Date of Award
Doctor of Philosophy (PhD)
Joseph W. Kolis
Colin D. McMillen
O. Thompson Mefford
Novel modern materials are constantly being discovered as humanity seeks constantly better improvement to the optics and electronics around us, from lasers used in medical therapies to the magnets and supercomputing chips in our phones. Inorganic oxides commonly draw inspiration from naturally occurring minerals to template new discoveries through substitution of similarly behaving elements with the goal of inducing certain desired properties, such as ferroelectricity or creating the elusive quantum spin liquid. While many minerals are silicates, its periodic table neighbor germanium(IV) has a rich and under-explored crystal chemistry that could contain many new structures and magnetic materials. Another common tetrahedral building block forms with molybdenum(VI), whose different oxidation state leads to other exciting frustrated magnetic compounds not seen with germanium.
This dissertation details the synthesis of novel tetrahedral germanate and molybdate transition metal oxides using hydrothermal crystal growth and high-quality single crystal x-ray diffraction data. Tetrahedra have an inherent threefold symmetry which can induce magnetic frustration in certain choice cations, such as first-row transition metals like Co2+. Germanium and molybdenum are both magnetically silent compared to the utilized transition metals, and therefore provide an excellent framework for the study of the transition metal magnetic behavior in various complex crystal structures. Additionally, all the germanates were categorized by their synthetic products compared to known and naturally occurring silicate minerals to better elucidate how the crystal chemistry of the two cations deviate.
In this study, new germanates in the mineral family of richterite (A(A,B)Mn5Ge8O22-x(OH)x, A = Na, K, Rb; B = Na, Sr, Ba), vesuvianite (Sr19Fe12Ge19O72(OH)6), greenwoodite (Ba2Fe11Ge2O22), and aenigmatite (ideally Na2M5GeGe6O20, M = Mn, Co) are discussed with thorough reference to their crystallographic nature. A comparative study was then performed between silicon and germanium to determine if the same synthesis would produce the same results. The vastly different chain-like structure K2Mn2Si3O9 in space group Pn and the germanate superstructure of K11Mn21Ge32O86(OH)9(H2O) in Fddd were created, demonstrating that even identical reaction conditions do not guarantee the same isomorphs will form. A novel germanate cluster was also shown in Ba4Mn3Ge7O21, which theoretically could produce a silicate isomorph but was shown not to in another comparative study. The germanate study culminated with the new structure type of Rb2Ba7Co6Ge18O50 and analogue K2Sr7Cu6Ge17.3O47.2(OH)2.8, whose space group assignment of P63cm gives these two samples potential as ferroelectrics. Their preliminary magnetic data is discussed within.
The exploratory molybdate work into three triangular magnetic systems of the half-delta chain (K2M3(MoO4)3(OH)2, M = Co, Mn), full-delta chain (AM2(MoO4)2(OH), A = Cs, Rb; M = Co, Cu), and Kagomé lattice (AM2(MoO4)2O(OH), A = K, Rb; M = Co, Mn, Cu) produced new members of each of these structure types through a systematic study of stoichiometry and mineralizer identity/concentration. This study also produced two novel structure types with the formula of M2(MoO4)(OH)2 (M = Co, Mn) which could produce magnetic frustration due to the molybdate tetrahedron corner-sharing at each vertex to magnetically active transition metal cations. When alkali chloride mineralizers were used, many members of the MMoO4 (M = Mn, Co, Cu) structural spectrum formed, indicating that mineralizer pH may prove an essential factor in the synthesis of desired molybdate triangular phases. Several new structure types are presented which further explore the rich synthetic chemistry of germanium and molybdenum as building blocks for novel transition metal oxide materials.
Smart, Megan, "Hydrothermal Synthesis of First-Row Transition Metal Polyanions Towards the Design of Frustrated Magnetic Materials" (2023). All Dissertations. 3521.
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