Rare-earth iodides

Annealing of stoichiometric amounts of the binary iodides or, alternatively, a one-batch synthesis from AI, R, and I2 yields satisfyingly pure ternary rare-earth iodides. 

Babai, A. and Mudring, A.-V, Rare-earth iodides in ionic liquids the crystal stiucture of [SEt3l3[Lnl6] (Ln = Nd, Sm), Inorg. Chem. 44, 8168-8169 (2005). 

Scheme 6) with three equivalents of MesSil led to the formation of the corresponding rare earth iodide and p7(SiMe3)3 (Huang and Diaconescu,... 

A summary of the available data on vaporization thermodynamics of rare-earth iodides is presented in table 10. Scandium iodide is by far the most extensively studied system due to its wide use in lamps. However, it was from a mass spectrometric study of Hirayama et al. (1976) on Lal3, where the detection of appreciable amounts of La2le(g) made evident that chemical interaction occurred in the gas phase between the monomers and that the presumption of congruent vaporization for the rare-earth iodides should... 

The lack of experimentally determined vibrational frequencies is critical also in the group of rare-earth iodides. Values for the asymmetric stretching frequency, V3, have been reported from matrix-IR work for Ndl3, GdH, H0I3 and LuL (Loktyushina and Mal tsev 1984). The only high-temperature spectroscopic work concerns the IR spectra of NdL (Wells et al. 1977) and the observation of the Vi(Scl3) mode from a vapor Raman study of the Csl-Scl3 binary system (Metallinou et al. 1991). Table 11 summarizes the available experimental and estimated vibrational frequencies of the rare-earth iodides. 

Almost all of the rare-earth metal/rare-earth metal tri-iodide systems, R/RI3, contain binary phases with the rare-earth element in an oxidation state lower than -1-3 ( reduced rare-earth metal iodides) [3, 7, 10-13]. More common is the oxidation state -i-2. Elements that form di-iodides RI2 are illustrated in Fig. 4.1. 

Fig. 4.1 Rare-earth elements that form di-iodides, Rl2-Light grey Metallic di-iodides, Rlz= (R )(O(l )2- grey Salt-like di-iodides, Rl2= (R )(r)2- Ambient conditions.

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Procedure 10% aqueous solution of potassium iodide, KI, when exposed to sunlight, liberated I2 due to the photolytic decomposition and gave blue colour with freshly prepared starch solution. The intensity of blue coloured complex with the starch increased many fold when the same solution was kept in the ultrasonic cleaning bath. As an extension of the experiment, the photochemical decomposition of KI could be seen to be increasing in the presence of a photocatalyst, Ti02, showing an additive effect of sonication and photocatalysis (sono-photocatalysis) However, the addition of different rare earth ions affect the process differently due to the different number of electrons in their valence shells. 

There appear to be no enthalpies of solution of rare-earth tribromides published in the available literature.2 However, Bommer and Hohmann reported a value of -230.5 kj mol-1 (at 20°C) for the enthalpy of solution of scandium tribromide in water (180). This value may be compared with -197.1 kj mol-1 for the chloride, and an estimate (from the published Lnl3 series, q.v.) of —240 to -250 kj mol-1 for the iodide. The markedly more negative values for the heavier ha-... 

Various methods [282] have been used to prepare anhydrous chlorides of the rare earths. Taylor and Carter [283] describe a general method for the preparation of high purity anhydrous halides in good yield. This method involves heating in vacuo, a molecularly dispersed mixture of hydrated rare earth halide with proper ammonium halide until the water and ammonium halide are expelled. All the trihalides except the iodides of Sm and Eu can be obtained using this proceedure. In the case of Sm and Eu the divalent iodides, Sml2 and Eul2 are obtained. 

Additional factors used to screen candidate iodine fixation compounds were thermal and chemical stability and volatility. Of the low solubility iodides, those of Ag, Cu(I), Pb, Pd, and T1 meet the arbitrary 250°C stability requirement. Several, such as those of Bi and Hg, have excessive vapor pressures. Many of the iodates show excellent thermal stability including those of the alkaline earths, rare earths, Ag, Cu, Pb, Zn, Hg, Th, and U. Several, including AglOo and Hg(I03)2 convert to the iodide on heating(19). 

Lanthanide bromides and iodides have found important applications in a completely different field. They are added as additives in high-pressure discharge lamps in the lighting industry to improve the arc stability and the colour quality. The latter is due to the contribution of the multiline spectrum of the doped rare earths which are added to the salt mixture. Lanthanide trihalides of dysprosium, holmium, thullium, gadolinium and lutetium are used frequently for this purpose (Hilpert and Niemann, 1997). 

Copper (I) iodide, 6 3 Crystallization, apparatus for, of tetrachloro (diethylene)di-platinum(II), 5 213 fractional, of magnesium rare earth nitrates, 2 52, 53 of rate earth bromates, 2 62... 

It is known in inorganic chemistry that all refractory and rare earth elements tend to form subvalent halides from their iodides, bromides and chlorides. 

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

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