Nouveaux matériaux ABi 2B 5O 16 (A = Na, Ag, K, Rb, Tl et B = Nb, Ta) de type pyrochlore déficitaire

Abstract

New defect pyrochlore oxides ABi 2B 5O 16 (A = Na, Ag, K, Rb, Tl and B = Nb, Ta) have been isolated in the pseudo-binary systems BiBO 4 — AB 3O 8. The crystal structures of six of these oxides with respective compositions ABi 2B 5O 16 (A = K, Tl and B = Nb, Ta) and ABi 2Ta 5O 16 (A = Na, Ag) have been determinated by Rietveld profile refinement of their X-ray diffraction powder patterns. According to the size of the monovalent A + cations, two different defect pyrochlore structures have been found. When the A + cations are Na + and Ag +, the occupied sites are those of the ideal pyrochlore structure: (A,Bi) on 16d site, Ta on 16c site, O on 48f site and O' on 8b site. When the A + cations are bigger, Tl + or K + cations, the structure is of the type proposed for the homologous compounds K 0.51Sb 0.67(Sb 2O 6)O' 0.26 [1], Tl 0.51Sb 0.71(Sb 2O 6)O' 0.32 [2] and A 0.4Bi 0.8(B 2O 6)O' 0.4 (A = Rb, Cs and B = Nb, Ta) [3] phases: Bi on 96g site, B (Nb, Ta) on 16c site, O on 48f site, O' on 8b site and A on 32e site.