Stability, Crystal Chemistry, and Magnetism of U 2+ x Ni 21– x \B 6 and Nb 3– y Ni 20+ y B 6 and the Role of Uranium in the Formation of the Quaternary U 2– z Nb z Ni 21 B 6 and U δ Nb 3-δ Ni 20 B 6 Systems
In this work, 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, $Fm\bar{3}m$)] 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 ( 32f and 48h 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.