Local structural changes associated with relaxor behavior in Sn-doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>BaTi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></m

Polar nanoregions (PNRs) play an important role in relaxor ferroelectrics. Despite decades of research, structural understanding of their formation is lacking. In this study, we investigate how PNRs form by using Ti -edge x-ray absorption fine structure spectroscopy to observe local structural changes of polar sites when Sn is doped in the order-disorder-type ferroelectric <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>BaTi</a:mi><a:msub><a:mi mathvariant="normal">O</a:mi><a:mn>3</a:mn></a:msub></a:mrow></a:math>. Sn doping, which induces local lattice distortion, suppresses the vibronic coupling caused by the pseudo-Jahn-Teller effect. As a result, the displacement of Ti at local polar sites is decreased and the energy barrier of the double-well potential is lowered, thus promoting the ordering of the electric dipole moments of polar domains. Our local structure study suggests that the PNR formation process and emergence of relaxor properties in Sn-doped <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>BaTi</c:mi><c:msub><c:mi mathvariant="normal">O</c:mi><c:mn>3</c:mn></c:msub></c:mrow></c:math> are closely related to the evolution of the local structural symmetry determined by the competition between local lattice distortion and vibronic coupling.