Size effects in nanocrystalline SrTiO3

P. Lupetin and G. Gregori

The grain size plays a fundamental role in the determination of the electrical properties of electroceramic oxides. Notably, at the nanoscale the grain boundaries are predominant over the bulk and they fully control the overall conduction properties. Therefore, the conduction mechanism may be completely different if one compares the nanoscale with the macroscale.

We study strontium titanate (SrTiO3) which is an excellent model material for electroceramic oxides and perovskites, thanks to its pronounced stability and its well explored defect chemistry at the macroscale and we investigate its conduction properties over a broad range of oxygen partial pressures via impedance spectroscopy, since the oxygen non-stoichiometry is the key parameter in determining whether this perovskite is p-type, n-type or ionic conductor.

Several exciting size-induced phenomena have already been observed in the undoped material: increase of n-type conductivity by several orders of magnitude, an equally great depression of p-type conductivity and a shift of the p-n transition of 12 orders of magnitude in terms of oxygen partial pressure (Fig. 1).

Fig. 1  Conductivity as a function of the oxygen partial pressure P measured at 544°C for various nominally undoped SrTiO3 samples. The symbols are assigned as follows: red open squares refer to the bulk of the microcrystalline SrTiO3 obtained by annealing the nanocrystalline sample at 1250°C for 4 hours; blue open triangles to the grain boundaries of the microcrystalline SrTiO3; grey diamonds to nano SrTiO3 50 nm; solid red nano to SrTiO3 with the effective grain size of ~30 nm. The green line correspond to the expected bulk conductivity calculated according to the bulk defect chemistry of SrTiO3 for the same acceptor impurities content (m = 0.01 at.%). The green dashed line shows the purely electronic conductivity and hence the p-n transition (conductivity minimum). The horizontal line represents the sum of p and n-type conductivity at  corresponding to . Reproduced by permission of John Wiley & Sons, Inc.

Publications:

P. Lupetin, G. Gregori, and J. Maier, “Mesoscopic Charge Carriers Chemistry in Nanocrystalline SrTiO3”, Angew. Chem. Int. Ed. 49 [52], 10123–10126 (2010). DOI: 10.1002/anie.201003917

G. Gregori, P. Lupetin, and J. Maier, “Huge Electrical Conductivity Changes in SrTiO3 upon Reduction of the Grain Size to the Nanoscale”, ECS Transactions 45 [1], 181-187 (2012). DOI: 10.1149/1.3701289

G. Gregori, S. Heinze, P. Lupetin, H.-U. Habermeier, and J. Maier, “Seebeck coefficient and electrical conductivity of mesoscopic nanocrystalline SrTiO3”, J. Mater. Sci. 48 [7], 2790-2796 (2013). DOI: 10.1007/s10853-012-6894-0

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