"Leapfrog Thermodynamics" among Binary Magnetic Phases Competing for Stability

• Published in: Journal of solid state chemistry Vol. 116; no. 1; pp. 92 - 94
• Main Author: Parks, T.C.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.04.1995; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Heat capacities of solids; Materials science; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Phase diagrams of metals and alloys; Physics; Thermal properties of condensed matter; Thermal properties of crystalline solids; Thermodynamic properties; Samarium alloys; Phase diagrams; Intermetallic compounds; Theoretical study; Rare earth alloys; Binary systems; Thermodynamic properties; Cobalt alloys; Transition element alloys
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1006/jssc.1995.1187

Relative to Sm2Co7 plus Sm2Co17, SmCo5 is stable below 700 K and above 1000 K. Contrary to previous thinking, this is thermodynamically consistent. The free energies of formation of these phases seem to be marginal when estimated from data reported for the analogous compounds of other lanthanons. Above its Curie point at 713 K, Sm2Co7 is stabilized by its gain in entropy from magnetic disordering, so that SmCo5 loses the competition for stability. The Curie point of SmCo5 is 1020 K, above which SmCo5 experiences a similar gain in entropy, stabilizing it relative to Sm2Co7 plus Sm2Co17. Such "leapfrog thermodynamics" are especially evident in binary systems like Sm-Co, in which each discrete compound is subject to an order-disorder transition, for which the critical temperatures follow a monotonic trend across the phase diagram.