PhD Dissertation Defense

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 V₂O₃, 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 V₂O₃ matrix in toward the Cr dopants resulting in a lengthening of the V-V bonds in the surrounding matrix.

We measured infrared (IR) reflectivity for the system. Since a band gap starts to open up in the IR when (Cr₂O₃)_x(V₂O₃)_1-x 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.

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 V₂O₃ could propagate through the lattice so that the transition could be some mixture of percolation with a “triggering” of extended regions undergoing the MIT.