A high, rounded peak of the permittivity, ${\mathrm{\varepsilon }}_{\max }^{\prime }$≈6×${10}^4$, at T≈18 K evidence a percolative-type phase transition into a random-field-induced ferroelectric domain state of the doped system ${\mathrm{Sr}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{TiO}}_3$ (x=0.007). Dispersion measurements of the permittivity reveal strong polydispersivity of the system. Deviations from Arrhenius behavior below T=11 K indicate the onset of dissipative quantum tunneling of the ${\mathrm{Ca}}^{2+}$ dipoles between their different off-center positions. Anomalies in both the linear and the nonlinear dielectric permittivity are explained by the presence of two different dissipative cluster contributions. They are attributed to the reorientation of ${\mathrm{Ca}}^{2+}$-centered polar nanometer-scale regions within ferroelastic single domains of the system and to their interaction with coherent acoustic quantum fluctuations of the host lattice, respectively.

• Received 5 December 1994

DOI:https://doi.org/10.1103/PhysRevB.51.8737