Kiiamov Airat, Kuznetsov Maxim, Croitori Dorina, Filippova Irina, Tsurkan Vladimir, Krug von Nidda Hans-Albrecht, Seidov Zakir, Mayr Franz, Widmann Sebastian, Vagizov Farit, Tayurskii Dmitrii, Tagirov Lenar
Institute of Physics, Kazan Federal University, 420008 Kazan, Russia.
Institute of Applied Physics, MD-20208 Chisinau, Moldova.
Molecules. 2022 Apr 20;27(9):2663. doi: 10.3390/molecules27092663.
Ternary potassium-iron sulfide, KFeS, 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 KFeS 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 Fe Mössbauer spectroscopy, respectively. An ab initio approach was developed to calculate the thermodynamic properties of KFeS. 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 KFeS 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.
三元硫化钾铁（KFeS）属于高度各向异性的准一维反铁磁体家族，具有不寻常的“反居里 - 外斯”磁化率，在温度升至700 K时准线性增长，对比热的磁贡献几乎消失，磁矩大幅减小等。虽然一些测量结果可以得到令人满意的描述，但磁转变时熵变的不足以及铁离子的自旋态仍然是进一步理解磁性的挑战。在这项工作中，通过布里奇曼法生长了高质量的KFeS单晶样品，并分别利用波长色散X射线电子探针微分析、粉末X射线衍射和Fe穆斯堡尔谱检查了它们的化学计量比、晶体结构以及不存在外来磁相。开发了一种从头算方法来计算KFeS的热力学性质。应用DFT + U版本的密度泛函理论计算了元素特定的声子模式及其态密度（PDOS），该理论明确考虑了电子的在位库仑排斥U及其交换相互作用J。通过与非弹性核散射实验得到的实验铁PDOS进行比较，对频率标度进行了必要的校准。红外吸收测量证实了存在与计算的PDOS一致的两个高频峰。校准后的PDOS允许通过对声子模式进行直接求和而无需近似和可调参数来计算KFeS对比热的晶格贡献。通过从实验比热中减去计算出的声子贡献来评估的温度依赖磁比热为估计铁离子的降低自旋态提供了一个下限。
