The Rare Earths: Their Occurrence, Chemistry, and Technology

Many double carbonates are obtained by dissolving the normal carbonate in excess of the precipitant. The absorption spectra of these solutions, which have a blue colour, are abnormal and very intense, and have been suggested as a basis of quantitative estimation.[277]

The acetylacetone derivative forms violet crystals, melting at 144°-145°.

A large number of organic salts of neodymium have been prepared by James, Hoben, and Robinson.[278]

Atomic Weight.

—The earlier determinations of this constant were carried out by the sulphate method, the synthetic process being usually employed. Auer von Welsbach, at the time of the discovery of praseodymium and neodymium,[279] gave the values 143·6 and 140·8 respectively for their atomic weights. Brauner, who carried out a determination in 1898,[280] showed that these numbers should be interchanged, and gave the value 143·63 for neodymium. Boudouard,[281] employing the analytical sulphate method, obtained the value 143·05, whilst in the same year Jones[282] gave the value 143·6. A second determination by Brauner[283] gave the value 143·89. All these values are undoubtedly too low, the material being probably contaminated with other earths.

In his second determination in 1908, Auer von Welsbach[284] gave the value 144·54 as the mean of three determinations. Feit and Przibylla,[285] using their volumetric method, gave the value 144·52, whilst Holmberg,[286] using material which he considered to have been the purest obtained up to that time, obtained the figure 144·11. More recently, Baxter and Chapin[287] have made determinations by treating the chloride with pure silver nitrate, and weighing the precipitated silver chloride, as well as by titration. The mean value obtained by the first method—ratio NdCl₃  : 3AgCl—was 144·272 (extremes 144·250 and 144·298), and by the second method—ratio NdCl₃  : 3Ag—was 144·268 (extremes 144·249 and 144·283), giving the mean value for the whole series of 144·270.

The value adopted by the International Committee is 144·3.

Samarium, Sa = 150·4

The samarium of the earlier chemists (see p. 168) contained a large proportion of the terbium elements, from which a fairly complete separation was first effected by Demarçay in 1900.[291] By the fractional crystallisation of the double magnesium nitrate in presence of bismuth magnesium nitrate, Urbain and Lacombe[292] succeeded in preparing samarium compounds, which were shown by spectroscopic examination[293] to be free from other earths. The element is intermediate in electropositive character and in the solubility relations of its salts between neodymium and the terbium earths; its salts are topaz-yellow in colour, and in concentrated solutions show absorption in the blue and violet regions. The oxide is almost white in colour, with only a faint yellow tinge. A systematic investigation of samarium compounds was carried out by Cleve,[294] but his work was vitiated by the fact that his material was very impure. More recently, the pure salts have been examined by Matignon and his pupils.

The melting-point of the metal lies between 1300° and 1400°C., so that its preparation by the electrolytic method is a matter of great difficulty. A mixture of the chloride with one-third of its weight of barium chloride is electrolysed by means of a current of 100 ampères, using a cathode of only 2·5 mm. thickness; the metal so obtained is greyish white in colour, and is the hardest of the cerium elements.

The chloride separates from aqueous solution as the hexahydrate, SaCl₃,H₂O, in large tabular yellow crystals. The anhydrous chloride is white, but fuses to a chocolate-brown liquid; it forms a large number of additive compounds with ammonia. When heated in an atmosphere of dry hydrogen or ammonia, air and moisture being carefully excluded, it yields the subchloride,[295] SaCl₂, as a dark brown crystalline solid, insoluble in alcohol and all organic solvents. Samarous chloride dissolves in water, forming a deep brownish-red solution, which rapidly becomes colourless, with evolution of hydrogen, and precipitation of the oxide and oxychloride. Samarous iodide, SaI₂, may be obtained by a similar process, and closely resembles the chloride.

The bromate, Sa(BrO₃)₃,9H₂O, melts at 75°, and closely resembles the corresponding compounds of the didymium metals. The sulphate crystallises with 8, and the nitrate with 6 molecules of water. The carbonate, Sa₂(CO₃)₃,3H₂O, can be obtained only by passing carbon dioxide through an aqueous suspension of the hydroxide; addition of alkali carbonate to a solution of a samarium salt precipitates hydrated double carbonates.

The acetylacetone compound melts at 146°-147°C.

Many organic salts have been prepared by James, Hoben, and Robinson (loc. cit.).

Atomic Weight.

—The earlier determinations of this constant were carried out with material not entirely free from europium. Demarçay[296] carried out a synthetic sulphate operation with the material which he obtained free from europium in 1900, and found values between the limits 147·2 and 148·0. The International Committee has adopted the value 150·4, which is based on the work of Urbain and Lacombe[297] in 1904. These authors made determinations of three series of ratios, obtained by (a) conversion of sulphate octohydrate to anhydrous sulphate, (b) conversion of anhydrous sulphate to oxide, and (c) conversion of sulphate octohydrate to oxide; these gave the values 150·314, 150·533, and 150·484 respectively, from which the mean atomic weight is 150·44.[298]

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]

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.

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.

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.

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.

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]

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.

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]

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.

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.

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.