STRUCTURAL BUILDING PRINCIPLES IN 2:3 OXIDES DERIVED FROM A FLUORITE-TYPE ANIONIC ARRAY
M.O. Figueiredo1 , A. Correia dos Santos2
1 Crystallography & Mineralogy
Center, IICT, & CENIMAT, Mater. Sc. Dpt., FCT/UNL, 2825 Monte
de Caparica, Portugal
2 Chem. Dpt., CITecmaT, Faculty of
Sciences, Univ. Lisboa, Campo Grande, 1700 Lisboa, Portugal
Fluorite structure-type is based on a simple cubic anionic array - that is, a superposition (s-type stacking) of square layers (Q-type) of anions - and has the structural formula Ca.cb [Ft2]Qs where cb stands for a cubic interstitial site in the anionic packing and t for the tetrahedral coordination of each anion by the surrounding cations [1].
This structure is the parental arrangement of a large and important group of RE oxides with composition ranging from MO2 to M2O3 . Recalling that dioxides are strictly isostructural with fluorite (Strukturbericht/SB symbol C1 ), four other arrangements with increasing degree of deviation from the ideal simple cubic anionic packing may be foreseen. This communication deals with the structural building principles for deriving 2:3 oxides from the fluorite-type anionic array:
- Disordered distribution of anionic vacancies (one vacant packing site in each four sites, still SB symbol C1 ), giving rise to the structural formula M2cb [Ot3 o]Qs . The proto-type compound is d-Bi2O3 , rare-earth representatives being a low temperature form of La2O3 and, eventually, non-stoichimetric oxides of REE with 3+/4+ valencies (e.g., a defective Ce-oxide).
- Ordered anionic vacancies, C-type sexquioxides (SB symbol D53 ) with cubic bixbyite structure-type, ß-(Mn,Fe)2O3 . There are two crystallographic sites for the metal ions, corresponding to different coordination geometries for CN 6, the structural formula being M6 Mo [Ot3 o]Qs . - Bonding percolation and distortion of the simple cubic anionic array to form cationic layers [M2O2]2+ that pack together with O2- anions and build up the crystal structure. This is the so-called A-type sexquioxide (SB symbol D52 ) typified by La2O3 . Strong RE-O bonds are developed within the cationic complex layers and, as a consequence, the c/a ratio of the trigonal structure deviates significantly from the ideal value that would correspond to a simple cubic packing. - Severe distortion of the anionic array and splitting of crystallographic sites in the monoclinic structure of B-type sexquioxides, typified by Sm2O3 . Two thirds of the cations fill monocapped trigonal prisms, the remaining distorted octahedra. A ful account on the morphotropic (chemical) domain of each structure type will be presented.