TY - JOUR
T1 - Hidden disorder in the α ′→δ transformation of Pu-1.9 at.% Ga
AU - Jeffries, J. R.
AU - Manley, M. E.
AU - Wall, M. A.
AU - Blobaum, K. J.M.
AU - Schwartz, A. J.
PY - 2012/6/6
Y1 - 2012/6/6
N2 - Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal (ΔH=TΔS), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at.% Ga alloy, the δ phase is retained as a metastable state at room temperature, but at low temperatures, the δ phase yields to a mixed-phase microstructure of δ- and α ′-Pu. The previously measured sources of entropy associated with the α ′→δ transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the α ′→δ transformation that corroborates the theoretical prediction, and implies that only about 65% of the entropy stabilizing the δ phase is accounted for, leaving a missing entropy of about 0.5 k B/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the δ phase. This hidden disorder represents multiple accessible states per atom within the δ phase of Pu that may not be included in our current understanding of the properties and phase stability of δ-Pu.
AB - Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal (ΔH=TΔS), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at.% Ga alloy, the δ phase is retained as a metastable state at room temperature, but at low temperatures, the δ phase yields to a mixed-phase microstructure of δ- and α ′-Pu. The previously measured sources of entropy associated with the α ′→δ transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the α ′→δ transformation that corroborates the theoretical prediction, and implies that only about 65% of the entropy stabilizing the δ phase is accounted for, leaving a missing entropy of about 0.5 k B/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the δ phase. This hidden disorder represents multiple accessible states per atom within the δ phase of Pu that may not be included in our current understanding of the properties and phase stability of δ-Pu.
UR - http://www.scopus.com/inward/record.url?scp=84862183651&partnerID=8YFLogxK
U2 - 10.1103/PhysRevB.85.224104
DO - 10.1103/PhysRevB.85.224104
M3 - Article
AN - SCOPUS:84862183651
SN - 1098-0121
VL - 85
JO - Physical Review B - Condensed Matter and Materials Physics
JF - Physical Review B - Condensed Matter and Materials Physics
IS - 22
M1 - 224104
ER -