Phase rare earth-carbon

Carbonates, Thiocarbonates, and Related Anions, The syntheses of carbonates, hydroxide carbonates, and oxide carbonates have been reported. Those factors which influence the crystallization of rare-earth carbonates have been investigated the carbonates were precipitated at various temperatures from aqueous solutions by using sodium carbonate (20—80 C), sodium bicarbonate (20—80 °C), trichloroacetic acid (40—120 °C), and urea (50—150 °C) as precipitants. The carbonates so formed were characterized by chemical analysis and A"-ray powder diffraction techniques. It was found that they could be classified into several phases (Table 19) according to the ionic radii... 

Binary rare-earth-carbon phase diagram 63... 

Two prototypes of binary rare-earth-carbon phase diagrams... 

Here, the so-called heavy lanthanides include the elements from samarium to-lutetium, except for ytterbium and europium which behave like bivalent metals and have unique properties. For these heavy-lanthanide-carbon systems, no complete phase diagram was found, only some information about the formation and the crystal structure of the carbides is available. On the basis of these data the general characteristics of the phase diagrams of the heavy-rare-earth-carbon systems can be summarized. In this case the yttrium-carbon phase diagram may be regarded as the best prototype available for compounds of the heavy lanthanide systems with carbon. 

According to the reported information on the binary rare-earth-carbon phase diagrams, a survey on formation of the binary rare earth carbides has been made and is given in table 2. 

The thermal decomposition of anhydrous rare earth carbonate to oxide occurs via intermediate oxycarbonate phases. The particular oxycarbonate formed depends on the experin iental conditions. The stoichiometric oxycarbonates ate monoxydi-carbonate, R26(063)2, and dioxymonocarbonate, R262C63. Monoxydicarbonate can also be prepared in hydrated form at higher temperatures in rare earth carbonate solutions (Nagashima et al., 1973). 

The solid precipitates formed after supercritical reaction consist of rare earth carbonates and unreacted rare earth oxides and/or hydroxides. Addition of 0.5 M HCl to the solids at ambient temperature and pressure solubilizes the rare earth carbonates. The unreacted rare earth oxides and/or hydroxides are left in the solid phase. The solution containing the rare earths can then be further separated into individual rare earths by conventional solvent extraction or ion exchange, Separation of rare earth carbonates is carried out in dilute acid. Th02, Zr02 and Ce02 either did not react or gave very low yields under the experimental conditions. 

Cyclopentadienyl compounds have been thoroughly investigated as suitable precursors to rare earth doped semiconductors in MOCVD (metal-organic chemical vapor deposition) or MOVPE (metal-organic vapor phase epitaxy) processes [283]. The use of btsa complexes for the same purpose has appeared in the literature very recently [285]. Typical process conditions are shown in Scheme 14. It was found that the carbon contamination of the deposited metal is less in the btsa case. 

None of the phases formed upon intercalation shows signs of being metallic. Even upon loading the carbon cage with a total of eight electrons [in the K6(M C82) systems], the valency of the encapsulated rare-earth ion is not reduced. 

Roesky introduced bis(iminophosphorano)methanides to rare earth chemistry with a comprehensive study of trivalent rare earth bis(imino-phosphorano)methanide dichlorides by the synthesis of samarium (51), dysprosium (52), erbium (53), ytterbium (54), lutetium (55), and yttrium (56) derivatives.37 Complexes 51-56 were prepared from the corresponding anhydrous rare earth trichlorides and 7 in THF and 51 and 56 were further derivatised with two equivalents of potassium diphenylamide to produce 57 and 58, respectively.37 Additionally, treatment of 51, 53, and 56 with two equivalents of sodium cyclopentadienyl resulted in the formation of the bis(cyclopentadienly) derivatives 59-61.38 In 51-61 a metal-methanide bond was observed in the solid state, and for 56 this was shown to persist in solution by 13C NMR spectroscopy (8Ch 17.6 ppm, JYc = 3.6 2/py = 89.1 Hz). However, for 61 the NMR data suggested the yttrium-carbon bond was lost in solution. DFT calculations supported the presence of an yttrium-methanide contact in 56 with a calculated shared electron number (SEN) of 0.40 for the yttrium-carbon bond in a monomeric gas phase model of 56 for comparison, the yttrium-nitrogen bond SEN was calculated to be 0.41. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

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