FeAs: Heat capacity, enthalpy increments, other thermodynamic properties from 5 to 1350 K, and magnetic transition

• Published in: The Journal of chemical thermodynamics 1989-04, Vol.21 (4), p.363-373
• Main Authors: Gonzalez-Alvarez, Domingo, Grønvold, Fredrik, Falk, Bengt, Westrum, Edgar F, Blachnik, R, Kudermann, G
• Format: Article
• Language: English
• Subjects: Chemical thermodynamics; Chemistry; Elements, mineral and organic compounds; Exact sciences and technology; General and physical chemistry; General. Theory; Metals and alloys; Mixtures; Thermodynamic properties
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1016/0021-9614(89)90137-7

The heat capacity of iron monoarsenide has been determined by adiabatic calorimetry from 5 to 1030 K and by drop calorimetry relative to 298.15 K over the range 875 to 1350 K. A small λ-type transition is observed at T N = (70.95±0.02) K. It is related to the disappearance of a doubly helically ordered magnetic-spin structure on heating. The obviously cooperative entropy increment of transition is only Δ trs S m o R = 0.021 . The higher-temperature heat capacity rises considerably above lattice expectations. Part of the rise is ascribed to low-spin electron redistribution in iron, while the further excess above 800 K presumably arises from a beginning low- to high-spin transition, possibly connected with interstitial defect formation in the MnP-type structure. FeAs melts at about 1325 K with Δ fus H m o = 6180 R · K. Thermodynamic functions have been evaluated and the values of C p, m ( T), S m o( T), H m o( T), and Φ m o( T), are 6.057 R, 7.513 R, 1177 R · K, and 3.567 R at 298.15 K, and 8.75 R, 16.03 R · K, and 9.745 R at 1000 K.