Oxyfluorides systems, oxyfluoride

Molybdenum hexafluoride [7783-77-9] MoF, is a volatile liquid at room temperature. It is very moisture sensitive, hydrolysing immediately upon contact with water to produce HF and molybdenum oxyfluorides. MoF should therefore be handled in a closed system or in a vacuum line located in a chemical hood. The crystals possess a body-centered cubic stmcture that changes to orthorhombic below —96° C (1,2). The known physical properties are Hsted in Table 1. 

Niobium dioxyfluoride, Nb02F, and tantalum dioxyfluoride, Ta02F, can be successfully used as precursors for the synthesis of many oxyfluoride compounds of niobium and tantalum. Systematic investigations performed on MeC>2F - M2CO3 systems, in which Me = Nb or Ta and M = alkali metal, provided necessary information on optimal synthesis procedures and imparted some conformity on the mechanism of the chemical interaction between the components. 

Predominant formation of either complex fluoride or complex oxyfluoride depends on the interaction rates ratio of processes (25) and (26). The relatively high interaction rates of (27) and (28) lead to the synthesis of simple fluorides or oxyfluorides, respectively. With the availability of two or more cations in the system, the ammonium complex fluorometalates interact forming stable binary fluorides or oxyfluorides or mixtures thereof. 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

Senegas and Galy obtained the same type of structure for Ni2Nb03F3, while investigating solid solutions in a NiF2 - NiNb2C>6 system [263]. Niobium and nickel cations are randomly located in the oxyfluoride octahedrons, which are linked via their sides. 

Analysis of the physicochemical properties of fluoride and oxyfluoride melts reveals that the complex ions are characterized by coordination numbers that do not exceed seven. Fluoride melts consist of the complex ions MeF72 and MeFe. Molten chloride-fluoride systems initiate the formation of heteroligand complexes of the form MeFgCl2 . Oxyfluoride and oxyfluoride-chloride melts can contain oxyfluoride complexes MeOF63 at relatively low concentrations. The behavior of the more concentrated melts can be attributed to the formation of oxyfluorometalate polyanions. 

Oxyfluoride compounds and highly densified ceramics that are related to lithium niobate, LiNbOs, were obtained in the system ... 

Some of the earliest examples of modulated structures to be unraveled were the fluorite-related vernier structures. These structures occur in a number of anion-excess fluorite-related phases and use a modulation to accommodate composition variation. They can be illustrated by the orthorhombic phases formed when the oxyfluoride YOF reacts with small amounts of YF3 to give composition YOxF3 with x in the range 0.78-0.87, but similar phases occur in the Zr(N, O, F) system with x taking values of 2.12-2.25 and other systems in which the Zr is replaced by a variety of lanthanides. 

Similarly, the chemical reactivity of these two chlorine oxyfluorides differs vastly whereas ClFsO is extremely reactive and cannot be handled even in a well-dried glass vacuum system, FCIO3 reacts only slowly with water. 

Although the literature often indicates catastrophic reactions of fluorine with oxides, nitrides, carbides, silicides, borides and the like, many such species may be reacted under controlled conditions to produce oxyfluorides, nitro-fluorides such as TINF, ZrNF, HFNF, ThNF, and UNF, and other interesting ternary systems. 

Single-crystal precession data indicate orthorhombic symmetry with the crystallographic space group Fddd. This system is not isostructural with any other known metal oxyfluoride or metal dioxide. The cell dimensions, determined from Guinier data, are a = 8.370 1 A. b = 10.182 1 A. and c = 7.030 1 A. The indexed powder data are given in reference 6. 

The typical structures of I(+7) involve a distorted octahedral configuration (type e) about iodine in most periodates and oxyfluoride, IOF5, and the heptacoordinated, pentagonal bipyramidal species (type f) for the IF7 and IOFg anions. The pentagonal bipyramidal structure can be described as two covalent collinear axial bonds between iodine and ligands in the apical positions and a coplanar, hypervalent 6c-10e bond system for the five equatorial bonds. 

Since all these materials have a certain solubility in the electrolyte, some contamination of the metal will occur. The Eltech Systems Corporation announced [246,247] a method to protect the substrate anode material by forming a Ce(IV) oxyfluoride layer on the anode. The protective layer is maintained by adding a Ce(III) compound to the electrolyte. However, cerium contaminates the aluminum metal so that it must be removed and recirculated to the electrolyte. 

In many of the lanthanide oxyfluorides and related phases, different cluster geometries, (labelled [1 0 2], [1 0 3] and [1 0 4] in the above notation), are found. The type of cluster depends npon the size of the lanthanide present. Large cations, La Nd, form only [1 0 3] clusters, while smaller cations, such as Sm, form [1 0 2] clusters, found in SmOo.yFi.e. In all of these systems, the clnster distribntion seems to be a function of temperature. 

The magnetic susceptibility of the oxyfluoride follows the Curie-Weiss law (6 = —6°) over the temperature range 88—294°K. The magnetic moment, /left = 2-46 B.M. at 20 . is compatible with two unpaired electrons, one associated with an 02 ion, the other with the d system of an octahedral hexafluoro-platinate(v) ion. 

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