Chemists and condensed matter physicists alike have long searched for compounds that can shed light on electronic behavior in solids. Electronic behavior is usually assessed by two straightforward ways: conductivity and magnetism. The interactions that determine magnetic states give clues as to the lattice contribution and the atomic orbital interactions. This thesis investigates three systems for their electronic, magnetic, and structural properties: firstly, three double perovskites with very similar structures but different magnetic properties; secondly, a family of compounds with a cubic structure that theoretically should superconduct but doesn't; and lastly, the effects of molybdenum on the structure, magnetic, and electronic properties of VO2.
Two new compounds, La2NiVO6 and La2CoVO 6 were synthesized along with the previously studied La2CoTiO 6. While all three compounds have the double perovskite structure, they exhibit very different magnetic properties. Only La2CoTiO 6 was found to have an ordered magnetic structure, the result of the transition metals ordering. The other two compounds had antiferromagnetic interactions, but with Ni and V mixed on a site and Co and V mixed on a site, neither exhibited long-range magnetic ordering.
From theory, M6Ni16Si7 (M=Mg, Sc, Ti, Nb, or Ta), should be superconducting. These five compounds were synthesized, and their magnetic and electronic properties were measured with surprisingly consistent magnetic behavior over the wide range of electron counts. Measurements revealed no superconductivity, contrary to expectations.
VO2 has a rather unique metal-insulator transition that occurs just above room temperature, which has been studied for decades. The insulator phase of VO2 contains V-V dimers and little magnetic activity is expected. By adding Mo, local magnetic states are created by disrupting these V-V dimers. For every Mo4+ added, an equal number of V 4+ ions displayed a magnetic moment, indicating the breaking up of V-V pairs. Doping also results in an increase in the density of states coinciding with a decrease in the number of magnetic moments on the lattice. These results suggest that chemical manipulation of simple systems, like VO2, provide an excellent framework for the development and testing of modern ideas about complex electronic matter and state-of-the-art theoretical treatments of correlated electron systems.