Konstantin Glukhov

The ferrielectric properties of layered CuInP₂S₆ (CIPS) are highly sensitive to stoichiometry and growth-induced defects. Here we systematically compare crystals grown by chemical vapor transport and the Bridgman–Stockbarger method with Cu- and In-enriched samples, combining compositional analysis, dielectric spectroscopy, uniaxial-pressure measurements, Raman spectroscopy, piezoresponse force microscopy and first-principles calculations. The transition temperature increases systematically from 307 K (Bridgman) to 312 K (Cu-enriched), 317 K (gas transport) and 322 K (In-enriched). In enrichment is consistent with spontaneous CIPS/In₄/₃P₂S₆ phase segregation that generates effective positive chemical pressure, stabilizing the ferrielectric state and producing a strongly biased, pinned domain configuration. Cu enrichment produces the opposite response—lower TC, enhanced dielectric loss and a fragmented domain morphology—characteristic of a negative-pressure-like perturbation of the Cu-ion energy landscape. Landau–Ginzburg–Devonshire analysis and density functional calculations provide a unified microscopic picture based on Cu-ion configurational freedom and interlayer coupling. Growth defects act as an independent source of domain-wall pinning. Together, these results establish stoichiometry and synthesis conditions as practical tools for engineering ferrielectric order in van der Waals CIPS.

The ferrielectric properties of layered CuInP₂S₆ (CIPS) are highly sensitive to stoichiometry and growth-induced defects. Here we systematically compare crystals grown by chemical vapor transport and the Bridgman–Stockbarger method with Cu- and In-enriched samples, combining compositional analysis, dielectric spectroscopy, uniaxial-pressure measurements, Raman spectroscopy, piezoresponse force microscopy and first-principles calculations. The transition temperature increases systematically from 307 K (Bridgman) to 312 K (Cu-enriched), 317 K (gas-transport) and 322 K (In-enriched). In enrichment is consistent with spontaneous CIPS/In₄/₃P₂S₆ phase segregation that generates effective positive chemical pressure, stabilizing the ferrielectric state and producing a strongly biased, pinned domain configuration. Cu enrichment produces the opposite response—lower TC, enhanced dielectric loss and a fragmented domain morphology—characteristic of a negative-pressure-like perturbation of the Cu-ion energy landscape. Landau–Ginzburg–Devonshire analysis and density-functional calculations provide a unified microscopic picture based on Cu-ion configurational freedom and interlayer coupling. Growth defects act as an independent source of domain-wall pinning. Together, these results establish stoichiometry and synthesis conditions as practical tools for engineering ferrielectric order in van der Waals CIPS.

The experimental studies of the paramagnetic-antiferromagnetic phase transition through Mössbauer spectroscopy and measurements of temperature and field dependencies of magnetic susceptibility in the layered Cu0.15Fe0.85PS3 crystal are presented. The peculiar behavior of the magnetization - field dependence at low-temperature region gives evidence of a weak ferromagnetism in the studied alloy. By the ab initio simulation of electronic and spin subsystems, in the framework of electron density functional theory, the peculiarities of spin ordering at low temperature as well as changes in interatomic interactions in the vicinity of the Cu substitutional atoms are analyzed. The calculated components of the electric field gradient tensor and asymmetry parameter for Fe ions are close to the ones found from Mössbauer spectra values. The Mulliken populations show that the main contribution to the ferromagnetic spin density is originated from 3d-copper and 3p-sulfur orbitals. The estimated total magnetic moment of the unit cell (8.543 emu/mol) is in reasonable agreement with the measured experimental value of ∼9 emu/mol.

The elastic and vibrational properties of the TIInS₂ and TlIn(S_0.75Se_0.25)₂ crystals were first calculated applying DFT/PBE method augmented by dispersion correction (DFT/PBE-D). The elastic constants C_ij, bulk modulus B, Young's modulus E, shear modulus G, Poisson's ratio σ, and coefficient B/G were calculated for the studied crystals. The phonon energy dispersion curves and partial densities of the phonon states for the TIInS₂ and TlIn(S_0.75Se_0.25)₂ were also calculated. The obtained data are comparable with the experimental results. Anomalies occurring in the phonon energies observed for the TlIn(S_0.75Se_0.25)₂ crystal were explained.

Detailed first-principles calculations were performed to explain the physical nature of the thermoelectric properties occurring in Sn₂P₂S₆(SPS) crystals with deviation from stoichiometry.

The Raman scattering spectra for the MnPS₃ single crystal have been studied at frequencies ranging from 5 to 1200 cm⁻¹, for temperatures ranging from 5 to 295 K. This is the first time that studies of low-temperature polarized Raman spectra in a magnetically ordered phase are presented for MnPS₃. Energy splitting between some of A_g- and B_g-modes, which were previously believed to be energy equivalent in crystals from this family, has been found. The nature of the additional phonon lines observed in the Raman scattering spectra is discussed. It is found that at low temperatures, the magnetic scattering spectrum has a complex form that is related to the presence of both two-magnon and a phonon-magnon bands. The vibrational properties of the MnPS₃ crystal were calculated using the LDA approximation with the DFT-D (OBS) dispersion correction implemented in the CASTEP computation package. Phonon dispersion curves and the total and partial phonon densities of states have been calculated in the antiferromagnetic phase of the MnPS₃ crystal. The calculated results are in good agreement with the experimental Raman spectra.

In this paper, an investigation of the electronic and optical properties of the TlGaSe₂ pure layered crystal and TlGaSe₂ doped with substitutional impurities in the framework of the density functional theory have been carried out. Based on the calculations of the band structure, energy dependencies for the optical characteristics such as the real and imaginary parts of the dielectric function, the refractive index, and the absorption coefficient were obtained for TlGa_1–хIn_хSe₂, TlGa(Se_1–хS_х)₂ compounds. The effect of the impurities on the band gap and optical parameters have been analyzed.

The thermal properties of the layered compounds CuInP₂(S,Se)₆, AgInP₂(S,Se)₆, and CuBiP₂Se₆ have been studied. Thermal diffusivity has been measured in the temperature range 30–350 K by means of a high resolution ac photopyroelectric calorimeter, showing an important thermal anisotropy, while specific heat has been calculated through the evaluation of the phonon spectra using density-functional perturbation theory, where it was not available in literature. Combining both variables, thermal conductivity has been obtained in the full temperature range, showing that heat is effectively transferred by phonons; very low values have been found for the thermal conductivity at high temperature, due to the disorderly placed Cu ions in the lattice. Ferroelectric phase transitions have been found for CuInP₂(S,Se)₆ and CuBiP₂Se₆ whereas AgInP₂(S,Se)₆ compounds do not show any polar phase.