Chemical Bonding and Atomic Structure in Y₂O₃:ZrO₂‐SrTiO₃ Layered Heterostructures

Abstract

nanosized layers of two different metal oxides upon one another produces layered heterostructures with a wide range of new physical properties. [1] For example, a conducting two-dimensional electron gas was discovered at the interface between the two insulators LaAlO 3 and SrTiO 3 . [2] The atomic arrangements within heterostructures of complex functional oxides determine the resulting properties and are not necessarily straightforward to construct based on the bulk crystal structures of the independent components.García-Barriocanal et al. reported that an epitaxial heterostructure of nanometer-sized layers of Y 2 O 3 -stabilized ZrO 2 (YSZ) and [001]-oriented SrTiO 3 (STO) has an up to eight orders of magnitude higher conductivity than bulk YSZ at room temperature. [3] YSZ is a commonly used electrolyte in solid oxide fuel cells (SOFCs) run at high temperatures (> 800 8C), and such an increase in ionic conductivity at lower temperatures would be expected to have a substantial impact on the performance of SOFCs. [4] However, considerable discussion has followed the original publication, with the suggestion that the increased conductivity may be partly electronic in origin. [5] The possibility remains that the conductivity is entirely ionic [6] with indications that the increased conductivity arises from disorder in the oxygen lattice induced in the YSZ layers. [7,8] Similar, though smaller, increases in conductivity have been found for other layered oxide heterostructures. [9] The exact atomic structure of the YSZ-STO heterostructures remains undetermined. Knowledge of the structure at the boundary between the two component oxide units is crucial for understanding the increase in conductivity, which has been assigned to reconstruction at this interface within the structure. [3] It is difficult to probe the structure of interfaces within thin films experimentally, and previous attempts for YSZ-STO films seem inconsistent. The relative orientation of the YSZ and STO regions has been determined by X-ray diffraction [10] and scanning transmission electron microscopy, [3,8] with YSZ rotated by 458 about the [001] axis relative to STO. However, electron energy loss spectroscopy suggests that the STO is terminated with a TiO 2 layer [3] in layered YSZ-STO, whereas experimental attempts to grow YSZ on STO substrates with different terminations show that [001]-oriented YSZ can only be reliably grown epitaxially on SrOterminated STO, [10] leaving the preferred termination of the STO in question. Previous calculations, carried out using only TiO 2 -terminated STO, [7] determined that oxide ions in the first YSZ layer adopted positions completing the TiO 6 octahedra. We assess the stability and electronic structure of different possible atomic arrangements in the YSZ/STO heterostructure using density functional theory (DFT) calculations, focussing on reconstruction at the repeating boundaries between the component YSZ and STO units. We find that the most stable structures, and those which correspond to known oxide crystal ...

3D elemental mapping with nanometer scale depth resolution via electron optical sectioning

“…In this paper, we report the combined EELS and ADF optical sectioning of thin film heterostructures composed of yttria-stabilized zirconia (Y 2 O 3 ) x (ZrO 2 ) 1−x (YSZ) and SrTiO 3 (STO), a system which has sparked intense debate due to the observation of colossal ionic conductivity at coherent YSZ/STO interfaces [17,18,19,20,21,22,23,24,25,26,27]. Observing a sample with significant islanding, and hence a high level of overall inhomogeneity, we found regions which EELS shows to contain the elements of both YSZ and STO.…”

Oxygen Disorder, a Way to Accommodate Large Epitaxial Strains in Oxides

“…The formation of new phases is a possible alternative to accommodate large strains. For the STO/YSZ/STO structure, Dyer et al pointed out that, near the YSZ‐STO interface, the structure of the disordered O sublattice in the YSZ film is similar to the baddeleyite structure . We compared oxygen disorder with the formation of a new phase for the ZrO 2 .…”

Outstanding Atomic Order in Ruddlesden–Popper Oxide Microcrystals

“…This situation is different from that previously reported for La 2−2x Ca 1+2x Mn 2 O 7 (0.6 ≤ x ≤ 1) since iodometric titrations indicate a slight oxygen deficiency. 23 The corresponding XRD pattern can be indexed on the basis of a RP n = 2 term with space group (SG) Cmc2 1 (No 36). Several space groups were tested in the fitting of the powder XRD data of La 0.5 Ca 2.5 Mn 2 O 7 at room temperature.…”

“…In recent years, there has been a burst of activity to manipulate strongly correlated phenomena based on oxide heterostructures. 35,36 Among these, particular efforts have been devoted to grow RP thin films. 13,14 The synthesis of such layered oxides has been a major challenge owing to the occurrence of disordered intergrowths that result in poor materials behavior in the higher members.…”