Date of Award

5-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Hwu, Shiou J

Committee Member

Pennington , William T

Committee Member

Smith , Rhett

Committee Member

Marinescu , D C

Abstract

In this research, focus has been placed on the study of magnetic anomalies due to geometric frustration of magnetic ions. The goal of the study presented in this dissertation was to synthesize pseudo&ndashlow&ndashdimensional (PLD) compounds, those structures showing zero (dot), one (chain), and two (sheet) dimensional lattices observed in extended (3D) host frameworks with enhanced anisotropic physical properties, in an attempt to elucidate some structure&ndashproperty correlations arising from magnetic spin confinement. The scope of this research involved 1) exploratory synthesis of novel, low&ndashdimensional transition metal oxide lattices, specifically Mn and V, embedded in non&ndashmagnetic oxyanion frameworks, either Ge, P, or As, employing various solid state techniques, 2) the characterization of structures and physical properties of these materials, including by not limited to the X&ndashray crystallographic structures and dc magnetic susceptibilities, and 3) the investigation of structure&ndashproperty correlations to possibly identify the origins of any interesting magnetic properties. The ability to correlate specific structural features to targeted magnetic behaviors is of great technological importance, particularly in the fields of data storage and information processing, with the outlook of quantum computing and ever-increasing information storage capacity needs.
In the following studies, typical reactions employed the combination of transition metal oxides and oxyanions of the type XOmn- or X2Omn- mixed with alkali halide salts heated to above either the melting point of the salts and/or the decomposition points of the oxides. The use of the halide salt flux is important for various reasons. First, the corrosive nature of the molten salt is very effective at digesting the oxides and increasing their reactivity with one another at reasonable temperatures (&le900°C). With the high melting and decomposition points of many oxide materials, reactivity can be fairly slow below very extreme temperatures (&ge1000°C). The salt essentially acts as a solvent for the reaction of the oxides. Secondly, due to the solvent&ndashlike nature, crystal growth from the medium can be controlled to some extent based on the salts employed and the cooling of the reaction. Just as in crystal growth mechanisms in hydrothermal or solvothermal reactions at low temperatures (&le300°C), the slow cooling of the molten salt solution can allow for crystal nucleation and growth, depending on the amount of time involved in the digestion and cooling processes and the nature of the salt flux employed. For example, a single halide salt versus a eutectic mixture of different alkali halides can produce different products or product distributions due to the varying interactions of the reactants with the different salt lattices and components. Finally, the salt flux can be a source of alkali metal ions that otherwise would typically be difficult or impossible to include in the reaction mixture. The most notable of these is that of Rb and Cs for the fact that one cannot purchase the oxide forms of these materials. The only way to incorporate them is to generate them in situ using either decomposition products from hydroxides or carbonates or from the metathesis reaction between the halide salt and another alkali oxide.
The research presented is divided according to the metal and oxyanion employed in the reactions. Initially, examination of germanates was performed due to the favorable coordination properties of Ge. First, germanates have been shown to occupy multiple coordination environments, including 4&ndashcoordinate tetrahedral, 5&ndashcoordinate square pyramidal, and 6&ndashcoordinate octahedral, and to occupy mixtures of these within the same compound. The ability of Ge to occupy these different sites means that it can play various roles in the structure, from discrete spacing ions to mixed site occupancy with typically 6&ndashcoordinate metal ions. Second, germanates have shown tendencies to form clustered secondary building units (SBU), including but not limited to metagermanate chains, rings, and clusters ( &ge7 Ge ), which add additional insulating oxyanions between magnetic centers. These SBUs aid in one of the ultimate goals of this research to design structures with isolated magnetic sublattices. The tendency of Ge to form SBUs drives the transition metal oxide lattice into smaller units within the overall extended structure, breaking down long range ordering of the magnetic interactions.
The change to vanadium phosphates and arsenates was due to two initial factors. First, V shows similar coordination anomalies as Ge, specifically the abilities to occupy multiple coordination environments. Vanadium is seen in tetrahedral, square pyramidal, and octahedral, however, these coordinations can be controlled to a greater extent than that of Ge due to oxidation states. Various oxidation states of V demand particular coordination geometries, and therefore, can drive the structural formations. For example, V3+ is typically octahedral, while V5+ is typically tetrahedral or square pyramidal; V4+ is the exception being able to occupy any of the three coordination environments. Second, reactions using both V and Ge were less than successful, so P and As oxides were employed. Simply put, the reactions performed showed much better reactive compatibility and control between the V and P and As than with the Ge.
Structures resulting from these various studies included 1D chain compounds of Na3-x(Mn3-xGex)O2(Ge4O12) (x = 0.24(1) - 0.30(3)) (Chapter 3) and AV2O3(PO4) (A = Rb, Cs) (Chapter 4), synthesized containing confined triangular lattices of vanadium and manganese oxides linked through closed&ndashshell, nonmagnetic oxyanion linkers, [PO4]3- and [GeO4]4-, respectively. These materials offer an insight into the interchain vs. intrachain interactions of frustrated equilateral triangle 1D chains that are of fundamental importance for design synthesis of extended solids with quantized properties. The 2D layered Kagomé compound AMn3O2(Ge2O7) (A = K, Rb) (Chapter 5) is another example of a low&ndashdimensional compound allowing exploration of the complicated ground states of a fully frustrated triangular sheet. Finally, as an intermediate between 1D chains and 2D sheets is the compound Na2Mn5(Ge4O11)2 (Chapter 3). This compound is constructed of 3&ndashwide manganese octahedra slabs separated from one another by chains of metagermanate rings and channels containing Na cations. Other synthesized compounds have taken advantage of the inherent properties of germanates and vanadates to generate low&ndashdimensional structures or salt&ndashincluded compounds. In this dissertation, we will present the synthesis, structure and properties of some selected PLD solids that show novel magnetic properties.

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