First-principles calculations for the ferromagnetic systems (Co,Fe)MnGe and Co(Mn,Fe)Ge show the hexagonal cell volume decreases as an amount of Fe increases mainly because of the reduction of the lattice constant c. The Fe substitution produces a reduction in the distance between adjacent atoms along the direction of the c axis and an increase in charge density between those atoms. This enhancement of the covalent bond is responsible for the hexagonal-structure stabilization or a decrease of the structural transition temperature from hexagonal to orthorhombic phases.

First-principles calculations were performed to investigate the effect of Fe substitution on the structural transformation of (Mn,Fe)CoGe and Mn(Co,Fe)Ge. The activation energy (barrier) between orthorhombic and hexagonal structures was estimated from the total energy of each of several virtual structures between them.Fe substitution reduces the activation energy, and movements of both of Co and Mn are closely related to the reduction. Moreover, the calculation result for the Fe substitution at the sites of both Mn and Co indicates that Fe atoms randomly occupy Mn and Co sites.