A structural study of the layered perovskite-derived Sr-n(Ti, Nb)(n)O3n+2 compounds by transmission electron microscopy

A series of Sr-n(Nb, Ti)nO(3n+2) structures with n = 4, 4.5, 5, 6 and 7 wer e studied by transmission electron microscopy. These structures are compose d of infinite two-dimensional slabs of the distorted perovskite structure t hat are n (Ti,Nb)O-6 octahedra thick and extend parallel to the {110}(perov skite) plane. The slabs are displaced with respect to each other by the tra nslation vector 1/2[011](perovskite). All members of the Sr-n(Nb,Ti)nO(3n+2 ) series have an orthorhombic basic lattice with a approximate to a(perovsk ite) and c approximate to 2(1/2)a(perovskite), while the long b axis increa ses systematically with increasing n value. The compounds with n = 4, 5, 6 and 7 were observed to undergo a commensurate --> incommensurate phase tran sition on cooling in the temperature range 150-250 degrees C. The wave-vect or of the incommensurate modulation is parallel to [100] direction of the b asic orthorhombic lattice and is close but not exactly equal to 1/2 a*. The n = 5 incommensurate phase further transformed at 180 degrees C to a monoc linic structure with the space group P112(1)/b (No. 14). For the n = 4, 6 a nd 7 compounds, no lock-in transition was observed down to -170 degrees C. For the compound with n = 4.5, the transition from orthorhombic to monoclin ic structure P112(1)/b occurred on cooling at 390 degrees C. All transition s observed in the Sr-n(Nb,Ti)(n)O3n+2 compounds were attributed to tilting of the (Ti, Nb)O-6 octahedra.