High-T_c Layered Ferrielectric Crystals by Coherent Spinodal Decomposition

Abstract

Research in the rapidly developing field of 2D electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cation-deficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In4/3P2S6 and ferrielectrically active CuInP2S6 are realized by controllable chemical phase separation in van der Waals bonded single crystals. CuInP2S6 by itself is a layered ferrielectric with a ferrielectric transition temperature (Tc) just over room temperature, which rapidly decreases with homogeneous doping. Surprisingly, in our composite materials, the ferrielectric Tc of the polar CuInP2S6 phase increases. This effect is enabled by unique spinodal decomposition that retains the overall van der Waals layered morphology of the crystal, but chemically separates CuInP2S6 and In4/3P2S6 within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or synthesis conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D in-plane heterostructures with periodically alternating polar and nonpolar phases.

Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric

“…In contrast to CIPS, the response of IPS has negligible hysteresis in-field (on-field Dac, blue curve, circle data points) and negligible magnitude off-field (remanent Dac, orange curve, square data points). The IPS response is therefore most consistent with electrostatic forces acting on the tip and the IPS phase shows nearly zero off-field converse piezoelectric response [14].…”

“…To probe the effects of negative electrostriction on the electromechanical response, we applied quantitative piezoresponse force microscopy (PFM) and voltage spectroscopy [25,26] that measures deformation of the surface in applied electric fields (Dac). We chose a several μm thick composite crystal where CIPS is interspersed with a non-piezoelectric IPS phase [14]. The presence of the non-ferroelectric phase has provided an unambiguous reference for the observed response below and above the ferroelectric Curie temperature TC, enabling the observation of intrinsic electromechanical behavior of the CIPS phase.…”

“…To ascertain the properties of local polarization CIPS in applied field, the measurements were repeated above the phase transition temperature of CIPS, around 40°C-70°C depending on specific composition [14], PFM images obtained at 100°C show negligible Dac (see Supplemental Material Figure S3 [20]), except for weak topographic crosstalk [31]. In voltage spectroscopy, the off-field response is close to zero for CIPS and IPS, corroborating loss of remanent polarization in CIPS (see Supplemental Material Figure S4 [20]).…”

Metal Thio‐ and Selenophosphates as Multifunctional van der Waals Layered Materials

“…and H 2 storage (e.g.,). However, we also observe that a large number of papers date back to the 1980s to late 1990s, and that only very recently has interest in these materials reemerged (e.g …”

“…Maisonneuve et al found that using In 2 S 3 as a precursor resulted in less impurity phase in CuInP 2 S 6 compounds. Indeed, we have used the same precursor to produce AgInP 2 S 6 , CuInP 2 S 6 , In 4/3 P 2 S 6 , and CuInP 2 S 6 ‐In 4/3 P 2 S 6 heterostructures with little to no impurity phases present . In addition, we have also utilized this technique, using Sc 2 S 3 as a precursor, to produce CuScP 2 S 6 and AgScP 2 S 6 crystals.…”