Observation of room-temperature ferroelectricity in spark-plasma sintered ${\mathrm{GdCrO}}_3$

A spark-plasma sintered ${\mathrm{GdCrO}}_3$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up–negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\approx 170\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\approx 170\mathrm{K}$ in ${\mathrm{GdCrO}}_3$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_1$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_3$ and found that the orthorhombic $Pna{2}_1$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_1$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\approx 450\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

Figure 1

The unit cells corresponding to the (a) $Pbnm$ and (b) $Pna{2}_1$ structures of ${\mathrm{GdCrO}}_3$. (c) Schematic visualization of the room-temperature (RT) structural change from $Pbnm$ (in GCO) to $Pna{2}_1$ (in SPS-GCO) after the spark-plasma sintering process. Upon heating beyond 450 K, this RT $Pna{2}_1$ phase gets converted to the high-temperature (HT) $Pbnm$ phase, which upon subsequent cooling remains in the $Pbnm$ phase.

Figure 2

Room-temperature synchrotron XRD data of GCO and SPS-GCO recorded with $\lambda =0.8172$ $\AA$. GCO-$Pbnm$ XRD (simulated using vesta for same wavelength) has been plotted as a reference. The inset highlights a representative XRD peak comparison.

Figure 3

(a) Room-temperature electric polarization ($P$)–electric field ($\mathit{E}$) loop (collected at a frequency of 200 Hz) of SPS-GCO. (b) Room-temperature PUND data for the 6 kV/cm applied electric field on SPS-GCO. Panels (c) and (d) show enlarged views of the electric polarization responses under positive and negative electric pulses of PUND data.

Figure 4

(a) Temperature ($\mathit{T}$) dependencies of magnetization ($\mathit{M}$) of SPS-GCO collected in zero-field-cooled heating (ZFC) and field-cooled cooling (FCC) modes with an applied magnetic field ($\mathit{H}$) of 100 Oe (the upper inset shows $\frac{dM}{dT}$ vs $\mathit{T}$ for both ZFC and FCC curves, which clearly highlights the antiferromagnetic transition at $\approx 170\mathrm{K}$ and the lower inset shows the linear dependence of $\mathit{M}$ on $\mathit{H}$ at room temperature). $\mathit{T}$ dependencies of (b) dielectric permittivity (${\epsilon }_{\mathrm{r}}^{\prime }$) (the inset shows the $\mathit{T}$ dependence of ${\epsilon }_{\mathrm{r}}^{\prime }$ collected in the heating and subsequent cooling cycles with electric field frequencies of 0.5 and 10 kHz) and (c) dielectric loss for SPS-GCO (the inset shows the strong relaxations in the dielectric loss data for GCO for different applied electric field frequencies). The right axis of panel (d) presents the $\mathit{T}$-dependent DSC data (collected in the heating cycle) along with the $\mathit{T}$-dependent ${\epsilon }_{\mathrm{r}}^{\prime }$ (with frequency 100 kHz) of SPS-GCO (shown in the corresponding left axis). The inset in panel (d) presents the $\mathit{T}$-dependent DSC data of SPS-GCO collected in the subsequent cooling cycle.

Figure 5

(a) The Rietveld refinement of room-temperature synchrotron XRD spectra of SPS-GCO considering the $Pna{2}_1$ space group. The left inset shows the (221) peak of SPS-GCO, refined with $Pbnm$ ($R_{\mathrm{p}}=6.93$, $R_{\mathrm{wp}}=9.87$, ${\chi }^2=2.82$). The right inset shows the corresponding peak, refined with $Pna{2}_1$ ($R_{\mathrm{p}}=6.49$, $R_{\mathrm{wp}}=9.23$, ${\chi }^2=2.47$). (b) The Rietveld refinement of room-temperature synchrotron XRD spectra of GCO with the $Pbnm$ space group. The left inset shows the (221) peak of GCO, refined with $Pbnm$ ($R_{\mathrm{p}}=6.34$, $R_{\mathrm{wp}}=9.68$, ${\chi }^2=2.81$). The right inset shows the corresponding peak, refined with $Pna{2}_1$ ($R_{\mathrm{p}}=6.70$, $R_{\mathrm{wp}}=9.98$, ${\chi }^2=2.99$).