Phase transition and magnetocaloric effect of LaFe11.5Si1.5Cx compounds: experiments and density functional theory calculations

Abstract

The itinerant-electron metamagnetic (IEM) transition is a crucial phenomenon in first-order transition magnetocaloric materials. However, most magnetocaloric materials experience a transition from first to second order, weakening their IEM transition and magnetocaloric effect. This study uses experiments and density functional theory (DFT) calculations to elucidate phase transition and magnetocaloric effect in C-doped La(Fe,Si)₁₃ compounds. The LaFe_11.5Si_1.5C_x alloys exhibit a cubic NaZn₁₃-type 1:13 primary phase. The critical phase transition from first order to second order occurs at x = 0.14. As x increases from 0 to 0.6, the maximum magnetic entropy change (${\mathrm{\Delta }S}_{\mathrm{M}}^{\max }$${\mathrm{\Delta }S}_{\mathrm{M}}^{\max }$) decreases from 19.6 to 4.5 J∙kg^-1∙K^-1 at ${\mu }_0∆H=2\mathit{T}$${\mu }_0∆H=2\mathit{T}$, while the Curie temperature (T_c) rises from 193 to 251 K. DFT calculations reveal that interstitial C-24d atom reduces the magnetic moment of 4 adjacent Fe-96i atoms, contracts the B1, B2 and B3 intra-icosahedron Fe-Fe bonds, but elongates the B4 and B5 inter-icosahedron bonds. Furthermore, the phase transition from first order to second order is attributed to the formation of Fe-C covalent bonds and localization of Fe 3d electrons, due to the strong electronic hybridization of the C 2p and Fe 3d states. C-doping weakens the IEM transition by shifting the minimum in the minority density of states (DOS) channels from the Fermi level to lower energies. This study offers valuable insights into the phase transition and IEM transition behaviors and paves the way for designing high performance magnetocaloric alloys.