Single crystal studies on Co-containing τ-borides Co23−xMxB6 (M=Al, Ga, Sn, Ti, V, Ir) and the boron-rich τ-boride Co12.3Ir8.9B10.5

Stability, Crystal Chemistry, and Magnetism of U2+xNi21-xB6 and Nb3-yNi20+yB6 and the Role of Uranium in the Formation of the Quaternary U2-zNbzNi21B6 and UδNb3-δNi20B6 Systems

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 U₂Ni₂₁B₆ and Nb_3-yNi_20+yB₆ [ternary derivatives of the cubic Cr₂₃C₆-type (cF116, Fm3̅m)] have been studied; the formation of the hypothetical "U₃Ni₂₀B₆" and "Nb₂Ni₂₁B₆" has been disproved. U₂Ni₂₁B₆ [a = 10.6701(2) Å] crystallizes in the ordered W₂Cr₂₁C₆-type, whereas Nb_3-yNi_20+yB₆ [a = 10.5842(1) Å] adopts the Mg₃Ni₂₀B₆-type. Ni in U₂Ni₂₁B₆ can be substituted by U, leading to the solid solution U_2+xNi_21-xB₆ (0 ≤ x ≤ 0.3); oppositely, Nb in Nb₃Ni₂₀B₆ is partially replaced by Ni, forming the solution Nb_3-yNi_20+yB₆ (0 ≤ y ≤ 0.5), none of them reaching the limit corresponding to the hypothetically ordered "U₃Ni₂₀B₆" and "Nb₂Ni₂₁B₆". These results prompted us to investigate quaternary compounds U_2-zNb_zNi₂₁B₆ and U_δNb_3-δNi₂₀B₆: strong competition in the occupancy of the 4a and 8c sites by U, Nb, and Ni atoms has been observed, with the 4a site occupied by U/Ni atoms only and the 8c site filled by U/Nb atoms only. U₂Ni₂₁B₆, U_2.3Ni_20.7B₆, and Nb₃Ni₂₀B₆ are Pauli paramagnets. Interestingly, Nb_2.5Ni_20.5B₆ 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 4a 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 U₂Ni₂₁B₆ and Nb₃Ni₂₀B₆ is energetically preferred. The various electronic states generating ferromagnetism on Nb_2.5Ni_20.5B₆ and Pauli paramagnetism on U₂Ni₂₁B₆ and Nb₃Ni₂₀B₆ have been identified.

Rediscovering the Crystal Chemistry of Borides

For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Hierarchical and chemical space partitioning in new intermetallic borides MNi21B20 (M = In, Sn)

MNi₂₁B₂₀ (M = ln, Sn): [Ni₆@B₂₀@Ni₂₄] triple shell clusters with M atoms centering the cuboctahedra [M@Ni₁₂].