Tuning the phase transition in transition-metal-based magnetocaloric compounds

Neutron-diffraction experiments on the (Mn, Fe)(2)(P,Si)-type compounds have shown a site preference of Si atoms in the hexagonal structure. The degree of ordering of Si depends on the Si/P ratio, while it is independent of the Mn/Fe ratio. The ferromagnetic-paramagnetic magnetoelastic transition is closely related to the size of the magnetic moment on the 3f site. A preferred occupation of Si atoms on the 2c site stabilizes and decreases the magnetic moment on the 3f and 3g site, respectively, which is supported by our first-principles density functional theory calculations. This effect, together with the contribution from the Si substitution-induced changes in the interatomic distances, leads to a phase transition that is tunable in temperature and degree of first order in Mn1.25Fe0.70P1-xSix compounds. These results provide us with further insight into the relationship between the magnetoelastic phase transition and the local atomic coordination.