When heating over 300-800 K in a calorimeter, Nd 2 Fe 14 BH x , xр5, microcrystals desorb the H atoms in six irreversible endothermic signals. They appear at temperatures T p ϳ340, 439 ͑doubly degenerate͒, 567, 605, and 653 K at the heating rate ␤ϭ15 K/min, following the modified Kissinger relation ln(T p /␤)ϭ(E a /R)T p Ϫ1 ϩconst, with R the gas constant. The 340 signal has an extremely weak intensity at a slow heating, ␤ р40 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 8 j1 crystallographic interstitial sites ͑between the Fe atoms͒ 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 ␤ϭ15 K/min͒ determines n i (H)Х0, 6, 5, 2, 5, and 2 H atoms in the respective sites per Nd 2 Fe 14 BH x , xϳ5, crystal unit cell with a total of 4xϭ20 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 8 j2 and 8 j1 sites at 616 and 684 K ͑at ␤ϭ50 K/min͒ 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 relaxation͒ signal of the rate of the change of the enthalpy, (‫ץ‬H/‫ץ‬t) T , with time t, after the desorption at tϭ0, following the primary endothermic desorption signal. The results are discussed with simulations of related processes.

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