Tantalum oxyfluorides

It seems that structural irregularities that cause spontaneous polarization are a relatively common property of niobium and tantalum oxyfluoride crystals. Fig. 100 shows the temperature dependence of SHG signals for several compounds that form island-type and chain-type structures. 

Sodium reduction development directions, 336 diluted melts, 331-332 of K-Salt, 327-328 principals, 326 Solid-phase interaction mechanism, 34-37 niobium oxyfluorides, 26-31 tantalum oxyfluorides, 32-34 Solubility diagrams (NH4)5Nb3OF18, 22 K2NbF7 in HF solutions, 14 K2TaF7 in HF solutions, 14 RbsNbjOF,, 22-23 Solubility of peroxides, 307 Specific conductivity, 153, 164 Spontaneous polarization, 223 Structural characteristics for X Me=8, 61,... 

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta405F14 (see p. 132), is thrown down when a solution of potassium tantalum fluoride, KaTaF7, is boiled.7 Powell and Schoeller 8 find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly add solution) for the detection and estimation of traces of tantalum in niobium compounds. 

The potassium tantalum fluoride first precipitated is a fusible substance. Its aqueous solutions on being boiled predpitate a very insoluble potassium tantalum oxyfluoride, 4KF.Ta2Os.2TaFs or K4Ta4OfiFu, as a white powder. This reaction is stated to constitute a sensitive test for tantalum.1... 

Metallic tantalum and tantalum pentoxide are both dissolved by hydrofluoric add, but evaporation of the solutions yields a residue which consists either of a tantalum oxyfluoride of variable composition or of the hydrated pentoxide. 

Ammonium Tantalum Oxyfluorides or ammonium fluoroxytantalates. —A substance which has the composition 3NH4F.TaOF3 or... 

Potassium Tantalum Oxyfluoride, 4KF.2TaFB.TaaOB or TaaOB. 2(2KF.TaFB).—(See under Potassium Tantalum Fluoride, 2KF.TaFB, on p. 190.) Two other potassium tantalum oxyfluorides have been reported, 2KF.TaOF3 2 and 8KF.TaOFs.s... 

Tantalum Peroxyfluorides or Fluoroxypertantalates.—The alkali tantalum oxyfluorides also have the property of taking up oxygen by reaction with hydrogen peroxide. 

Publications on bromination are very limited. Olah (1985) used antimony and tantalum oxyfluorides on alumina as catalysts and reported a 77% conversion at 260 C. Although no "oxybromination" was mentioned in the literature, its reactions can be defined, as similar to the oxychlorination reactions, by combining Reactions (19) to (22) with Reaction (23). 

T. M. Alam, J. S. Clawson, F. Bonhomme, S. G. Thomas, M. A. Rodriguez, S. Zheng, J. Autschbach, A solid-state NMR, X-ray diffraction, and ah initio investigation into the structures of novel tantalum oxyfluoride clusters, Chem. Mater., 20, 2205-2217 (2008). 

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. 

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. 

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. 

Among the different families of tantalum and niobium complex fluorides and oxyfluorides, the family of compounds with an X Me ratio equal to 6 is the largest. Table 22 presents the main structural characteristics of hexafluoroniobates and hexafluorotantalates. All known cases of niobium- and tantalum-containing formulary analogs have the same crystal structure type, at least at ambient temperature. 

Since the coordination number of tantalum or niobium in fluoride and oxyfluoride compounds cannot be lower than 6 due to steric limitations, further decrease of the X Me ratio (lower than 6) leads to linkage between complex ions in order to achieve coordination saturation by sharing of ligands between different central atoms of the complexes. The resulting compounds have X Me ratios between 6 and 4, and form crystals with a chain-type structure. 

The crystal structure of tantalum and niobium dioxyfluorides, TaC F and Nb02F, consists of oxyfluoride octahedrons linked via their vertexes to form a three-dimensional lattice with a Re03 type structure, as demonstrated by Andersson and Astrom [233] and by Frevel and Rinn [234]. Fig. 39 shows the structure of NbC F. 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

The lowest coordination number of tantalum or niobium permitted by crystal chemistry formalism is 6, which corresponds to an octahedral configuration. X Me ratios that equal 3, 2 or 1 can, therefore, be obtained by corresponding substitutions in the cationic sub-lattice. A condition for such substitution is no doubt steric similarity between the second cation and the tantalum or niobium ion so as to enable its replacement in the octahedral polyhedron. In such cases, the structure of the compound consists of oxyfluoride octahedrons that are linked by their vertexes, sides or faces, according to the compound type, MeX3, MeX2 or MeX respectively. Table 37 lists compounds that have a coordination-type structure [259-261]. 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

The proposed model of the structure of oxyfluoride melts corresponds with the conductivity results shown in Fig. 69. The specific conductivity of the melt drops abruptly and asymptotically approaches a constant value with the increase in tantalum oxide concentration. This can be regarded as an additional indication of the formation of oxyfluorotantale-associated polyanions, which leads to a decrease in the volume in which light ions, such as potassium and fluorine, can move. The formation of the polyanions can be presented as follows ... 

The formation of complexes in fluoride and oxyfluoride melts containing tantalum and niobium will be discussed later on in detail. 

The main problems encountered in the investigation of tantalum- and niobium-containing fluoride and oxyfluoride complexes are related to the tendency of the compounds to undergo hydrolysis, particularly at elevated/high temperatures. In addition, the interpretations of the observed effects are often nontrivial and unclear due to the relatively complicated inter-particular interactions and changes that occur under thermal treatment. From this point of view, vibration spectroscopy methods are of high importance due to the dependence of solid phase spectra on the temperature, which, above all, stems from the nature of such inter-ionic interactions [369]. 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

Stefanovich, Leonov and Venevtsev [417] describe a typical procedure of SHG measurement. The scheme of the SHG equipment enables to perform measurements at different temperatures, as shown in Fig. 96. SHG measurements of some tantalum and niobium complex fluoride and oxyfluoride compounds in the powdered form were reported in [206, 211] and some results are presented in Table 58. 

Table 58. SHG intensity values (hdho, (SiO2)) °f some tantalum and niobium fluorides and oxyfluorides after normalization by alpha-quartz signal fu (SiO]). Measurements were taken before and after thermal treatment, up to extinction of signal.

Thus, in cubic oxyfluorides of niobium and tantalum with rock-salt (NaCl) crystal structures, the formation and extinction of spontaneous polarization occurs due to polar ordering or disordering of Li+ - Nb5+(Ta5+) dipoles. 

An irreversible extinction of the SHG signal at 150-200°C is observed for a number of other fluoride and oxyfluoride compounds of tantalum and niobium that crystallize in centrosymmetric space groups. This phenomenon is especially typical for the compounds prepared by precipitation from solutions [206]. The appearance of the weak SHG signal for such compounds is related to imperfections in their crystal structure and the creation of dipoles. Nevertheless, appropriate thermal treatment improves the structure and leads to the disappearance of dipoles and to the irreversible disappearance of the corresponding SHG signal. 

A residual phase, usually consisting of insoluble fluorides and oxyfluorides of alkali earth and rare earth metals, is separated from the solution by filtration. The mechanism of the chemical decomposition of raw materials of the tantalum- and niobium-containing oxide type seems to be complicated, and unfortunately, the process has yet to be adequately investigated. 

The cake is leached with water in order to dissolve tantalum and niobium (and other related compounds) in the form of fluoride salts of ammonium. Ammonium fluoroferrate and fluoromanganate are unstable in aqueous solutions of low acidity. It is assumed that iron and manganese will form precipitates of insoluble fluorides or oxyfluorides that can be separated from the solution by filtration. 

The way in which ammonia solution is added to tantalum or niobium strip solutions is also important for the quality of the precipitated hydroxides and final oxides. The traditional method by which ammonia is poured into a container of strip solution and the mixture agitated is not optimal. According to this method, the first portion of ammonia is added to a solution of high acidity, the pH of which continues to drop gradually with each addition of ammonia, until the final addition of ammonia is made into a low-acidity solution. This procedure leads to a relatively slow increase in pH that can cause contamination of the hydroxide with crystalline oxyfluoride compounds. 

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