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PbTiO3-based ferroelectrics have impressive electroactive properties, originating from the Pb2+ 6s2 electron lone-pair, which cause large elastic distortion and electric polarization due to cooperative pseudo Jahn-Teller effect. Recently, tin-based perovskite oxide (SnTiO3) containing Sn2+ and a chemistry similar to that of the 6s2 lone-pair has been identified as a thermally stable, environmentally friendly substitute for PbTiO3-based ferroelectrics. However experimental attempts to stabilize Sn2+ on the A-site of perovskite ATiO3 have so far failed. In this work, we report on the growth of atomically smooth, epitaxial, and coherent Sn-alloyed SrTiO3 films on SrTiO3 (001) substrates using a hybrid molecular beam epitaxy approach. With increasing Sn concentration, the out-of-plane lattice parameter first increases in accordance with the Vegard’s law and then decreases for Sn(Sr+Ti+Sn) at. % ratio > 0.1 due to the incorporation of Sn2+ at the A-site. Using a combination of high-resolution X-ray photoelectron spectroscopy and density functional calculations, we show that while majority of Sn is on the B-site, there is a quantitatively unknown fraction of Sn being consistent with the A-site occupancy making SrTiO3 polar. A relaxor-like ferroelectric local distortion with monoclinic symmetry, induced by A-site Sn2+, was observed in Sn-doped SrTiO3 with Sn(Sr+Ti+Sn) at. % ratio = 0.1 using optical second harmonic generation measurements. The role of growth kinetics on the stability of Sn2+ in SrTiO3 is discussed.
Bibliographical noteFunding Information:
The authors would like to acknowledge Dr. S. Chambers for helpful discussion and Dr. Bing Luo for the technical help with XPS measurements. This work was supported primarily through the U.S. National Science Foundation under Award No. DMR-1607318. Part of this work was carried out in the College of Science and Engineering Characterization Facility, and Minnesota Nano Center at the University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program. T.W. would also like to thank UMN DDF fellowship for the support. Computational work at the University of Connecticut was supported by the National Science Foundation under Award No. DMR-1309114. Y.Y. and V.G. acknowledge the NSF MRSEC Center for Nanoscale Science at the Pennsylvania State University, through Grant No. DMR-1420620.
© 2016 Author(s).
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- 3 Finished
11/1/14 → 9/30/21
Project: Research project