Expanding Well-Studied Structural Families of Materials through Exploratory Synthesis

Flynn, Steven Matthew

doi:https://doi.org/10.21985/n2-djxp-ah44

Work

Expanding Well-Studied Structural Families of Materials through Exploratory Synthesis

Public

Download PDF

Download All Files (.zip)

Continuous and coordinated materials discovery efforts have amassed a wealth of knowledge concerning many general classes of materials. The number of known phases of all structure-types, however, is far less than number of possible materials dictated by the elements on the periodic table. Recently, with improved computational abilities and well-developed theory-based methods, the focus in the materials discovery community has shifted away from exploratory approaches towards targeted synthesis of functional materials predicted by theory. However,several types of materials are ill-suited for a computational approach, such as phases that are metastable, exhibit significant disorder, or have new structure types. This thesis demonstrates that exploratory synthesis complements computational strategies by identifying previously unknownsynthetic insights in the well-studied fluorite, rutile, and α-UO3 structural families. Anion-deficient fluorite materials exhibit a number of commercially useful properties arising from the specific arrangement of anion vacancies in each structure. Cu, Mn, Fe, Co, and Ni substitutions were performed in a new member of this family, Zn0.456In1.084Ge0.46O3, to attempt to tune its properties and probe the generalizability of its unique structure. While substitution wasfound to greatly lower both transparency and conductivity, a surprising flexibility for transition metals and tendency for single site substitution revealed the extent to which order and disorder may coexist in fluorite-related materials. Rutile-related materials are widely applied and well-studied, with tunnels that are beneficial to ionic transport applications. Single-crystal growth of LiIn2SbO6 revealed an unprecedented variation on the rutile-related structure in which the chains and channels exhibit an alternating-width configuration. Substitution of In with Sc or Fe appears to affect Li ordering, potentially changing the space group with implications for Li mobility and noncentrosymmetric properties. A relationship between alkali metal size and chain structure is suggested which may lead to design principles for targeting phases with alternating-width channels. Materials in the anion deficient α-UO3 family are known for their large, complex structures. A new, phase pure member of the family was synthesized by the reaction of HfO2 and NH4HF2, potentially the Hf analogue to Zr7O9F10. Another distinct unreported hafnium oxyfluoride phase, which could not be isolated, was obtained by fluorination of HfO2 with polymer-based fluorinating agents. Comparison to the hydrolysis of β-HfF4 under identical conditions shows that the NH4HF2 route produces the oxyfluoride with greater selectivity and at lower temperatures. Potential reaction pathways for the NH4HF2 fluorination of HfO2 are discussed. These new discoveries are substantive additions to the already expansive body of knowledge about these three structural families and they suggest that there is still more information that can be gained by exploring well-studied systems. Furthermore, as disordered phases and new structures, these additions would also be difficult to predict with a computational approach. Together, these results demonstrate how exploratory synthesis can complement theory-based predictions to generate fundamental insights. As such, both strategies should be employed by future materials discovery initiatives for optimal outcomes.

Creator