Calorimetric study of the desorption of the interstitial hydrogen atoms in ferromagnetic Nd2Fe14BHx (x<∼5) microcrystals

Ram, S. ; J-Fecht, H. ; Haldar, S. ; Ramachandrarao, P. ; Banerjee, H. D. (1997) Calorimetric study of the desorption of the interstitial hydrogen atoms in ferromagnetic Nd2Fe14BHx (x<∼5) microcrystals Physical Review B: Condensed Matter and Materials Physics, 56 (2). pp. 726-737. ISSN 1098-0121

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When heating over 300–800 K in a calorimeter, Nd2Fe14BHx, x<∼5, microcrystals desorb the H atoms in six irreversible endothermic signals. They appear at temperatures Tp∼340, 439 (doubly degenerate), 567, 605, and 653 K at the heating rate β=15K/min, following the modified Kissinger relation. ln(Tp/β)=(Ea/R)Tp-1+const, with R the gas constant. The 340 signal has an extremely weak intensity at a slow heating, β<∼40K/min, because the H atoms in the involved interstitial site in the sample slowly tunnel to neighboring higher-transition-energy sites. The thermogram at 439 K is the most prominent. It contains two signals which could be resolved by selective isothermal desorptions. These six different identified thermal signals are assigned to the desorptions of the H atoms from six specific 4c, 16k2, 16k1, 4e, 8j2, and 8j1 crystallographic interstitial sites (between the Fe atoms) in the sample, characterized by six different activation energies Ea between 48 and 123 kJ/mol, taking into account their H occupancies ni(H). The distribution of partial enthalpies in the thermal signals (at β=15K/min) determines ni(H)≅0, 6, 5, 2, 5, and 2 H atoms in the respective sites per Nd2Fe14BHx, x∼5, crystal unit cell with a total of 4x=20H atoms. It is found that a significant portion of the thermally excited H atoms in these sites, in the process to the desorption, redistributes over neighboring sites of modified energies to keep the H atoms at high temperatures. The redistribution reaction is exothermic. It has been observed separately in 8j2 and 8j1 sites at 616 and 684 K (at β=50K/min) in a partially H-desorbed sample in the lower-transition-energy sites by heating it at 583 K. A local redistribution of the interatomic distances and/or the electronic charges occurs within the lattice following the thermal desorption of the H atoms. It results in a monotonically decreasing exothermic (structural relaxation) signal of the rate of the change of the enthalpy, (∂H/∂t)T, with time t, after the desorption at t=0, following the primary endothermic desorption signal. The results are discussed with simulations of related processes.

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