Sr87NMR of phase transitions inSrTi16O3andSrTi18O3

Abstract

$^{87}\mathrm{Sr}$ quadrupole perturbed NMR data of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ show that local symmetry breaking takes place also at the A site of the $\mathrm{AB}{\mathrm{O}}_{3}$ perovskite lattice in the cubic phase and not only at the B site as observed before. This is true for $\mathrm{Sr}{\mathrm{Ti}}^{16}{\mathrm{O}}_{3}$ (STO-16) and $\mathrm{Sr}{\mathrm{Ti}}^{18}{\mathrm{O}}_{3}$ (STO-18). The onset of the antiferrodistortive transition around ${T}_{a}=105\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is accompanied by a huge splitting of the $^{87}\mathrm{Sr}$ NMR line due to formation of macroscopic 90\ifmmode^\circ\else\textdegree\fi{} domains and an anomalous shortening of the spin-spin relaxation time ${T}_{2}$ of the quadrupole satellite transitions which persist in the tetragonal phase down to $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This shows the presence of very low frequency fluctuations of the phase transition order parameter. Rhombohedral polar clusters appear in the tetragonal matrix both in STO-16 and STO-18 below $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and percolate in STO-18 below $T\ensuremath{\approx}24\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ leading to an inhomogeneous ferroelectric state. This is also supported by measurements of the field cooled $^{87}\mathrm{Sr}$ NMR spectra.