The thermodynamic characterization of the H-induced amorphization of CeFe2 and the determination of the energy site distribution using reaction calorimetry

The enthalpy of reaction for the hydrogen-induced amorphization of CeFe2 has been determined as a function of n = H/CeFe2. At 323 K the enthalpy for this amorphization reaction is constant equal to -37 kJ/mol 1/2H(2) from n = 0 to 1.8; it then declines in magnitude to about -24 kJ/mol 1/2H(2) at n = 3.0. The hydrogen pressure is too small to measure at 323 K except for n greater than or equal to 2.5. Hydrogen can be partially removed from the amorphized hydride at, e.g. 573 K, permitting the determination of the reaction enthalpy for the reintroduction of the hydrogen into the hypostoichiometric amorphous hydride. Enthalpies could be determined using this technique from about n = 2 to 3. From the relationship between these calorimetrically measured Delta H-H values and n for the reintroduction of hydrogen into the hypostoichiometric hydride, i.e. n < 3.0, a part of the energy distribution function for the occupation of the interstices with hydrogen can be determined. The distribution function can be closely described by a Gaussian function with sigma = 14 kJ/mol 1/2H(2) with an average value of Delta H-H(av) = -54 kJ/mol 1/2H(2). If the distribution is indeed a Gaussian, the Delta H for the following reaction can be obtained, i.e. CeFe2(c) --> CeFe2(a) Delta H(c --> a) = 51 kJ/mol CeFe2. Copyright (C) 1996 Acta Metallurgica Inc.