Low Dimensional Electron Correlated Materials

This PhD thesis is devoted to investigation of inorganic materials with low-dimensional structures. Such materials attract significant fundamental and applied interest due to the potential to achieve highly anisotropic properties, which can be controlled by various external stimuli. After brief introduction to this research area in Chapter 1 and description of experimental and theoretical methods in Chapter 2, we describe the study of ternary manganese selenides, Na2Mn2Se3 and Na2Mn3Se4, in Chapter 3. The relationship between the layered crystal structure of these materials and their magnetic behavior is discussed based on the arrangement of magnetic sites in each layer, and results of quantum chemical calculation on Na2Mn2Se3 and Na2Mn3Se4 indicate both compounds are semiconductors. Na2Mn2Se3 does not exhibit magnetic ordering down to 1.8 K, but strong antiferromagnetic correlations were confirmed by field-dependent magnetization measurements. Neutron diffraction study on powder and single crystal samples also did not reveal clear signatures of magnetic ordering. The strong magnetic frustration was confirmed by calculating the frustration factor. A new antiferromagnetic compound, Na2Mn3Se4, was also discovered. It exhibits antiferromagnetic transition at 27 K. Additional magnetic transition is observed at 50 K in single crystal and powder neutron diffraction studies. The magnetic structure of this material shows a complex combination of ferromagnetic and antiferromagnetic interactions that alternate along the crystallographic a-axis. Strong magnetic frustration due to triangulated spin arrangement is confirmed. Synthesis, crystal structure, and magnetic properties of layered materials ACeSe2 (A = Li, Na) are described in Chapter 4. The unconventional magnetism of these compounds is due to the arrangement of magnetic sites on the regular triangular lattice, which causes strong magnetic frustration. A detailed investigation of magnetic properties reveals no magnetic ordering down to 1.8 K. The Weiss constant, determined from fitting experimental data to the Curie-Weiss law, is –71 K, suggesting a very large magnetic frustration factor for this system. The relationship between the structural and magnetic properties of the geometrically frustrated magnetic system is discussed. The extension of studies of layered structures to potentially two-dimensional magnetic materials, Fe3GeTe2 and Bi2MnSe4, is described in Chapter 5. These crystal structures contain stacks of 2D magnetic sheets interacting through weak interlayer van der Waals forces. Chemical vapor transport method is employed to grow high-quality single crystals. The structural and magnetic properties of these materials, including magnetocrystalline anisotropy of bulk samples, were confirmed by temperature dependent magnetic susceptibility measurement. The potential to modify magnetic behavior for few-layer materials, down to the monolayer limit obtained by mechanical or liquid phase exfoliation methods, is yet to be investigated. Chapter 6 describes the study of one-dimensional structures that contain tubular or ribbon-shaped polyphosphide fragments. Thin nano-ribbons were obtained by mechanical and liquid-phase exfoliation of bulk KP15, due to the weak van der Waals interactions between the polyphosphide tubes. Optical bandgap slightly increases as the thickness of nano-ribbons decreases which is an indication of bandgap tunability in the material. Nevertheless, the results of Raman spectroscopy, high resolution transmission electron microscopy, and selected area electron diffraction studies suggest the decreased stability of the samples that contain thinner nanoribbons (<20 nm).