Pressure-driven phase transformations and phase segregation in ferrielectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6\text{−}{\mathrm{In}}_{4/3}{\mathrm{P}}_2{\mathrm{S}}_6$ self-assembled heterostructures

Layered multiferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ spontaneously phase segregates to form ferrielectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ and paraelectric ${\mathrm{In}}_{4/3}{\mathrm{P}}_2{\mathrm{S}}_6$ (IPS) domains in a two-dimensional self-assembled heterostructure. Here, we study the effect of hydrostatic pressure on the structure of Cu-deficient $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ by Raman spectroscopy measurements up to 20 GPa. Detailed analysis of the frequencies, intensities, and linewidths of the Raman peaks reveals four discontinuities in the spectra around 2, 10, 14, and 17 GPa. At ∼2 GPa, we observe a structural transition initiated by the diffusion of IPS domains, which culminates in a drastic reduction of the number of peaks around 10 GPa. We attribute this to a possible monoclinic-trigonal phase transition at 10 GPa. At higher pressures (∼14 GPa), significant increases in peak intensities and sharpening of the Raman peaks suggest a band gap lowering and an isostructural electronic transition, with a possible onset of metallization at pressures above 17 GPa. When the pressure is released, the structure again phase separates into two distinct chemical domains within the same single crystalline framework—however, these domains are much smaller in size than the as-synthesized material resulting in suppression of ferroelectricity through nanoconfinement. Hydrostatic pressure can thus be used to tune the electronic and ferrielectric properties of Cu-deficient layered $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$.

Figure 1

Top and lateral views of the (a) CIPS and (b) IPS crystal structures. (c) Refinements of synchrotron XRD data showing the temperature-dependent evolution of the unit cell dimensions of pure-phase CIPS, pure-phase IPS, and the two respective phases in the CIPS-IPS in-plane heterostructure.

Figure 2

Room temperature ambient pressure Raman spectra (633 nm excitation) from CIPS-IPS and CIPS.

Figure 3

Raman spectra from CIPS-IPS single crystals upon (a) compression and (b) decompression. All spectra are normalized with respect to the highest-intensity peak and vertically offset for clarity. (c) Optical microscope images of the CIPS-IPS crystals inside the diamond anvil cell during compression up to 5.1 GPa. The color change indicates a piezochromic effect due to a change in the electronic structure.

Figure 4

Comparison between Raman spectra from CIPS-IPS at low pressure (and room temperature) and across the ferrielectric-paraelectric transition temperature $\left(T_C\sim 340\mathrm{K}\right)$. The dotted (dashed) lines indicate peaks with discernible changes in intensities upon heating (compression).

Figure 5

2D heat map showing the shift in Raman peak frequencies from CIPS-IPS upon compression. The color scale corresponds to peak intensities (counts) normalized to the diamond Raman peak.

Figure 6

Evolution of peak frequencies, normalized intensities, and linewidths with pressure.

Figure 7

Structural analysis before and after compression. (a) Raman spectrum collected before compression and after decompression back to ambient pressure, and EDS (or AFM) maps showing the evolution of the microstructure in the as-prepared single crystals and after decompression. (b) Secondary electron image of the CIPS-IPS heterostructured crystal showing smooth surface. The striations in the EDS image of the as-grown CIPS-IPS crystal (c) correspond to the CIPS (red) and IPS (green) sublattices. These are significantly reduced in size upon decompression (d,e), leading to a loss of ferroelectric order. (f) AFM phase image from a freshly cleaved surface of the as-prepared CIPS-IPS crystal. The domains of the CIPS and IPS can be clearly distinguished. (g) AFM phase image of the decompressed crystal showing a loss of domain structure.