Polymorphism of Fluoroargentates(II): Facile Collapse of a Layered Network of α‐K₂AgF₄ Due to the Insufficient Size of the Potassium Cation

Abstract

We report the crystal structure and magnetic properties of a novel β polymorph of K2AgF4. β‐K2AgF4 is paramagnetic above 20 K and exhibits a low Curie temperature (θ < 5 K). Solid state DFT (GGA and GGA+U) calculations were performed to analyze the electronic and magnetic structure of β‐K2AgF4 at 0 K/0 GPa, reproducing correctly the ferromagnetic (FM) semiconductor ground state with the band gap at the Fermi level of approximately 1.65 eV. Furthermore, we show that the novel β form is thermodynamically favoured over the previously reported two‐dimensional α form and can be formed either by slow spontaneous exothermic α to β phase transition occurring on heating or direct synthesis from KF and AgF2 at 300 °C. The relative stability of the α and β phases is rationalized in terms of the size of the M+ cation in the M2M′F4 series (M = Na, K, Cs, M′ = Cu, Ag) and the mismatch between [MF] and [M′F4/2] sublattices in the layered perovskite α form.

Silverland: the Realm of Compounds of Divalent Silver—and Why They are Interesting

“…Double perovskite, K 2 AgF 4 , turned out to be metastable, and it readily collapsed to quasi-1D polymorph upon heating [61]; see also [50]. The 1D polymorph was not useful from the viewpoint of fluoroargentate(II)/oxocuprate(II) analogy, and our attention has turned to KAgF 3 .…”

“…Moreover, in 2010, my laboratory was among those which greatly benefited from large structural funds coming from the European Union (CEPT project). The combined experimental/theoretical team (senior researcher Dr. P. Leszczyński; postdoctoral associates Dr. M. Derzsi, Dr. A. Budzianowski; PhD students P. Malinowski, D. Kurzydłowski, and several MSc students) focused their attention on compounds of Ag 2+ in F [61,62], O [63,64], N [65], and Cl [66] environment, by reinvestigating several poorly known systems and preparing or theoretically hypothesizing a few novel ones. The continued funding was later assured by the newly formed (2010) national state agency for funding of scientific research, Polish National Science Center (NCN), via grants OPUS and HARMO-NIA, with more enthusiastic young researchers onboard (P. Połczyński, T. Gilewski, T. Michałowski, J. Gawraczyński, A. Grzelak, and others).…”

α-K2AgF4: Ferromagnetism induced by the weak superexchange of different eg orbitals from the nearest neighbor Ag ions

“…A new layered perovskite-related fluoroargentates(II), K 2 AgF 4 , has been synthesized by Z. Mazej et al 3,4 Here after, as described in Ref. 4, we will refer to the quasi-two-dimensional polymorph of K 2 AgF 4 as α-K 2 AgF 4 , which is isostructural to the parent compound of high-T C cuprate La 2 CuO 4 .…”

“…4, we will refer to the quasi-two-dimensional polymorph of K 2 AgF 4 as α-K 2 AgF 4 , which is isostructural to the parent compound of high-T C cuprate La 2 CuO 4 . La 2 CuO 4 is an antiferromagnetic (AFM) insulator.…”

“…La 2 CuO 4 is an antiferromagnetic (AFM) insulator. However, in contrast to La 2 CuO 4 , α-K 2 AgF 4 was reported as a ferromagnetic (FM) insulator with T C ∼ 26 K. 3,4 In general, magnetic ions are often antiferromagnetically coupled through superexchange interaction in insulators, while ferromagnetically coupled through Stoner itinerant electrons band model in metal. Therefore, FM in insulator is abnormal in transition metal compounds.…”