Structure and magnetism of Fe-substituted MnNiSi0.95Al0.05

MM'X compounds such as MnNiSi 0.95 Al 0.05 are potential candidates for room temperature magnetocaloric cooling prototypes due to a magnetostructural transition occurring between TiNiSi orthorhombic and Ni 2 In hexagonal structure-types. Here, Fe substituted MnNiSi 0.95 Al 0.05 is synthesized by arc melting and subsequent heat treatment. Mn 1-x Fe x NiSi 0.95 Al 0.05 and Mn 1-y Ni 1-y Fe 2y Si 0.95 Al 0.05 substitutions feature both the TiNiSi and Ni 2 In structure-types, and undergo martensitic transitions around room temperature, while MnNi 1-z Fe z Si 0.95 Al 0.05 compositions display MgZn 2 and Mn 5 Si 3 hexagonal structure -types. The stronger covalent character induced by Fe substitution impacts the structural transitions, shifting them to room temperature for Mn 0.5 Fe 0.5 NiSi 0.95 Al 0.05 and Mn 0.68 Ni 0.68 Fe 0.64 Si 0.95 Al 0.05 , which undergo transitions during heating at 305 and 302 K, respectively. The orthorhombic structures display anisotropic thermal expansion, and we observe a negative thermal expansion for Mn 0.45 Fe 0.55 NiSi 0.95 Al 0.05 and a null expansion for Mn 0.68 Ni 0.68 Fe 0.64 Si 0.95 Al 0.05 . Finally, density functional theory calculations predict a larger magnetic moment in Mn 0.68 Ni 0.68 Fe 0.64 Si 0.95 Al 0.05 , which is confirmed both from neutron diffraction and macroscopic measurements. However, due to inhomogeneity attributed to the heat treatment condition, Mn 1-y Ni 1-y Fe 2y Si 0.95 Al 0.05 compounds show hindered magnetocaloric potential compared to Mn 1-x Fe x NiSi 0.95 Al 0.05 compounds.