Electrical conduction in semiconducting NbO2 at hydrostatic pressures up to 6000 atm

We have measured the dc resistance, R, of n-type NbO₂ single crystals in the semiconducting, distorted rutile-structured phase at temperatures from T = 196 K to 410 K using hydrostatic pressures, P, from one to 6000 atm. The resistivity, divided by the temperature, increases exponentially with $\text{1}\text{T}$ and the resistance increases linearly with $\text{P}\text{T}$ at different rates along the a- and c-axes. We find that conduction is due to thermally-activated, adiabatic hopping of small polarons and obtain values and pressure dependences for the vibrational frequency and polaronic energies involved. A discussion of our results is given.