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

Abstract

When heating over 300-800 K in a calorimeter, Nd2Fe14BHx, x less than or equal to 5, microcrystals desorb the H atoms in six irreversible endothermic signals. They appear at temperatures T-p similar to 340, 439 (doubly degenerate), 567, 605, and 653 K at the heating rate beta=15 K/min, following the modified Kissinger relation ln(T-p)=(E-a/R)T-p(-1) +const, with R the gas constant. The 340 signal has an extremely weak intensity at a slow heating, beta less than or equal to 40 K/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 E-a between 48 and 123 kJ/mol, taking into account their H occupancies n(i)(H). The distribution of partial enthalpies in the thermal signals (at beta=15 K/min) determines n(i)(H)congruent to 0, 6, 5, 2, 5, and 2 H atoms in the respective sites per Nd2Fe14BHx, x similar to 5, crystal unit cell with a total of 4x=20 H 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 beta=50 K/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, (partial derivative H/partial derivative 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.