Lanthanum compounds

Mono(guanidinate) complexes of lanthanum and yttrium have been synthesized as illustrated in Scheme 58.The lanthanum compounds were made starting from La[N(SiMe3)2]3 and dicyclohexylcarbodiimide. Both mono (guanidinate) derivatives are monomeric in the solid state with a four-coordinate La center. ... 

There is no essential difference between quenching via a MMCT state or a LMCT state. The latter occurs, for example, in Eu(III) if the LMCT state is either at low energy or if this state shows a large offset in the configurational coordinate diagram [23, 35]. The latter occurs in glasses [123], certain cryptates [124] and lanthanum compounds [125]. 

Some rare earth compounds are used in glassmaking. Cerium is the most abundant, and its compounds are used to polish glass. Lanthanum compounds are used in making glass lenses, and praseodymium compounds color glass green. 

Lanthanum sulfate is used to prepare many lanthanum compounds. Physical Properties... 

Torardi et al, 1987 David et al, 1987). The structure is closely related to that of the superconducting copper oxide YBa2Cu307, consisting of square-pyramidally coordinated copper (Cu-O sheets) and square-planar copper (Cu-O chains). Excess oxygen in the lanthanum compound is located interstitially converting partially the chain copper to square-pyramidal and/or octahedral coordination. 

Rare-earth compounds lanthanum compounds Not only increase burn rate of DB propellants but also exhibit plateau or mesa combustion in the middle or high pressure range [244]. 

It is unexpected that the catalytic activity and the proton acidity do not depend on the lanthanum content. This result cannot be related to the schemes of hydrolysis of the zeolitic rare earth cations reviewed in Ref. 13. On the other hand, acidity measurements in solution (15) have shown that in the lanthanum zeolites studied in this work the La3+ ions have replaced the NH4+ ions and have not formed a lanthanum compound (13). Finally, the variations in the sodium content of these lanthanum zeolites do not seem to be the dominant factor in contrast to the alkaline earth zeolites (26). 

To get further evidence about which model is more appropriate, Webster and Drickamer (1980a) have measured the luminescence efficiency of La202S Eu3+ and Y202S Eu3+ and the lifetimes of the lanthanum compound under pressures up to 12 GPa. Intensities and life-... 

The ternary systems display a variety of structural chemistry depending on the sizes of the alkaline and lanthanide metals (Scheme 3 Fig. 3 [43, 45-57]). The smaller alkali cations determine the expected coordination structures as found in salt-like compounds, e.g., Na3Y (NH2)6 or KY(NH2)4. Layer structures are observed in alkali metal poor systems like MLa2(NH2)7 while cesium derivatives, apart from the lanthanum compounds, form perowskit-like arrangements as in CsEu(NH2)3 and Cs3Ln2(NH2)9. The mono ammoniates of some Cs-systems are probably metastable. Preparation of analogous ternary systems with Li were unsuccessful in contrast to, e.g., LiAl(NH2)4 [58]. 

The effect of CO2 on the stability of the membranes was tested, because of the possible instability of lanthanum compounds towards CO2 (formation of La2(CC>3)2). CO2 treatment at 600°C for 100 hours did not result in a measurable pore-growth the pore-size increased from 6.0 nm to 6.3 nm after treatment, which is within the measurement error. 

C (190-196) and by the thermal decomposition of the trifluoride at 800°C in air (191, 197, 198). The lanthanum compound itself may also be prepared by hydrolysis of the trifluoride (199) and by the reaction of the oxide with molten sodium fluoride (200). On treatment with CFCI3 (201), it is converted back to the trifluoride. The cerium analog has been prepared from Ce02 by reaction with CeF3 at 2750°C (202) or with CeF3 and cerium metal at 900°C in a nickel tube (203). The infrared spectra of these solids have been reported (204). 

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

The crystal structures of actinium compounds, where they have been studied, for example, in AcH3, AcF3 AczS3i and AcOCl, are the same as those of the analogous lanthanum compounds. 

In practice, both processes can occur there can be some decrease of oxygen and some conversion to Cu + ions. The concentration of holes on the copper oxide planes can be calculated when the parameter x in the formula (Lao.9,Sro.i)2Cu04 j is known. This superconductor is often called the lanthanum compound. The superconducting transition temperature depends on the hole concentration in... 

The discoveries that generated the extensive news coverage in early 1987 were the initial report of superconductivity in the lanthanum compound by Bednorz and Muller,and the observation of superconductivity in the compound YBa2Cu307 at 92-94 K, well above the boihng point (77 K) of liquid nitrogen. The latter result was the outcome of a collaboration between the research groups of C. W. Chu of the University of Houston and M. K. Wu of the University of Alabama. 

Like the lanthanum compound that we just described, the present material is deficient in oxygen, and the x in the chemical formula denotes the possibility of different amounts... 

There is much interest in this superconductor for two reasons. Measurements of the effect of pressure on the material indicate that the onset transition temperature increases to 147 K when the pressure is raised to 230000 times atmospheric pressure (23.5giga Pascal). The result excited many researchers because pressure on a material can be created chemically by replacing some fraction of an ion by a similar ion of smaller radius. The obvious choice in this case was to replace the larger barium with smaller strontium. This had worked with the lanthanum compound discussed earlier. Unfortunately nature is not always so predictable. Replacement of barium by strontium reduced Tc to 127 K instead of increasing it beyond 133 K. The other reason for the interest in this material seems much more important. Resistance measurements in dc magnetic fields have shown... 

A few months later, at a Boston meeting on superconductivity, a somewhat different response was indicated by Shoji Tanaka, who had led the University of Tokyo team that confirmed IBM s discovery of superconductivity in the lanthanum compound. Asked to comment on recent reports that the Japanese had made great progress in firing a ceramic into a serviceable wire, Tanaka said, My work is not in that direction, adding, after a fairly long pause, but in any event that might be a secret, I think. ... 

For example, in the determination of the atomic weight of lanthanum the salt lanthanum ammonium nitrate was recrystallized from water 126 times, eadi crystallization involving the separation of die material into as many as 12 fractions. Analysis of the final fraction showed constancy of composition to 1 part in 20,000. Lanthanum compounds, like other rare-earth compounds, are hard to purify because corresponding compounds of the rare-earth metals differ only slightly from one another in properties. 

Samarium Tungstate.—double salt of composition SNa O. 2Sni203.9W03 has been prepared in the same way as the corresponding lanthanum compound (see p. 219). 

The first structure of a [Ln(phen)2(N03)3] complex was reported in 1992 for the lanthanum compound.It closely resembled the established bipy analogue in that the three nitrate groups were bidentate and the lanthanum was 10-coordinate. The structural information was complemented by a multinuclear solution ( H-, C-, O-, and La) NMR study. The structure of the other extreme member of the series, the lutetium complex, was reported in 1996. Unlike the La complex, but like [Lu(bipy)2(N03)3], the study was not complicated by disorder. The complexes appear to form an isomorpohous and isostructural series. On moving from the lanthanum to the lutetium compound, the Ln—N distances decrease from 2.646(3)-2.701(3) A (La) to 2.462(8)-2.479(8) A (Lu), and the range of Ln—O distances decreases from 2.580(3)-2.611(3) A for the lanthanum compound to 2.364(8)-2.525(6) A for the lutetium complex. Several structures have subsequently been reported of other [Ln(phen)2(N03)3] systems. [Ln(phen)2(N03)3] (Ln = Pr, Lu, Dy, are isostructural the individual complex... 

One of the most important uses of lanthanum compounds is in carbon arc lamps. In a carbon arc lamp, an electrical current is passed through the lamp electrode. The electrode is made of carbon and traces of other materials that have been added. The current causes the carbon to heat up and give off a brilliant white light. The exact color of the light depends on the other materials that have been added to the carbon. Lanthanum fluoride (Laf3) and lanthanum oxide (La203) are usually used for this purpose. 

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