High-pressure synthesis, crystal structure, and magnetic properties of Mn_1-xT_xGe₄ (T: Co, Fe)

Mn_1-xT_xGe₄ (T: Co, Fe) solid solution was synthesized at 6 GPa and 650-750°C using a belt-type high-pressure apparatus. The crystal structure of the Mn_1-xT_xGe₄ solid solution is related to the β-NiHg₄ type. The following structural changes were observed in the Mn_1-xCo_xGe₄ system: tetragonal (x = 0) to orthorhombic (0 < x < 0.2) and orthorhombic to pseudotetragonal (0.2 ≦ x ≦ 0.4). Similar structural changes were also observed at 0 < x < 0.4 (tetra. → ortho.) and x = 0.4 (ortho. → pseudotetra.) in the Mn_1-xFe_xGe₄ system. These results indicate that the structural changes in Mn_1-xT_xGe₄ strongly depend on the average valence electron number of the transition metal atoms. Mn_1-xT_xGe₄ solid solutions are itinerant electron ferromagnets and the Curie temperature decreases linearly with decreasing unit-cell volume.