Exotic Ground States of VANADIUM(N)OXYGEN(2N-1) Vanadium Oxides.
Abstract
For the past several decades the nature of the V_{rm N}O _{rm 2N-1} Magneli series has been of theoretical and experimental interest. The metal-to-insulator and antiferromagnetic phase transitions seen in the members of the series coupled with their systematically changing crystal structure have been the primary reasons for this interest, but, despite this attention, the unifying trends that have been expected have failed to emerge. In this work, a large body of data on the V_ {rm N}O_{rm 2N-1} series has been collected, and through the analysis of this data, trends that span the whole series can be found. Single crystal of V_{rm N}O_{rm 2N-1} N = 4-8 and (V_{.98}Cr ^{.02})_8O _{15} have been grown and crystal structures for V_6O_ {11}, V_7O _{13}, and V_8O _{15} have ben determined via single crystal X-ray diffraction in both above and below their phase transitions. Magnetic susceptibility measurements as a function of temperature have been preformed at atmospheric pressure, and electrical resistance measurements as a function of temperature have been preformed at pressures of up to 20 kbar. Analysis of this data leads to several unifying trends. The curious temperature dependent magnetic susceptibility seen in all members of the series is attributed to fluctuating regions of ferromagnetic correlation. The metallic antiferromagnetic ground state seen only in V_7O _{13} at atmospheric pressure appears in V_8O_{15 } and (V_{.98}Cr _{.02})_8O _{15} under pressure, and it is proposed that this transition is a spin density wave transition. And the metal-to-insulator transitions seen in the majority of the members of the series can be unified into a single composity pressure-temperature phase diagram. A simple model is proposed to explain the N and pressure dependence of the metal-to-insulator transition temperature, and a mechanism (a spin-Peierls transition) for this transition is discussed.
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1990
- Bibcode:
- 1990PhDT.......102C
- Keywords:
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- Physics: Condensed Matter