| 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-low-dimensional (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-property correlations arising from magnetic spin confinement. The scope of this research involved 1) exploratory synthesis of novel, low-dimensional transition metal oxide lattices, specifically Mn and V, embedded in non-magnetic 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-ray crystallographic structures and dc magnetic susceptibilities, and 3) the investigation of structure-property correlations to possibly identify the origins of any interesting magnetic properties.;In the following studies, typical reactions employed the combination of transition metal oxides and oxyanions of the type XOm n- 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 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--coordinate tetrahedral, 5--coordinate square pyramidal, and 6--coordinate octahedral, and to occupy mixtures of these within the same compound. Second, germanates have shown tendencies to form clustered secondary building units (SBU), including but not limited to metagermanate chains, rings, and clusters ( > 7 Ge ), which add additional insulating oxyanions between magnetic centers.;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, V 3+ 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(Ge4 O12) (x = 0.24(1) - 0.30(3)) (Chapter 3) and AV 2O3(PO4) (A = Rb, Cs) (Chapter 4), synthesized containing confined triangular lattices of vanadium and manganese oxides linked through closed-shell, nonmagnetic oxyanion linkers, [PO4] 3- and [GeO4]4-, respectively. The 2D layered Kagome compound AMn3O2(Ge2O7 ) (A = K, Rb) (Chapter 5) is another example of a low-dimensional 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--wide 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-dimensional structures or salt-included compounds. In this dissertation, we will present the synthesis, structure and properties of some selected PLD solids that show novel magnetic properties. (Abstract shortened by UMI.)... |
