Order-Disorder Component in the Phase Transition Mechanism ofO18Enriched Strontium Titanate

Abstract

Ti and Sr nuclear magnetic resonance spectra of ${}^{18}\mathrm{O}$ enriched ${\mathrm{SrTiO}}_3$ (STO-18) provide direct evidence for Ti disorder already in the cubic phase and show that the ferroelectric transition at $T_C=24\mathrm{K}$ occurs in two steps. Below 70 K rhombohedral polar clusters are formed in the tetragonal matrix. These clusters subsequently grow in concentration, freeze out, and percolate, leading to an inhomogeneous ferroelectric state below $T_C$. This shows that the elusive ferroelectric transition in STO-18 is indeed connected with local symmetry lowering and implies the existence of an order-disorder component in addition to the displacive soft mode one. Rhombohedral clusters, Ti disorder, and a two-component state are found in the so-called quantum paraelectric state of STO-16 as well. The concentration of the rhombohedral clusters is, however, not high enough to allow for percolation.