We herein provide a combined experimental investigation and theoretical calculations on the impact of Mn doping and Fe off-stoichiometry on the magnetoelastic transition and the magnetocaloric properties of Laves phase Hf_0.82Ta_0.18Fe₂ alloys. Mn substitution led to an increase in unit-cell volume while Fe vacancies induced lattice contraction. By adjusting the Mn and Fe content, we achieved a table-like magnetocaloric response with a magnetic entropy change of 1.7–2.2 J/(kg K) at a magnetic field change of 2 T over a wide temperature range from 190 to 260 K. Mössbauer spectroscopy, neutron powder diffraction and density functional theory calculations all reveal that both Mn atoms and Fe vacancies preferentially occupy the 6h crystallographic site of the lattice structure with space group P6₃/mmc, and that the shortest intralayer Fe-6h interatomic distance governs the magnetoelastic transition in (Hf, Ta)Fe₂ Laves phases. The tunable magnetic transition is ascribed to the slight change of the electronic state of the Fe-6h site and limited hybridization between Mn and Fe atoms. These findings offer new insight into the site-specific control for optimizing the magnetocaloric properties of Fe-based Laves phase alloys and inspire the design of other promising magnetocaloric materials with magnetoelastic transitions.

Zero thermal expansion (ZTE) materials with the advantage of an invariable length with varying temperatures are in high demand for modern industry but are relatively rare for metals. Fe-based Laves phases attract significant attention due to the rich and intriguing physical properties resulting from the coupling between crystal, electric, and magnetic structures. In this work, the structural, magnetic transition, thermal expansion, and magnetocaloric effect of single-phase Fe_2-xHf_0.80Nb_0.20 Laves phase alloys were investigated by means of macroscopic magnetic measurements, Mössbauer spectroscopy, and X-ray diffraction at the temperature range of 4.2-400 K. With the introduction of Fe vacancies, the ZTE coefficient of −1.2 ppm/K is smaller than that (1.7 ppm/K) of stoichiometric Fe₂Hf_0.80Nb_0.20 alloy. Meanwhile, the magnetic entropy change experiences an enhancement from 0.39 to 0.50 J/kg K at a magnetic field change of 2 T. These improved properties are attributed to the vacancy-induced coexistence of ferromagnetic and antiferromagnetic phases, as evidenced by variable-temperature X-ray diffraction and Mössbauer spectroscopy. This work unveils a promising avenue for new zero thermal expansion materials by controlling the vacancies at magnetic atom positions in Fe-based Laves phase alloys.