The new oxofluoride FeSeO3F, which is isostructural with FeTeO3F and GaTeO3F, was prepared by hydrothermal synthesis, and its structure was determined by X-ray diffraction. The magnetic properties of FeSeO3F were characterized by magnetic susceptibility and specific heat measurements, by evaluating its spin exchanges on the basis of density functional theory (DFT) calculations, and by performing a quantum Monte Carlo simulation of the magnetic susceptibility. FeSeO3F crystallizes in the monoclinic space group P21/n and has one unique Se(4+) ion and one unique Fe(3+) ion. The building blocks of FeSeO3F are [SeO3] trigonal pyramids and cis-[FeO4F2] distorted octahedra. The cis-[FeO4F2] octahedra are condensed by sharing the O-O and F-F edges alternatingly to form [FeO3F]∞ chains, which are interconnected via the [SeO3] pyramids by corner-sharing. The magnetic susceptibility of FeSeO3F is characterized by a broad maximum at 75(2) K and a long-range antiferromagnetic order below ∼45 K. The latter is observed by magnetic susceptibility and specific heat measurements. DFT calculations show that the Fe-F-Fe spin exchange is stronger than the Fe-O-Fe exchange, so each [FeO3F]∞ chain is a Heisenberg antiferromagnetic chain with alternating antiferromagnetic spin exchanges. The temperature dependence of the magnetic susceptibility is well-reproduced by a quantum-Monte Carlo simulation.
Compounds within the solid solution (Co 1−x Ni x ) 3 Sb 4 O 6 F 6 were prepared by the hydrothermal method. The compounds crystallize in the noncentrosymmetric cubic space group I 4̅ 3m with unit cell parameters a = 8.176(1) Å for M = Co and a = 8.0778 (1) Å for M = Ni. The crystal structure is made up by corner sharing [MO 2 F 4 ] octahedra via the fluorine atoms. [Sb 4 O 6 E 4 ] supertetrahedra (T2) consisting of four [SbO 3 E] groups (E being the stereochemically active lone-pair on Sb) that share O atoms with the [MO 2 F 2 ] n network.Magnetic ordering phenomena are observed with two characteristic temperatures, T N and T*, in the range from 67 to 170 K, that evolve gradually with composition and collapse for M = Co (x = 0) to one transition. T N is assigned to a transition into a long-range ordered antiferromagnetic phase, and T* marks a temperature in the range of 45 to 65 K where field cooled (FC) and zero field cooled (ZFC) susceptibility splits. The latter is tentatively attributed to a canting of the spin moments. ■ INTRODUCTIONSeveral structural studies have been carried out on compounds with an asymmetric coordination due to the presence of lonepairs. 1−3 The stereochemically active lone-pair around a pelement cation such as Se 4+ , Te 4+ , Sb 3+ , Bi 3+ etc. results in such an asymmetric coordination within the respective polyhedra. Thus, this effect can be utilized to design new noncentrosymmetric-or low-dimensional compounds. Consequently, several inorganic compounds of this class exhibit interesting physical properties such as nonlinear optical second harmonic generation (SHG) or magnetic frustration, e.g. It has been found for several compounds of the M-L-O-X family (M = transition metal, L = p-element lone-pair cation, X = Cl or Br) that the majority of Cl/Br members act as terminating species that reside together with the lone-pairs in large nonbonding regions giving rise to 2D layered crystal structures with only weak van-der Waals interactions in between the layers 9,10 or 3D framework structures with channels or voids that the-lone pairs and halides are facing. 11,12 To further explore the M-L-O-X system we have included fluorine ions instead of chlorine or bromine. Fluorine tends to act more like oxygen in terms of bridging different kinds of [MO n F 6−n ] m-octahedra and thus leads to the formation of frameworks rather than layered structures. This difference is attributed to the smaller radius and stronger electronegativity of fluorine compared to chlorine/bromine.There are still only a few oxofluoride compounds reported containing lone-pair elements. One reason for the difficulty to synthesize such compounds is related to the enhanced reactivity of fluorine ions. Hence, it is reasonable to expect a rich variety of intriguing crystal structures in the M-L-O-F system. So far only a few compounds with various interesting structures have been successfully synthesized:
Two new isostructural Co(2+) containing tellurium and selenium oxofluoride compounds Co(2)TeO(3)F(2) and Co(2)SeO(3)F(2) are synthesized and their structures determined by single crystal X-ray diffraction. They crystallize in the orthorhombic space group Pnma with the unit cell parameters a = 7.3810(5) Å, b = 10.1936(7) Å, c = 5.3013(3) Å and a = 7.2655(8) Å, b = 10.0013(13) Å, c = 5.3564(6) Å, respectively. The Co(II) ion has octahedral coordination [CoO(3)F(3)] and builds up a 3D framework by corner- and edge sharing. The Se(IV) and the Te(IV) ions have the coordinations [SeO(3)E] and [TeO(3)E] respectively where E is the lone-pair electrons. The Se(IV) and Te(IV) ions are isolated from each other and bond only to the [CoO(3)F(3)] polyhedra. The electronegative element fluorine takes the role of a network builder like oxygen and helps to form the 3D framework structure. This is a difference compared to many oxohalide compounds containing Cl and Br where the halide ions are terminating ions preventing a 3D network from being formed. Long range antiferromagnetic interactions dominate at temperatures < 20 K. The magnetic susceptibility follows the Curie-Weiss law above 25 K with the Curie constant C = 5.62 emu K mol(-1), the Weiss temperature θ = -56 K and the effective magnetic moment μ(eff) = 4.74 μ(B) per cobalt atom.
A porous oxohalide, Fe6Ca2(SeO3)9Cl4, has been synthesized by solid state reactions using concentrated HCl as the Cl-source. It crystallizes in the hexagonal space group P63/m with unit cell parameters a = 12.118(2) Å, c = 12.703(4) Å, Z = 2. The crystal structure is an open framework having one-dimensional channels extending along [001] that the chlorine atoms and lone pairs on Se(4+) are facing. The channels in this framework structure are unusually large compared to other oxohalide compounds and also accessible to guest molecules. Water vapor sorption measurements show an uptake of 9 wt% at 293 K.
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