Searching for Laves Phase Superstructures: Structural and ²⁷Al NMR Spectroscopic Investigations in the Hf-V-Al System

Laves phases exhibit a plethora of different structures and a multitude of physical properties. Investigations in the ternary system Hf-V-Al led to the discovery of numerous members of the solid solution Hf(V_1-Al)₂, which adopt the hexagonal MgZn₂ type (C14) for medium to high amounts of Al (x = 0.2-1) and the cubic MgCu₂ type (C15) for small Al amounts (x = 0.05-0.1). While all members exhibit Pauli-paramagnetic behavior due to the absence of localized magnetic moments, the V-rich cubic member Hf(V_0.95Al_0.05)₂ additionally exhibits a superconducting state below T_C = 7.6(1) K. All synthesized compounds were characterized by powder X-ray diffraction, and selected samples were furthermore investigated by ²⁷Al solid-state magic-angle spinning (MAS) NMR. HfAl₂ exhibits two Al resonances, one rather sharp and one significantly broadened signal, in line with the crystal structure and respective coordination environments. The members of the solid solution exhibit extremely broadened resonances due to the mixing of V and Al on the same crystallographic sites. For nominal Hf(V_0.125Al_0.875)₂, however, two distinct sharp NMR signals were observed. This contrasts with the description of a solid solution. Therefore, single-crystal X-ray studies were conducted, showing that Hf(V_0.125Al_0.875)₂ really is an ordered compound with the sum formula Hf₄VAl₇ (P3̅m1), which exhibits an, thus far, unknown superstructure of MgZn₂.