Oxyfluorides

Oxyhalides have a halide ion substituted for an oxygen ion in the perovskite structure. As with the oxynitrides, addition of a halogen requires charge compensation, typically via a change in the nominal cation valence. Oxyhalides with a formula ABOjF are generally approximated as ionic compounds, so that 

Ensuring charge balance is maintained gives the possibilities  

By far the commonest of these phases contain Ca, Sr or Ba as the A-site ion and F as the halide (Table 2.3), for example, SrFeOjF. As with the oxynitrides, it appears that the and F anions occupy the X-sites at random to give a cubic perovskite unit cell, although short-range order is not mled out in these phases. 

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Reactions With Inorganic Compounds. In an investigation of the reactions of BrF with oxides (71—73), Httle or no reaction was found with the oxides of Be, Mg, Ce, Ca, Fe, Zn, Zr, Cd, Sn, Hg, Th, and the rare earths, whereas the oxides of Mo and Re formed stable oxyfluorides. 

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. 

Three other binary compounds of molybdenum and fluorine are known to exist molybdenum trifluoride [20193-58-2] MoF, molybdenum tetrafluoride [23412-45-5] MoF, and molybdenum pentafluoride [13819-84-6] MoF. Also known are the two oxyfluorides, molybdenum dioxydifluoride [13824-57-2] M0O2F2, and molybdenum oxytetrafluoride [14459-59-7] MoOF. The use of these other compounds is limited to research appHcations. 

Dioxygea difluoride has fouad some appHcatioa ia the coaversioa of uranium oxides to UF (66), ia fluoriaatioa of actinide fluorides and oxyfluorides to AcF (67), and in the recovery of actinides from nuclear wastes (68) (see Actinides and transactinides Nuclear reaction, waste managel nt). 

The compounds phosphoms trifluoride [7783-55-3] PP35 phosphoms pentafluoride [7647-19-0] PP55 phosphoms oxyfluoride [13478-20-17, POF and phosphoms thiofluoride [2404-52-6] were prepared prior to 1900. The most widely studied of these are PF and PF. Physical properties are... 

Phosphoms oxyfluoride is a colorless gas which is susceptible to hydrolysis. It can be formed by the reaction of PF with water, and it can undergo further hydrolysis to form a mixture of fluorophosphoric acids. It reacts with HF to form PF. It can be prepared by fluorination of phosphoms oxytrichloride using HF, AsF, or SbF. It can also be prepared by the reaction of calcium phosphate and ammonium fluoride (40), by the oxidization of PF with NO2CI (41) and NOCl (42) in the presence of ozone (43) by the thermal decomposition of strontium fluorophosphate hydrate (44) by thermal decomposition of CaPO F 2H20 (45) and reaction of SiF and P2O5 (46). 

Rhenium also forms several important oxyfluorides rhenium oxytetrafluoride [17026-29-8], ReOF rhenium oxypentafluoride [23377-53-9], ReOF rhenium dioxytrifluoride [57246-89-6], Re02F2 and perrhenyl fluoride [25813-73-4], ReO F. AH are soHds at room temperature. Properties are summari2ed in Table 1. 

The pH of a freshly prepared 0.4% solution of stannous fluoride is between 2.8 and 3.5. Initially clear aqueous solutions become cloudy on standing owing to hydrolysis and oxidation. The insoluble residue is a mixture containing stannous and stannic species, fluoride, oxide, oxyfluorides, and hydrates. 

Titanium(lV) fluoride dihydrate [60927-06-2] TiF 2H20, crystals can be prepared by the action of aqueous HF on titanium metal. The solution is carefully evaporated to obtain the crystals. Neutral solutions when heated slowly hydroly2e and form titanium(lV) oxyfluoride [13537-16-17, TiOF2 (6). Upon dissolution in hydrogen fluoride, TiF forms hexafluorotitanic acid [17439-11-17, ll]TiF. ... 

Alumina dissolves at low concentrations by forming oxyfluoride ions having a 2 1 ratio of aluminum to oxygen (Al2 0F2 ),... 

Oxyfluoride ions are discharged at the anode, forming carbon dioxide [124-38-9] and aluminum fluoride. The first oxygen can be removed more readily from Al2 02F4 than either the second or the oxygen from Al2 0Fg ... 

Tantalum Compounds. Potassium heptafluorotantalate [16924-00-8] K TaF, is the most important tantalum compound produced at plant scale. This compound is used in large quantities for tantalum metal production. The fluorotantalate is prepared by adding potassium salts such as KCl and KF to the hot aqueous tantalum solution produced by the solvent extraction process. The mixture is then allowed to cool under strictiy controlled conditions to get a crystalline mass having a reproducible particle size distribution. To prevent the formation of oxyfluorides, it is necessary to start with reaction mixtures having an excess of about 5% HF on a wt/wt basis. The acid is added directiy to the reaction mixture or together with the aqueous solution of the potassium compound. Potassium heptafluorotantalate is produced either in a batch process where the quantity of output is about 300—500 kg K TaFy, or by a continuously operated process (28). 

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

Bismuth pentafluoride is an active fluorinating agent. It reacts explosively with water to form ozone, oxygen difluoride, and a voluminous chocolate-brown precipitate, possibly a hydrated bismuth(V) oxyfluoride. A similar brown precipitate is observed when the white soHd compound bismuth oxytrifluoride [66172-91 -6] BiOF, is hydrolyzed. Upon standing, the chocolate-brown precipitate slowly undergoes reduction to yield a white bismuth(Ill) compound. At room temperature BiF reacts vigorously with iodine or sulfur above 50°C it converts paraffin oil to fluorocarbons at 150°C it fluorinates uranium tetrafluoride to uranium pentafluoride and at 180°C it converts Br2 to bromine trifluoride, BrF, and bromine pentafluoride, BrF, and chlorine to chlorine fluoride, GIF. It apparently does not react with dry oxygen. 

Other methods of preparation, eg, the reaction of CIF or BrF and CrO, yield the oxyfluoride contaminated with reactants and side reaction products. 

Antimony trifluoride [7783-56-4] M 178.8, m 292 . Crystd from MeOH to remove oxide and oxyfluoride, then sublimed under vacuum in an aluminium cup on to a water-cooled copper condenser [Woolf J Chem Soc 279 1955]. 

CARBON OXYFLUORIDE see CARBONYL FLUORIDE CARBON REMOVER, LIQUID CARBON TETRABROMIDE CARBON TETRACHLORIDE... 

Other finite-complex anions occur in the oxyfluorides. For example the hydrated salts K2[- S2F]q0].H20 and Rb2[As2F]oO].H20... 

IF7 is a stronger fluorinating agent that IF3 and reacts with most elements either in the cold or on wanning. CO enflames in IF7 vapour but NO reacts smoothly and SO2 only when warmed. IF7 vapour hydrolyses without violence to HIO4 and HF with small amounts of water at room temperature the oxyfluoride can be isolated ... 

Oxygen and fluorine ions have very similar ionic radii. This steric similarity ensures relatively easy substitution of the oxygen in the compounds, by fluorine ions. Such substitution opens up huge possibilities for the synthesis of oxyfluoride compounds with desired crystal structure and properties. 

Investigation of the ferroelectric properties of oxyfluoride compounds is also a relatively new but fast-developing area in solid state chemistry [44-46]. 

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. 

Ammonium complex fluorometalates are generally susceptible to hydrolysis by water formed in the course of the fluorination process (see 25). Such interactions, resulting in the formation of oxyfluoride metalates can be described as follows ... 

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. 

Simultaneous fluorination of niobium oxide and oxides of trivalent metals using an ammonium hydrofluoride melt leads only to oxide-type compounds, MinNbC>4 due to low thermal stability of fluoride or oxyfluoride compounds that contain both niobium and trivalent metals. 

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. 

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