Off-plane polarization ordering in metal chalcogen diphosphates from bulk to monolayer

Abstract

Vertically (off-plane) ferroelectric ordering in ultra-thin films has been pursued for decades. We predict the existence of intrinsic vertical polarization orderings in ultra-thin metal chalcogendiphosphates (MCDs). Taking CuInP2Se6 as an example, the first-principles calculation and electrostatic-energy model show that, under the open-circuit boundary condition, the ground state of bulk CuInP2Se6 is ferroelectric (FE) while that of monolayer is antiferroelectric (AFE), and the critical thickness for this FE/AFE transition is around 6 layers. Interestingly, under the closedcircuit boundary condition, the FE state can hold even for monolayer. Particularly, because of the small energy difference but the large barrier between FE and AFE orderings, the FE state can be stabilized in free-standing monolayer, giving rise to intrinsic, off-plane two-dimensional ferroelectrics. Applying Monte Carlo simulations, we further calculate the ferroelectric Curie temperature (Tc) and electric hysteresis.

van der Waals forces control ferroelectric–antiferroelectric ordering in CuInP2S6 and CuBiP2Se6 laminar materials

“…Hence the van der Waals force is perceived as being very sensitive to the details of inter-layer arrangements, with the overall bulk structure emerging in part as a result of the way these contributions act to cancel each other out. Improved treatments of the electrostatic interactions 26 will not affect this analysis.…”

“…Also, the energies of the ferroelectric crystals at these reference geometries are 26 meV more stable compared to the antiferroelectric structures than one would expect simply by adding bilayer energies, an effect arising from long-range attractive dipole–dipole interactions present in ferroelectric structures. Proper treatment of these effects, 26 plus use of the most accurate computational methods possible, will be critical to making robust predictions for properties of specific nanoflakes. For example, the calculated 8 meV energy difference between A ii FF and A ii A oo A ii tetralayers is too small and too susceptible to method variations for this to be regarded as a robust prediction of tetralayer properties.…”

“…In the related system CuInP 2 Se 6 , small changes associated with these calculation effects have been shown to have influence on ferroelectric structure. 26 …”

Recent Progress in Two‐Dimensional Ferroelectric Materials

“…As shown in Figure 2c, the anti-ferroelectric distortion is induced by the offcentering Ag + and Bi 3+ ions, resulting in two stable ferroelectric phases with opposite directions. [51] Similar to AgBiP 2 Se 6 , other TMTP materials, including CuInP 2 S 6 , [40,52,53] CuInP 2 Se 6 , [54,55] etc., are also typical and promising materials for researchers to study. Among them, CuInP 2 S 6 is a typical member of the TMTP family with intralayer ferroelectric ordering, which has already been proven to be a ferroelectric material experimentally and will be discussed in the next section.…”

Piezoelectric domain walls in van der Waals antiferroelectric CuInP2Se6

“…Indeed, recently, Song et al 12 proposed that ultrathin films of CuInP 2 Se 6 develop an antiferroelectric ground state, with the ferrielectric-antiferroelectric crossover occurring at a thickness of 6-8 layers. The primary driving force for the crossover is the depolarizing field that favors the antiferroelectric with net zero polarization.…”