Temperature dependence of electron delocalization in mixed balance freudenbergite

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dc.contributor.authorCashion, JDen_AU
dc.contributor.authorVance, ERen_AU
dc.contributor.authorRyan, DHen_AU
dc.date.accessioned2022-07-12T02:19:02Zen_AU
dc.date.available2022-07-12T02:19:02Zen_AU
dc.date.issued2020-02-04en_AU
dc.date.statistics2021-10-13en_AU
dc.description.abstractSince our last reports, we have carried out further experiments on the mixed valence sodium iron-titanate freudenbergite [1, 2]. Freudenbergite has the nominal formula Na2FexTi8-xO16, with x = 1 being ferrous and x = 2 being ferric. However, a ferrous composition sample, calcined in air, became mixed valence with a closely 50:50 valence split. It is normally considered that the Fe and Ti ions are randomly distributed in the two (Fe,Ti)O6 octahedra. However neutron diffraction showed that the Ti:Fe ratio was 0.82:0.18 in the larger M(1) site and 0.93:0.07 in the smaller M(2) site compared to the average 0.875:0.125. It is expected that all the iron in the smaller M(2) site will be ferric. The sample turned out to have very unusual Mössbauer spectra as the temperature was varied. At low temperatures, well resolved ferric and ferrous doublets were observed. But as the temperature was increased, the ferrous doublet slowly collapsed and had to be fitted with up to three doublets to match the envelope. The ferric doublet remained unchanged in intensity and hyperfine parameters. The collapse of the ferrous spectrum is due to a thermally driven electron delocalization of the sixth d-electron. The electric field gradient in ferrous materials is mainly due to this electron and its removal causes the quadrupole splitting to more closely resemble that of the ferric ions, which is due entirely to the lattice. We have tried unsuccessfully to manufacture more of these electron mobile samples with various compositions and calcining in air and argon. However, pure ferrous and pure ferric samples do not display any dynamic behaviour, and even a ferrous sample with 3% ferric iron did not display any dynamics [2]. Spectra of the present sample taken at 6K and 10 K showed evidence of broadening, presumably due to the inset of magnetic ordering. However, it was not clear whether both ions were ordering or only one. There is no record of a magnetic ordering temperature for freudenbergite in the literature, and any such observation will undoubtedly be strongly sample dependent. The electron dynamics can be caused by intervalence charge transfer between ions or by crystal field effects or electron delocalisation in single ions, and are the primary cause of the colour in popular minerals. Other Fe-Ti minerals which exhibit such behaviour in the Mössbauer spectra include sapphire, kyanite, fassaite, omphacite, aenigmatite, and Ti andradite.en_AU
dc.identifier.citationCashion, J. D., Vance, E. R., & Ryan, D. H. (2020). Temperature dependence of electron delocalization in mixed balance freudenbergite. Poster presented to the 44th Condensed Matter and Materials Meeting, Holiday Inn, Rotorua, New Zealand, 4-7 February 2020. (pp. 58). Retrieved from: https://physics.org.au/wp-content/uploads/cmm/2020/CMM20_ConferenceHandbook(04Feb2020).pdfen_AU
dc.identifier.conferenceenddate7 February 2020en_AU
dc.identifier.conferencename44th Condensed Matter and Materials Meetingen_AU
dc.identifier.conferenceplaceRotorua, New Zealanden_AU
dc.identifier.conferencestartdate4 February 2020en_AU
dc.identifier.otherPoster 18en_AU
dc.identifier.pagination58en_AU
dc.identifier.urihttps://physics.org.au/wp-content/uploads/cmm/2020/CMM20_ConferenceHandbook(04Feb2020).pdfen_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/13354en_AU
dc.language.isoenen_AU
dc.publisherAustralian Institute of Physicsen_AU
dc.subjectAlkali metal compoundsen_AU
dc.subjectCoherent scatteringen_AU
dc.subjectGamma spectrometersen_AU
dc.subjectMeasuring instrumentsen_AU
dc.subjectOxygen compoundsen_AU
dc.subjectDiffractionen_AU
dc.subjectTitanium compoundsen_AU
dc.subjectTransition element compoundsen_AU
dc.titleTemperature dependence of electron delocalization in mixed balance freudenbergiteen_AU
dc.typeConference Posteren_AU
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