Thermochemical study of the phase reaction (1/7)Pr/sub 7/O/sub 12/ + (1/7-x/2)0/sub 2/ = PrO/sub 2-x/

Journal Article · Sat Mar 14 23:00:00 EST 1981 · J. Solid State Chem.; (United States)

DOI:https://doi.org/10.1016/0022-4596(81)90009-8· OSTI ID:6776266

Inaba, H; Navrotsky, A; Eyring, L

The heat of reaction and equilibrium pressure for both the oxidation and reduction reactions, (1/7)Pr/sub 7/O/sub 12/ + (1/7 - x/2)O/sub 2/ = PrO/sub 2-x/, have been measured by means of a Tian-Calvet-type calorimeter and thermal balance. The results of equilibrium pressure measurements under isothermal conditions show a reproducible and unsymmetrical hysteresis loop. The unsymmetrical hysteresis loop is interpreted as due to a different pattern of intergrowth formed during oxidation and reduction. The ordered intermediate phases (Pr/sub n/O/sub 2n-2/) are intergrown coherently at the unit cell level with each other but not with the disordered ..cap alpha.. phase (PrO/sub 2-x/). The role of coherent intergrowth in both symmetric and unsymmetric hysteresis loops has been discussed. The partial molar enthalpy (-..delta.. anti H /sub 0//sub 2/) is about 58 kcal/mole of O/sub 2/ in the Pr/sub 7/O/sub 12/ phase; it increases slightly as O/Pr increases, then increases sharply to about 85 kcal/mole of O/sub 2/ around PrO/sub 1/ /sub 78/. No difference in partial molar enthalpy is observed between the oxidation and reduction paths within the experimental error, even though different equilibrium pressures are observed depending upon the path. The slight increase in the partial molar enthalpy in the two-phase region is attributed to the interaction term of the different domains using the same regular solution model used to account for the hysteresis observed previously. The sharp change in -..delta.. anti H /sub 0//sub 2/ around PrO/sub 1/ /sub 78/ is believed associated with the phase change from the coherently intergrown ordered phases to the disordered ..cap alpha.. phase. The difference between the partial molar entropy on oxidation and that on reduction is explained by both the regular solution model and the usual thermodynamic treatment using the data of isothermal hysteresis.

• Research Organization: Arizona State Univ., Tempe
• OSTI ID: 6776266
• Journal Information: J. Solid State Chem.; (United States), Journal Name: J. Solid State Chem.; (United States) Vol. 37:1; ISSN JSSCB
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
CHALCOGENIDES
CHEMICAL REACTIONS
ENTHALPY
HYSTERESIS
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
PRASEODYMIUM COMPOUNDS
PRASEODYMIUM OXIDES
RARE EARTH COMPOUNDS
REACTION HEAT
REDOX REACTIONS
THERMODYNAMIC PROPERTIES