Quenching processes typically produced disordered metastable phases, which can be changed into the ordered phase via the post-annealing process. For instance, YFe12 fabricated by the quenching process is among the phases whose structure can be transformed from unordered to ordered states. Similar trends in structural changes have also been reported in as-spun Y2Fe18.9, Sm(Fe, Si)9, and Sm(Fe, Co)12. Unlike TbCu7 structure, which is configured by randomly substituting a pair of iron atoms (iron dumbbells) for rare-earth (R) atoms on CaCu5 structure, ThMn12 structure is generally configured by substituting the iron dumbbells for only half of the R atoms in the CaCu5 structure. To date, different structural models for treating continuous changes from one structure to another, i.e., CaCu5 to TbCu7 or TbCu7 to Th2Ni17, by substituting the iron dumbbells for R atoms have been developed. Unfortunately, the application of the available models is limited to lattice distortions with the same rotation axis and symmetry. Therefore, the development of more reliable structural models for analysis and treatment of hexagonal and tetragonal systems is highly desirable.
To this note, Dr. Hiroyuki Suzuki from the Magnetic Materials Research Laboratory at Hitachi Metals, Ltd. (/ the Research & Development Group at Hitachi, Ltd.) in Japan developed a new structural model for analyzing the magnetic properties of lattice distortions from hexagonal to tetragonal systems in non-equilibrium Y – Fe alloys. The model was specialized by space group Immm and denoted by substituting the iron dumbbells for R atoms. Specifically, the treatment of lattice distortions from hexagonal TbCu7 to tetragonal ThMn12 structures was analyzed. The model was also used to examine the peculiar trajectories of lattice distortions due to annealing, depending on the compositions of the Y – Fe alloys. The work is currently published in the research journal, Intermetallics.
The author reported that the model could treat continuous distortions from hexagonal to tetragonal via orthorhombic structures. The trajectories of the lattice distortions could be classified into four categories depending on the composition of the Y – Fe alloys. For instance, type-I distortion was transformed from a hexagonal structure with an axial ratio (bortho/aortho) of 1.002 to a tetragonal one with bortho/aortho = 1.000 via one with large distortion of bortho/aortho = 1.008. On the other hand, type-II distortion transformed from a hexagonal structure with bortho/aortho = 1.001 to an orthorhombic structure close to a tetragonal structure. Both type-I and type-II distortions exhibited random substitutions of the iron dumbbells for yttrium atoms. Moreover, it was worth noting that the developed continuous-lattice-distortion model is versatile and could also be used to analyze the magnetic properties and structural stability of compounds with modulated CaCu5 structures which are also configured by substituting the iron dumbbells for R atoms.
In summary, Dr. Suzuki devised a continuous-lattice-distortion model for analyzing the structural stability and magnetic properties of lattice distortions from hexagonal to tetragonal systems in non-equilibrium Y – Fe alloys. Results showed that the model could treat continuous distortions from hexagonal to tetragonal via orthorhombic structure, while the trajectories of lattice distortions were classified into four categories depending on the composition ratio of the Y – Fe alloys. Furthermore, the presented model could also bring geometric insights on the magnetic properties and structural stability of previously reported compounds with modulated-CaCu5 structures. Overall, the study findings would advance both theoretical and experimental analysis of different metaphases.
Suzuki, H. (2020). Structural analysis and magnetic properties of lattice distortions from hexagonal to tetragonal systems in non-equilibrium Y–Fe alloys. Intermetallics, 119, 106713.