A structural study of the layered perovskite-derived Sr_n(Ti, Nb)_nO_3n+2 compounds by transmission electron microscopy

Abstract

A series of Sr _n (Nb, Ti) _n O_3n+2 structures with n = 4, 4.5, 5, 6 and 7 were studied by transmission electron microscopy. These structures are composed of infinite two-dimensional slabs of the distorted perovskite structure that are n (Ti, Nb)O₆ octahedra thick and extend parallel to the {110}_perrovskite plane. The slabs are displaced with respect to each other by the translation vector ½[011]_perovskite. All members of the Sr _n (Nb, Ti) _n O_3n+2 series have an orthorhombic basic lattice with a ≈ a _perovskite and c ≈ 2 1/2 a _perovskite, while the long b axis increases systematically with increasing n value. The compounds with n = 4, 5, 6 and 7 were observed to undergo a commensurate ← incommensurate phase transition on cooling in the temperature range 150–250°C. The wave-vector of the incommensurate modulation is parallel to [100] direction of the basic orthorhombic lattice and is close but not exactly equal to ½a∗. The n = 5 incommensurate phase further transformed at 180°C to a monoclinic structure with the space group P112₁/b (No. 14). For the n = 4. 6 and 7 compounds, no lock-in transition was observed down to −170°C. For the compound with n = 4.5, the transition from orthorhombic to monoclinic structure P112₁/b occurred on cooling at 390°C. All transitions observed in the Sr _n (Nb, Ti) _n O_3n+2 compounds were attributed to tilting of the (Ti, Nb)O₆ octahedra.