Étude structurale comparée des chromates: M(I)M(III)(CrO4)2; M(I) = Li+, Na+, K+, Rb+, Cs+, NH4+, Ag+ et Tl+; M(III) = Al3+, Fe3+, Cr3+, Tl3+ et Bi3+

International audience; Comparison of crystal structures of chromates: M(I)M(III)(CrO4)2; M(l) = Li+, Na+,K+, Rb+, Cs+, NH4+, Ag+ and TI+; M(III) = Al3+, Fe3+, Cr3+, Tl3+ and Bi3+ . Among compounds of formula type M(I)M(III)(CrO4)2 , the structures of trivalent bismuth compounds are always different from the others, because the surrounding of bismuth is never well-defined. Besides, nearly all these compounds have two-dimensional structures with the exception of Tl(I)Tl(III)(CrO4)2 , which is three-dimensional. This structural type can exist only when the ionic radius of the monovalent cation has a value near that of O2- ion (1.40 Å ), as for Rb+, NH4+ and Tl+.structure / crystal / cation + / cation 3+ / chromate; Parmi les nombreux composés de formule type M(I)M(III)(CrO 4) 2 , le bismuth trivalent se distingue des autres cations à cause de son environnement particulier peu organisé. En outre, la quasi-totalité de ces composés possèdent des structures en feuillets. Tl(I)Tl(III)(CrO 4) 2 , fait exception à la règle en ayant une structure tridimensionnelle. Ce type structural n'existe que si le rayon ionique du cation monovalent M(I) est proche du rayon ionique de l'oxygène O 2- voisin de 1,40 Å, ce qui est le cas pour Rb + , NH 4 + et Tl +.