Mixed metal fluoride systems

Mixed metal fluoride systems where the cations of the minor component can principally substitute cations of the major component in the lattice therefore open an easy way to... 

J. Krishna Murthy, U. GroB, St. Rudiger, E. Unveren and E. Kemnitz, Mixed metal fluorides as doped Lewis acidic catalysts systems a comparative study involving novel high surface area metal fluorides, J. Fluorine Chem., 125, 937-949 (2004). 

Since fluoro-bridges have been identified in both Nb - and Ta -fluoride systems, the possibility of preparing discrete fluoroanions containing both these metals is feasible. This has been achieved by mixing solutions of (M = Nb or Ta) and (Bu4N)[M F6] (M = Ta, Nb, or Sb) in either SO2 or CH2CI2. The mixed-metal anions were characterized by their... 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

The lanthanide and actinide halides remain an exceedingly active area of research since 1980 they have been cited in well over 2500 Chemical Abstracts references, with the majority relating to the lanthanides. Lanthanide and actinide halide chemistry has also been reviewed numerous times. The binary lanthanide chlorides, bromides, and iodides were reviewed in this series (Haschke 1979). In that review, which included trihalides (RX3), tetrahalides (RX4), and reduced halides (RX , n < 3), preparative procedures, structural interrelationships, and thermodynamic properties were discussed. Hydrated halides and mixed metal halides were discussed to a lesser extent. The synthesis of scandium, yttrium and the lanthanide trihalides, RX3, where X = F, Cl, Br, and I, with emphasis on the halide hydrates, solution chemistry, and aspects related to enthalpies of solution, were reviewed by Burgess and Kijowski (1981). The binary lanthanide fluorides and mixed fluoride systems, AF — RF3 and AFj — RF3, where A represents the group 1 and group 2 cations, were reviewed in a subsequent Handbook (Greis and Haschke 1982). That review emphasized the close relationship of the structures of these compounds to that of fluorite. 

A large number of mixed metal or complex fluorides have been characterized for the rare earths. In part, these systems have been extensively studied because of their interesting optical, electrical and magnetic properties and because of their potential as host materials for ions of spectroscopic interest. The M(I)X-RfllDXj, the M(I)X-R(IV)X4 and the M(II)X2-R(III)X, (M(I)-aIkali metal, M(II) alkali earth or divalent rare earth) systems have been most extensively investigated however, a limited number of M(III)X3-R(III)X3 and M(IV)X4-R(III)X3 systems have been described. The mixed metal halides have interesting complex vapor species and several of the solid phases have structures which are closely related to those of binary halides. Phase equilibria of mixed metal systems have been determined primarily by thermal and X-ray diffraction analyses. The preparative procedures and many properties of the mixed halides have been reviewed by Brown (1968) and the mixed fluorides have been reviewed by tsanova (1971). 

The mixed metal phases of the MX-RX3 (M = alkali metal, X-Cl, Br, I) systems are presented in table 32.10. Early work and new data for the chloride systems are described by Drobot et al. (1968). Additional results for the chlorides and data for the bromides and iodides are described by Kutscher and Schneider (1974). Investigations for all the rare earths are complete only for the NaCl and KCl systems. The absence of data for the lithium halide systems may indicate that intermediate phases are not formed data for the Rb systems are limited (Drobot,( l968). Comparison of the compositions in table 32.10 with those of the fluorides in table 32.9 show the formation of identical compositions except... 

The equilibrium vapor of the mixed fluorides has been most extensively studied, but the work is limited primarily to alkali metal systems. A series of reports by Sidorov and coworkers is reviewed by Sidorov and Shol ts (1972). Mass spectrometric data for equilibrium species of the LiF-ScFs, NaF-ScFs,... 

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