Abstract
Using first-principles calculations and group-theory-based models, we study the stabilization of ferrielectricity (FiE) in ${\mathrm{CuInP}}_{2}{\mathrm{Se}}_{6}$. We find that the FiE ground state is stabilized by a large anharmonic coupling between the polar mode and a fully symmetric Raman-active mode. Our results open possibilities for controlling the single-step switching barrier for polarization by tuning the Raman-active mode. We discuss the implications of our findings in the context of designing next-generation optoelectronic devices that can overcome the voltage-time dilemma.
Highlights
Ferroelectricity in layered van der Waals materials has attracted a lot of attention because of its applications in high-density nonvolatile memory devices [1,2]
We find that the polar displacements of the Cu and In atoms create most of the polarization in the FiE phase
We report that a strong anharmonic coupling between the polar mode and a fully symmetric Raman-active mode is necessary to stabilize the polar phase
Summary
Ferroelectricity in layered van der Waals materials has attracted a lot of attention because of its applications in high-density nonvolatile memory devices [1,2]. Very few layered materials demonstrate switchable out-of-plane polarization [3–8]. The transition metal thiophosphates which includes CuInP2Se6 (CIPSe) are a promising family of materials that host out-of-plane ferrielectricity (FiE) with large values of polarization [4,9,10]. The FiE phases in this family is characterized by a negative piezoelectric response [11,12] and a negative electrostriction [13], along with large dielectric tunability [14]. This led to device applications such as ferroelectric (FE) tunnel junctions [15] and FE field-effect transistors [16,17]. Similar to other materials in this family [20], the phase transition is expected to have a large order-disorder character in addition to a displacive character [21]. While considerable order-disorder character is expected for the phase transition near the critical temperature, the switching under
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