Description of CuInP2S6 ferrielectrics in a mixed Ising model

R. Yevych; V. Liubachko; V. Hryts; M. Medulych; A. Kohutych; Yu. Vysochanskii

doi:10.5488/cmp.27.14701

Authors

R. Yevych Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0000-0001-6971-3429
V. Liubachko Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0000-0002-5114-2821
V. Hryts Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0009-0007-0836-7367
M. Medulych Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0000-0003-2128-0235
A. Kohutych Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0000-0001-9138-2267
Yu. Vysochanskii Institute for Solid State Physics and Chemistry, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine https://orcid.org/0000-0002-2501-1780

DOI:

https://doi.org/10.5488/cmp.27.14701

Keywords:

ferrielectrics, CuInP2S6, mixed Ising model, quantum anharmonic oscillator model

Abstract

The appearance of spontaneous polarization in CuInP₂S₆ ferrielectrics is related to the second order Jahn-Teller effect for copper cations located in a double-well local potential, the stereoactivity of indium cations located in a three-well local potential, as well as the valence fluctuations of phosphorus cations. The paraelectric to ferrielectric phase transition is primarily determined by the coupling of indium cations with their surroundings. This transition can be analyzed using the mixed Ising model with spins s = 1/2 and S = 1. The spectrum of pseudospin fluctuations at different temperatures was calculated using a mean-field approach for a set of quantum anharmonic oscillators. The results were then compared with Raman spectroscopy data for CuInP₂S₆ crystal. The analysis indicates that the lattice anharmonicity below 150 K, is mainly determined by the indium sublattice, leading to the coexistence of the glassy state and ferrielectric phase. Above 150 K, the anharmonicity of the copper sublattice activates the ionic conductivity and results in the existence of a long-ranged fluctuated cluster of spontaneous polarization in a temperature interval of the paraelectric phase above T_C.

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