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.

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