HYSTERESIS BEHAVIOUR AND THERMODYNAMIC PROPERTIES OF THE PA - H SYSTEM
THE ENTHALPIES AND ENTROPIES OF FORMATION OF THE non-stoichiometric protactinium hydride phases were obtained from temperature - pressure - composition data. Severe hysteresis observed in the pressure - composition isotherms measured in the hydriding and dehydriding directions correlates with the occurrence of sluggish structural transitions during hydriding. The dehydriding pressures are path independent and were adopted for the evaluation of the thermodynamic properties of the MgCu-2(C15)- related phase II (1.3<or=(H)/Pa<or=1.7) and the beta-UH-3 related phase III (2.0<or=(H)/(Pa)<or= 3.0) which form at temperatures above 500 K. DELTA-H degrees f- (II, PaH-1.3, 675 K) is 62.2 +- 2.0 kJ mol**-1 and DELTA-H degrees f- (III, PaH-3.0, 657 K) is 128.6 +- 6.9 kJ mol**- 1-, while the corresponding DELTA-S degrees f- values are 80.3 +- 2.9 J K**-1 mol**-1 and 180.8 +- 9.9 J K**-1 mol**-1 respectively. Only hydriding pressures and the associated thermodynamic values were obtained for the alpha- UH-3 related phase IV (1.3 <or= (H)/(Pa) <or= 3.0) which forms at temperatures below 500 K. In the absence of equilibrium thermodynamic results for phase IV, the relative stabilities of phases II and IV and of phases III and IV at low temperatures cannot be established.
Bibliographic Reference: JOURNAL OF THE LESS-COMMON METALS, VOL. 107 (1985), PP. 159-177
Record Number: 1989123112000 / Last updated on: 1987-01-01
Available languages: en