Properties of incipient ferroelectrics in the vicinity of quantum critical point

Quantum paraelectricity is a type of incipient ferroelectricity where the ferroelectric order is suppressed by quantum fluctuations. The prominent quantum paraelectric, SrTiO₃, is a material situated in close proximity to a quantum critical point (QCP) of ferroelectric transition in which the critical temperature to ferroelectric state is suppressed down to 0 K. However, the understanding of the behaviour of the phase transition in the vicinity of this QCP remains challenging. Here we investigate the Pb-doped SrTiO3 solid solutions, approaching the pre-critical regions of the phase diagram and study the outcome of the coexistence of quantum fluctuations and thermal motion. Using the Raman spectroscopy and dielectric measurements, we demonstrate that all samples at low doping concentration undergo the structural transition from cubic to non-polar tetragonal phase at temperature ~90 K, and to the ferroelectric orthorhombic phase at lower temperatures. This transition occurs gradually through the emergence of the polar nanoregions inside the non-polar tetragonal phase with their further expansion on cooling. Our results reveal the existence of novel structural phases in the vicinity of QCP occurring due to competition between quantum and classical regimes.