The exploratory synthesis of low-dimensional inorganic solids: Structure and property correlation studies of novel mixed-metal niobium oxo compounds

Abstract

The search for a new class of compounds with quasi-low-dimensional frameworks has become important for the development of new transition metal oxides that are of magnetic and electronic importance. The exploratory synthesis of low-dimensional inorganic solids, in particular reduced niobium oxophosphates and oxosilicates containing extended, but confined, M-O-M and M-M bond interactions, has been investigated. These bonding types are common structural features observed in metallic oxides that possess unusual phenomena associated with charge density waves, superconductivity, and magnetic ordering. Ultimately, these new compounds should lead to structural models for experimental and theoretical studies of the behavior of delocalized electrons in a confined lattice. A number of significant solid state compounds with interesting structures and physical properties have been prepared through our exploratory synthesis. Four tetragonal tungsten bronze-related (TTB) mixed-metal oxides have been made; namely BaNb$\sb2\rm O\sb{6-\rm x},$ orthorhombic NbPO$\sb5,$ and ANb$\sb3\rm P\sb3O\sb15$ (A = Cs, and Ba). The niobium oxophosphate compounds, BaNb$\sb7\rm P\sb6O\sb{33}$ and CaNb$\sb2$P$\rm\sb2O\sb{11},$ with mixed-frameworks and fused NbO$\sb6$ octahedra have also been prepared and studied. Finally, a series of niobium-rich oxosilicates with the general formula (Ba$\sb3\rm Nb\sb6Si\sb4O\sb{26})\sb{n}(Ba\sb3Nb\sb8O\sb{21}$) has been identified with quasi-one-dimensional transition metal oxide chains. Various synthetic techniques have been employed to prepare the title compounds, in particular a conventional high-temperature solid-state ceramic method and a molten salt flux method. To characterize these new materials, a variety of analytical techniques have been utilized including both single crystal and powder X-ray diffraction, Weissenberg X-ray photographs, thermal analysis (TGA and DTA), magnetic susceptibility and four-probe conductivity measurements. Additional relevant techniques for characterization of inorganic solids were also used. Our study of structure/property correlations should prove insightful towards a better understanding of the conducting mechanism in technologically important metallic oxides and in the development of new materials with desired applications.