DC PIEZORESISTANCE AND AC CONDUCTANCE OF NIOBIUM DIOXIDE (SMALL POLARON, ELECTRICAL TRANSPORT PROPERTIES)

CLAUDIO GUERRA VELA, Purdue University

Abstract

We have measured the resistance, R, of monocrystalline n-type NbO(,2) in the semiconducting, distorted rutile-structured phase at temperatures from T = 196 K to 410 K and hydrostatic pressures, P, from one to 6,000 atm. We find that R/T increases exponentially with 1/T and (DELTA)R/R increases linearly with P/T at different rates along the a- and c-axes. We deduce that conduction is due to adiabatic hopping of small polarons and obtain values for phonon, electron transfer, and polaron binding energies, the pressure dependences of these energies, and of the small polaron activation energy. An electronic phase diagram is presented also. We also measured the complex ac conductivity using frequencies from 5 to 92 kHz between 1.5 K and 300 K along the a- and c-axes of NbO(,2). Above 200 K the real part of the conductivity (sigma)(,a) and (sigma)(,c) were independent of frequency, f, and strongly activated like the dc conductivity. Below 200 K, (sigma)(,a) decreased ever less rapidly until 120 K where a weakly activated regime began in which (sigma)(,a) varied about like f('0.5) implying transitions of polarons between centers with a characteristic energy difference. The conduction mechanism seems to involve impurities randomly distributed over a range of distances. A temperature-independent transition probability is found which indicates carrier tunneling. Below 160 K, (sigma)(,c) exhibited a number of peaks accompanied by inflections in the dielectric permittivity indicating that thermally activated relaxation processes are involved. It is suggested that each of the processes is due to polaronic electrons hopping between the Nb('+4) ions in pairs located along the c-axis. The electron is thought to have been thermally activated from the site of an oxygen vacancy. We deduce and report the energies and relaxation times involved in the ac conduction processes.

Degree

Ph.D.

Subject Area

Condensation

COinS