Lanthanum trihalides

Of the various crystal matrices, the cubic calcium fluoride (CaF2), the lanthanum trihalides (LaFs, LaCl3, LaBr3) and ethylsulphate are popular host lattices. The spectra of rare earths with partly filled /-shells in doped crystals consist of very sharp lines, similar to those in atomic spectra, of closely spaced groups. Fig. 20 gives a summary of the crystal... 

The calculated barrier to the transition from a pyramidal to a planar configuration and in the opposite direction is very low and varies depending on the calculation procedure. In particular, according to Vetere et al. (2000), the DFT/PBEO procedure yields barrier values of 0.71, 0.33, 0.08, and 0.04 kj/mol for lanthanum trihalides from trifluoride to triiodide, respectively. Of greater importance is another conclusion made by Kovacs et al. (1997) and substantiated by theoretical calculations in more recent works. The low out-of-plane bending vibrational frequencies V2 obtained in calculations are caused by the use of the harmonic approximation. In reality, these vibrations are anharmonic, and the V2 values are much higher. Unfortunately, taking into account anharmonic effects is currently impracticable for lanthanide trichlorides. 

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

Metallic samarium is obtained by heating the oxide, Sm203 with lanthanum turnings or cerium in slight excess amounts in a tantalum crucible under high vacuum. The metal is recovered by condensation of its vapors at 300 to 400°C. The metal cannot be obtained by reduction of its halides, SmFs or SmCls, or by heating with calcium or barium. In such reduction, trihalides are reduced to dihalides, but not to the metal. 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

In contrast, the anhydrous trichlorides, tribromides and triiodides and the reduced halides are extremely air sensitive materials and are rapidly hydrated or hydrolyzed in air. The triiodides are especially moisture sensitive and deliquesce. Unlike the trifluorides, the other trihalides are very soluble in water at 25°C. Solubilities of the trichlorides vary from 3.89 moles/ for lanthanum, to a minimum of 3.57 moles/ at terbium and back up to 4.10moles/ at lutetium with pH values of the saturated solutions in the range 1.0-2.0 (Spedding et al., 1974a). At high temperatures, the trichlorides, tribromides and triiodides also... 

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