From a Dense Structure to Open Frameworks: The Structural Plethora of Alkali Metal Iron Fluorophosphates

Abstract

By employing the pyridinium hexafluorophosphate task-specific ionic liquids 1-butyl-4-methylpyridinium hexafluorophosphate ([C 4 mpyr][PF 6 ]) and 1-ethylpyridinium hexafluorophosphate ([C 2 pyr][PF 6 ]) as the reaction medium, mineralizer, structure-directing agent, and, in the case of the smaller pyridinium cation, even a structural component, it was possible to obtain five new alkali metal iron phosphates featuring interconnected FeX 6 octahedra and PX 4 (X = F, O, or OH) tetrahedra. NaFe(PO 3 F) 2 ( 1 ) is a dense 3D structure, RbFe(PO 3 F)(PO 2 (OH)F)(PO 2 (OH) 2 ) ( 2 ) features 1D strands, (C 2 pyr)LiFe(PO 3 F) 3 (PO 2 F 2 )F ( 3 ) has 2D layers, and LiFe(PO 3 F)(PO 2 F 2 )F ( 4 ) as well as Cs 0.75 Fe(PO 2.75 (OH) 0.25 F)(PO 2 F 2 ) 2 ( 5 ) are 3D open frameworks. While in 1–2 as well as in 4 and 5 , FeX 6 octahedra and PX 4 (X = F, O, or OH) tetrahedra alternate, 3 features octahedra dimers, Fe 2 X 11 (X = F, O, or OH). The magnetic behavior of all compounds is governed by antiferromagnetic interactions. Interestingly, 3 exhibits a broad maximum in the temperature dependence of the magnetic susceptibility, characteristic of a low-dimensional magnetic system consistent with the presence of Fe–Fe dimers in its crystal structure.

Frustration and 120° Magnetic Ordering in the Layered Triangular Antiferromagnets AFe(PO3F)2 (A = K, (NH4)2Cl, NH4, Rb, and Cs)

“…The samples were synthesized via ionothermal reactions, a more recent approach in which ILs are used as a solvent, structure-directing agent, and mineralizer. − In contrast to other methods, such as hydrothermal syntheses, the ionothermal approach allows reactions under mild conditions, particularly as no high pressures build up from the solvent. As ILs are generally composed of weakly coordinating ions, the competition of coordination of the metal ions with solvent constituents is reduced; together with a templating effect, this helps to reduce the otherwise commonly encountered problem of the formation of dense phases.…”

Oxohydroxo‐Tellurates(VI) K2[TeO2(OH)4] and K2[Fe2TeO6(OH)2] ⋅ 2H2O from Alkaline Hydroflux

“…Broad maxima are observed around 160 K, but no distinct magnetic transition is noticeable. Such kind of magnetic behavior has been recently reported for the pseudo two‐dimensional system (C 2 pyr)LiFe 2 (PO 3 F) 3 (PO 2 F 2 )F [77] . As mentioned above, an overall antiferromagnetic behavior is expected by analyzing the structure of K 2 [Fe 2 TeO 6 (OH) 2 ] ⋅ 2H 2 O according to the Goodenough‐Kanamori‐Anderson rules [69] .…”

“…Broad maxima are observed around 160 K, but no distinct magnetic transition is noticeable. Such kind of magnetic behavior has been recently reported for the pseudo two-dimensional system (C 2 pyr)LiFe 2 (PO 3 F) 3 (PO 2 F 2 )F. [77] As mentioned above, an overall antiferromagnetic behavior is expected by analyzing the structure of K 2 [Fe 2 TeO 6 (OH) 2 ] • 2H 2 O according to the Goodenough-Kanamori-Anderson rules. [69] The coexistence of ferromagnetic exchange in the pairs of edge-sharing iron(III)- centered octahedra and antiferromagnetic coupling via the corners that these pairs share is consistent with the observations, although the first aspect cannot be derived from the available data.…”