Heat capacity of MnAs 0.88P 0.12 from 10 to 500 K: Thermodynamic properties and transitions

Abstract

The heat capacity of MnAs 0.88P 0.12 has been measured by adiabatic shield calorimetry from 10 to 500 K. It is shown that very small energy changes are connected with two magnetic order-order transitions, indicating that these can be regarded as mainly “noncoupled” magnetic transitions. At higher temperatures contributions to the excess heat capacity arises from a magnetic order-disorder transition, a conversion from low- to high-spin state for manganese, and a MnP- to NiAs-type structural transition. The observed heat capacity is resolved into contributions from the different physical phenomena, and the character of the transitions is discussed. In particular it is substantiated that the dilational contribution, which includes magnetoelastic and magnetovolume terms as well as normal anharmonicity terms, plays a major role in MnAs 0.88P 0.12. The entropy of the magnetic order-disorder transition is smaller than should be expected from a complete randomization of the spins, assuming a purely magnetic transition. Thermodynamic functions have been evaluated and the respective values of C p , { S O m ( T) - S O m (0)}, and -{G O m(T) - H O m(0)} T at 298.15 K are 68.74, 72.09, and 32.30 J K −1 mole −1, and at 500 K 56.05, 108.12, and 56.64 J K −1 mole −1.