Terbium oxidation states

The principle just outhned has two parts. The first part deals with redox processes and was developed here by examining the relative stabihties of the -i-2 and -i-3 oxidation states of the lanthanides. It can be extended in a variety of ways. Thus if the f variation is shifted one element to the right, it tells us the nature of the f variations, and accounts for the distribution of the -i-4 oxidation states of the lanthanides [2, 10, 15]. Their stability shows maxima at cerium(IV) and terbium(IV), decreasing rapidly as one moves from these elements across the series. 

In conclusion I should like to consider a few of the chemical investigations which might be accomplished in the rare earth field by Mossbauer spectroscopy. The study of nonstoichiometric oxides has been discussed earlier, but there is the problem of finding an appropriate doping nuclide for the praseodymium oxide system. The element most capable of following the changes in oxidation state of the praseodymium is terbium-159, which does have a Mossbauer state, however, with a rather broad resonance (58,0 k.e.v., = 0.13 nsec.). Nevertheless, with a sufiiciently... 

Terbium has several oxidation states, but +3 is the most common, as in the following examples ... 

The oxide terbium peroxide has a unique formula and exhibits a very rare oxidation state for terbium that is not a whole number. It is one of the rare cases in which the valence is not a whole integer 4Te + 70 —> Tb O ... 

The rates of internal conversion from the 5Z)3 to the 5D4 states were also measured. The backup oxide in this case was yttrium. This information was obtained by determining the rise time of the 5Z)4-state green fluorescence as a function of time, when the 5Z>3 state was excited. The rise time of the 5Z)4 state is, of course, the decay time of the 5Z>3 state. It was assumed that the decay of the 5Z)3 was predominantly due to an efficient internal conversion process to the 5D4. Measurements of the decay time of the 5Z)3 state directly were not possible, since the emission from this state is very weak if not, indeed, absent. The result of this study is shown in Fig. 23, where it can be seen that the internal-conversion time remains constant at about 17 fxsec up to a terbium oxide concentration of 1 mole per cent. At higher concentrations, the internal conversion time falls rapidly, until at 10 mole per cent terbium oxide the value is about 1.7 /xsec. This is down by a factor of 10 over samples containing 1 mole per cent or less of terbium oxide. 

The so-called sesquioxide (PuOi 5 i.75) is a typical mixed oxidation state oxide, similar to those formed by uranium, praseodymium, terbium, titanium, and many other metals. Its composition shows continuous variation with changes in temperature and pressure of oxygen above the oxide. 

The values for the redox potential for the couple M3 + /M2+ have been estimated57 using a simple ionic model and available thermodynamic data. The results (Table 2) correlate closely with the ionization potentials for the M2+ ions, and are in good agreement with both chemical observations and other estimates obtained by spectroscopic correlations. Irreversible oxidation of terbium(m) to terbium(iv) in aqueous K2C03-K0H solutions has been observed electrochemically 58 the discovery of an intermediate of mixed oxidation state explains partly the reduction behaviour of terbium(iv) deposits. Praseodymium(iv) and terbium(iv) have also been detected in nitrate solutions. 

Berkelium exhibits both the III and IV oxidation states, as would be expected from the oxidation states displayed by its lanthanide counterpart, terbium. Bk(III) is the most stable oxidation state in noncomplex-ing aqueous solution. Bk(IV) is reasonably stable in solution, undoubtedly because of the stabilizing influence of the half-filled Sf7 electronic configuration. Bk(III) and Bk(IV) exist in aqueous solution as the simple hydrated ions Bk3+(aq) and Bk4+(aq), respectively, unless com-plexed by ligands. Bk(III) is green in most mineral acid solutions. Bk(IV) is yellow in HC1 solution and is orange-yellow in H2S04 solution. A discussion of the absorption spectra of berkelium ions in solution can be found in Section IV,C. 

The lanthanides, unlike the transition metals and the actinides, tend not to form compounds over a range of oxidation states. The +3 oxidation state is characteristic of all of the lanthanides, and the oxide fluorides of formula LnOF (Ln = lanthanide metal) are well known. The less stable oxidation states of + 2 and + 4 are known, but the latter is represented only by the dioxides and tetrafluorides of cerium, praseodymium, and terbium, and no tetravalent oxide fluorides have been reported. 

This oxidation state is very important for cerium and of minor, though not negligible, importance for praseodymium and terbium. With the possible exception of a few fluorides, for example, Cs3LnF7 for Nd and Dy, no other Ln compounds are known. 

These elements are usually terpositive, forming salts such as La(N03)g 6H20. Cerium forms also a w ell-defined series of salts in which it is quadripositive. This oxidation state corresponds to its atomic number, 4 greater than that of xenon. Praseodymium, neodymium, and terbium form dioxides, but not quadrivalent salts. 

The mixture of these effects described above is obtained by the doping of rare earths elements with variable oxidation state. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies than those of pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is to the existence of irivalent terbium and praseodymium ions. In addition, a similar effect is also provided by the ternary oxides Cco6Zro4.iM 02. >2 (M = La, and Ga ), ... 

Cerium, praseodymium, and terbium oxides display homologous series of ordered phases of narrow composition range, disordered phases of wide composition range, and the phenomenon of chemical hysteresis among phases which are structurally related to the fluorite-type dioxides. Hence they must play an essential role in the satisfactory development of a comprehensive theory of the solid state. All the actinide elements form fluorite-related oxides, and the trend from ThOx to CmOx is toward behavior similar to that of the lanthanides already mentioned. The relationships among all these fluorite-related oxides must be recognized and clarified to provide the broad base on which a satisfactory theory can be built. 

All the rare-earth elements occur in the HI oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable n oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state. 

The series of 15 elements, lanthanium to lutetium, is known as the lanthanide series. These elements all form trivalent ions in solution quadrivalent oxidation states of cerium, praseodymium, and terbium, and bivalent states of samarium and europium are also obtained. 

For these elements, the correspondence of the actinide and the lanthanide series becomes most clearly revealed. The position of curium corresponds to that of gadolinium where the / shell is half-filled. For curium, the +3 oxidation state is the normal state in solution, although, unlike gadolinium, a solid tetrafluoride, CmF4, has been obtained. Berkelium has +3 and +4 oxidation states, as would be expected from its position relative to terbium, but the +4 state of terbium does not exist in solution whereas for Bk it does. 

Stabilisation of the oxidation state -t-4 of terbium has also been described for the complex [Tb (PWii039)2]made from the oxidation of [Tb (PW 1039)2]"" with aqueous persulphate. " In these 1 2 complexes the central metal ion can accept electrons from the highly negatively charged anions and is shielded from the external environments. ... 

Yoshihara measured Tb in terbium oxide Tb O, and in terbium chloride solution by the sum peak pair method , and pointed out that the difference of the sum peak intensity ratio between two states could be enlarged by using comparison of a sum peak pair. This method has an advantage that the dependence of the sum peak pair ratio on the soun -detector distance is not as noticeable as that in the ordinary sum peak method. 

For most lanthanides, the 3+ oxidation state is the most stable, and therefore almost all REE-oxides are presented as REE2O3. However, some of the lanthanides may have several valences in one and the same oxide, so formulas are given to express this phenomenon. Praseodymium oxide usually contains 3+ and 4+ praseodymium in a somewhat variable ratio, depending upon the conditions of formation. Its formula is rendered as PreOn. Similarly, Xb407, one of the main commercial terbium compounds, contains some Xb4+ along with the more stable Xb3+. Ce has the 3+ state as most stable oxidation state, and the oxide is represented as 6203. 

Europium has not many applications. Commercial applications of europium almost always take advantage of the phosphorescence, either in the 2+, or 3+ oxidation state. Industrial application started in the 1960s (Wickersheim and Lefever 1964). Many advances have been made since, leading to the discoveiy of europium activated yttrium oxysulfide (YaOaSiEu " ). This is a red phosphor. Other main industrial phosphors doped with europium are Sr5(P04)3Cl Eu and BaMgAliiOiviEu " for blue, and Y203 Eu " for red. Green phosphors involve terbium (Tb (Caro 1998 Nazarov et al. 2004). The phosphors are for instance used in flat screen monitors and televisions. 

The problem is not symmetrical. Oxidation of the reduced state is generally facile and nonactivated. Adsorption of O2 is exothermic and, on most reduced oxides, occurs readily at low temperature. Reduction of the oxidized state, on the other hand, is temperature dependent and much more difficult. It is here that cerium oxide and, perhaps, the oxides of terbium and praseodymium, excel. Cerium oxide, however, has advantages in terms of cost and availability. It was first introduced as an oxygen storage component in 1981 and has been an essential part of the TWC catalyst ever since. 

The close similarities between Pr(IV) and terbium(IV) in mixed oxides were already mentioned. In solutions of SiWuOj and BWnO g , Saprykin, Spitsyn and Krot (cf. Pope 1983) found in 1976 that the unusual oxidation state americium(IV) could be obtained by oxidizing Am(III) with peroxodisulfate OjSOOSOf to Am(SiWii039)2 and Am(BW, 1039)2Only in a narrow pH interval close to 7 was it possible to obtain a 30-50% yield of the terbium(IV) complexes of analogous composition. 

To identify the new nuclide, a rapid cation-exchange separation technique using ammonium citrate as an eluant was employed. Early experiments indicated that element 97 had two oxidation states 3+ and 4+. The actinide concept provided the guidance to search for these two oxidation states, by analogy with the homolog element, terbium (Tb). The chemically separated samples were subjected to the measurement of radiation. Characteristic Cm X-rays associated with the electron capture (EC) decay and low-intensity a particles with a half-life of 4.5 h were detected. Berkelium was named after the city of Berkeley, California where it was discovered, just as the name terbium derived from Ytterby, Sweden. 

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