Large Exchange Bias, High Dielectric Constant, and Outstanding Ionic Conductivity in a Single‐Phase Spin Glass

Abstract

as an interdisciplinary research line of chemistry, physics, and materials engineering. [1,2] Numerous scientific efforts have focused on designing new materials with the desired electronic and magnetic features by tuning the chemical composition, structural parameters, and physical modifications. [3,4] In recent years, defect variant structures have been considered as promising candidates to provide new, targeted properties and property combinations. The interdependence of magnetic and electronic structures to the partial vacancies on anion positions has been reported for a few ferrite and ferrate compounds indicating significant effects of vacancies on the Fermi level, orbital splitting, and geometrical interactions in the crystal structure, thereby changing the magnetization and the ionic conductivity. [5,6] The change in properties originates from the different vacancies of isolated cations and anions, as well as defects in the complex anionic motifs. For compounds including iron, depending on the structure, the iron vacancies can be considered either as vacancies of single cations, or as defects in the anionic motifs. Although the impact of iron The multigram synthesis of K 2 [Fe 3 S 4 ] starting from K 2 S and FeS is presented, and its electronic and magnetic properties are investigated. The title compound obtains a defect variant of the K[Fe 2 Se 2 ] structure type. Dielectric and impedance measurements indicate a dielectric constant of 1120 at 1 kHz and an outstanding ionic conductivity of 24.37 mS cm -1 at 295 K, which is in the range of the highest reported value for potential solid-state electrolytes for potassium-ion batteries. The Seebeck coefficient of the n-type conductor amounts to −60 µV K −1 at 973 K. The mismatch of the measured electrical resistivity and the predicted metal-like band structure by periodic quantum chemical calculations indicates Mott insulating behavior. Magnetometry demonstrates temperature-dependent, large exchange bias fields of 35 mT, as a consequence of the coexistence of spin glass and antiferromagnetic orderings due to the iron vacancies in the lattice. In addition, the decreasing training effects of 34% in the exchange bias are identified at temperatures lower than 20 K. These results demonstrate the critical role of iron vacancies in tuning the electronic and magnetic properties and a multifunctional material from abundant and accessible elements.

Low-Cost, Multifunctional, and Sustainable Sodium Sulfido Ferrate(II)

“…We have recently reported a 2:3:4 stoichiometric ratio in K 2 [Fe 3 S 4 ] with space group type I 4/ mmm . However, the only similarity of 1 with the potassium analogue is the layered anionic structure and the 75% occupation of iron atoms within the structure . Despite the similarities in iron vacancies and cationic layers, these two compounds crystallize in two different lattice structures.…”

“…However, the only similarity of 1 with the potassium analogue is the layered anionic structure and the 75% occupation of iron atoms within the structure. 11 Despite the similarities in iron vacancies and cationic layers, these two compounds crystallize in two different lattice structures. In K 2 [Fe 3 S 4 ], the edge-sharing tetrahedra create the anionic layers without any inherent structural voids, while in 1, the edge-and corner-sharing tetrahedra induce honeycomb cavities in the anionic layers.…”

“…We have recently reported on the high dielectric constant and the second highest bulk ionic conductivity of K-ion containing materials in K 2 [Fe 3 S 4 ], a ternary sulfido ferrate containing iron vacancies in the anionic substructure. 11 We were, therefore, interested in the sodium analogue, Na 2 [Fe 3 S 4 ] (1), which should combine high dielectric constants and high ionic conductivity values with abundant and nontoxic elemental components. Owing to the abundant sources of sodium compared to those of potassium, as well as further potentials and an increasing interest in sodium-ion batteries, 1 is a promising candidate to synthesize and investigate in comparison with the potassium analogue.…”

“…We have recently reported on the high dielectric constant and the second highest bulk ionic conductivity of K-ion containing materials in K 2 [Fe 3 S 4 ], a ternary sulfido ferrate containing iron vacancies in the anionic substructure . We were, therefore, interested in the sodium analogue, Na 2 [Fe 3 S 4 ] ( 1 ), which should combine high dielectric constants and high ionic conductivity values with abundant and nontoxic elemental components.…”

Large-scale synthesis of mixed valence K3[Fe2S4] with high dielectric and ferrimagnetic characteristics

“…Synthesis of K 2 S. The synthesis process was carried out using 20 g (0.512 mol, 2 eq., Acros Organics, 98%) potassium and 8.2 g (0.258 mol, 1 eq., abcr, 99%) sulphur in liquid NH 3 , according to the reported procedure. 49 Caution: the reaction of sulphur and potassium is exothermic! Cooling and a constant pressure release need to be ensured!…”