Enhancement of Ionic Conductivity in Sm‐Doped Ceria/Yttria‐Stabilized Zirconia Heteroepitaxial Structures

Abstract

Recent developments in the fi eld of thin-fi lm growth technologies have allowed control at an atomic level of deposited layers, thus opening new perspectives in the fi eld of engineering of multilayers and heterostructures based on complex oxides. [ 1 ] In particular, it is expected that oxide heterostructures, with almost ideal interfaces, may lead to interesting artifi cial materials with novel properties. Artifi cial thin-fi lm oxide structures make the already complex individual bulk properties even more interesting through their interaction at the interface. Following such an approach, a number of heterostructures have been tailored which show extraordinary properties that do not belong to the individual layers. These range from superconductivity at the interface between nonsuperconducting layers to high-mobility 2D conductivity at the interface between insulating oxides. [ 2 , 3 ] The number of possible combinations of these oxides is enormous, and the potential for novel behavior having practical applications represents a strong motivation for this research.The same approach can be applied to heterostructures based on oxide ionic conductors provided that the issues concerning structural match at the interface are solved. The interest in heterostructures based on oxide ionic conductors is driven by the space-charge-zone effects at the interface, which can increase the charge-carrier concentration locally, and by interface mobility effects, the latter being of particular relevance in the case of materials with high defect density and relatively low mobility. The potential impact of oxide ionic conductor superlattices has been shown for superlattices based on CaF 2 and BaF 2 layers, grown by molecular beam epitaxy, which exhibited an increase in ionic conductivity

Ab initioinvestigation of defect formation at ZrO2-CeO2interfaces

“…Interestingly, even if the (111) interface is predicted here to be thermodynamically more stable than the (100) interface, from a technological point of view the (100) interface is much more relevant, being the preferential growth orientation triggered by the experimentally used substrates (STO buffered MgO single crystals). 11 Starting from these junction structures, we investigated how the presence of a ZrO 2 /CeO 2 interface affects the equilibrium of intrinsic (oxygen vacancies V O ) and extrinsic defects (Gd and Y doping). First, we focused our attention on the possible effect of the epitaxial strain on the oxygen vacancy formation energy (E V O ) by comparing E V O in relaxed ZrO 2 and CeO 2 bulk phases with their values in bulk structures tetragonally deformed to reproduce the interfacial strain.…”

“…10 A significant enhancement of the ionic conductivity through the interface plane up to 2 orders of magnitude with respect to bulk materials has been reported for a junction formed by thin layers of YSZ and samarium-doped ceria, deposited on a MgO substrate with a buffer layer of SrTiO 3 (STO). 11 Furthermore, the influence of interfaces on the ionic conductivity of heterostructures made of cubicstabilized zirconia and different insulating oxides has been investigated by Janek's group with the aim of correlating the interfacial ion conductivity in the multilayer with the interface microstructural properties. 12,13 An enhancement of 2 orders of magnitude in the ionic conductivity was obtained for samples with incoherent interfaces, while coupling oxide materials with the same crystal structure but slightly different lattice parameters forming semicoherent interfaces led to a slight conductivity increase and reduction for tensile and compressive strains, respectively.…”

“…Above all, the strain at the interface, induced by the lattice mismatch between the two materials, seems to play a key role in triggering the formation of a preferential migration channel for the ionic transport and in affecting the defect formation energy, as also pointed out by Botez et al 14 In this work we use ab initio calculations within the density functional theory (DFT) framework to investigate the structure of two CeO 2 -ZrO 2 interfaces and how they affect the formation energies of intrinsic and extrinsic defects. In particular, we analyze ZrO 2 -CeO 2 junctions built along the (100) direction, as in the experiments by Sanna et al, 11 and along the (111) direction.…”

Strain induced ionic conductivity enhancement in epitaxial Ce0.9Gd0.1O2−δ thin films

“…18 However, MgO(001) (a ¼ b ¼ c ¼ 4.219 Å ) is incompatible for the epitaxial growth of CGO (a ¼ b ¼ c ¼ 5.418 Å ) thin films due to its large lattice mismatch ja CGO À a MgOð110Þ j=a CGO Â 100 ¼ 9:1% resulting in blocking effects due to large grain boundary network. 13,19 For CGO films deposited on MgO(001), occurrence of dislocations starts soon after film nucleation to accommodate the large mismatch with typically large mosaic spread accompanied by columnar growth. 9,20 The critical thickness at which dislocations begin to form varies (approximately) inversely with lattice mismatch.…”

“…Moreover, in this work, the conductivity of epitaxial CGO films are measured in air (high P O 2 ) where the ionic conductivity has been demonstrated to be dominant. 9,19 Pulsed laser deposition (PLD) technique was used to deposit the thin films in this work. X-ray diffraction (XRD) technique was used not only to understand the epitaxial nature of CGO thin films but also to measure the strain in them.…”