Seeing oxygen disorder in YSZ/SrTiO₃ colossal ionic conductor heterostructures using EELS

¹ Department of Physics and Astronomy, Vanderbilt University, Nashville, 37235 TN, USA
² Materials Science and Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, 37831-6071 TN, USA
³ Grupo de Fisica de Materiales Complejos, Universidad Complutense, 28040 Madrid, Spain

^a e-mail: pennycooksj@ornl.gov

Received: 20 October 2010
Revised: 27 January 2011
Accepted: 9 February 2011
Published online: 7 June 2011

Abstract

Colossal ionic conductivity was recently discovered in YSZ/SrTiO₃ multilayers and was explained in terms of strain- and interface-enhanced disorder of the O sublattice. In the present paper we use a combination of scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) and theoretical EELS simulations to confirm the presence of a disordered YSZ O sublattice in coherent YSZ/SrTiO₃ multilayers. O K-edge fine structure simulated for the strained disordered O sublattice phase of YSZ possesses blurred-out features compared to that of ordered cubic bulk YSZ, and experimental EELS fine structure taken from the strained YSZ of coherent YSZ/SrTiO₃ thin films is similarly blurred out. Elemental mapping is shown to be capable of resolving ordered YSZ O sublattices. Elemental mapping of O in the coherent YSZ/STO multilayers is presented in which the O sublattice is seen to be clearly resolved in the STO but blurred out in the YSZ, indicating it to be disordered. In addition, we present imaging and EELS results which show that strained regions exist at the incoherent interfaces of YSZ islands in STO with blurred out fine structure, suggesting these incoherent regions may also support high ionic conductivities. Recently, Cavallaro et al. reported electronic conductivities in samples of incoherent disconnected islands embedded in STO that are similar to the islands described herein. The presence of a region of O depleted STO at the interface with incoherent YSZ islands is revealed by EELS elemental mapping, implying the n-type doping of STO/YSZ nanocomposites with disconnected incoherent YSZ islands.