Synthesizing New [(SnSe)1.15]m(TSe2)n, [(SnSe)1.16]m(VSe2)n[(SnSe)1.16]p(TaSe2)q, and (SnSe)1.16(V.51Ta.49Se2) Intergrowth Compounds (T = V and Ta)
Date
2013-10-03
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University of Oregon
Abstract
A modification of the modulated elemental reactants synthetic technique was developed and used to synthesize several new layered compounds. Several TSe2, [(SnSe)1+y]m(TSe2)n, [(SnSe)1+y]m(TSe2)n[(SnSe)1+y]p(T'Se2)q, and (SnSe)1+y(V1-xTaxSe2) layered compounds were synthesized by the new modulated elemental reactant (MER) technique with T = V, Ta, and Ti. The MER approach is a low-temperature synthesis that allows the kinetic trapping of metastable compounds, allowing a designed synthesis with control over the value of m, n, p, and q. These layered compounds were structurally characterized by X-ray diffraction and transmission electron microscopy studies. Each integer increase of m, n, p, and q resulted in a linear increase in thickness, representing the single structural units of SnSe (a Sn/Se atomic bilayer) and TSe2 (an X-T-X trilayer sandwich). All of the compounds contained preferentially oriented layering, with the layer's (00l) plane parallel to the substrate surface. From Bragg-Brentano and in-plane X-ray diffraction studies, the layers are found to be regularly spaced along c with abrupt interfaces and contain crystallinity in their ab-plane. Many of these layered compounds were found to exhibit turbostratic disorder, a random rotational disorder that is usually present in materials made by the MER technique. The presence of turbostratic disorder was found to be dependent on the polytypes that exist in the bulk form of the TSe2 constituent. The electrical properties of the layered compounds were studied by means of temperature-dependent resistivity and Hall measurements. Interesting electrical properties were found as a result of the turbostratic disorder, including a charge density wave transition found in the [(SnSe)1.15]m(VSe2) ferecrystal. The onset temperature of the CDW transition was found to be a sensitive function of the layering sequence, increasing with higher m values. The CDW transition was attributed to the VSe2 constituent and was found in all the composite crystals that contained VSe2. It was found that the [(SnSe)1+y]m(VSe2)n and [(SnSe)1+y]m(TaSe2)n intergrowths could be combined into an ABCB layered [(SnSe)1+y]m(VSe2)n[(SnSe)1+y]p(TaSe2)q intergrowth, with chemical separation of the VSe2 and TaSe2 constituents.
This dissertation includes previously published and unpublished co-authored material.