Superspace description of NaCa4Nb5O17 (a perovskite-related compound of the type AnBnO3n+2) as a modulated layered structure.
The recently determined structure of NaCa(4)Nb(5)O(17) is reanalyzed within the superspace framework. The material follows the general features of the superspace model proposed for the perovskite-related compounds of the general formula A(n)B(n)O(3n+2) [Elcoro et al. (2001). Acta Cryst. B57, 471-484]. It can be described as a commensurate modulated structure with discontinuous atomic domains given by occupational crenel functions. These atomic domains automatically introduce the layered configuration of the actual structure in real space. However, the displacive modulations follow a quite different pattern from that observed in the Sr(n)(Nb,Ti)(n)O(3n+2) series, discussed in the above-mentioned reference. The superspace group relevant in this new case has been identified through a systematic search of all the possible groups and a comparison of the resulting superspace embeddings of the experimental three-dimensional structure. Being a commensurate structure, the fundamental ambiguity of the superspace description was broken by choosing the highest possible symmetry that yields smooth displacive atomic modulations and hence minimizes the number of parameters. The efficiency of the proposed superspace model is demonstrated with a new refinement of the structure. Assuming the model's general relevance, symmetry properties for the whole [Ca,Na](n)Nb(n)O(3n+2) series are predicted.
A structural study of the layered perovskite-derived Sr n (Ti, Nb) n O3n +2 compounds by transmission electron microscopy
Abstract A series of Sr n (Nb, Ti) n O3n+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 O3n+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 O3n+2 compounds were attributed to tilting of the (Ti, Nb)O6 octahedra.
Phase stability and interfacial structures in the SrO-SrTiO3system
Abstract The phase stability of the subsolidus region in the composition regime SrO–SrTiO3 has been studied using a combination of atomistic computer simulation techniques and high-resolution electron microscopy (HREM). Both predictions from atomistic calculations and HREM observations suggest that at low concentrations (less than 50mol% excess SrO), strontium-rich non-stoichiometry is accommodated by formation of a specific Ruddlesden-Popper defect phase (Sr3Ti2O7) with overall composition maintained by a coherent intergrowth of Sr3Ti2O7 and SrTiO3 phases. A model is proposed for the atomic structure of the Sr3Ti2O7-SrTiO3 interface.