Lithiation of the Fe₂P-based magnetocaloric materials: A first-principles study

The physical properties of the extensively studied Fe₂P material family, well-known for its promising magnetocaloric qualities are greatly influenced by the unit-cell parameters of this hexagonal system. This sensitivity of the various magnetocaloric properties to structural parameters is particularly important for developing a material suitable for room-temperature magnetic refrigeration. A change in the unit cell, due to added elements can induce pronounced changes in the Curie temperature and the nature of the magnetic phase transition. Li belongs to a yet unexplored group of possible dopant elements – alkali metals, and exhibits an unusual behavior upon introduction to Fe₂P. We observe a preference to replace iron atoms, as opposed to the common tendency of non-magnetic dopants to replace phosphorus, leading to a strong influence on the magnetic structure. The addition of Li introduces a deformation of the unit cell with a small change in volume and a decrease in c/a ratio, while the same crystallographic phase is maintained over a relatively wide concentration range. We show that lithium has an exceptionally strong effect on the Curie temperature of Fe₂P reaching 800 K at 20% Li compared to 240 K for the undoped material.