Rare earth impurities

The superconducting state can coexist with magnetic moments of localized electrons (e.g. of 4f type). It was experimentally found by Matthias et al. (1958a) that for rare-earth impurities substituted into a superconductor Tc rapidly decreases with increasing impurity concentration and that superconductivity is completely destroyed beyond a... 

Often, however, in processes of this nature even the perturbed intrinsic emission does not appear in TL. Instead emission characteristic of an impurity is all that is seen. The interpretation generally placed on this is that the energy is transferred to the impurity. CaF2 Ce provides us with a useful example. In CaF2 the simple F-center is unstable at room temperature and does not form as a product of irradiation. However, when trivalent rare-earth impurity ions are present impurity/vacancy pairs are formed following irradiation. These are of C v symmetry with the vacancy... 

The Elc polarized emission lifetime is reduced at low temperatures in the presence of rare-earth impurities. One observes a non-exponential decay. The long-living component equals the intrinsic one (without quencher) and the short-living component is attributed to regions emitting near the impurities (Ref. 93, see also Sect. G.). 

Molnar, F. Anion exchange concentration of rare-earth impurities in rare-earth materials for analytical purposes. Anal. AppL Rare Earth Mater., NATO Adv. Study Inst. 1972 (Publ 1973), 55... 

Lead, indium, aluminum-rare earth systems (dilute rare earth impurities) Schwidtal (1960) has also studied the influence of a paramagnetic impurity like... 

Fig. 15. Characteristic energies of quadrupolar interaction (E ) and of isotropic exchange (E.xch) for heavy rare-earth impurities in gold. is proportional to Kr, exch is proportional to (g - 1)J (Fert et al. 1977).

Preparation of high purity rare earth metals st U"ts with the pure oxide. Since most of the cation impurities which are present in the oxide will be present in the final metal product, it is imperative that the oxide be as pure as possible with respect to other cations. The exception is any cation which is volatile and will evaporate during vacuum melting of the rare earth metal, e.g. the alkali and alkaline earth metals. The oxides prepared in the Ames Laboratory for our studies were 99.999 wt.% pure with respect to all rare earth impurities and all other naturally occurring cation impurity elements. The separation and purification of the oxides are discussed in ch. 22. The cation impurities in the... 

Section 4 presents some selected topics on superconducting rare earth alloys and compounds. In this chapter we do not discuss the influence of paramagnetic rare earth impurities on the Tc of a superconducting host metal. There is an excellent recent review of this subject by Maple (1973). The chapters by Fulde and Maple will also cover this topic. Finally, section 5 summarizes our conclusions. 

Paramagnetic impurities lower the of a superconductor (e.g. Maple, 1973). This can be a serious problem for the study of a superconductor in the millikelvin range. The investigated Ce sample contained magnetic rare earth impurities at the level of lOOat.ppm. If the depression of Tc in Ce is as large as in La (Matthias et al., 1958), one expects a reduction of Tc of the order of 20 mK. This means that the T -P curve for ideally pure Ce probably lies higher than the data in the insert of fig. 10.8. 

Rare-earth impurities in high-purity Ce02 were determined by ICP/MS after separation by liquid-liquid extraction, avoiding interferences of CeH" with the detection of Pr and CeO and CeOH with Gd and Tb isotopes (B. Li et al. 1997). The Ce was oxidized to Ce with KMn04 at pH 4 and all of the rare-earth elements were extracted into 0.05 M ethylhexyl(ethylhexyl)phosphonic acid/cyclohexane. The trivalent rare earths were stripped with 1.5 M HNO3, while 99.81 0.01% of the Ce ", which is more strongly extracted, remained in the organic phase. The separation was sufficient to allow detection of each rare-earth impurity at 20-90 ppb, despite some interference of CeOH+ in the determination of the only stable isotope of terbium, Tb. 

Goldner, P., Mortier, M., 2001. Effect of rare earth impurities on fluorescent cooling in ZBLAN glass. J. Non-Cryst. Solids 284 (1-3), 249-254. 

Elements that require higher accuracies are not rare earth impurities and can be determined much more accurately by other techniques (e.g. iron by wet chemical methods). 

Further analytical work by SSMS on the sample is considered if certain rare earth impurities require lower error limits. Internal referencing is employed in which the reference is an appropriately selected rare earth. The rare earth levels, determined with internal referencing, are then compared to the determinations for these same elements in the original sample. The ratios of these determinations should be constant and this constant can be used as a correction factor for all of the determinations in the original sample. The Laboratory is presently changing the internal standardization step to incorporate the use of isotope dilution when it is practical. 

The application of SSMS to the semiquantitative survey of all impurities in rare earth matrices is very useful. All of the recent innovations for improvement of performance (Ahearn, 1972) are applicable to the analysis of rare earth matrices. The determination of rare earth impurities in rare earth matrices presents no unusual difficulties and the quantification of these impurity levels can be performed with relative ease if standards are prepared. Quantification of non-rare earth impurities in rare earth matrices by comparison with specially prepared standards has not been reported. These determinations are based frequently upon RSC s determined from non-rare earth metal standards or upon RSC s determined from dry blended preparations of rare earth oxide samples. 

Accurate analyses also require that the selected analytical line be free of interfering spectral lines originating from other elements in the sample. Unless rare earth matrices free of the suspected impurities are available, it is difficult to establish whether the presence of spectral lines at the characteristic wavelength of persistent lines of the impurity are really caused by the rare-earth impurity, or by very weak lines of the matrix rare earth, or both. In the past, it has not been possible to place full reliance on the term spectroscopically pure because these tests may have been based on the absence of weaker Impurity lines, whereas stronger impurity lines may still be detected but are assumed to be weak lines of the matrix rare earth. 

Determination of rare earth impurities in a rare earth... 

Rare earth impurity Detection limit (ppm) Matrix Rare earth impurity Detection limit (ppm) Matrix... 

The application of an inductively coupled plasma system to the determination of rare earth impurities in a pure rare earth has not to our knowledge been reported. However, preliminary experiments in our laboratories indicate that trace rare earth spectral line intensities are essentially independent of the major constituent. Thus, a calibration curve for the determination of one rare earth in a matrix may well be valid for other rare earth matrices so long as spectral line interferences do not occur. Additional comments on these observations may be found in section 4. 

The determination of non-rare earth impurities in rare earth matrices is complicated not only by the complex spectra of many of the rare earth elements but also by the very different volatilization rates of the impurities to be determined. Simple dc arc excitation methods are usually found to suiter from poor precision if a matrix element spectral line is used for internal standardization, because the impurity and matrix elements will likely be present in the arc column at different times, and will therefore experience different arc fluctuations. Four approaches have been used to improve precision. The first is to add reference elements to the sample with vaporization rates similar to the rates of the impurity elements to be determined. 

The separation of most non-rare earth impurities from a rare earth matrix is not particularly difficult, and this fourth method has been used to improve detection limits or to eliminate the complex rare earth spectrum (Slyusareva et al., 1965 Osumi et al., 1971d Sato et al., 1971). 

Without prior separation of preconcentration steps, the detection limits shows in Table 37D.4 have been reported for non-rare earth impurities in various rare earth oxides when arc or spark excitation is used. 

A general procedure for the DC carbon arc determination of trace rare earth impurities in highly purified rare earth materials... 

Herman, E., W3, Determination of Rare Earth Impurities in Pure Rare Earths by Means of Extraction Chromatography, in hfichelsen, O.B. ed.. Analysis and Application of Rare Earth Materials (Uni-versitetsforlaget, Oslo) pp. 39-53. 

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