Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6H1.6

Abstract

We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 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) lowtemperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between non-hydrogenated and hydrogenated samples. This shows that hydrogen does not only shift 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 ∆Siso. 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 Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. arXiv:1911.11087v1 [cond-mat.mtrl-sci]

Flexible Magnetocaloric Fiber Mats for Room-Temperature Energy Applications

“…Rietveld refinement was performed on the diffractogram of the LaFe 11.83 Mn 0.32 Si 1.28 H powder, and the result is presented in Figure c (Rietveld refinement parameters: R p : 1.51, R ωp : 2.34, and R exp : 0.02). The Rietveld refinement shows that LaFeSi grains crystallized in a cubic NaZn 13 -type structure with a minor amount of α-Fe phase (∼3%) as typically occurs in these materials. ,, The estimated volume and lattice parameters are 1546.115 (±0.006) Å 3 and 11.56327 (±0.00003) Å, respectively, and they are in agreement with the literature . It is important to remark that XRD was performed at room temperature (∼297 K), that is, at a temperature above LaFeSi’s T c .…”

Mechanochemical Synthesis and Hydrogen Sorption Properties of a V-Ni Alloy

“…In this case, the presence of one hydrogen atom in interstitial sites of the metal lattice prevents other hydrogen atoms from occupying next-neighbor interstitial sites [31]. Some other cases of entropy variations with hydrogenation were reported for magnetic systems, such as LaFe11.4Si1.6H1.6 [32], where a large change in the phonon density occurs during hydrogenation, and consequently, a large variation of the lattice entropy is observed.…”

Viable Materials with a Giant Magnetocaloric Effect

“…There are thousands of magnetocaloric publications, including hundreds of those mentioning a giant caloric effect, see Figure 2b. However, most considered materials are not economically viable, because they contain precious metals (Rh, Pd) [21,22,24], rare earth [12,[38][39][40][41], expensive germanium [42,43] or gallium [28,44], toxic elements [45][46][47][48][49][50][51], or hydrogen [52][53][54][55], which is released in a gas phase, see Figure 2c. Before 2005, a large MCE (exceeding that in metallic Gd) was claimed in precious FeRh [21,22], expensive Gd 5 Si 2 Ge 2 [28,[56][57][58][59][60][61][62][63][64][65][66][67][68] and related materials (GdGe 4 [69,70], GdSn 4 [71,72], Tb 5 Si 2 Ge 2 [73]), Heusler alloys…”

“…Examples with toxic antimony include Mn 1.9 Co 0.1 Sb [211], Mn 2−x Cr x Sb [236], NiMn 0.9 Sb 0.1 [254], Ni 0.5−x Co x Mn 0.38 Sb 0.12 [92], etc. Hydrides emit H 2 gas [52][53][54][55]. A giant MCE was found in cobalt hydroxides Co(OH) 2 [212,259,280].…”

“…Magnetic shape memory alloys [148,204] include Ni-Mn-In [167,175,192,261,283] doped with Co [136,172,241] and Si [122]; Ni-Mn-Sn [115,226]; NiMn-based B2 (Ni-Co)(Mn-Ti) [30], Ni-Co-Mn-Al films [189,190] [293], and metallic glasses [274]. Significant interest was devoted to the potentially viable LaFe 13−x Si x [137,200,284,288,290,292], doped with Co [249] and other additives [35], or partially hydrated [52][53][54][55]. For completeness, we mention Mn 3 CuN 1−x C x [147] and molecular magnets [128].…”