Phase Composition of the Cathodic Products Obtained in Alkali Chloride Melts Containing Potassium Monooxyfluoride Complexes of Tantalum V. V. Grinevitch a , S. A. Kuznetsov b , A. V. Arakcheeva a , and M. Gaune-Escard c a Baikov Institute of Metalurgy and Materials Science, Moscow 11911, Russia b Institute of Chemistry Kola Science Centre RAS, Apatity 184209, Russia c Ecole Polytechnique Universitaire de Marseille, IUSTI-CNRS UMR 6595, Marseille 13453, France The electrochemical behavior of molten salts systems where LiCl, NaCl, NaCl-KCl, KCl and CsCl were used as solvent for K 2 TaF 7 and K 3 TaOF 6 was investigated. In particular, the influence of the electrolysis parameters, such as concentration of tantalum species in electrolyte, temperature and current density on the cathodic products phase composition was determined by X-ray diffraction and scanning electron microscopy. Several phases generally crystallized at the cathode simultaneously. Among identified phases were metallic tantalum in cubic (α) and tetragonal (β) crystal modifications, tantalum oxide bronzes (hexagonal, tetragonal, cubic) and TaO. The most characteristic phases for the CsCl melt were two isostructural compounds with pyrochlor type crystal lattice CsTa 5 2+z O 5+y F 1-y and CsTa 5-x 2+z O 5+y F 1-y . The former deposited in the form of transparent dielectric crystals and the later – in the form of metal-like conductive ones. This latter compound seems to be synthesized for the first time. Introduction Oxygen-containing refractory metal species have occupied the attention of many researchers in recent years (1-4). Such interest stems from the need to obtain reliable information on the electrochemical behavior of this harmful impurity in niobium, tantalum, tungsten or zirconium. In addition, the problem of oxygen in electrolysis of refractory metal compounds has taken on a new significance over the last decade. This is associated with the possibility of electrochemical synthesis of novel compounds in the systems Nb-O, Nb-O-F, Nb-F-Me, Nb-O-F-Me (Me is an alkali metal). It is well known that the nature of the anion in a solvent has a profound effect on the electrochemical behavior of refractory metals in melts (4). At the same time, the role of the cation composition of a solvent in electrochemical processes has remained out of the focus. This may explain the limited and often non functional choice of solvent employed in electrochemical researches, most of the time the equimolar NaCl-KCl mixture or the ternary LiF-NaF-KF eutectic (FLINAK). However, in many cases such a simplified approach to the choice of solvent selection restricts the amount of information that could be obtained from electrochemical studies. From an electrochemical point of view of the above-listed salt systems they are essentially mixtures of several halides in random combination. Their use removes the possibility of determining the specific role played by each outer-sphere cation of the solvent in an electrochemical process. Meanwhile, our study of niobium-containing salt systems (5,6) showed that this role may be very important and even decisive. It was demonstrated that varying the outer-sphere cation ECS Transactions, 3 (35) 363-374 (2007) 10.1149/1.2798680, copyright The Electrochemical Society 363 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.82 Downloaded on 2019-04-26 to IP