Density Functional Theory Approach to the Vibrational Properties and Magnetic Specific Heat of the Covalent Chain Antiferromagnet KFeS2

Airat Kiiamov; Maxim Kuznetsov; Dorina Croitori; Irina Filippova; Vladimir Tsurkan; Hans-Albrecht Krug von Nidda; Zakir Seidov; Franz Mayr; Sebastian Widmann; Farit Vagizov; Dmitrii Tayurskii; Lenar Tagirov

doi:10.3390/molecules27092663

Density Functional Theory Approach to the Vibrational Properties and Magnetic Specific Heat of the Covalent Chain Antiferromagnet KFeS₂

Molecules. 2022 Apr 20;27(9):2663. doi: 10.3390/molecules27092663.

Authors

Affiliations

¹ Institute of Physics, Kazan Federal University, 420008 Kazan, Russia.
² Institute of Applied Physics, MD-20208 Chisinau, Moldova.
³ Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany.
⁴ Institute of Physics, Azerbaijan National Academy of Sciences, AZ-1143 Baku, Azerbaijan.
⁵ Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, 420029 Kazan, Russia.

Abstract

Ternary potassium-iron sulfide, KFeS₂, belongs to the family of highly anisotropic quasi-one-dimensional antiferromagnets with unusual "anti-Curie-Weiss" susceptibility, quasi-linearly growing with a rising temperature up to 700 K, an almost vanishing magnetic contribution to the specific heat, drastically reduced magnetic moment, etc. While some of the measurements can be satisfactorily described, the deficiency of the entropy changes upon the magnetic transition and the spin state of the iron ion remains a challenge for the further understanding of magnetism. In this work, high-quality single-crystalline samples of KFeS₂ were grown by the Bridgman method, and their stoichiometry, crystal structure, and absence of alien magnetic phases were checked, utilizing wave-length dispersive X-ray electron-probe microanalysis, powder X-ray diffraction, and ⁵⁷Fe Mössbauer spectroscopy, respectively. An ab initio approach was developed to calculate the thermodynamic properties of KFeS₂. The element-specific phonon modes and their density of states (PDOS) were calculated applying the density functional theory in the DFT + U version, which explicitly takes into account the on-site Coulomb repulsion U of electrons and their exchange interaction J. The necessary calibration of the frequency scale was carried out by comparison with the experimental iron PDOS derived from the inelastic nuclear scattering experiment. The infrared absorption measurements confirmed the presence of two high-frequency peaks consistent with the calculated PDOS. The calibrated PDOS allowed the calculation of the lattice contribution to the specific heat of KFeS₂ by direct summation over the phonon modes without approximations and adjustable parameters. The temperature-dependent magnetic specific heat evaluated by subtraction of the calculated phonon contribution from the experimental specific heat provides a lower boundary for estimating the reduced spin state of the iron ion.

Keywords: density functional theory; magnetic specific heat; phonon density of states; quasi-one-dimensional antiferromagnet; spin chain models; ternary potassium-iron sulfide.

Abstract

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