We explore ferroelectric properties of cleaved 2-D flakes of copper indium thiophosphate, CuInP2S6 (CITP), and probe size effects along with limits of ferroelectric phase stability, by ambient and ultra high vacuum scanning probe microscopy. CITP belongs to the only material family known to display ferroelectric polarization in a van der Waals, layered crystal at room temperature and above. Our measurements directly reveal stable, ferroelectric polarization as evidenced by domain structures, switchable polarization, and hysteresis loops. We found that at room temperature the domain structure of flakes thicker than 100 nm is similar to the cleaved bulk surfaces, whereas below 50 nm polarization disappears. We ascribe this behavior to a well-known instability of polarization due to depolarization field. Furthermore, polarization switching at high bias is also associated with ionic mobility, as evidenced both by macroscopic measurements and by formation of surface damage under the tip at a bias of 4 V-likely due to copper reduction. Mobile Cu ions may therefore also contribute to internal screening mechanisms. The existence of stable polarization in a van-der-Waals crystal naturally points toward new strategies for ultimate scaling of polar materials, quasi-2D, and single-layer materials with advanced and nonlinear dielectric properties that are presently not found in any members of the growing "graphene family".

Polar van der Waals chalcogenophosphates exhibit unique properties, such as negative electrostriction and multi-well ferrielectricity, and enable combining dielectric and 2D electronic materials. Using low temperature piezoresponse force microscopy, we revealed coexistence of piezoelectric and non-piezoelectric phases in CuInP 2 Se 6 , forming unusual domain walls with enhanced piezoelectric response. From systematic imaging experiments we have inferred the formation of a partially polarized antiferroelectric state, with inclusions of structurally distinct ferrielectric domains enclosed by the corresponding phase boundaries. The assignment is strongly supported by optical spectroscopies and density-functionaltheory calculations. Enhanced piezoresponse at the ferrielectric/antiferroelectric phase boundary and the ability to manipulate this entity with electric field on the nanoscale expand the existing phenomenology of functional domain walls. At the same time, phase-coexistence in chalcogenophosphates may lead to rational strategies for incorporation of ferroic functionality into van der Waals heterostructures, with stronger resilience toward detrimental size-effects.

The valence fluctuations which are related to the charge disproportionation of phosphorous ions P 4+ + P 4+ → P 3+ + P 5+ are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)2P2S6 semiconductors. They involve recharging of SnPS3 or PbPS3 structural groups which could be represented as half-filled sites in the crystal lattice. Temperature-pressure phase diagram for Sn2P2S6 compound and temperature-composition phase diagram for (PbySn1−y)2P2S6 mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb2P2S6 are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.

No abstract

For the first time in a bulk proper uniaxial ferroelectrics, double antiferroelectric-like hysteresis loops have been observed in the case of Sn2P2S6 crystal. The quantum anharmonic oscillator model was proposed for description of such polarization switching process. This phenomenon is related to three-well local potential of spontaneous polarization fluctuations at peculiar negative ratio of coupling constants which correspond to inter-site interaction in given sublattice and interaction between two sublattices of Sn2P2S6 modeled crystal structure. Obtained data can be used for development of triple-level cell type memory technology.PACS numbers: 64.60. De,77.80.Dj,77.84.Bw With the development of modern information technology, humanity is needed in ever larger volumes of digital information storage. Due to the fact, that the technological norms for reducing the size of memory cells gradually approach the limitations imposed by physical laws, memory manufacturers are looking for ways out of this situation. One of the methods for increasing the density of information storage is the use of memory cells with several logical states. In such systems, several bits of information are stored in one cell. A vivid example of this approach is a multi-level cell and triple-level cell (TLC) type flash memory [1]. However, flash-technology has a number of drawbacks [2], which leads to the search for new types of memory cells. One of the candidates for the role of the non-volatile universal memory of the future is ferroelectric memory (Fe-RAM) [3]. As it turned out, in these memory cells it is also possible to store several information bits [4]. However, the practical implementation of such systems is only at the level of theoretical calculations.In this Letter we propose the use of a room temperature proper uniaxial ferroelectric-semiconductor Sn 2 P 2 S 6 as an active material of a ferroelectric memory cell. The three-well local potential for the spontaneous polarization fluctuations [5] allows one cell to store three bits of information. This makes it possible to create on its base not only memory cells, but also non-Boolean information systems.In Sn 2 P 2 S 6 crystals the second order phase transition from paraelectric phase (P2 1 /n) into ferroelectric one (Pn) occurs at T 0 ≈ 338 K. At room temperature spontaneous polarization is oriented in (010) monoclinic symmetry plane near [100] direction [6]. Origin of spontaneous polarization is related to Sn 2+ cations electron lone pair stereoactivity together with valence fluctuations P 4+ + P 4+ ←→ P 3+ + P 5+ inside (P 2 S 6 ) 4− anions [5,7,8] which in whole can be considered in frame of second order Jahn-Teller effect [9]. Thermodynamics of this mixed "displacive -order/disorder" continuous transition can be described in the Blume-Emery-Griffiths model [10,11], which consider possibility of thermal fluctuations between three values of pseudospins ("-1", "0", "+1") in local three-well potential. This model predicts possibility of metastable nonpolar states inside of ferro...