Acentric Pseudo-Kagome Structures: The Solid Solution (Co_1–xNi_x)₃Sb₄O₆F₆

Abstract

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:

Direct Synthesis of Two Inorganic Catalysts on Carbon Fibres for the Electrocatalytic Oxidation of Water

“…Cobalt appears to change the valence in equilibrium with the applied potential to Co 3+ for Co 3 Sb 4 O 6 F 6 , however, no such redox event can be seen for Sb. The possible role for Sb 3+ to accommodate a forth bond to take a seesaw coordination is discussed in our previous study . The present results support that the Sb 5+ ‐containing compound Co 1− x Sb 1+ x O 4 would show less activity because the octahedral coordination cannot change and the compound is therefore not able to efficiently contribute in a catalytic cycle.…”

“…The overpotential with short‐term stability by using the RE with a porous glass frit was determined at 5 min to η =311 mV at j =1 mA cm −2 and at 30 min to η =467 mV at j =10 mA cm −2 (see the inset in Figure ). In a recently published study on the catalyst material as a fine‐grained powder dispersed in a conductive paste, Co 3 Sb 4 O 6 F 6 proved to perform with Faradaic efficiency and with a lower overpotential than an electrode composed of CoO/Co 3 O 4 in a ratio of 0.54/0.46 . The total surface area of the fine‐grained Co 3 Sb 4 O 6 F 6 powder is much larger than the surface area exposed in the present direct synthesis of electrode A.…”

“…In higher magnification the surfaces exhibit two different morphologies, that is, i) surfaces with thin cracks in perpendicular directions exhibit an irregular morphology (see Figures c and d) and ii) surfaces with spherical outgrowths that exhibit surface mesoporosity (see Figures e and f). Morphology i) was previously identified in electrodes based on powdered Co 3 Sb 4 O 6 F 6 crystals embedded in a conductive paste . The morphology change after electrolysis is different compared to the change induced by the electrolyte.…”

“…Twod ifferent production schemes were employed, henceforth referred to as electrode A and B, respectively. Electrode A:C oF 2 (2 mmol, 0.194 g, Sigma-Aldrich) was mixed with Sb 2 O 3 (1 mmol, 0.291 g, Sigma-Aldrich), H 2 O( 2mL, 11 vol %) was added [17] and the obtained solution was stirred with as patula followed by 2min of ultrasonication. Ac arbon fibre sheet cut to fit the cell inner diameter of 20 mm was placed horizontally at the bottom of the cell and bent leaving af ew cm above the water sur-face, followed by 1min ultrasonication.…”

“…[16] We have recently synthesised solid solutionsw ith an interchangeable transition-metal position in (Co x Ni 1Àx ) 3 Sb 4 O 6 F 6 and (Co y Mn 1Ày ) 3 Sb 4 O 6 F 6 . [17][18][19] In ap rimarys tudy,t he Co oxofluoride was shown to be capable of oxidisingw ater at an overpotential of 0.33 Vb yu sing the chemical electron acceptor [Ru(bpy) 3 ](PF 6 ) 3 (bpy = 2,2'-bipyridine)a nd that efficiency decreased with an increasingNicontent. [19] The material showed anotable stabilityt hat was confirmed in af ollowing study employing the Co-Ni-Mno xofluoridesi ne lectrochemical water oxidation.…”