Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe_(11.4)Si_(1.6)H_(1.6)

摘要

We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe_(11.4)Si_(1.6)H_(1.6) in terms of the vibrational (phonon) density of states (VDOS), which we determined with ~(57)Fe nuclear resonant inelastic x-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy △S_(lat) across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change △S_(lat) of (3.2 ± 1.9) J/kgK across the FM-to-PM transition contributes with ～35% significantly and cooperatively to the total isothermal entropy change △S_(iso), Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Dehye velocity of sound of LaFe_(11.4)Si_(1.6)H_(1.6) was determined from the NRIXS and the DFT data.