Deviation of congruent composition in Fe-Pd system

The first-principles calculation of the disordered L1₀ phase boundary for the Fe-Pd system is attempted by combining the FLAPW electronic structure total energy calculations and the cluster variation method via the cluster expansion method. The lattice vibration effects are taken into account based on the Debye-Gruneisen model within quasi-harmonic approximation. The transition temperature is reproduced with very high accuracy. However, the experimental congruent composition of disordered L1₀ phase that significantly deviates from 1:1 stoichiometry is not reproduced. Further calculations are attempted based on the phenomenological Lennard-Jones type pair potential, which is capable of introducing both tetragonality of the L1₀ ordered phase and additional configurational freedom because of the magnetic spins. The preliminary calculations indicate that the tetragonality enhances the stability of the L1₀ ordered phase and the magnetic contributions also change the transition temperature. Despite these findings, the shift of the congruent composition still remains as a subject that needs further research. The electronic origin of the shifting of the congruent composition is briefly discussed.