POINT-DEFECTS OF TEPHROITE - I - THE ELECTRICAL-CONDUCTIVITY OF MN2SIO4

The electrical conductivity a of single crystalline Mn2SiO4 (tephroite ) grown from the melt, was investigated by impedance spectroscopy betw een 1101 and 1197 degrees C as function of oxygen activity within the range similar to 10(-16) less than or equal to a(O2) less than or equa l to 0.21 (air). Mn2SiO4 is n-conducting within the low and p-conducti ng within the high oxygen-activity regime, independent whether the sam ple are buffered against MnO or MnSiO3. Based on our experimental resu lts we propose a point-defect model the majority defects of which are vacancies in the manganese sublattice V-Mn'', Mn3+-ions on Si-lattice sites Mn-Si', holes h(.) (= Mn-Mn(.)) and electrons e'. Unlike Fe2SiO4 (fayalite), wherein according to thermogravimetric results of Nakamur a and Schmalzried [1] associates (Fe-Fe. Fe-Si')(x) exist, our defect model works without postulating the analogous species {Mn-Mn. Mn-Si'}( x) to exist in tephroite. We drew this conclusion from the 1/5.5-power law which is followed by our sigma-a(O2)-plots measured on stoichiome tric tephroite in the high-a(O2) regime, whereas Nakamura and Schmalzr ied derived an 1/5.0 power law from their measurements in the range of similar oxygen activities on likewise stoichiometric fayalite. The eq uilibrium constants for formation reactions of the aforementioned defe cts, which emerged from our experiments were used to depict Kroger-Vin k diagrams;they describe the dependence of defect concentrations on ox ygen activities for both MnO resp. MnSiO3 buffered tephroite.