Structural and magnetic properties of Sc1-xNbxFe2 intermetallics showing anomalous zero thermal expansion

Abstract Ternary intermetallics deriving from ScFe2 Laves phases can present a relatively unique case of ferro-ferromagnetic transition originating from an iron moment instability, which is potentially interesting for several applications. Yet, at the exception of (Sc,Ti)Fe2, their structural and magnetic phase diagrams remain scarcely explored. Here, we present a systematic investigation of the crystal structure and magnetic properties of Sc1-xNbxFe2 intermetallics. This series is found to mostly crystallize in the C14 hexagonal structure. But, in contrast to former studies, a C14/C15 hexagonal/cubic dimorphism is observed for the binary parent ScFe2, with the C15 structure that can be brought to the edge of stability depending on heat treatments. This led us to revisit the properties of ScFe2 via first-principles calculations. The total energy difference between C14 (stable) and C15 structures is found small, and C15 ScFe2 is predicted to be ferromagnetic with a saturation magnetization close to that of C14 ScFe2, which is fully compatible with experimental observations. Experimentally, Sc1-xNbxFe2 intermetallics with x ≤ 0.35 are ferromagnetic with Curie temperatures decreasing with increasing the Nb content; while such a long range ferromagnetic order with net magnetization is no longer observed for x > 0.6. A Neel transition is observed around 250 K in 0.6 ≤ x ≤ 0.9 alloys, this antiferromagnetic order is coexisting with a minor ferromagnetic phase, in line with former results from Mossbauer spectroscopy. For 0.2 ≤ x ≤ 0.3 samples, an ─unusual─ ferro.-ferromagnetic transition with a change in magnetization from ~1.2 μB/Fe to ~0.9 μB/Fe upon heating is found around 150 K in addition to the ferro-paramagnetic transition. This broad ferro.-ferromagnetic transition is isostructural as the C14 hexagonal crystal structure is preserved, but it leads to a virtually zero thermal expansion up to 250 K.