Ionothermal Synthesis of the First Luminescent Open-Framework Manganese Borophosphate with Switchable Magnetic Properties

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

By employing the pyridinium hexafluorophosphate task-specific ionic liquids 1-butyl-4-methylpyridinium hexafluorophosphate ([C₄mpyr][PF₆]) and 1-ethylpyridinium hexafluorophosphate ([C₂pyr][PF₆]) 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 FeX₆ octahedra and PX₄ (X = F, O, or OH) tetrahedra. NaFe(PO₃F)₂ (1) is a dense 3D structure, RbFe(PO₃F)(PO₂(OH)F)(PO₂(OH)₂) (2) features 1D strands, (C₂pyr)LiFe(PO₃F)₃(PO₂F₂)F (3) has 2D layers, and LiFe(PO₃F)(PO₂F₂)F (4) as well as Cs_0.75Fe(PO_2.75(OH)_0.25F)(PO₂F₂)₂ (5) are 3D open frameworks. While in 1–2 as well as in 4 and 5, FeX₆ octahedra and PX₄ (X = F, O, or OH) tetrahedra alternate, 3 features octahedra dimers, Fe₂X₁₁ (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.

Application of ionic liquids as the reaction medium, structure templates, and mineralizer led to a series of active-metal-iron phosphates with structural motifs of varied dimensionality and, ultimately, an open-framework structure.

BiMnPO5 with ferromagnetic Mn2+-(μ-O)2-Mn2+ units: a model for magnetic exchange in edge-linked Mn2+O6 octahedra

Magnetic interactions within Mn-(μ-O)2-Mn pairs are crucial to the function of some essential enzymes and catalysts, but their nature is unclear. Neutron diffraction reveals that similar units in BiMnPO5 show ferromagnetic coupling which has been rationalized by density functional theory modelling and calculations of magnetic exchange energies. The results are important to many solid state and biological systems.

A soft chemistry approach to the synthesis of single crystalline and highly pure (NH4)CoF3 for optical and magnetic investigations

A new ionothermal synthesis utilizing 1-alkyl-pyridinium hexafluorophosphates [C_xPy][PF₆] (x = 2, 4, 6) led to the formation of highly crystalline single-phase ammonium cobalt trifluoride, (NH₄)CoF₃. Although ammonium transition-metal fluorides have been extensively studied with respect to their structural and magnetic properties, multiple aspects remain unclear. For that reason, the obtained (NH₄)CoF₃ has been investigated over a broad temperature range by means of single-crystal and powder x-ray diffraction as well as magnetization and specific heat measurements. In addition, energy-dispersive x-ray and vibrational spectroscopy as well as thermal analysis measurements were undertaken. (NH₄)CoF₃ crystallizes in the cubic perovskite structure and undergoes a structural distortion to a tetragonal phase at 127.7 K, which also is observable in the magnetic susceptibility measurements, which has not been observed before. A second magnetic phase transition occurring at 116.9 K is of second-order character. The bifurcation of the susceptibility curves indicates a canted antiferromagnetic ordering. At 2.5 K, susceptibility measurements point to a third phase change for (NH₄)CoF₃.

Ionothermal Synthesis, Structures, and Magnetism of Three New Open Framework Iron Halide-Phosphates

A set of different open framework iron phosphates have been synthesized ionothermally using a task-specific ionic liquid, 1-butyl-4-methylpyridinium hexafluorophosphate, that acts in the synthesis as the reaction medium and mineralizer: (NH₄)₂Fe₂(HPO₄)(PO₄)Cl₂F (1) and K₂Fe₂(HPO₄)(PO₄)Cl₂F (2) exhibit similar composition and closely related structural features. Both structures consist of {Fe₂(HPO₄)(PO₄)Cl₂F}^2- macroanions and charge balancing ammonium or potassium cations. Their open framework structure contains layers and chains of corner-linked {Fe(1)O₂Cl₄} and {Fe(2)F₂O₄} octahedra, respectively, interconnected by PO₄ tetrahedra forming 10-ring channels. KFe(PO₃F)F₂ (3) is built up by {Fe[(PO₃F)_4/3F_2/2]}{Fe(PO₃F)_2/3F_2/2F₂} layers separated by K⁺ cations. Chains of alternating {FeF₂O₄} and {FeO₂F₄} octahedra, which are linear for 1 but undulated for 2, are linked to each other via corner-sharing {PO₃F} tetrahedra with the fluorine pointing into the interlayer space. The compounds were characterized by means of single crystal and powder X-ray diffraction, infrared spectroscopy, and magnetic measurements. 1 reveals a strong ground state spin anisotropy with a spin 5/2 state and a magnetic moment of 5.3 μ_B/Fe³⁺. Specific heat and magnetic data unveil three magnetic transitions at 95, 50, and 3.6 K. Compound 2 has a very similar crystal structure as compared to 1 but exhibits a different magnetic behavior: a slightly lower magnetic moment of 4.7 μ_B/Fe³⁺ and a magnetic transition to a canted antiferromagnetic state below 90 K. Compound 3 exhibits typical paramagnetic behavior close to room-temperature (5.71 μ_B/Fe³⁺). There are no clear indications for a phase transition down to 2 K despite strong antiferromagnetic spin-spin interactions; only a magnetic anomaly appears at 50 K in the zero-field cooled data.

Crystal structure and spin-trimer magnetism of Rb2.3(H2O)0.8Mn3[B4P6O24(O,OH)2]

The novel borophosphate Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data obtained from a non-merohedral twin and refined against F² to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn, with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) ų, and Z = 4. The title compound is the first example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn²⁺-centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb⁺ cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO₄ and HPO₄ tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] reveals predominant antiferromagnetic exchange interactions and the high-temperature effective magnetic moment corresponding to the high-spin S = 5/2 state of Mn²⁺ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and specific heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.

Organotemplate-free synthesis of two open-framework metal borophosphates

Two new isostructural metal borophosphates, |K₂(H₂O)|[CoB₂P₃O₁₂(OH)] (1) and |K₂(H₂O)|[NiB₂P₃O₁₂(OH)] (2), have been hydrothermally synthesized by the “green” organotemplate-free route. Notably, the introduction of potassium ions in our synthetic system endows 1 and 2 with the K⁺ ion conductivity.

Ionothermal synthesis of open-framework metal phosphates with a Kagomé lattice network exhibiting canted anti-ferromagnetism†Electronic supplementary information (ESI) available: Cif files, atomic parameters, X-ray diffraction patterns, IR spectra, TG curves, and thermal ellipsoid plot and atomic label schemes of compound . See DOI: 10.1039/c4tc00290cClick here for additional data file

Four open-framework transition-metal phosphates; (NH4)2Co3(HPO4)2F4 (1), (NH4)Co3(HPO4)2(H2PO4)F2 (2), KCo3(HPO4)2(H2PO4)F2 (3), and KFe3(HPO4)2(H2PO4)F2 (4); are prepared by ionothermal synthesis using pyridinium hexafluorophosphate as the ionic liquid. Single-crystal X-ray diffraction analyses reveal that the four compounds contain cobalt/iron-oxygen/fluoride layers with Kagomé topology composed of interlinked face-sharing MO3F3/MO4F2 octahedra. PO3OH pseudo-tetrahedral groups augment the [M3O6F4] (1)/[M3O8F2] layers on both sides to give M3(HPO4)2F4 (1) and M3(HPO4)2F2 (2-4) layers. These layers are stacked along the a axis in a sequence AA…, resulting in the formation of a layer structure for (NH4)2Co3(HPO4)2F4(1). In NH4Co3(HPO4)2(H2PO4)F2 and KM3(HPO4)2(H2PO4)F2, the M3(HPO4)2F2 layers are stacked along the a axis in a sequence AA i … and are connected by [PO3(OH)] tetrahedra, giving rise to a 3-D open framework structure with 10-ring channels along the [001] direction. The negative charges of the inorganic framework are balanced by K+/NH4+ ions located within the channels. The magnetic transition metal cations themselves form layers with stair-case Kagomé topology. Magnetic susceptibility and magnetization measurements reveal that all four compounds exhibit a canted anti-ferromagnetic ground state (Tc = 10 or 13 K for Co and Tc = 27 K for Fe) with different canting angles. The full orbital moment is observed for both Co2+ and Fe2+.