Octahedral tilting and cation ordering in perovskite-like Ca4Nb2O9=3•Ca(Ca1/3Nb2/3)O3 polymorphs

Four Ca4Nb2O9 polymorphs with perovskite-related A(B'B-1/3'(2/3))O-3 structures and different arrangements of B-site cations were identified and characterized by X-ray powder diffraction and transmission electron microscopy. The B-cation ordering in three of these phases is combined with b(-)b(-)c(+) octahedral tilting. Two high-temperature forms of Ca4Nb2O9 are separated by a displacive phase transition which involves octahedral tilting and occurs between 1500 and 1600 degrees C. Above this transition, the structure is either disordered or exhibits chemical 1:1 ordering (ordering vector 1/2[111](c)*) of Ca and Nb cations on the B-sites; the correct structural model has yet to be determined. Below the transition, the structure exhibits 1:1 ordering combined with octahedral tilting. Upon cooling, the 1:1 ordered structure transforms to a metastable P1 triclinic structure with lattice parameters root 6a(c) x root 2a(c) x 2 root 2a(c), alpha approximate to delta approximate to 90 degrees, beta approximate to 125 degrees (a(c) refers to cubic perovskite) and ordering vector 1/4[111](c)*. This transformation occurs by reordering of cations on (111) planes. The metastable P1 structure transforms upon annealing to a stable 2:1 ordered P2(1)/c structure with lattice parameters root 6a(c) x root 2a(c) x 3 root 2ac, beta = 125 degrees (ordering vector 1/3[111](c)*), This first-order transition occurs by precipitation and growth of the 2:1 ordered phase, which is stable below 1400 degrees C. (C) 2000 Academic Press.