Trivalent erbium

In aqueous solution, erbium is always trivalent, Er3+. It forms water-insoluble trivalent salts, such as fluoride, ErFs, carbonate, Er2(COs)2, hydroxide, Er(OH)3, phosphate, ErP04, and oxalate Er2(C204)s. It also forms water-soluble salts, chloride, ErCls bromide, ErBrs iodide, Erls sulfate, Er2(S04)s and nitrate, Er(NOs)3. Evaporation of solutions generally yields hydrated salts. 

Johnson et al. (55) have reported a phonon-assisted energy exchange from trivalent erbium to trivalent thulium or to trivalent holmium. In this case, these authors were able to rule out resonance exchange completely Of some importance is that these systems are useful for laser oscillators, and the energy exchange results in a substantial decrease in threshold. 

Roesky introduced bis(iminophosphorano)methanides to rare earth chemistry with a comprehensive study of trivalent rare earth bis(imino-phosphorano)methanide dichlorides by the synthesis of samarium (51), dysprosium (52), erbium (53), ytterbium (54), lutetium (55), and yttrium (56) derivatives.37 Complexes 51-56 were prepared from the corresponding anhydrous rare earth trichlorides and 7 in THF and 51 and 56 were further derivatised with two equivalents of potassium diphenylamide to produce 57 and 58, respectively.37 Additionally, treatment of 51, 53, and 56 with two equivalents of sodium cyclopentadienyl resulted in the formation of the bis(cyclopentadienly) derivatives 59-61.38 In 51-61 a metal-methanide bond was observed in the solid state, and for 56 this was shown to persist in solution by 13C NMR spectroscopy (8Ch 17.6 ppm, JYc = 3.6 2/py = 89.1 Hz). However, for 61 the NMR data suggested the yttrium-carbon bond was lost in solution. DFT calculations supported the presence of an yttrium-methanide contact in 56 with a calculated shared electron number (SEN) of 0.40 for the yttrium-carbon bond in a monomeric gas phase model of 56 for comparison, the yttrium-nitrogen bond SEN was calculated to be 0.41. 

The term rare earth elements is sometimes applied to the elements La-Lu plus yttrium. The convenience of including La, which, strictly speaking, is not a lanthanide, is obvious. The reason for including Y is that Y has radii (atomic, metallic, ionic) that fall close to those of erbium and holmium and all of its chemistry is in the trivalent state. Hence it resembles the later lanthanides very closely in its chemistry and occurs with them in Nature. 

Later, he accepted the elemental nature of the rare earths and he tried to place them in the periodic table according to a homologous accommodation methodology, as Mendeleev did. In 1876, Meyer placed cerium, erbium, and yttrium in the boron group as trivalent elements, but he placed lanthanum in the column of the tetravalent elements. 

Trivalent cations of REE in aqueous solutions, acidified with HCl, HNO3, or HCIO4, absorb in the UV or VIS. The absorption bands are narrow, with sharp, non-overlapping peaks, but the molar absorptivities are rather small (1-10), and individual species of REE can be determined at concentrations of the order of 1 mg/ml [120]. Higher sensitivities are obtained after the ions have been converted into EDTA complexes [121]. The determination can be made more selective and sensitive by the use of the derivative spectrophotometry techniques [122-124]. Neodymium and erbium have been determined in the mixtures of REE by the derivative spectrophotometry technique using ferron and diethylamine [ 125]. 

The absorption and emission spectra of thulium and erbium were studied in phosphate and borate glass by Reisfeld and Eckstein (31, 32). Their objectives were 1) to study the influence of two different glass hosts on the transition probabilities of trivalent Er and Tm ions (henceforth R.E.), 2) to compare the intensities of the spectra of these R. E. in glcisses with those of liquid solutions and doped oxide lattices, and 3) to compare the broadening of the R. E. fluorescence bands in glasses with those in aqueous solutions and doped crystals. 

Also as a result of the lanthanide contraction, yttrium has an ionic radius comparable to that of the heavier REE species in the holmium-erbium region. If the effective ionic radius (Shannon 1976) of is plotted (0.90 A)., it plots in between element 67 (Ho) and 68 (Er). Scandium (effective ionic radius is 0.745 A), plots outside of the Lanthanide series. As also the outermost electronic arrangement of yttrium is similar to the heavy rare earths, the element behaves chemically like the heavy rare earths. It concentrates during (geo)chemical processes with the heavier REEs, and is difhcult to separate from the heavy REEs. Scandium, on the other hand, has a much smaller atomic radius, and the trivalent ionic size is much smaller than that of the heavy rare earths. Therefore, scandium does not occur in rare earth minerals, and in general has a chemical behavior that is significantiy different from the other rare earth elements (Gupta and Krishnamurthy 2005). 

The biggest surprise of their study was the discovery of the divalent character of metallic europium and ytterbium. Earlier studies of the light lanthanide (La, Ce, Pr and Nd) and erbium metals indicated that these lanthanides and yttrium were trivalent and so the expectation was that the remaining lanthanides should also be trivalent. Since divalent salts of europium and ytterbium were known in addition to the corresponding trivalent ones, the divalent nature was readily understood in terms of Hund s rules for stable half-filled and completely filled electronic levels (in these cases, 4f and 4f respectively). 

The high-temperature polymorphic form for most of the rare earth metals just before melting is the bcc structure. Four of the trivalent lanthanides (holmium, erbium, thulium and lutetium) are monomorphic and do not form a bcc structure before melting at atmospheric pressure (see fig. 4). However, the bcc phase can be formed in holmium and erbium by the application of pressure (< 1 GPa), see section 3.7.1. The existence of the bcc phase in the lanthanides has been correlated with the d occupation number, which decreases along the lanthanide series, but increases... 

Recently the luminescence properties of Pr ", Nd, Tm and Yb " ions in fluorite have been obtained by steady-state measurements. In addition, the luminescence spectra of Ce ", Sm ", Sm ", Dy ", Er and Yb were measured. It was pointed out that Xex = 415 nm is most suitable for measuring the Ho " emission beside the Er ". The emission of trivalent holmium and erbium ions was measured independently using time-resolved measurements and tentative assignment of luminescence lines to 3 and C4V symmetry sites was proposed. Besides for natural fluorite crystal, the transition between Stark energy levels of lanthanide ions were presented (Czaja et al. 2012). 

The most glaring discrepancy quoted at the begiiming of this section is the case of the H(2)ii/2 level of triply ionized neodymium. It persisted for a long time in spite of clear experimental evidence (see for instance Caro et al. 1981). Faucher and Garcia (1988) and Faucher et al. (1989a,b) showed that the anomaly touched the rank-four crystal field only they suspected that it concerned twin levels only and proposed an empirical rule to take it into account for any neodymium compound within the classical one-electron cfp fitting. The same type of correction applies to the H(2)n/2 level of trivalent erbium in the complementary 4f configuration (Moune et al. 1991). 

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