Dynamic Effects in a New Substitutional Center of Trivalent Iron in Quartz

Abstract

After exchange of Li against Na in a natural quartz of high quality by electrolysis a t higher temperatures a new center of Fe3+ with monoclinic site symmetry a t room temperature is detected by electron paramagnetic resonance (EPR). At temperatures below 200 K the linewidths of the EPR spectra increase considerably, and a t about 60 K a signal of a triclinic center with a wellresolved quartet hyperfine splitting appears instead. Thus the charge-compensating Na ion evidently changes rapidly between two equivalent positions a t higher temperatures. An estimate of the activation energy for this hopping process is obtained from the temperature dependence of the linewidths for the monoclinic spectrum. Nach Austausch von Li gegen Na in einem naturlichen Quarz hoher Qualitkit durch Elektrolyse bei hoheren Temperaturen wird ein neues Zentrum des dreiwertigen Eisens mit monokliner Punktsymmetrie bei Raumtemperatur durch Elektronenspinresonanz (EPR) entdeckt. Unterhalb200 K nehmen die Linienbreiten seiner EPR-Spektren stetig zu, und bei ca. 60 K taucht ein Signal eines triklinen Zentrums mit gut aufgeloster Quartett-Hyperfeinaufspaltung auf. Na+ als Ladungskompensator springt offenbar schnell zwischen zwei Lquivalenten Positionen bei hoheren Temperaturen. Die Aktivierungsenergie fur diesen Sprung kann aus der Temperaturabhkngigkeit der Linienbreiten fur das monokline Zentrum abgeschkitzt werden.

The structures of three centers of trivalent iron in ?-quartz

“…Measurements on crystals of high quality allowed determination of this hfs for the __ 3/2 transition at all orientations. The nature of the alkali ion could be determined by electrolytic exchange of the alkali ion: Whereas exchange against Li enhanced the intensity of the original EPR spectra of the $1 center, exchange against Na resulted in a completely different center for which details will be published separately (Stegger and Lehmann, 1989).…”

EPR of iron centres in silicon dioxide

“…Thus Stegger and Lehmann [74,75] discovered that in [FeO4/Li] ~ the EPR signals for ~Li (7.42% abundance, I= 1) and 7Li (92.58%, I--3/2) at certain crystal orientations in field B are actually separated (Fig. 5c).…”

“…With sodium as compensator, quite different behavior is observed [72,74,79]. Sodium replacement for lithium was achieved by electro-diffusion along the c-axis channels.…”