Preparation flux methods

Aluminum-containing compounds LUAIB4, YbAlB4 (YCrB4 type) as well as Yb2AlB5 (Y2ReBg type) are prepared from A1 flux methods A1 in these compounds behaves as a transition metal. ... 

Borides have been prepared as single crystals by making use of gas-phase, liquid-phase and flux methods, depending on the thermal stability of the boride and on the required size and perfection of the single crystals. 

The preparation of some polychalcogenide solids can be achieved at 200-450 °C by molten salt (flux) methods. The reaction of tin with alkali metal sulfides in the presence of Ss at 200-450 °C gives a variety of alkali metal tin sulfides depending on the ratio of the starting materials, the reaction temperature, and the alkah metals (Scheme 30) [90]. These alkali metal tin sul-... 

In the review by Kanatzidis et al. (2005), the preparation by the tin-flux method is mentioned also for several ternary phosphides and polyphosphides of rare-earth and transition metals. Typically the components (R metal, T metal, P and Sn in an atomic ratio of about 1 4 20 50) in sealed silica tubes were slowly heated, to avoid violent reactions, up to 800°C, annealed at that temperature for 1 week and slowly (2 K/h) cooled to ambient temperature. The tin-rich matrix was dissolved in diluted hydrochloric acid. The authors described the preparation of compounds corresponding for instance to the formula MeT4P12 (Me = heavy rare-earth metals and Th and U, T = Fe, Ru, etc.) and to the series of phases MeT2P2 (Me is a lanthanide or an actinide and T a late transition metal) having a structure related to the BaAl4 or ThCr2Si2 types. 

Kato, H., Kudo, A. 1999a. Photocatalytic decomposition of pure water into Hj and Oj overStTajO, prepared by a flux method. Chem Lett 11 1207-1208. 

Several other novel strategies have been employed for the synthesis of superconducting cuprates some of them were mentioned earlier while discussing the various methods. Especially noteworthy are the use of the combustion method and the alkali-flux method for cuprate synthesis. Superconducting infinite-layered cuprates seem to be possible only when prepared under high pressures because of bonding (structural) considerations [87, 88]. In Table 7 we list the various cuprate superconductors along with their properties and the preferred methods of synthesis. 

An attractive method to produce single crystals (in the dimension range from pm to several nun) is the high-temperature solution (flux) method, becanse of its simphcity and the low temperature required. The elements are dissolved in the solvent metal (often Al) and subseqnently the solntion is slowly cooled to room temperature. A prerequisite is, of course, that the solubility of the used solvent in the desired boride is insignificant. The solubility of Al in most boron-rich binary borides has been found to be extremely small. Crystals prepared in this manner are suitable for measurement of physical properties, for instance, microhardness, electrical resistivity, and so on. 

An interesting combination of methods is the polysulfide flux method, which can be usedforthe preparation of Ln202S (Ln trivalent rare-earth ion)-based luminescent materials (e.g. Y202S Eu or Gd202S Pr) [5.228]. In this method, the mixed oxides of the metals are mixed with excess sulfur and an alkali metal carbonate. On heating, the alkaline carbonate decomposes and reacts with sulfur to form a Uquid polysulfide flux. The oxides react with the polysulfide flux to form the oxysulfide. Flux residues can be removed by washing the reaction product in water. 

The flux method is a well-known method used for single crystal growth. It has not been applied to the synthesis of fine powders because usually high temperature heating is necessary to obtain molten salts. However, the modified flux method has been reported for the preparation of fine panicles of Ce,. Pr Oj solid solutions.In the preparation of the powders by the flux method, molten salts of alkali metal hydroxides, nitrates, and chlorides are used as solvents. The use of molten salts. 

Single crystals of the T1 (1223) phase of the Tl-Sr-Ca-Cu-0 system are prepared by a conventional flux method. An example of the starting composition is... 

R123 phase Method Preparation Flux Thermal treatment Size (mm ) r.(K) Remarks... 

In the case of Y123, variation of the initial flux composition is advantageous in the preparation of thick crystals by the self-flux method (sect. 5.1) or in the control of crystalline film orientation in the LPE method (sect. 5.4). However, for LRl 23 crystals the control of liquid composition is crucial even for their superconductivity enhancement. 

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