Local structure studies of SrTi¹⁶O₃ and SrTi ¹⁸O₃

In this work we report on the local structure of Ti in SrTi ¹⁶O₃ (STO16) and SrTi¹⁸O₃ (STO18) investigated in the low temperature range (6-300 K) by extended x-ray absorption fine structure and x-ray absorption near edge structure (XANES) spectroscopy at Ti K-edge and by optical second harmonic generation (SHG). By comparing XANES of STO16 and STO18 we have identified the isotopic effect which produces at T < 100 K a noticeable difference in the measured mean square relative displacements (MSRD) of Ti-O₁ bonds: while STO16 follow the expected Einstein-like behavior, for STO18 we have measured an increase of MSRD values with decreasing temperature. This is an indication of an increasing off-center position of the Ti atoms in the TiO₆ octahedra.