Lanthanum, properties

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Table 23-1. some properties of lanthanum and the lanthanide elements... 

Lanthanide elements, 411, 389 contraction, 413 electron configurations, 415 occurrence and preparation, 413 oxidation numbers, 414 properties, 412 Lanthanum... 

PZT (lead zirconate titanate) and PLZT (lead lanthanum zirconate titanate) combine ferroelectic, optical, and electronic properties and are used in optoelectronic and piezoelectric devices. Powders for hot pressing produced by CVD are being investigated. 

The transition elements comprise groups 3 to 12 and are found in the central region of the standard periodic table, an example of which is reproduced on the endpaper. This group is further subdivided into those of the first row (the elements scandium to zinc), the second row (the elements yttrium to cadmium) and the third row (the elements lanthanum to mercury). The term transition arises from the elements supposed transitional positions between the metallic elements of groups 1 and 2 and the predominantly non-metallic elements of groups 13 to 18. Nevertheless, the transition elements are also, and interchangeably, known as the transition metals in view of their typical metallic properties. 

It can therefore be concluded that the unexpected ferromagnetism obtained independently for CaB2C2 and for lanthanum-doped CaBg does not reflect an intrinsic property of these compounds [7]. 

Based on the results of our band-structure calculations we assume that the metal-like properties of lanthanum nitridoborates are related by B-B interactions between adjacent BNx units in structures. 

Mixed B-C-N compounds of lanthanum maybe subdivided into La-(BNx), La-(BCx), and La-(CNx) compounds (Fig. 8.15). The chemistry of lanthanide nitridoborates has been developed in some detail and some properties were studied. But still more work is necessary, especially in the field of quaternary Ln-metal-(BNx) compounds. 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

To finish with another trend for NO removal consisting in NO direct decomposition, we would like to depict the infrared study of NO adsorption and decomposition over basic lanthanum oxide La203 [78], In this case, the basic oxygens are proposed to lead to N02 and N03 spectator species, whereas the active sites for effective NO decomposition are described as anion vacancies, which are often present in transition metal oxides. This last work makes the transition with the study of DeNO, catalysts from the point of view of their ability to transfer electrons, i.e. their redox properties. 

Lanthanum and samarium show virtually no NO dissociation activity even in the presence of Pt. These supports are not reducible and have no OSC property. The intrinsic NO dissociation activity of platinum is very weak, probably in reason of the low metal dispersion. The behavior of terbium oxide is more surprising. Although it is reducible in H2, it is unable to dissociate NO except in the presence of Pt. 

This soft, silver white metal reacts with air and water. The oxide is applied in optical glasses with high refractive indices (special lenses for powerful cameras and telescopes). Used for special effects in optoelectronics and electronics. Lanthanum exhibits catalytic properties. It is a component of flint and battery electrodes. Lanthanum boride (LaB6) is the superior electron-emitter for electron microscopes. Lanthanum is the first of the series of 14 lanthanides, also called the "rare-earth" metals, whose inner N shells are filled with electrons. They do not belong on the "red list" of endangered species they are neither rare nor threatened with depletion. China is particularly rich in lanthanide ores. 

Ryaznov M, Keinle L, Simon A, Mattausch HU (2005) New synthesis route to and physical properties of lanthanum monoiodide. Inorg Chem 45 2068-2074... 

ZnO photocatalyst can also be coupled with other materials in order to improve its chemical and physical properties [183] and photocatalytic activity [184]. Nanosized ZnO was immobilized on aluminum foil for the degradation of phenol [185]. Lanthanum and ZnO were combined to degrade 2,4,6-trichlorophenol [186]. Compared with Ti02 nanomaterial, ZnO nanomaterial generally absorbs a significant amount of the solar spectrum in the visible range therefore, ZnO nanomaterials were combined with Ti02 nanomaterials used as a photocatalyst [187]. 

A. Eftekhari, Electrochemical properties of lanthanum hexacyanoferrate particles immobilized onto electrode surface by Au-codeposition method. Electroanalysis 16, 1324 (2004). 

Cheng Z, Zha S, Aguilar L, and Liu M. Chemical, electrical, and thermal properties of strontium doped lanthanum vanadate. Solid State Ionics 2005 176 1921-1928. 

Meadowcroft DB. Some properties of strontium-doped lanthanum chromite. Brit. J. Appl. Phys. 1969 D2 1225-1233. 

Tanasescu S, Orasanu A, Berger D, Jitaru I, and Shoonman J. Electrical conductivity and thermodynamic properties of some alkaline earth-doped lanthanum chromites. Int. J. Thermophysics 2005 26 543-557. 

Paulik SW, Baskaran S, and Armstrong TR. Mechanical properties of calcium- and strontium-substituted lanthanum chromite. J. Mater. Sci. 1997 33 2397-2404. 

Montross CS, Yokokawa H, Dokiya M, and Bekessy L. Mechanical properties of magnesia-doped lanthanum chromite versus temperature. J. Am. Ceram. Soc. 1995 78 1869-1872. 

Ding X, Liu Y, Gao L, and Guo L. Effects of cation substitution on thermal expansion and electrical properties of lanthanum chromites. J. Alloys Compounds 2006 425 318-3 22. 

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