Dielectric relaxation and phase transitions at cryogenic temperatures in $0.65\left[\mathrm{Pb}\left({\mathrm{Ni}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3\right]-0.35\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ ceramics

Dielectric measurements on $0.65\left[\mathrm{Pb}\left({\mathrm{Ni}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3\right]-0.35\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ ceramic in the temperature range $90–470\mathrm{K}$ show a relaxor ferroelectric transition around $350\mathrm{K}$ with a Vogel-Fulcher freezing temperature of $338\mathrm{K}$ and an appearance of a nonergodic relaxor ferroelectric phase of tetragonal structure at room temperature. This nonergodic phase reenters into the relaxor state at low temperatures as evidenced by the appearance of a frequency dependent anomaly in the imaginary part of the dielectric constant around $160\mathrm{K}$, similar to those reported in other relaxor ferroelectric based morphotropic phase boundary ceramics. The polarization relaxation time for the $160\mathrm{K}$ anomaly also follows Vogel-Fulcher type temperature dependence. Temperature dependent magnetization measurements show that this low temperature anomaly is not linked with any magnetic transition. Elastic modulus and low temperature x-ray diffraction measurements reveal a tetragonal to monoclinic phase transition around $225\mathrm{K}$. It is argued that the low temperature dielectric dispersion around $160\mathrm{K}$ results from the freezing of mesoscopic conformally miniaturized monoclinic domains formed inside the parent tetragonal domains below the structural phase transition temperature of $225\mathrm{K}$.

Figure 1

Variation of the real $\left({\epsilon }^{\prime }\right)$ and imaginary $\left({\epsilon }^{″}\right)$ parts of the dielectric constant at various frequencies: [(1) $100\mathrm{Hz}$, (2) $300\mathrm{Hz}$, (3) $500\mathrm{Hz}$, (4) $700\mathrm{Hz}$, (5) $1\mathrm{kHz}$, (6) $3\mathrm{kHz}$, (7) $5\mathrm{kHz}$, (8) $7\mathrm{kHz}$, and (9) $10\mathrm{kHz}$] in the temperature ranges (a) $90–470\mathrm{K}$ and (b) $90–300\mathrm{K}$. Inset (i) in Fig. 1a shows the zoomed portion of the real part of the dielectric constant near the peak. Inset (ii) in (a) and inset of (b) show the result for heating and cooling cycles at $10\mathrm{kHz}$. The arrow indicates the sense of shift of peak positions with increasing measuring frequency.

Figure 2

Vogel-Fulcher fit for relaxation time corresponding to (a) the high temperature dielectric anomaly and (b) the low temperature dielectric anomaly. The nonlinear nature of the ln $\tau$ vs $1∕T$ plot in the inset clearly rule out Arrhenius behavior.

Figure 3

Variation of the magnetic susceptibility $\left(\chi \right)$ with temperature for PNN-0.35PT and PNN. Inset shows the variation of $1∕\chi$ with temperature, which gives negative Curie-Weiss temperatures for both PNN-0.35PT and PNN.

Figure 4

Variation of elastic modulus $\left(E\right)$ with temperature for PNN-0.35PT ceramic. Inset shows the temperature dependence of $1∕{\epsilon }^{\prime }\left(T\right)[d\left({\epsilon }^{\prime }\left(T\right)\right)∕dT]$ with an anomaly around $225\mathrm{K}$.

Figure 5

Evolution of the powder x-ray diffraction profiles of the 200, 220, and 222 pseudocubic reflections with temperature for PNN-0.35PT after stripping $\mathrm{Cu}K{\alpha }_2$ contribution.

Figure 6

Observed (dots), calculated (continuous line), and difference (bottom line) powder diffraction profiles of the 200, 220, and 222 pseudocubic reflections obtained from full pattern Rietveld analysis of PNN-0.35PT in the $2\theta$ range 20°–120° using different structural models: (a) tetragonal $P4mm$ at $300\mathrm{K}$, (b) same at $15\mathrm{K}$, (c) monoclinic $Cm$ and tetragonal coexistence, and (d) monoclinic $Pm$ and tetragonal coexistence. The tick marks above the difference plot show the positions of the Bragg peaks. In (c) and (d), upper and lower tick marks are for tetragonal and monoclinic phases, respectively.

Figure 7

The observed (dots), calculated (continuous line), and difference (bottom line) profiles obtained from the Rietveld refinement of PNN-0.35PT using coexistence of monoclinic phase in the $Pm$ space group with the tetragonal phase at $15\mathrm{K}$. Inset shows the fit for higher $2\theta$ range. The upper and lower vertical tick marks denote the positions of the Bragg peaks for the tetragonal and monoclinic phases, respectively.

Figure 8

Temperature dependence of the $P\text{−}E$ hysteresis loop with temperature at constant electric field $12\mathrm{kV}∕\mathrm{cm}$ (scaling $y$ axis: 1 big $\mathrm{div.}=9\mu \mathrm{C}∕{\mathrm{cm}}^2$).

Figure 9

Saturated $P\text{−}E$ hysteresis loop at $123\mathrm{K}$ at an electric field of $22\mathrm{kV}∕\mathrm{cm}$.

Figure 10

The temperature variation of the remanent polarization and coercive field: The $P_r$ and $E_c$ correspond to constant field $(12\mathrm{kV}∕\mathrm{cm})$ measurements whereas $P_{rS}$ and $E_{cS}$ are for higher fields required to get saturated hysteresis loops below $225\mathrm{K}$.

Figure 11

Variation of the lattice parameters $a_t$, $c_t$ and $a_m$, $b_m$, $c_m$, of the tetragonal and monoclinic phases with temperature for PNN-0.35PT obtained by Rietveld refinement. The lattice parameter values of $c_t$ shown with stars below $225\mathrm{K}$ in the stability field of the monoclinic $Pm$ phase correspond to those obtained using the model of Jin et al. (Ref. 36). The vertical line marks the tetragonal to monoclinic phase transition temperature.