Ab initio theoretical study of magnetization and phase stability of the (Fe,Co,Ni)23B6 and (Fe,Co,Ni)23Zr6 structures of Cr23C6 and Mn23Th6 prototypes

We present an ab initio theoretical investigation of the magnetization and phase stability of two different complex cubic structures of prototype Cr23C6 and Mn23Th6, with an emphasis on the (Fe,Co,Ni)(23)B-6 and (Fe,Co,Ni)(23)Zr-6 compositions. These phases have recently been observed as secondary or even primary crystallization products of (Fe,Co,Ni)-Zr-B and related metallic glasses that have been studied for applications as soft magnets with nanocrystalline grain size. We first demonstrate the validity of the theoretical technique employed through a detailed comparison between the predictions of the calculations for the Co-Zr binary system and the experimentally stable phases. We then investigate the magnetization and stability of the binary phases. While the Fe-based binary Fe23Zr6 and Fe23B6 phases are expected to have the highest magnetization, the Co-based binary Co23Zr6 and Co23B6 structures are predicted to be the most stable of each prototype. The Co23Zr6 structure is the only binary 23:6 structure predicted to be a stable phase for the (Fe,Co,Ni)(23)B-6 and (Fe,Co,Ni)(23)Zr-6 systems investigated here. Small additions of Zr atoms to the (Fe,Co,Ni)(23)B-6 phases tend to substitutionally occupy the 8c Wykoff site and stabilize these structures. In contrast, small additions of B to the (Fe,Co,Ni)(23)Zr-6 phases have a much weaker site preference and tend to destabilize these structures. As a result, (Fe,Co,Ni)(23)B-6 structures are stabilized in (Fe,Co,Ni)-Zr-B systems relative to the binary (Fe,Co,Ni)(23)B-6 systems while the (Fe,Co,Ni)(23)Zr-6 phases are not. The results presented in this work are in good qualitative agreement with experimental observations of the compositional modifications tending to promote formation of the 23:6 phases in Fe-Co-Zr-B and related metallic glasses.