Atomic-scale structural and compositional analyses of Ruddlesden-Popper planar faults in AO-excess SrTiO₃ (A = Sr²⁺, Ca²⁺, Ba²⁺) ceramics

Abstract

The microstructure in AO-excess SrTiO3 (A = Sr2+, Ca2+, Ba2+) ceramics is strongly affected by the formation of Ruddlesden-Popper fault–rich (RP fault) lamellae, which are coherently intergrown with the matrix of the perovskite grains. We studied the structure and chemistry of RP faults by applying quantitative high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy analyses. We showed that the Sr2+ and Ca2+ dopant ions form RP faults during the initial stage of sintering. The final microstructure showed preferentially grown RP fault lamellae embedded in the central part of the anisotropic perovskite grains. In contrast, the dopant Ba2+ ions preferably substituted for Sr2+ in the SrTiO3 matrix by forming a BaxSr1−xTiO3 solid solution. The surplus of Sr2+ ions was compensated structurally in the later stages of sintering by the formation of SrO-rich RP faults. The resulting microstructure showed RP fault lamellae located at the surface of equiaxed BaxSr1-xTiO3 perovskite grains.

Investigation of surface Sr segregation in model thin film solid oxidefuel cell perovskite electrodes

“…Sr excess on SrTiO 3 has been extensively studied with three different species reported: SrO island precipitation, [21][22][23] SrO excess in a cubic perovskite phase (SrTiO 3 ), [24][25][26][27] and a reconstructed Sr excess phase (SrO$n(SrTiO 3 )). [25][26][27][28][29] Except for island formation, the other structures require a certain solubility of SrO in SrTiO 3 .…”

“…[25][26][27][28][29] Except for island formation, the other structures require a certain solubility of SrO in SrTiO 3 . Both experimental observations and numerical calculations demonstrate that the cubic perovskite structure can View Online accommodate only limited amounts of excess SrO in the form of a solid solution, [24][25][26][27]30 i.e., less than 0.2 mol% SrO in reference. 30 Likewise, the majority of literature suggests that the incorporation of planar defects in the form of rock salt structured layers is the likely mechanism for accommodating SrO excess.…”

Crystallographic Orientation Relationships Between SrTiO3 and Ruddlesden‐Popper Phase

“…1(d), the fault vector should be half of the lattice translation, i.e., R = 1/2 (111) ST . 10 Similarly, when expressed in the rock salt lattice, 14 it becomes R = 1/2(011) RS [ Fig. 1(c)] .…”

“…This is consistent with previous reports in the literature. 10,14 The interleaved lamellae of ST and the RP phases are discernible using high-resolution (HR) TEM imaging. Fig.…”

“…Alternatively, the structure seen as the (002) layer of the TiO 6 octahedra in SrTiO 3 perovskite along [001] ST is removed, and the remaining SrO layers are condensed on the (110) plane. A displacement vector R = 1/2[01] RS described in the rock salt lattice is formed at the (001) interface [Fig. (c)], or equivalently, R = 1/2[111] ST described in perovskite [Fig.…”