Electrical conductivity, thermopower and 57Fe Mossbauer spectroscopy of aegirine (NaFeSi2O6)

DC and AC electrical conductivities were measured on samples of two different crystals of the mineral aegirine (NaFeSi2O6) parallel (parallel to) and perpendicular (perpendicular to) to the [001] direction of the clinopyroxene structure between similar to200 and similar to600 K. Impedance spectroscopy was applied (20 Hz-1 MHz) and the bulk DC conductivity sigma(DC) was determined by extrapolating AC data to zero frequency. In both directions, the log sigma(DC) - 1/T curves bend slightly. In the high- and low-temperature limits, differential activation energies were derived for measurements parallel to [001] of E-A similar to0.45 and similar to0.35 eV, respectively, and the numbers perpendicular to [001] are very similar. The value of sigma(DC) parallel to [001] with sigma(DC)(300 K) similar to2.0 x 10(-6) Omega(-1)cm(-1) is by a factor of 2-10 above that measured perpendicular to [001], depending on temperature, which means anisotropic charge transport. Below similar to350 K, the AC conductivity sigma'(omega) (omega/2pi=frequency) is enhanced relative to sigma(DC) for both directions with an increasing difference for rising frequencies on lowering the temperature. An approximate power law for sigma'(omega) is noted at higher frequencies and low temperatures with sigma'(omega) proportional to omega(s), which is frequently observed on amorphous and disordered semiconductors. Scaling of sigma'(omega) data is possible with reference to sigma(DC), which results in a quasi-universal curve for different temperatures. An attempt was made to discuss DC and AC results in the light of theoretical models of hopping charge transport and of a possible Fe2+ --> Fe3+ electron hopping mechanism. The thermopower Theta (Seebeck effect) in the temperature range similar to360 K < T <similar to770 K is negative in both directions. There is a linear Theta - 1/T relationship above similar to400 K with activation energy E-Theta similar to0.030 eV parallel to [001] and 0.070 eV perpendicular to [001]. Fe-57 Mossbauer spectroscopy was applied to detect Fe2+ in addition to the dominating concentration of Fe3+.