Inhomogeneities in the (V1-xCrx) 2O3 system

Date of Completion

January 2010

Keywords

Physics, Solid State|Physics, Condensed Matter

Degree

Ph.D.

Abstract

The metal-insulator transition (MIT) in Cr doped V2O3 has been an interesting subject since it was discovered in 1969. The phase change at room temperature from paramagnetic metal (PM) to paramagnetic insulator (PI) depends on the percentage of Cr concentration in V 2O3, and the switch from PM to PI phase takes place at only about 1% Cr concentration. ^ Our previous EXAFS studies showed that Cr atoms go to substitutional sites. Also, there is a collapse of the V 2O3 matrix in toward the Cr dopants resulting in a lengthening of the V – V bonds in the surrounding matrix. While the sample reaches sufficient Cr concentrations, the insulating regions could link up to form an overall insulating phase. ^ We measured EELS spectrum of pure V2 O3. We developed our ability to thin oriented wafers of V2O3 and investigated the V L edges and the O K edges as a function of temperature. However, we are unable to resolve the fine differences in the O K edges. We believe soft x-ray XANES has better resolution than EELS method. ^ We measured infrared (IR)reflectivity for the system. Since a band gap starts to open up in the IR when (V1−xCrx) 2O 3 becomes insulating, one would expect there to be a pronounced difference between IR reflectivity between conducting and insulating regions. We also measured the Terahertz reflectivity in cooperation with CINT at Los Alamos. The results are interesting. Both IR and THz reflectivity shows similar spectrum. From the 1% through the 2.8%, and then to the 5.2% Cr concentrations, one has a similar curve shape but a systematic decrease in reflectivity. ^ However, the resistivity at room temperature of these PI samples is essentially the same. Our explanation to this drastic difference between resistivity and reflectivity is the percolation model. It could be that once a local region becomes insulating, the resultant distortion of the V 2O3 could propagate through the lattice so that the transition could be some mixture of percolation with a "triggering" of extended regions undergoing the MIT. ^

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