Studies of potassium ferrites. IV. Mössbauer effect and conductivity studies of mixed FeGa and FeAl compounds of formula K 1+ xM11O 17: Electron hopping and oxidation at room temperature

Abstract

The 57Fe Mössbauer spectra of a wide range of mixed FeGa and FeAl analogs of K-β-alumina have been obtained. Substitution with the nonmagnetic ions lowers the Néel temperature from 800 °K for the pure ferrite to less than 5°K for a 25% Fe-75% Ga sample. The spectra of the 50% substituted samples were paramagnetic at 295°K and consisted of a predominant octahedral Fe(III) doublet (IS = 0.33, QS = 0.55 mm sec −1), a partially averaged Fe(II)Fe(III) resonance caused by electron hopping at about 10 7 Hz, and, for some preparation conditions, a minor tetrahedral Fe(III) doublet (IS = 0.21, QS = 0.42 mm sec −1). There is thus a delicate balance of Fe(III) site preferences. The spectra at 77°K showed partial magnetic splitting consistent with superparamagnetic regions in the lattice, which may arise because of decoupling between successive blocks in the structure. The results provide direct evidence that Fe(II)Fe(III) electron hopping is responsible for the electronic conductivity, the activation energy of which was found to increase rapidly with dilution, from 6 kJ mole −1 for the pure ferrite to 41–65 kJ mole −1 for the 25% FeGa sample. For the 50% substituted samples the observed proportions of Fe(II) corresponded with the nonstoichiometric K + excess, showing that, at the high-temperature preparation conditions, compensation by oxygen interstitials or metal ion vacancies was not favored when Fe was present. Slow oxidation of the samples occurred in air at room temperature to produce a new doublet attributed to Fe(III) in tetrahedral sites close to the conduction planes. Two possible explanations are consistent with such results. Either Fe(II) is present on octahedral sites initially and forms Frenkel defects next to interstitial oxygens produced during oxidation, or alternatively, Fe(II) is present on both octahedral and bridging tetrahedral sites initially, which then accounts for the appearance of tetrahedral Fe(III) upon oxidation.