EFFECT OF HYDROGEN ON THE STABILITIES OF ORDERED STRUCTURES IN PD3MN

Thermodynamic stabilities of the ordered structures in Pd3Mn have been theoretically evaluated, both in the presence and absence of hydrogen . The internal energies of the L1(2) and L1(2-s) structures of Pd3Mn w ere found to be the same. The L1(2-s) phase is the stable phase at all temperatures, both in the presence and absence of hydrogen, according to the statistical thermodynamic calculations indicating that the hyd rogen-induced ordering phenomenon from the L1(2-s) to the L1(2) struct ure could be due to stress-assisted motion of atoms as a result of hyd rogen occupying interstitial sites in the L1(2-s), form. The differenc e in stabilities between the L1(2) and L1(2-s) structures is small for both of the cases. The difference in configurational entropies betwee n the two ordered forms of Pd3Mn decreases with increasing hydrogen co ncentration. The effect of configurational entropy on the stability of the L1(2) and L1(2-s) phases has been addressed.