The structural principle that accounts for the aniondeficient, fluorite-related homologous series of higher rare earth oxides has awaited the determination of a sufficient number of their structures to test hypotheses. Recent structure refinement of five additional members has permitted extraction of a number of generalizations concerning their crystallization behavior. These general principles are outlined. Furthermore, based on the fluorite structure itself, a phenomenological structural principle is outlined that (1) unifies all known and possible phases in these fluorite-related systems under one generic formula, (2) models all known structures correctly and (3) enables modeling of any unknown structure or polymorph in the series. All that is required are electron diffraction patterns adequate to determine the supercell and a knowledge of its composition.
Strukturelle Prinzipien bei den hoheren Oxiden der Seltenen Erden
Zur Aufklarung der strukturellen Prinzipien der Fluorit-verwandten homologen Serie hoherer Selten-Erd-Oxide mit Anionen-Unterschus war die Bestimmung einer hinreichenden Anzahl ihrer Strukturen vonnoten. Neuere Strukturverfeinerungen an funf weiteren Mitgliedern dieser Serie erlauben eine Reihe von allgemeingultigen Aussagen uber ihr Kristallisationsverhalten. Uber diese generellen Prinzipien wird berichtet. Daruber hinaus wird, auf der Grundlage des Fluorittyps, ein phanomenologisches Prinzip vorgestellt, das erstens alle bekannten und moglichen Phasen in diesem fluoritverwandten Systemen durch eine Formel miteinander verknupft, zweitens alle bekannten Strukturen richtig wiedergibt und drittens uns in die Lage versetzt, Modelle fur alle noch unbekannten Strukturen oder Polymorphe dieser Serie abzuleiten. Benotigt werden hierzu lediglich Elektronenbeugungsaufnahmen, die die Bestimmung der Uberstrukturzelle zulassen sowie eine Vorstellung uber die Zusammensetzung.
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