Low-temperature relaxor state induced by epitaxial compression in PbSc${}_{0.5}$Nb${}_{0.5}$O${}_3$ films

The use of high-performance perovskite-structure epitaxial relaxor ferroelectric films [S. H. Baek et al., Science 334, 958 (2011)] is hindered by the lack of knowledge of epitaxial effects therein. It is experimentally shown here that a biaxial epitaxial compression can favor the relaxor state over ferroelectricity. The absence of the ferroelectric transition and the existence of the low-temperature relaxor state are evidenced by a combination of x-ray diffraction, dielectric, polarization, and optical studies of PbSc${}_{0.5}$Nb${}_{0.5}$O${}_3$ films epitaxially grown on La${}_{0.5}$Sr${}_{0.5}$CoO${}_3$/MgO(001). This finding is beyond existing models of polarization in perovskite-structure epitaxial films and beyond the established ability to induce ferroelectricity by an epitaxial strain.

Figure 1

Crystal structure. Cu Kα radiation. (a) X-ray diffraction θ-2θ pattern of the PSN/LSCO/MgO heterostructure in the selected region around (001) and (002) reflections. The intensity of the substrate reflection is reduced by filtering (shown schematically). (b) In-plane epitaxy: ϕ scans through (022) and (112) reciprocal lattice points of MgO and LSCO, and PSN, correspondingly.

Figure 2

(a) The out-of-plane lattice parameter $c$ (solid diamonds) and the in-plane lattice parameter $a$ (solid circles) of the PSN film as a function of temperature. The lattice parameter of the cubic MgO substrate is also shown (open triangles). (b) The tetragonality ($c$/$a$ −1) and the unit cell volume $V$ of the PSN film as a function of temperature.

Figure 3

The apparent (a) real part ε of dielectric permittivity and (b) dielectric loss tan $D$ as a function of frequency $f$ at different temperatures in Pt/PSN/LSCO.

Figure 4

The apparent (a) and (c) dielectric permittivity ε and (b) loss tan $D$ as a function of temperature $T$ and (a) and (b) frequency $f$ $=$ 0.3–100 kHz in the Pt/PMN/LSCO. Arrows in (a) and (b) show the direction of frequency increase. Open and solid symbols in (c) show heating and cooling runs, correspondingly. $f$ $=$ 1 kHz.

Figure 5

Dielectric properties. (a) The relationship between the temperature $T_m$ of the dielectric maxima and the measurement frequency $f$. The straight line shows a fit to the Vogel-Fulcher law.

Figure 6

Derivative ξ of inverse permittivity as a function of temperature determined at $f$ $=$ 10 kHz in the PSN film in comparison with that in (a) the epitaxial PMN film[24] and in (b) the bulk PSN ceramic (dashed line, data reduced by factor 3) and the FE Pb${}_{0.5}$Sr${}_{0.5}$TiO${}_3$ film (open circles).[22] Lines are smoothed data.

Figure 7

Normalized change $\Delta \varepsilon /\varepsilon \left(0\right)$ of the dielectric permittivity as a function of amplitude $E_{\mathrm{AC}}$ of ac electric field determined at $f$ $=$ 10 kHz in the PSN film. Lines are third-order polynomial fits. For comparison also $\Delta \varepsilon /\varepsilon \left(0\right)$ in the 260-nm-thick FE Pb${}_{0.5}$Sr${}_{0.5}$TiO${}_3$ film and a linear fit are shown.[27]

Figure 8

(a) The room-temperature dynamic polarization-electric field ($P$-$E$) loops in Pt/PSN/LSCO recorded at different maximum applied voltages and $f$ $=$ 1 kHz. (b) The apparent coercive field $E_C$ as a function of frequency $f$ at maximum applied field of 37.5 × 10${}^6$ Vm${}^{-1}$.

Figure 9

Index of refraction $n$ as a function of temperature (wavelength 600 nm). In (a), line shows smoothed data. Dashed line shows the dielectric permittivity ε ($f$ $=$ 1 kHz). The high-temperature linear fits $n_{\mathrm{l}}$($T$) are shown by straight lines in (b) and (c). Dotted line in (c) shows estimated average random polarization $|P_R|$.

Figure 10

The temperature $T_m$ of the dielectric maxima as a function of magnitude $\left|s\right|$ of the in-plane biaxial strain in epitaxial films of (a) PSN and (b) PMN.[24, 44]