Lanthanides light

For TRFM of lanthanides light sources providing UV or near UV have been applied, either in pulsed form (xenon flash lamps) or as continuous radiation [xenon and mercury lamps, argon lasers, or light emitting diodes (LEDs)] in combination with light chopping modules. Importantly, for maximum emission the excitation... 

Usually lanthanides are divided into several subgroups the light lanthanides, from La to Nd, medium lanthanides, from Sm to Dy, and heavy lanthanides. Ho to Lu. Alternatively, nomenclature such as ceric RE, from La to Nd, and yttric RE, from Sm to Lu plus Y, is used. 

It is easy to reduce anhydrous rare-earth hatides to the metal by reaction of mote electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pt mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphoms, and hydrogen at elevated temperature and remove these elements from most other metals. 

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

RE(N0 )2 NH NO 4H20 for light lanthanide separation (La, Nd, Pr) 2RE(N02)3 3Mg(N03)2 24H20 for middle lanthanide separation (Sm, Eu, Gd). Bromates and ethylsulfates have been found useful. Fractional crystallization is particularly slow and tedious for the medium and heavy rare earths. 

Extraction by carboxyUc acids (qv) is carried out in a neutral or weaMy acidic medium. The most widely used carboxyUc acid is RR (CH2)CCOOH, where Rplus represents seven carbon atoms. Trade names are Versatic 10 (Shell Chemicals) and Neodecanoic acid (Exxon Chemicals). CarboxyUc acids can be used either in chloride or in nitrate media and have a better selectivity for light lanthanides than for heavy lanthanide separation. 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

A full discussion of thorium electrochemistry is available (3). Thorium is generally more acidic than the lanthanides but less acidic than other light actinides, such as U, Np, and Pu, as expected from the larger Th" " ionic radius (108 pm). 

Fluorescence and Glass Lasers. Some ions absorb light of a certain frequency emitting light of lower frequency. This is known as fluorescence. Examples of ions that fluoresce in glass are Mn(TV), Pb(II), and the lanthanide ions. 

An alternative process for opening bastnasite is used ia Chiaa high temperature roastiag with sulfuric acid followed by an aqueous leach produces a solution containing the Ln elements. Ln is then precipitated by addition of sodium chloride as a mixed sulfate. Controlled precipitation of hydroxide can remove impurities and the Ln content is eventually taken up ia HCl. The initial cerium-containing product, oace the heavy metals Sm and beyond have been removed, is a light lanthanide (La, Ce, Pr, and Nd) rare-earth chloride. 

Mischmetal. Mischmetal [62379-61-7] contains, in metallic form, the mixed light lanthanides in the same or slightly modified ratio as occurs in the resource minerals. It is produced by the electrolysis of fused mixed lanthanide chloride prepared from either bastnasite or mona2ite. Although the precise composition of the resulting metal depends on the composition of chloride used, the cerium content of most grades is always close to 50 wt %. 

The role of cerium in these lighting phosphors is not as the emitting atom but rather as the sensitizer. The initial step in the lighting process is the efficient absorption of the 254 nm emission Ce ", with broad absorption bands in the uv, is very suitable. This absorbed energy is then transferred to the sublattice within the crystalline phosphor eventually the activator ion is fed and emission results. Cerium, as a sensitizer ion, is compatible in crystal lattices with other lanthanide ions, such as Eu and Tb, the usual activator atoms. 

The minerals on which the work was performed during the nineteenth century were indeed rare, and the materials isolated were of no interest outside the laboratory. By 1891, however, the Austrian chemist C. A. von Welsbach had perfected the thoria gas mantle to improve the low luminosity of the coal-gas flames then used for lighting. Woven cotton or artificial silk of the required shape was soaked in an aqueous solution of the nitrates of appropriate metals and the fibre then burned off and the nitrates converted to oxides. A mixture of 99% ThOz and 1% CeOz was used and has not since been bettered. CeOz catalyses the combustion of the gas and apparently, because of the poor thermal conductivity of the ThOz, particles of CeOz become hotter and so brighter than would otherwise be possible. The commercial success of the gas mantle was immense and produced a worldwide search for thorium. Its major ore is monazite, which rarely contains more than 12% ThOz but about 45% LnzOz. Not only did the search reveal that thorium, and hence the lanthanides, are more plentiful than had previously been thought, but the extraction of the thorium produced large amounts of lanthanides for which there was at first little use. 

Figure 3. The lattice parameter for the family of rock-salt structure actinide-antimonide compounds is shown where the line is for the corresponding lanthanide compounds. The metallic radii for the light actinide elements are plotted. The smooth line simply connects Ac to the heavy actinides. In both cases the smooth line represents the ideal tri-valent behavior.

As in aqueous solution, the lanthanide contraction favors a change from nine-coordination for the light lanthanides to eight-coordination for the light lanthanides such that [Ln(DMF)8]3+ is the major species when Ln3+ = Ce3+-Nd3+, and that this becomes the only detected species when Ln3+ = Tb3+-Lu3+ in dimethylformamide perchlorate solution (11, 92, 93, 321-323). Thus, Nd3+ is characterized by AH° = -14.9 kJ mol-1, AS0 = -69.1 J K"1 mol-1, and AV° = - 9.8 cm3 mol-1 for the equilibrium shown in Eq. (25) (93). The molar volume of DMF is 72 cm3 mol- and it therefore appears that the substantially smaller magnitude of AV° is a consequence of significant... 

Apart from d- and 4f-based magnetic systems, the physical properties of actinides can be classified to be intermediate between the lanthanides and d-electron metals. 5f-electron states form bands whose width lies in between those of d- and 4f-electron states. On the other hand, the spin-orbit interaction increases as a function of atomic number and is the largest for actinides. Therefore, one can see direct similarity between the light actinides, up to plutonium, and the transition metals on one side, and the heavy actinides and 4f elements on the other side. In general, the presence or absence of magnetic order in actinides depends on the shortest distance between 5f atoms (Hill limit). 

---