Nb-stabilized locally broken symmetry below and above $T_c$ in a $\mathrm{PbZr}{\mathrm{O}}_3$ single crystal

The influence of a small amount of niobium ions (0.6 mol% $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$) introduced into $\mathrm{PbZr}{\mathrm{O}}_3$ single crystals on the structural, dielectric, optical, and electromechanical properties was studied. The main goal was to search for the influence of this doping on the phase transition sequence. It was found that addition of Nb leads to stabilization of the known intermediate phase and to formation of a new one. It is interesting that locally broken symmetry could be observed both below and far above the Curie temperature. All these results are analyzed in terms of charge compensation phenomena.

Figure 1

Diffraction pattern of $\mathrm{PbZr}{\mathrm{O}}_3$:Nb at room temperature.

Figure 2

(a) Real part of permittivity ε vs temperature for $\mathrm{PbZr}{\mathrm{O}}_3$:Nb crystal, for heating and cooling (1 K/min). (b) The arrow indicates another anomaly of ε${}^{\prime }$ on cooling for the $\mathrm{PbZr}{\mathrm{O}}_3$:Nb crystal, not observed in ceramics of the same chemical composition. One can see clearly the disappearance of dielectric thermal hysteresis towards 200 °C.

Figure 3

Permittivity versus temperature for $\mathrm{PbZr}{\mathrm{O}}_3$:Nb crystals. The arrow indicates a point $T_{\mathrm{BH}}$, below which the Curie-Weiss law (red line) is not obeyed. $T_{\mathrm{BH}}$ is the Bussmann-Holder temperature equal to $1.1T_C$. It represents a theoretical limit of noncentrosymmetric clusters that start growing rapidly on approaching $T_C$ from the high-temperature side [7].

Figure 4

(a) Reciprocal permittivity as a function of temperature above $T_C$. The black line is a fit to the Curie-Weiss law $C/\left(T-T_0\right)$. (b) The difference between reciprocal permittivity ${\varepsilon }_{1\mathrm{MHz}}^{-1}$ detected and fitted ${\varepsilon }_{1\mathrm{MHz}\mathrm{f}\mathrm{it}}^{-1}$ as a function of temperature. The point $T_{\mathrm{pd}}$ is the temperature at which piezoelectric signals disappear (see Fig. 6 further in the text), and—as in Fig. 3—$T_{\mathrm{BH}}$ is the Bussmann-Holder temperature. The difference in reciprocal permittivity becomes negligible at temperatures higher than $T_{\mathrm{BH}}$ because it reflects the existence of competing lattice instabilities at the Γ and $R$ points, which are present at all temperatures above $T_C$ [7], and to which the low-frequency permittivity is sensitive enough, even at 300 °C. Such dependence is observed on both cooling and heating.

Figure 5

Birefringence maps of a single $\mathrm{PbZr}{\mathrm{O}}_3:\mathrm{Nb}$ crystal registered on cooling: (a) above $T_C$, at 235 °C, (b) below $T_C$, within the first intermediate phase, at 229 °C, (c) within the second intermediate phase, at 215 °C, and (d) within the antiferroelectric phase, at 186 °C. The bar with gradient of colors represents values of birefringence from 0 (pink color) to nonzero values (the red color represents the highest value in a defined investigated region).

Figure 6

Birefringence above $T_C$ for a single $\mathrm{PbZr}{\mathrm{O}}_3:\mathrm{Nb}$ crystal. The nonzero birefringent phase has been evolving up to temperature $T_{\mathrm{pd}}$, at which piezoelectric signals vanish (see Fig. 7 further in the text).

Figure 7

(a) Piezoelectric coefficient $d_{33}$ versus temperature for a $\mathrm{PbZr}{\mathrm{O}}_3:\mathrm{Nb}$ single crystal. (b) Temperature dependence of the piezoelectric coefficient $d_{33}$ above $T_{{\varepsilon }^{\prime }\max }$. $T_{\mathrm{pd}}$ is the temperature at which piezo activity vanishes.

Figure 8

Temperature changes of the electrostrictive strain $\eta \sim E^2$ for a $\mathrm{PbZr}{\mathrm{O}}_3:\mathrm{Nb}$ single crystal, on cooling.