Nanocomposites and nanomaterials

Pseudospin modelling and mean field analysis of ferrielectricity 
in CuInP2S6 van der Waals crystals
Vysochanskii Yu., Liubachko V., Yevych R., Hryts V., Medulych M., 
Kohutych A., Glukhov K.  
Institute for Solid State Physics and Chemistry, Uzhhorod University, Pidgirna Str. 46, 88000 Uzhhorod, Ukraine
E-mail: vysochanskii@gmail.com

The CuInP2S6 ferroics are already involved into van der Waals heterostructures. Their functionality is determined by antiparallel polar ordering of Cu+ and In3+ sublattices. For 2D CuInP2S6 crystals, the phase diagram and the spectrum of the spontaneous polarization fluctuations can be described in the quantum anharmonic oscillators (QAO) model that has already been applicated for ferroelectrics [1]. At this, the pseudospins s = +1/2, -1/2 are related to Cu+ cations, that flip in two-well potential, and pseudospins S = +1, 0, -1 describe local dynamics of In3+ cations in three-well potential. Thermodynamics of such a system in the mixed Spin – 1/2 and Spin – 1 Ising model [2] demonstrates the evolution of temperature – pressure (composition) phase diagram topology at variation of the local crystal field anisotropy around In3+ cations: the tricritical point on the paraelectric phase stability boundary transforms into a triple point, that is joined by the first order phase transitions line with bicritical end point inside the ferrielectric phase. Such evolution can be realized by mechanical compression of CuInP2S6 crystals or at cooper by silver substitution in (AgxCu1-x)InP2S6 mixed crystals, and it is originated from the change of the In3+ cations local three-well potential. In the continuous mean-field approximation, with Landau – Devonshire thermodynamic functional containing invariants till eight order, has been predicted [3] a presence of a special point on temperature – pressure diagram of CuInP2S6 ferrielectrics, which can be related to the bicritical end point on temperature – electric field – pressure diagram.   

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