Chemistry, growth kinetics, and epitaxial stabilization of Sn2+ in Sn-doped SrTiO3 using (CH3)6Sn2 tin precursor

Tianqi Wang,Venkatraman Gopalan,Krishna Chaitanya Pitike,Serge M Nakhmanson,Yakun Yuan,Bharat Jalan

doi:10.1063/1.4972995

Abstract

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 &gt; 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.

Highlights

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
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
Sn in II-IV perovskite A2+B4+O3 structure can occupy both A- and B-sites, which results in a number of significant synthesis challenges along the route to novel Sn-based perovskite compounds that have recently generated a lot of interest in the functional materials community

Summary

Introduction

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. Experimental efforts dedicated to the bulk synthesis of Sn-alloyed SrTiO3 have established that incorporation of Sn4+ at the Ti-sites can readily be achieved using extreme oxidation conditions, while stabilization of Sn2+ at the Sr-sites remains highly challenging.[15,17,18,19] On the other hand, thin-film synthesis approaches offer additional degrees of freedom to tailor the solubility of Sn with epitaxial strain, as well as control over its growth kinetics.

Results

Conclusion