Transition metal oxides and chalcogenides have been the major focus of studies in condensed matter physics. The complexity of the system, involving spin and orbital effects, as well as
lattice degree of freedom, makes them intriguing subjects not only because of these individual effects, but also the effects due to the interaction among them. In AB2X4 materials (A = Mn2+,
Co2+, Fe2+; B = V3+, Cr3+; X = O2-, S2-) which crystallize in spinel structure (space group F d -3 m), these effects and their interactions manifest in their transport properties, magnetic
ordering, itinerant electron magnetism, structural distortion, and geometrical frustration effect due to the antiferromagnetically coupled B-sites. These effects are dependent on the distance
between the interacting cations, which can be varied by chemical substitution or pressure. The main objective of this dissertation is to study the physical properties of Mott-insulator
spinels in approaching their critical inter-cationic distances where an insulator-metal transition occurs. Studying the insulator-metal transition in Mott insulators is important in advancing
our understanding, especially in the field of fundamental physics and materials engineering, on the intricate relationships between the transport and magnetic properties and the emergence of
new behaviors that arise from such properties in these materials. In this dissertation, the behavior of the physical properties of Mn1-xCoxV2O4, AV2O4 (A = Cd, Mg, Zn), and the transport
properties of FeCr2S4 in approaching the insulator-metal transition are reported. Mn1-xCoxV2O4, AV2O4, and FeCr2S4 are chosen for this study due to their dominant V-V or Cr-Cr interactions,
which are responsible for their transport properties. In Mn1-xCoxV2O4, the vanadium-vanadium distance is varied by means of chemical pressure (chemical substitution) to bring the system
closer to the itinerant electron limit given by the critical V-V distance of 2.94 Å. In Mn1-xCoxV2O4, the structural distortion temperature and transport activation energy decreases with
decreasing V-V distance, while the magnetic ordering temperature increases. The results of the transport and structural studies are in agreement with the critical V-V distance scenario of
electronic delocalization. Next, a comparative structural study on AV2O4 with non-magnetic A-site ions (A = Cd, Mg, Zn) and Mn1-xCoxV2O4 is also reported. The study indicates that while the
V-V interactions are dominant, the A-site ions and their magnetism produce a considerable effect on the passage from the localized to delocalized electron limit. This is proven by the two
paths that emerge in the V-V distance dependence of the transport and structural properties where one path includes only the AV2O4, whereas the other includes only Mn1-xCoxV2O4. The transport
property of FeCr2S4 under high pressure was also studied. Due to the t2g electronic configuration of Cr3+, the Cr-Cr interaction is also dominant. A high pressure measurement using a cubic
anvil press up to 8 GPa was performed to induce an insulator-metal transition. The decrease in the Cr-Cr distance with increasing hydrostatic pressure was confirmed by x-ray diffraction
measurements. The Bloch parameter of FeCr2S4 was found to be -2.4, which suggests that FeCr2S4 lies in the localized regime. The high pressure transport measurement on FeCr2S4 shows a
decrease in the activation energy and an increase in the magnetic transition temperature with increasing hydrostatic pressure. An insulator to metal transition was observed at a pressure of
7.5 GPa with a possible onset at 7 GPa, at which the Cr-Cr distance is 3.44 Å. In the case of Cr-oxides, it was predicted that the critical Cr-Cr distance is 2.84 Å, but it should be higher
for a less electronegative anion. Therefore, the difference in the anion species is responsible for the difference of 0.6 Å between the critical Cr-Cr distance in oxides and the actual Cr-Cr
distance where the insulator-metal transition occurs. The insulator-metal transition is followed by a structural transformation at P = 8 GPa. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2014. / November 3, 2014. / Includes bibliographical references. / Haidong Zhou, Professor Co-Directing Dissertation; Vladimir Dobrosavljević, Professor Co-Directing Dissertation; Theo M. Siegrist, University
Representative; Christianne Beekman, Committee Member; Volker Credé, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_252852 |
| Contributors | Kiswandhi, Andhika O. (authoraut), Zhou, Haidong, 1978- (professor co-directing dissertation), Dobrosavljević, Vladimir (professor co-directing dissertation), Siegrist, Theo M. (university representative), Beekman, Christianne, 1980- (committee member), Credé, Volker (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Physics (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (121 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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