing ternary systems with p-block elements. The existence of the series is strongly restricted by both geometric and electronic factors. R 3 Au 9 Pn compounds crystallize in the hexagonal crystal system with space group P6 3 /m (a = 8.08-8.24 Å, c = 8.98-9.08 Å). All compounds feature Au-Pn, formally anionic, networks built up by layers of alternating edge-sharing Au@Au 6 and Sb@Au 6 trigonal antiprisms of overall composition Au 6/2 Pn connected through additional Au atoms and separated by a triangular cationic substructure formed by R atoms. From a first look, the series appears to be isostructural with recently reported R 3 Au 7 Sn 3 (a ternary ordered derivative of the Cu 10 Sn 3 -structure type), but no example of R 3 Au 9 M is known when M is a triel or tetrel element. R 3 Au 9 Pn also contains Au@Au 6 Au 2 R 3 fully capped trigonal prisms, which are found to be isostructural with those found in the well-researched R 14 Au 51 series. This structural motif, not present in R 3 Au 7 Sn 3 , represents a previously unrecognized link between Cu 10 Sn 3 and Gd 14 Ag 51 parent structure types. Magnetic property measurements carried out for Ho 3 Au 9 Sb reveal a complex magnetic structure characterized by antiferromagnetic interactions at low temperature (T N = 10 K). Two metamagnetic transitions occur at high field with a change from antiferromagnetic toward ferromagnetic ordering. Density functional theory based computations were performed to understand the materials' properties and to shed some light on the stability ranges. This allowed a better understanding of the bonding pattern, especially of the Au-containing substructure, and elucidation of the role of the third element in the stability of the structure type.