Hydrothermal Synthesis of Single-Crystal Magnetic Oxides

Multifunctional materials are systems that have two or more properties of interest. Some of these features are polar axes, chirality, magnetic ions, triangular arrangements, and/or magnetically silent building blocks. Various combinations of these properties can result in chiral ferromagnetism, multiferroic magnetoelectric behavior, antiferromagnetism, and quantum spin liquids and spin ices. By tuning these structure-property relations, quantum computing, memory, and sensors may be created. Large single crystals allow for a better understanding of the magnetic behavior of these crystals since they can be oriented in a magnetic field. Inorganic single crystals have a wide range of applications but growing high-quality single crystals can be challenging. Some methods cannot solubilize refractory oxides, create crystals without defects, or produce large single crystals for good-quality magnetic data collection. However, the hydrothermal method can minimize or eliminate these issues due to the lower thermal energy within the system, therefore, hydrothermal synthesis was utilized to grow a range of magnetic materials. This has been demonstrated through analysis using various techniques such as powder X-ray diffraction, single-crystal X-ray diffraction, electron dispersive spectroscopy, infrared spectroscopy, and physical property measurements. The first goal of this dissertation was to exploit the hydrothermal method to learn how to optimize various promising frustrated triangular arrangement systems such as half-delta, delta, and Kagome materials. There were some preliminary magnetic studies with Na2Co3(VO4)2(OD)2, K2Co3(VO4)2CO3, K2Co3(MoO4)3(OH)2, Co2(MoO4)(OH)2, and CsCo2(MoO4)2(OH) showing complex, and sometimes, ambiguous magnetic ordering. Therefore, fine-tuning hydrothermal conditions to grow larger crystals for oriented magnetic measurements and for neutron scattering studies was necessary. The MoO42- and VO43- ions served as magnetically silent building blocks while the magnetic TM ions were assembled in a triangular arrangement in relation to each other. The structures of various oxyanion building blocks such as phosphates, vanadates, arsenates, molybdates, and bismuthates are well-known and vast in literature, but there is limited descriptive and structural chemistry of antimonates. Antimony, another magnetically silent element, led to a series of large single crystals of a known system that lacked anisotropic magnetization data. The second goal studied the magnetic interactions within the TMSb2O4 (TM = Mn, Co, Ni) system after the large growth of well-ordered single crystals. The relatively simple crystal structure displayed various magnetic pathways between TM2+ ions, including intrachain direct exchange, superexchange, and interchain superexchange. Within the TM–Sb–O system, two other structures also arose: TMSb2O6 and TM2Sb2O7. Exploring the Mn–Sb–O phase space resulted in new compounds and structure types: BiMnFe2O6-type MnSbO3, Mn5Sb2O10, Mn11Sb5O24, spinel-type Mn(Mn1.5Sb0.5)O4, and Wadsleyite-type (Mn1.5Sb0.5)MnO4. The pyrochlore structure type was of particular interest as this type displays unusual magnetic and quantum material behavior when rare earth elements are used, therefore, rare earth ruthenates were pursued. The third goal focused on growing large single crystals of another magnetically interesting class of compounds, namely the pyrochlores with emphasis on heavy transition metal ion building blocks. For second-row elements, the orbital overlap and spin-orbit coupling are comparable in energy, so they have the potential to have fascinating magnetic and spectroscopic properties. Furthermore, ruthenium can exist in multiple oxidation states, so its reaction chemistry can be very extensive when combined with the lanthanides. Hydrothermal synthesis is particularly required for single crystal growth of lanthanide ruthenates since many of the starting materials are refractory and have only been grown by traditional solid-state synthesis as powders with site disorder and defects. In addition to pyrochlore (Ln2Ru2O7 where Ln= lanthanides) formation, this work also led to the growth of ruthenium lanthanide perovskites and other interesting structure types (LnRuO3, Ln2RuO5(OH), and Ln5Ru2O12) with minimal site disorder and defects. Complex magnetic interactions were demonstrated with magnetization data for Ln2Ru2O7 (Ln = Pr, Nd) and LnRuO3 (Ln = La, Pr, Nd). This work has contributed significantly to advancing the understanding of these fascinating systems, but there is much more exploration and studies needed to gain insight into their structure-property relationships to reveal attractive multifunctional properties.