Thermodynamics of intermetallics through emf measurements under effusion conditions

摘要

A general treatment is performed for a complete description of the thermodynamics of galvanic cells Pt/Me", Me"_zF_j/CaF_2s.c./Me_hMe'_k, Me_lMe'_m, Me_xF_y/Pt type working under effusion conditions where the vaporization of Me" is considered. The treatment has been applied to three Me_2Ni_(17)/Ni_(ss) systems with Me ident to Eu, Tb. Ho and Me_hMe'_k is the solid solution, Ni_(ss), produced by the eutectic reaction L reversible Me_2Ni_(17)(s) + Ni_(ss). From the emf vs T experimental data, DELTA _f H_(298)~(theta) and DELTA _f S_(T-bar)~(theta) have been calculated. The DELTA _f H_(298)~(theta) values for Tb_2Ni_(17) (s) and Ho_2Ni_(17)(s) are, respectively, -16.76 +- 0.06 and -18.8 +- 0.1 kJ/mol atoms. In the same order, the DELTA _f S_(T-bar)~(theta) values are -1.27 +- 0.06 and -1.81 +- 0.09 JK~(-1)/mol atoms, respectively, at T = 1023 and 1085 K. For Eu_2Ni_(17)(s), DELTA _f H_(T-bar)~(theta) and DELTA _f S_(T-bar)~(theta) are, respectively, -48 +- 4 kJ /mol atoms and 2.0 +- 0.2 JK~(-1)/mol atoms, both the values at T = 910 K. The latter results should be considered unexpected and the reason for this is extensively discussed.