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Average and local structure of the Pb-free ferroelectric perovskites (Sr,Sn)TiO 3 and (Ba,Ca,Sn)TiO 3 Geneva Laurita ∗ Materials Research Laboratory, University of California, Santa Barbara, CA 93106 Katharine Page Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Shoichiro Suzuki Murata Manufacturing, Co., Ltd., Nagaokakyo, Kyoto 617-8555, Japan Ram Seshadri Materials Department and Materials Research Laboratory, University of California, Santa Barbara, CA 93106 (Dated: December 9, 2015) The characteristic structural off-centering of Pb 2+ in oxides, associated with its 6s 2 lone pair, al- lows it to play a dominant role in polar materials, and makes it a somewhat ubiquitous component of ferroelectrics. In this work, we examine the compounds Sr 0.9 Sn 0.1 TiO 3 and Ba 0.79 Ca 0.16 Sn 0.05 TiO 3 using neutron total scattering techniques with data acquired at different temperatures. In these compounds, previously reported as ferroelectrics, Sn 2+ appears to display some of the characteris- tics of Pb 2+ . We compare the local and long-range structures of the Sn 2+ -substituted compositions to the unsubstituted parent compounds SrTiO 3 and BaTiO 3 . We find that even at these small substitution levels, the Sn 2+ lone pairs drive the local ordering behavior, with the local structure of both compounds more similar to the structure of PbTiO 3 rather than the parent compounds. I. INTRODUCTION The perovskite oxides AB O 3 form a class of highly- studied and technologically relevant materials, with many exhibiting ferroelectric behavior. 1 Of the ferroelec- tric perovskites, examples include BaTiO 3 and PbTiO 3 , where the ferroelectric polarization is associated with the off-centering of Ti in TiO 6 octahedra due to so-called second-order Jahn-Teller effects. 2 In PbTiO 3 , the Pb 2+ ions on the A site additionally drive off-centering, due to the characteristic 6s 2 lone pair. 3,4 The off-centerings can cooperatively give rise to spontaneous polarization and ferroelectric behavior. First principles studies have sug- gested that SnTiO 3 should similarly display off-centering due to the Sn 2+ 5s 2 lone pair, and this as yet experimen- tally unrealized compound is expected to exhibit a larger polarization than PbTiO 3 . 5,6 This is in keeping with ex- pected trends in lone-pair behavior of divalent carbon group compounds. 3 The environmentally benign nature of Sn 2+ would make SnTiO 3 -derived compounds an at- tractive alternative to Pb-containing ferroelectrics. SrTiO 3 is an incipient ferroelectric that does not ex- hibit a ferroelectric transition above 0 K due to quan- tum fluctuations. 7 However, substitution of Sn 2+ into the Sr 2+ site of SrTiO 3 induces a relaxor-like ferroelec- tric transition. 8,9 Raman spectroscopic observations of polar transverse optic modes at room temperature sug- gest the ferroelectric behavior arises due to the formation of percolative polar nanoregions formed in the material. 10 A maximum dielectric permittivity was observed with a nominal 10 % Sn-substitution, and is therefore the com- position chosen for this study. 8 In contrast to SrTiO 3 , BaTiO 3 is a room-temperature ferroelectric material that is commonly employed in multilayer ceramic capacitors. However, a relatively low ferroelectric transition temper- ature, T c , of approximately 400 K limits the use of this material. 11 Efforts to increase the T c of BaTiO 3 have been facilitated through the substitution of Pb 2+ into the Ba 2+ site, 12 but the toxicity of Pb leaves room for im- provement for technological applications. To increase the solubility of the smaller Sn 2+ into the lattice of BaTiO 3 , the lattice volume can be decreased by the addition of Ca 2+ into the A-site. It has been shown in the literature that the T c of (Ba,Ca)TiO 3 remains relatively constant up to the solubility limit of Ca 2+ (approximately 20 %), making this an accessible system to study the effects of Sn 2+ substitution. 13,14 Successful Sn 2+ substitution into perovskite titanates has been completed through the co- substitution of Ca 2+ and Sn 2+ into the Ba 2+ site of BaTiO 3 and exhibits an increase of 20 K in the T c for the nominal composition Ba 0.79 Ca 0.16 Sn 0.05 TiO 3 (fur- ther studied here) compared to that of the unsubstituted compound. 15–18 The presence of the lone pair of electrons bound to Sn 2+ can induce off-centering, and since these off- centerings need not be correlated, it is necessary to utilize a structural technique that will analyze not only the aver- age crystallographic structure but also the local structure in a material. This can be accomplished through the ap- plication of total scattering techniques, which utilize both the Bragg and diffuse components to provide information on the average structure (obtained from Rietveld refine- ment of the Bragg scattering) and the local structure

physical review. b, condensed matter and materials physics

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Average and local structure of the Pb-free ferroelectric perovskites(Sr,Sn)TiO3and(Ba,Ca,Sn)<…