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Non-Stoichiometry And The Electrical Activity Of Grain Boundaries In SrTiO3

M. Kim,2 G. Duscher,2,3 N. D. Browning,2 K. Sohlberg,4 S. J. Pennycook1, S.T. Pantelides5,1

Physical Review Letters, 86, 4056 (2001)

Full Article (PDF 428 KB)

The electrical activity of grain boundaries profoundly affects many properties in perovskites and related materials, including the nonlinear I-V characteristics of capacitors and varistors,  poor critical currents in the high temperature superconductors and low field colossal magnetoresistance in manganites.  While many effects are attributed to the presence of charge on the grain boundaries a microscopic understanding of the phenomenon has been lacking.

Most theoretical studies of grain boundaries assumed them to be stoichiometric.  We used a combination of experimental and computational results to show that grain boundaries in SrTiO3 are intrinsically non-stoichiometric.  The basic structural arrangement was determined from Z-contrast images, which revealed two types of dislocation cores with either Sr or Ti columns at the center.  Then electron energy-loss spectroscopy (EELS) showed that the ratio of Ti to O atoms in the grain boundary is larger than in the bulk (oxygen deficiency or Ti excess).  A series of first-principles theoretical results showed non-stoichiometry to be energetically favorable, with the lowest-energy structures consistent with the Z-contrast images.  The Sr cores were found to be deficient in O whereas the Ti cores were rich in Ti.  Both effects produce a larger Ti:O ratio in the grain boundary, as found by EELS.  Calculations of the electron density of non-stoichiometric grain boundaries showed the origin of the grain boundary charge.  Excess electrons reside on the undercoordinated Ti atoms (see Fig. 1) and reside in the conduction band.  In the usual p-type bulk, these electrons leave the boundary to fill nearby acceptors, leaving the boundary positively charged and a surrounding space charge layer.  The so-called double-Schottky barrier is better described as a pnp double junction since the boundary is not metallic.

The microscopic origin of the grain boundary electrical activity is therefore shown to be non-stoichiometry.  The energies involved are 1-3 eV implying that these effects will be difficult to avoid if they are unwanted, as in the case of the superconductors.  This microscopic understanding of the electrical activity should enable new means of doping grain boundaries for improved electronic, magnetic or superconducting properties.

Figure 1
  Figure 1:  Model structure to illustrate how non-stoichiometry induces electrical activity.  The grain boundary was constructed of Ti cores with one column containing excess Ti.  The resulting conduction band charge density is sharply peaked around the non-stoichiometric column, but perfectly normal on the stoichiometric column.
  1. Solid State Division, ORNL, and University of Illinois at Chicago, IL
  2. Joint faculty North Carolina State University, Raleigh, NC
  3. Now at Drexel University, Philadelphia, PA
  4. Dept. of Physics and Astronomy, Vanderbilt University, Nashville, TN.

 Oak Ridge National Laboratory