Calorimetric study of the desorption of the interstitial hydrogen atoms in ferromagnetic Nd<SUB>2</SUB>Fe<SUB>14</SUB>BH<SUB>x</SUB> (x&lt;&#x223C;5) microcrystals

When heating over 300–800 K in a calorimeter, Nd<SUB>2</SUB>Fe<SUB>14</SUB>BH<SUB>x</SUB>, x&#60;&#8764;5, microcrystals desorb the H atoms in six irreversible endothermic signals. They appear at temperatures T<SUB>p</SUB>&#8764;340, 439 (doubly degenerate), 567, 605, and 653 K at the heating rate &#946;=15K/min, following the modified Kissinger relation. ln(T<SUB>p</SUB>/&#946;)=(E<SUB>a</SUB>/R)T<SUB>p</SUB><SUP>-1</SUP>+const, with R the gas constant. The 340 signal has an extremely weak intensity at a slow heating, &#946;&#60;&#8764;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 E<SUB>a</SUB> between 48 and 123 kJ/mol, taking into account their H occupancies n<SUB>i</SUB>(H). The distribution of partial enthalpies in the thermal signals (at &#946;=15K/min) determines n<SUB>i</SUB>(H)&#x2245;0, 6, 5, 2, 5, and 2 H atoms in the respective sites per Nd<SUB>2</SUB>Fe<SUB>14</SUB>BH<SUB>x</SUB>, x&#8764;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 &#946;=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, (&#8706;H/&#8706;t)<SUB>T</SUB>, with time t, after the desorption at t=0, following the primary endothermic desorption signal. The results are discussed with simulations of related processes