Macroscopic response and directional disorder dynamics in chemically substituted ferroelectrics

Abstract

Using temperature-resolved dielectric spectroscopy in the range 25-320 K we investigate the macroscopic response, phase symmetry, and order/disorder states in bulk ferroelectric K 1−y Li y Ta 1−x Nb x (KLTN). Four longrange symmetry phases are identified with their relative transitions. Directional analysis of the order/disorder states using Fröhlich entropy indicates global symmetry breaking along the growth axis and an anisotropic dipolar effective thermodynamic behavior, which ranges from disordered to ordered at the same temperature for different directions in the sample. Results indicate that the macroscopic polarization, driven by nanosized polar regions, follows a microscopic perovskite eight-sites lattice model.

Evidence of double-loop hysteresis in disordered ferroelectric crystal

“…Dielectric spectroscopy is performed measuring the relative real dielectric permittivity ε r as a function of the temperature T through a precision LCR meter (Agilent-4284A) by applying a probing 1 V/cm field between plane-parallel electrodes deposited on the crystal facets (x direction). Temperature variation in the 255-295 K range (straddling the nominal phase transition 8 ) is achieved using a T-controlled closed two-stage helium cryostat and is monitored through a calibrated silicon diode sensor (0.01 K in precision). Slow cooling and heating rates (jΔT=Δtj ≃ 0:01 K/s) are employed in these measurements.…”

“…where s ¼ s(T) is the Fröhlich entropy for the unitary volume and field. 8,43,44 Equation ( 1) shows that the application of E increases the entropy if s(T) . 0 and decreases the entropy if s(T) , 0.…”

“…At high temperatures, the studied KLTN crystal has a cubic symmetry with a paraelectric behavior. 8 When the temperature T is lowered, at a given so-called Burns temperature T B , 36 the disordered distribution of inherent off-center ions, which occupy energetically equivalent lattice sites, leads to the formation of mesoscopic polar regions (PNRs) that below a temperature T * , T B are permanent and distributed in the highly polarizable lattice. They are uncorrelated and free to reorient, while both their number and size increase on further lowering temperature.…”

“…Here, the EO response can be more complex and dependent on previous history, i.e., previous pressure, bias electric field, and temperature cycles. Specific thermal trajectories have been shown to lead to, for example, the enhancement of the electro-optic response, 3,4 enhanced nonlinear response supporting scale-free optics and subwavelength beam propagation, [5][6][7] marked order-disorder anisotropies, 8,9 programable hysteretic effects, [10][11][12]17 aging and rejuvenation, 18,19 along with the presence of intrinsic nonlinearities and effective temperatures. 20 One particularly striking and interesting signature of hysteretic phenomenology is the so-called double-loop isothermal paths as a function of the bias field.…”

“…5,6,35 Experiments in disordered ferroelectrics have identified a range of temperatures spanning across T C where a non-ergodic evolution is found. [8][9][10][11]18,19,25,26,30,31 However, a comparative study of these features as a function of both temperature and electric-field bias has never been reported. In this work, we simultaneously investigate the temperature-dependent dielectric response and fielddependent electro-optic (EO) response.…”