The crystal structures and magnetic properties of TiFeSi coexisting in hexagonal and orthorhombic symmetries

Abstract The intertwisted helix framework of the chiral FeSi-type structure with the space group P213 (S.G. P213) inspired us to investigate other non-centrosymmetric compounds, particularly 111-type phases with different FeSi frameworks. Various FeSi-containing 111-type phases have been summarized according to their valence electron counts and space group. In this work we have focused on the non-centrosymmetric TiFeSi phase. The crystal structure analysis of the hexagonal Fe2P-type and the reported orthorhombic TiFeSi-type (α-TiFeSi) indicate a superlattice relationship exists between the two structures, which drove us to perform the total energy calculations of the hexagonal and orthorhombic models of TiFeSi. Following the theoretical predictions, a hexagonal TiFeSi phase (β-TiFeSi) is proposed and successfully synthesized at high temperature using arc melting. The crystal structure of the high-temperature hexagonal phase was determined by powder and single crystal X-ray diffraction. After the annealing of the arc-melted samples, TiFeSi crystallizes into an orthorhombic structure with the space group Ima2. The systematic magnetic characterizations indicate ferromagnetic properties are present in the non-centrosymmetric orthorhombic α-TiFeSi phase with possible helical magnetic ordering at low temperatures. Moreover, to compare with the centrosymmetric cases, the electronic and magnetic properties of the antiferromagnetic TiNiSi-type MFeSi (M = Zr and Hf) compounds were also examined using both density functional theory and experimental magnetic measurements.