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

Abstract

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 [lambda]-type transition is observed at TN = (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 [Delta]trsSmo/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 [Delta]fusHmo = 6180R [middle dot] K. Thermodynamic functions have been evaluated and the values of Cp, m(T), Smo(T), Hmo(T), and [Phi]mo(T), are 6.057R, 7.513R, 1177R [middle dot] K, and 3.567R at 298.15 K, and 8.75R, 16.03R [middle dot] K, and 9.745R at 1000 K.