Microstructure study of the rare-earth intermetallic compounds R5(SixGe1-x)4 and R5(SixGe1-x)3
The unique combination of magnetic properties and structural transitions exhibited by many members of the R5(SixGe1-x)4 family (R = rare earths, 0 ≤ x ≤1) presents numerous opportunities for these materials in advanced energy transformation applications. Past research has proven that the crystal structure and magnetic ordering of the R5(SixGe1-x)4 compounds can be altered by temperature, magnetic field, pressure and the Si/Ge ratio. Results of this thesis study on the crystal structure of the Er5Si4 compound have for the first time shown that the application of mechanical forces (i.e. shear stress introduced during the mechanical grinding) can also result in a structural transition from Gd5Si4-type orthorhombic to Gd5Si2Ge2-type monoclinic. This structural transition is reversible, moving in the opposite direction when the material is subjected to low-temperature annealing at 500 &^°C.
Successful future utilization of the R5(SixGe1-x)4 family in novel devices depends on a fundamental understanding of the structure-property interplay on the nanoscale level, which makes a complete understanding of the microstructure of this family especially important. Past scanning electron microscopy (SEM) observation has shown that nanometer-thin plates exist in every R5(SixGe1-x)4 (“5:4”) phase studied, independent of initial parent crystal structure and composition. A comprehensive electron microscopy study including SEM, energy dispersive spectroscopy (EDS), selected area diffraction (SAD), and high resolution transmission electron microscopy (HRTEM) of a selected complex 5:4 compound based on Er rather than Gd, (Er0.9Lu0.1)5Si4, has produced data supporting the assumption that all the platelet-like features present in the R5(SixGe1-x)4 family are hexagonal R5(SixGe1-x)3 (“5:3”) phase and possess the same reported orientation relationship that exists for the Gd5Ge4 and Gd5Si2Ge2 compounds, i.e. (10-2)m || [10-10](1-211)p. Additionally, the phase identification in (Er0.9Lu0.1)5Si4 carried out using X-ray powder diffraction (XRD) techniques revealed that the low amount of 5:3 phase is undetectable in a conventional laboratory Cu Kαdiffractometer due to detection limitations, but that extremely low amounts of the 5:3 phase can be detected using high resolution powder diffraction (HRPD) employing a synchrotron source. These results suggest that use of synchrotron radiation for the study of R5(SixGe1-x)4 compounds should be favored over conventional XRD for future investigations.
The phase stability of the thin 5:3 plates in a Gd5Ge4 sample was examined by performing long-term annealing at very high temperature. The experimental results indicate the plates are thermally unstable above 1200&^°C. While phase transformation of 5:3 to 5:4 occurs during the annealing, the phase transition is still fairly sluggish, being incomplete even after 24 hours annealing at this elevated temperature. Additional experiments using laser surface melting performed on the surface of a Ho5(Si0.8Ge0.2)4 sample showed that rapid cooling will suppress the precipitation of 5:3 plates.
Bulk microstructure studies of polycrystalline and monocrystalline Gd5Ge3 compounds examined using optical microscopy, SEM and TEM also show a series of linear features present in the Gd5Ge3 matrix, similar in appearance in many ways to the 5:3 plates observed in R5(SixGe1-x)4 compounds. A systematic microscopy analysis of these linear features revealed they also are thin plates with a stoichiometric composition of Gd5Ge4 with an orthorhombic structure. The orientation relationship between the 5:3 matrix and the precipitate 5:4 thin plates was determined as [10-10] (1-211)m ||  (10-2)p.