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.
[362] Compt. rend. 1906, 142, 785.
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.
[363] Compt. rend. 1908, 146, 922.
[364] Jantsch and Ohl, Ber. 1911, 44, 1274.
Several other salts are described by Urbain, and by Jantsch and Ohl (loc. cit.).