The Rare Earths: Their Occurrence, Chemistry, and Technology

Terbium, Tb = 159·2

Terbia occurs among the rare earth oxides in exceedingly small quantities, and its separation has in consequence presented such great difficulties that only within the last few years have terbium compounds been completely freed from gadolinium and neighbouring elements. In 1886 Lecoq de Boisbaudran,[338] by fractional precipitation of the hydroxides with ammonia, and subsequent fractional crystallisation of the double sulphates, obtained an oxide much richer in terbia than any specimen previously prepared; it was dark yellow in colour. In 1902 Marc[339] obtained from monazite a very dark oxide containing about 15 per cent. of terbia, whilst Feit[340] in 1905 obtained a dark brown oxide consisting of gadolinia with about 13 per cent. of terbia. Pure terbium compounds were obtained by Urbain in 1904,[341] by fractional crystallisation of the nitrate from nitric acid, in presence of bismuth nitrate, and by crystallisation of the double nickel nitrates, and precipitation with ammonia; he showed that the element was identical with the Z_δ and Z_β of de Boisbaudran,[342] with the Γ of Demarçay,[343] and with the G_β and possibly the G_ζ of Crookes[344] (see p. 193).

The element gives the white sesquioxide, Tb₂O₃, and colourless salts.[345] The peroxide, of which the composition corresponds approximately to the formula Tb₄O₇, is obtained as a brownish-black powder by ignition of suitable salts. Its presence, even in small quantities, gives so deep a colouration to the other earths that some kind of salt formation seems probable. It is insoluble in cold acids; it dissolves in hot nitric acid with evolution of oxygen, forming a solution from which the nitrate, Tb(NO₃)₃,6H₂O, melting at 89·3°, separates on cooling. In hot hydrochloric acid, the peroxide dissolves with evolution of chlorine, forming solutions from which the chloride, TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely deliquescent, and easily forms supersaturated solutions. The sulphate, Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of the oxide by addition of considerable quantities of alcohol; it is isomorphous with the other sulphate octohydrates, and is completely dehydrated at 360°.

Atomic Weight.

—The value adopted by the International Committee is 159·2, which was obtained by Urbain in 1905 (loc. cit.) from the ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on which reliance can be placed, as the material of the earlier workers was seldom even approximately pure.

CHAPTER XIV
THE ERBIUM AND YTTERBIUM GROUPS—YTTRIUM AND SCANDIUM

In his examination of the ‘Yttria’ of Gadolin and Ekeberg, during the years 1839 to 1843, Mosander, by methods based on differences in strength of the oxides as bases, separated the earth into three new oxides, yttria proper, the most strongly basic, terbia, intermediate in strength, and erbia,[348] the least basic. No further separation was effected until 1878, when Marignac, by fractional decomposition of the nitrates, separated from erbia a new oxide, for which he proposed the name Ytterbia; the new oxide was the least basic of the erbia earths. In the following year, Nilson[349] isolated from ytterbia a still less basic oxide, by the same method; he proposed the name Scandia, to recall the fact that it occurred in gadolinite and euxenite, which up to that time had been found only in Scandinavia. In 1879 also, Soret[350] announced the discovery of a new element X, evidence for the existence of which he had obtained during a spectroscopic examination of a mixture of erbia and terbia earths; the oxide of X was isolated in the same year by Cleve[351] from the old erbia, by fractional decomposition of the nitrates, and the name Holmium, from the town of Stockholm, was proposed for the new element. The same investigation led to the discovery of Thulium, which derives its name from Thule, an old name for Scandinavia.

Lecoq de Boisbaudran[352] in 1886 showed Cleve’s Holmia to be a mixture of at least two oxides; he retained the name Holmium for the element which gave the most characteristic absorption bands of the old holmium, and proposed the name Dysprosium (from δυσπροσιτος, difficult of access) for the second element. The name Erbia was retained for the oxide remaining after the removal of holmia, thulia, and dysprosia from the old erbia; the homogeneity of this erbia has been called in question, but is now fairly firmly established. The individuality of dysprosium[353] and holmium[354] may also be regarded as definitely established; that of thulium remains doubtful (see p. 204).

The homogeneity of ytterbia was questioned by Auer von Welsbach[355] in 1906; by fractionation of the ammonium double oxalates, that author isolated the oxides of two new elements, for which he proposed the names Aldebaranium and Cassiopeium. By fractionation of the nitrates from nitric acid solution, Urbain[356] arrived at the same result, and proposed the names Ytterbium (Neoytterbium) and Lutecium, which have been adopted by the International Committee. The latter author, employing the same method in the fractionation of the gadolinite earths, has recently obtained very strong evidence of the existence in this group of another element, for which he proposes the name Celtium;[357] the discovery, however, awaits confirmation.

Separation

In the separation of the yttrium elements, methods based on differences in electropositive character are of much greater importance than in the separation of the cerium and terbium groups, and the method of nitrate fusion has been very largely employed even in comparatively recent work. This method, which was introduced by Berlin in 1860, has been of great value in the separation of yttrium and the ytterbium elements from the erbium group; it was employed in the isolation of ytterbium by Marignac, and of scandium by Nilson.

If a concentrated solution of the nitrates be evaporated down, and the syrupy residue subjected to gradually increasing temperature, the nitrates of the ytterbium elements and scandium are converted first into the basic nitrates; at somewhat higher temperatures the erbium salts are decomposed, whilst yttrium nitrate and the nitrates of any cerium elements present are the last to break up. If the mixture of basic and neutral nitrates be dissolved in boiling water, the former, being less soluble, crystallise out on cooling, and may be separated by this means, the process being repeated with the filtrate containing the unchanged nitrates. In this way, the weakly basic scandia and ytterbia quickly collect in the first fractions, whilst the oxides of the erbia group are easily separated from the more strongly basic yttria. The presence of the intermediate terbium group renders the process much less easily workable.

The process may be modified by raising the temperature to such an extent that the soluble basic nitrates are converted into insoluble superbasic nitrates, the temperatures at which this change occurs increasing from element to element as the positive character becomes more marked; the mixture of basic and superbasic salts is then extracted with dilute nitric acid which leaves that latter undissolved and removes the more positive elements in solution.

Fractional precipitation of the hydroxides by means of ammonia, alkalies, or alkaline earths has also been frequently employed. A modification of this process is the precipitation with aniline, carried out by Kruss;[358] in this method, the solution of the chloride in warm dilute alcohol is treated with an alcoholic solution of the organic base. Another modification is the ‘Oxide process’ employed by Auer von Welsbach[359] for the separation of the cerium elements, and by Drossbach[360] in the yttrium group. The concentrated solution of the mixed salts is thoroughly digested with the oxides obtained by precipitating a fraction of the earths; the more strongly basic oxides tend to displace the less basic, so that these accumulate in the insoluble part. The solution is filtered from the undissolved oxides, another fraction precipitated, and the oxides obtained from the precipitate digested with the concentrated solution as before.

The more modern methods of separation combine the above processes with the methods of fractional crystallisation, for which the bromates and alkylsulphates of these elements are well adapted. The procedure[361] which experience shows will lead to a fairly rapid separation is roughly represented in Fig 9. The double sulphates (B), left in solution after removal of the cerium and part of the terbium group, are transformed into the bromates, which are separated by fractional crystallisation into five main fractions. The least soluble portion, fraction 1, contains the terbium elements with some dysprosium; in the fractionation of the terbium group by means of the nitrates and double nitrates, the dysprosium, with some terbium, collects in the final fractions (fraction 6). Fraction 2 contains terbium, dysprosium, holmium, and yttrium as the bromates; these are converted into the anhydrous chlorides, from which, by treatment with sodium ethylsulphate in alcoholic solution, the ethylsulphates are obtained. By fractional crystallisation, dysprosium may be obtained in a fairly pure condition (fraction 7), the least soluble part (fraction 6) containing the terbium with some dysprosium. Holmium and yttrium collect in the most soluble part (fraction 8), from which pure holmium can be obtained by the method of nitrate fusion. Fraction 3 contains yttrium and erbium, with small quantities of dysprosium and holmium; the latter are readily separated by the nitrate fusion, which will also allow of a fairly complete separation of yttrium (fraction 9). Fraction 4 contains yttrium and erbium; scandium if present will also collect here. Erbium can be obtained pure by the nitrate fusion; the second fraction from this process contains both yttrium and erbium, and may be further worked up with the fraction of similar composition (fraction 10) from fraction 3.

The mother-liquors from the bromate separation (fraction 5) contain thulium and the ytterbium elements; the crystallisation is continued, and allows of complete separation of thulium and ytterbium, and probably of lutecium, though the most soluble fractions do not seem to have been fully separated.

The Erbium Group

The oxides of this group, as contrasted with the ytterbia oxides, give rise to coloured salts, which in solution show definite absorption spectra in the optical region; the spectrum of erbium salts is particularly definite and characteristic. Erbium has among the yttrium elements the place of neodymium among the cerium elements; after yttria, erbia is the commonest oxide of the yttria group, though on account of the difficulties of separation the chemistry of erbium is by no means so complete and definite as that of neodymium. The oxides in order of decreasing basicity, as shown by the order in which they are thrown down by ammonia, are: dysprosia, holmia, erbia, thulia; the electropositive character becomes weaker, therefore—as generally in the rare earth series—as the atomic weight of the elements increases.

Dysprosium, Dy = 162·5

Compounds of this element were probably prepared in the pure state for the first time by Urbain[362] in 1906, by the fractional crystallisation of the ethylsulphate. He showed that after fourteen recrystallisations, the absorption spectrum of the salts and the mean atomic weight of the element remain unaltered, and that after removal of terbium by the very efficient ethylsulphate method, all remaining traces of yttrium could be rapidly removed by crystallisation of the nitrate. The salts have generally a more or less pronounced yellow colour.

The oxide, Dy₂O₃, is a white powder which does not alter in composition when strongly heated in reducing or oxidising atmospheres. It is remarkable in that it is the most strongly paramagnetic oxide known, having a coefficient of susceptibility much greater than that of ferric oxide.[363] The chloride crystallises with 6, the sulphate with 8, and the nitrate with 5 molecules of water of crystallisation. The bromate, Dy(BrO₃)₃,9H₂O,[364] obtained by double decomposition, melts at 78°. The platinocyanide, Dy₂[Pt(CN)₄]₃,21H₂O, forms bright red cubic crystals, with greenish fluorescence.

Several other salts are described by Urbain, and by Jantsch and Ohl (loc. cit.).

Atomic Weight.

—Urbain and Demenitroux[365] determined this constant from the ratio Dy₂(SO₄)₃,8H₂O : Dy₂O₃. The mean value of six determinations carried out with material obtained by fractional crystallisation of the nitrate was 162·52; with material purified by the ethylsulphate crystallisation, the mean of six determinations gave the value 162·54. The International Atomic Weight is 162·5.

Holmium, Ho = 163·5

The individuality of this element can hardly be regarded as perfectly established, though Holmberg[371] has prepared salts which in solution show only faint indications of erbium and dysprosium, when tested spectroscopically. That author fractionated the yttrium elements obtained from euxenite by a long process of separation, which involved crystallisation of the m-nitrobenzenesulphonates, of the simple nitrates (two series), of the double ammonium oxalates, and finally fractional precipitation of the hydroxides by ammonia.

He determined the Atomic Weight as 163·5, which is the value accepted by the International Committee, and mapped the absorption spectrum. The oxide, Ho₂O₃, is a pale yellow powder; the salts are yellow, with a faint orange tinge.

Erbium, Er = 167·7

Although erbia was separated by Mosander seventy years ago, it is doubtful if the perfectly pure oxide has ever been prepared. Whilst the individuality of the element is well established, its homogeneity has frequently been called in question. The name ‘Neo-Erbia’ was given by Cleve[372] to the residue left after the separation from the old erbia of ytterbia, scandia, thulia, and holmia (with which dysprosia (q.v.) was also separated), but the spectrum examination of Kruss and Nilson[373] led them to regard Cleve’s oxide as still complex. Their results, however, were explained by the work of Hofmann and his pupils,[374] who consider erbia to be a homogeneous product; the homogeneity of the element, therefore, may be considered as established, though it would be strengthened by a more complete knowledge of the neighbouring elements, holmium and thulium.

The element forms a rose-coloured oxide, and rose-coloured salts, which give to the compounds of the mixed erbia earths their characteristic colour. The oxide gives a very definite and characteristic reflection spectrum, but the salts do not possess this property;[375] the reflection spectrum remains unchanged in the presence of foreign oxides, provided no combination occurs. From the atomic weight determinations, it seems clear that the salts described by Cleve and his pupils[376] were not pure erbium compounds; a few salts only appear to have been recently obtained in the pure state for the atomic weight determination (q.v.).

The sulphate separates from aqueous solutions at ordinary temperatures as the octohydrate, Er₂(SO₄)₃,8H₂O, which forms rose-coloured monoclinic crystals isomorphous with the corresponding sulphates of the whole group. The anhydrous sulphate is formed by long heating at 400°, more quickly at 475°, and can be heated to 630° without decomposition. At 845° a basic salt, Er₂O₃,SO₃, is formed, which begins to decompose at 950°; at 1055° the transformation to the oxide is complete. The ammonium and potassium double sulphates are easily soluble in cold water.

The oxalate is thrown down in rosettes of bright rosy plates, which according to Hofmann[377] have the formula Er₂(C₂O₄)₃,10H₂O, even when dried in the air. Cleve believed the salt to be thrown down as the enneahydrate. When kept in vacuo over phosphoric anhydride, the decahydrate passes into the trihydrate, which when heated decomposes, passing into the oxide at a temperature of 575°. The nitrate, Er(NO₃)₃,5H₂O, separates from aqueous solution as the pentahydrate, in large stable red crystals. The platinocyanide, Er₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour with green fluorescence. The formate, Er(HCOO)₃—Cleve, loc. cit.—is a red powder, obtained by dissolving the oxide in formic acid; it crystallises from water as the dihydrate.

Atomic Weight.

—The determinations of the earlier workers, being carried out with impure material, gave results which differ very widely, and are quite unreliable. Cleve’s value of 1880, for material free from ytterbia, but not apparently free from earths of lower equivalent, was 166·25; Brauner,[378] using the same material in 1905, obtained the much higher value 167·14. The determinations of Hofmann and Burger[379] in 1908 gave the mean value 167·38; with purer material, Hofmann in 1910[380] obtained the mean value 167·68, on which is based the value accepted by the International Committee, 167·7.