CHAPTER XIII

THE TERBIUM GROUP

In his examination of the yttria earths in 1842, Mosander described two new oxides isolated from the old yttria. To one of these, an orange-yellow earth which yielded colourless salts, he gave the name Erbia; the second earth, which was colourless and gave rose-coloured salts, he called Terbia. Bahr and Bunsen examined the yttria oxides in 1866, and obtained only the latter earth, which gave rose-coloured salts; to this they applied Mosander’s name Erbia, and stated that the earth to which Mosander had given that name had no existence. Delafontaine, however, confirmed Mosander’s work, showing that the orange-yellow earth which yielded colourless salts (Mosander’s Erbia) had been fractionated out of their material by Bahr and Bunsen in the double sulphate separation of the cerium group; to avoid further confusion, however, he proposed to give to this oxide (Mosander’s Erbia) the name Terbia, leaving for the colourless oxide, which forms rose-coloured salts (which Mosander had called Terbia) the name Erbia applied to it by Bahr and Bunsen. This reversed nomenclature has been generally accepted.

Delafontaine,[300] continuing his work on the earths from samarskite (see p. 168) announced in 1878 the discovery of a new oxide, Philippia, intermediate between terbia and yttria; but this was subsequently shown to be a mixture of yttria and terbia (see p. 133). In the same year, Lawrence Smith[301] announced the discovery of another oxide, Mosandria, from the samarskite earths; this was afterwards shown by Lecoq de Boisbaudran to be a mixture of terbia with gadolinia.[302] In 1880 Marignac[303] announced the discovery of two more new oxides, Y_{α} and Y_{β} from the same mineral; Y_{β} was afterwards found to be identical with samaria, whilst Y_{α} was subsequently separated from the old terbia earths by Lecoq de Boisbaudran, who proposed, with the assent of Marignac, the name Gadolinium.[304] The terbia left after removal of the erbia earths and gadolinia was believed by that author to be still a mixture, a conclusion supported by the work of Hofmann and Kruss in 1893.[305]

[300] _Compt. rend._ 1878, ~87~, 559.

[301] _Ibid._ 1878, ~87~, 146.

[302] _Ibid._ 1886, ~102~, 647.

[303] _Compt. rend._ 1880, ~90~, 899.

[304] _Loc. cit._

[305] _Zeitsch. anorg. Chem._ 1893, ~4~, 27.

In 1886 Demarçay[306] isolated from samaria a new oxide, which he designated S₁. From his work on this oxide in 1892-1893, de Boisbaudran[307] concluded that samaria consisted of at least three oxides, samaria proper, and two new oxides Z_{ξ} and Z_{ε}. In 1896, Demarçay[308] separated an earth Σ, which showed the spark-spectrum of Z_{ε} and the reversal spectrum of Z_{ξ}, and finally in 1901[309] he obtained the new oxide in a fairly pure condition, and gave it the name Europia.

[306] _Compt. rend._ 1886, ~102~, 1551.

[307] _Ibid._ 1892, ~114~, 575; _ibid._ 1893, ~116~, 611 and 674.

[308] _Ibid._ 1896, ~122~, 728.

[309] _Ibid._ 1901, ~132~, 1484.

The complicated history of the terbium group has been entirely cleared up by the work of Urbain and his co-workers during the early years of the present century, and processes have been devised by which the separation of the three members of the group from one another, and from the related elements of the erbium group on the one side, and samarium on the other, can be satisfactorily accomplished. The chemistry of this group, therefore, may be regarded as satisfactorily settled, though relatively little is known of the properties of the elements and their compounds.

In their general chemical relations, elements of the terbium group occupy an intermediate position between the cerium group and the elements of the yttrium group in the narrower sense. In the solubility relations of the double salts, they are bounded on the one side by samarium and the less soluble cerium group, on the other by dysprosium and holmium and the more soluble yttrium group. They show only very slight differences in electropositive character, and methods based on differences in basic strength of the oxides, therefore, are of very little use for separating them from one another. Fractional precipitation with ammonia separates them in the order terbium, samarium, gadolinium, and europium--samaria being less strongly basic than the oxides of gadolinium and europium; this constitutes an exception to the general rule regarding the solubilities of the double nitrates and sulphates with increasing electropositive character.[310] The difficulties of separation are greatly increased by the very small proportions in which the elements are usually found in rare earth minerals. Gadolinium usually occurs in the largest quantities; in consequence of this, there is little doubt that most of the material described by the earlier workers as terbia consisted very largely of gadolinia.

[310] See Lecoq de Boisbaudran, _Compt. rend._ 1890, ~111~, 394.

The group is not characterised by well-marked absorption spectra; europium and terbium show weak absorption in the blue region. Terbium, of which the salts are colourless, forms a very strongly coloured peroxide, analogous to that of praseodymium; small quantities of this give to the mixed oxides obtained by ignition the characteristic yellow colour, whilst mixtures richer in the peroxide become correspondingly darker and darker.

SEPARATION

In the double sulphate separation of the yttrium and cerium groups, the terbium elements divide themselves between the soluble and the insoluble portions; if the separation is made as complete as possible by addition of a large excess of alkali sulphate under suitable conditions, the larger part of the compounds of the group will be precipitated with the cerium elements. In the separation of the cerium elements the terbium elements collect in the most soluble fractions, and the mother-liquors of the double nitrate crystallisations therefore form a very convenient source of these elements. A considerable proportion, however, will usually remain in solution with the double sulphates of the yttrium group; in the bromate separation of these (see p. 198), the terbium elements collect in the least soluble fractions. By careful fractionation under suitable conditions, the double sulphate method may be used to separate the terbium group completely from the cerium and yttrium elements. A very convenient method of separating the terbium group from a rare earth mixture is the ethylsulphate process of Urbain. By fractional crystallisation of these salts from alcohol or water, the separation into three groups can be satisfactorily accomplished.

For the separation of the terbium elements from one another, the nitrate and double nitrate methods are most suitable. Samarium can readily be separated by crystallisation of the double magnesium nitrates in presence of bismuth magnesium nitrate; by continuing the fractionation, europium magnesium nitrate can be separated in a pure state, as there is a considerable difference between the solubility of this salt and the corresponding compound of gadolinium;[311] the process, however, is somewhat long and tedious. For the separation of gadolinium and terbium, the double nitrates are converted into the simple nitrates, and these fractionated from nitric acid in presence of bismuth nitrate. The gadolinium nitrate separates before the bismuth nitrate, and may be obtained fairly pure in this way, though the process is extremely tedious, and several thousand recrystallisations are required.[312] Terbium nitrate has almost the same solubility as bismuth nitrate, and the two separate together in the middle fractions. The more soluble nitrates of the erbia earths collect in the mother-liquors.

[311] James (_J. Amer. Chem. Soc._ 1912, ~34~, 757) employs at this stage the fractional crystallisation of the double nickel nitrates.

[312] See Urbain, _Compt. rend._ 1904, ~139~, 736.

~Europium~, Eu = 152·0

This element is one of the rarest of the whole group, and occurs only in extremely small quantities. Monazite sand is said to contain about 0·002 per cent. of the oxide, though on account of the remarkable intensity of some of the stronger lines in the arc spectrum, Eberhard[313] was able to detect europium with ease in a mixture of rare earth oxides from that mineral, after the separation of cerium. The _oxide_ has a pale rose colour; the salts are also faintly coloured, and in solution show weak absorption bands.

[313] _Zeitsch. anorg. Chem._ 1905, ~45~, 378.

_Europium sulphate_, Eu₂(SO₄)₃,8H₂O, separates in pink crystals, which are completely dehydrated at 375°; _europic chloride_, EuCl₃, in the anhydrous state forms fine yellow needles; _europium oxychloride_, EuOCl, prepared by heating europic chloride in dry air to 600°, is a white solid, insoluble in water, but soluble in strong acids; _europous chloride_, EuCl₂, prepared by reduction of the higher chloride in hydrogen, is a white amorphous solid, soluble in water to a neutral solution, which on boiling throws down the oxide, Eu₂O₃.[314] Several organic salts have been prepared by James and Robinson.[315]

[314] Urbain and Bourion, _Compt. rend._ 1911, ~153~, 1155.

[315] _J. Amer. Chem. Soc._ 1913, ~35~, 754.

~Atomic Weight.~--Using the material isolated from samaria, Demarçay[316] in 1900, by the synthetic sulphate method, found the atomic weight of europium to be about 151. Urbain and Lacombe[317] determined the value in 1904, with material free from gadolinium and samarium, using the three ratios which they employed in the case of the latter element (see p. 182); their values, corrected by Brauner, were 152·00, 151·93 and 151·94 respectively. Another series of determinations was carried out by Jantsch[318] in 1908, the same method being employed; he obtained the mean value 152·03, with an error of ±·02. The International Committee have adopted the value 152·0.

[316] _Compt. rend._ 1900, ~130~, 1469.

[317] _Ibid._ 1904, ~138~, 627.

[318] _Ibid._ 1908, ~146~, 473.

~Detection.~--The absorption spectrum was determined by Demarçay,[319] but is not sufficiently intense or characteristic for ordinary purposes of detection. The spark spectrum has been investigated by the same author (_loc. cit._); it is very bright, and shows the three blue rays which characterised Lecoq de Boisbaudran’s Z_{ε}. The reversal spectrum shows the characteristic band of Z_{ξ}.

The pure oxide, according to Urbain,[320] shows no luminescence under the influence of cathode rays, but when impure, or very largely diluted with lime or gypsum, it gives very bright and characteristic spectra.

[319] _Ibid._ 1900, ~130~, 469.

[320] _Ibid._, 1906, ~142~, 205, 1518.

The arc spectrum[321] is very characteristic, and contains some exceedingly intense lines, by means of which Lunt[322] has detected europium in the sun and in many stars. The lines most suited for identification of the element are the following:

[321] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 31.

[322] _Proc. Roy. Soc._ 1907, ~79~; A, 118.

3688·57 3725·10 3819·80 3907·28 3930·66 3972·16 4129·90 4205·20 4435·75 4522·76 4594·27 4627·47 4662·10 6645·44

~Gadolinium~, Gd = 157·3.

Gadolinia is the commonest of the terbia oxides, and occurs in considerable quantities in some of the rare earth minerals, notably in samarskite and gadolinite; its separation from the neighbouring oxides, europia and terbia, is, however, exceedingly difficult, and has only been satisfactorily accomplished in recent times. The gadolinium compounds prepared and examined by the earlier workers, as appears from the atomic weight determinations, must have been associated with earths of lower atomic weight, and undoubtedly also with small quantities of terbium. After the isolation of Marignac’s Y_{α}, and the examination of the element by Lecoq de Boisbaudran, to whom the name gadolinium is due, further investigations were carried out by Bettendorff[323] and by Benedicts.[324] Pure gadolinia was probably first obtained by Demarçay,[325] by fractional crystallisation of the magnesium double nitrate; the oxide obtained by Urbain and Lacombe[326] by crystallisation of the nitrates in presence of bismuth nitrate, was proved to be spectroscopically pure by Eberhard.[327]

[323] _Annalen_, 1892, ~270~, 376.

[324] _Zeitsch. anorg. Chem._ 1900, ~22~, 393.

[325] _Compt. rend._ 1900, ~131~, 343; _ibid._ 1901, ~132~, 1484.

[326] _Ibid._ 1905, ~140~, 583, etc.

[327] _Zeitsch. anorg. Chem._ 1905, ~54~, 374.

The gadolinia obtained by ignition of the salts of volatile acids should be perfectly white; presence of terbia causes it to assume a yellow colour.[328] The salts are colourless, and their solutions show no absorption in the visible region, though Urbain[329] has shown that there are four strong bands in the ultraviolet.

[328] Eberhard (_loc. cit._) has shown that even in the perfectly white oxide, traces of terbia can be distinguished by spectroscopic examination.

[329] _Compt. rend._ 1905, ~140~, 1233.

The _hydroxide_, Gd(OH)₃, is a gelatinous precipitate with strongly basic properties, rapidly absorbing carbon dioxide from the air. The _oxide_, Gd₂O₃, also absorbs carbonic anhydride from the air, and is easily soluble in acids, even after strong ignition. The element is therefore strongly electropositive. Its position among the yttrium elements, however, is justified by the properties of the _platinocyanide_, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red crystals, with a green metallic lustre, belonging to the rhombic system, and isomorphous with the corresponding yttrium and erbium salts; the cerium elements, on the other hand, give yellow platinocyanides, with a blue metallic lustre, which crystallise in the monoclinic system.

The _nitrate_, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the ordinary temperatures in large crystals belonging to the anorthic system, and is isomorphous with the corresponding compounds of praseodymium and neodymium.[330] From solutions in strong nitric acid, a pentahydrate is obtained, which melts at 92°; the hexahydrate melts at 91°. The _sulphate_ separates from aqueous solution as the octohydrate, Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups. The anhydrous sulphate is much less soluble in water at 0° than the corresponding compounds of the cerium elements. The _selenate_ forms hydrates with 10 and 8 molecules of water of crystallisation respectively; these are isomorphous with the corresponding selenates of yttrium and the erbium metals.

[330] Lang and Haitinger, _Annalen_, 1907, ~351~, 450.

~Atomic Weight.~--The determinations of this constant made by the earlier workers were all carried out with impure material and gave results which were considerably too low. The International Committee have adopted the value 157·3, which is based on the work of Urbain.[331] In employing the analytical sulphate method, that author observed that the anhydrous sulphate did not remain constant in weight when allowed to remain in a desiccator, and that it could not be accurately weighed. He therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the octohydrate directly to oxide, and obtained the mean value 157·24.

[331] _Compt. rend._ 1905, ~140~, 583.

~Detection.~--Pure gadolinium compounds show no absorption in the visible spectrum, but there are four strong bands[332] in the ultraviolet, viz. 311·6-310·5; 306·0-305·7; 305·6-305·5; and 305·4-305·0. The arc spectrum[333] is very rich in lines, of which the most intense are the following:

[332] Urbain, _ibid._ 1905, ~140~, 1233.

[333] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 21.

3082·15 3100·66 3422·62 3545·94 3549·52 3585·12 3646·36 3671·39 3719·63 3743·68 3768·60 3796·62 3814·18 3852·65 3916·70 4037·49 4050·05 4063·62 4070·51 4073·99 4085·73 4098·80 4130·59 4184·48 4251·90 4262·24 4325·83 4327·29 4342·35 6114·26

The spark spectra have been examined by Demarçay,[334] Baur and Marc,[335] Urbain[336] and Crookes.[337]

[334] _Compt. rend._ 1900, ~131~, 343.

[335] _Ber._ 1901, ~34~, 2460.

[336] _Loc. cit._

[337] _Proc. Roy. Soc._ 1905, ~74~, 420.

~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).

[338] _Compt. rend._ 1886, ~102~, 395, 483.

[339] _Ber._ 1902, ~35~, 2382.

[340] _Zeitsch. anorg. Chem._ 1905, ~43~, 267.

[341] _Compt. rend._ 1904, ~139~, 736; 1905, ~141~, 521; 1909, ~149~, 37.

[342] _Ibid._ 1895, ~121~, 709; 1904, ~139~, 1015.

[343] _Ibid._ 1900, ~131~, 343.

[344] _Trans. Chem. Soc._ 1889, ~55~, 258.

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°.

[345] The terbium compounds here described have been prepared by Urbain (_loc. cit._) from carefully purified material; other compounds have been described by Potratz (_Chem. News_, 1905, ~92~, 3), but her material contained a large proportion of gadolinium.

~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.

~Detection.~--Solutions of terbium salts show only one band in the visible spectrum, at 487·7 in the blue. This band was observed by Lecoq de Boisbaudran in a specimen of terbia containing dysprosia, and assumed by him to belong to a new element, Z_{δ} (_loc. cit._) In the ultraviolet nine absorption bands have been observed (Urbain, _loc. cit._)

The spark spectrum shows the lines observed by Demarçay in 1900, and attributed by him to the new element Γ. Lecoq de Boisbaudran’s element Z_{β} showed a green fluorescence with the reversed spark, a phenomenon which Urbain has found to be exhibited by pure terbium compounds.

The arc spectrum of Urbain’s pure terbia was examined by Eberhard[346]--see also Exner and Haschek, and Eder and Valenta.[347] The element may be detected in minerals and earth mixtures by the following lines:

3523·82 3676·52 3703·05 3704·01 4005·62 4278·71

The chief lines in the arc spectrum (Exner and Haschek) are the following:

3324·53 3509·34 3531·86 3561·90 3568·69 3600·60 3628·53 3650·60 3659·02 3704·10 3711·91 3848·90 3874·33 3899·34 3925·60 3939·75 3977·01 3982·07 4005·70 4012·99 4278·70 4752·69

Pure terbia does not exhibit the phenomenon of cathode luminescence, but gadolinia containing a trace of terbia shows a marked green fluorescence, which was attributed by Crookes to a new Meta-element, G_{β}. A trace of terbia in aluminium oxide causes the latter to exhibit a highly characteristic intense white luminescence.

[346] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1906, ~18~, 385.

[347] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 14.