Erbium(III) oxalate is an inorganic compound of erbium and the oxalate ion, with the formula Er2(C2O4)3. It forms several hydrates, including well-characterized hexa-, deca- and octadecahydrates. On heating, erbium oxalate decomposes through carbonate-containing intermediates to erbium(III) oxide.
Preparation Hydrated erbium(III) oxalate can be precipitated from aqueous solutions of erbium(III) salts using oxalic acid or soluble oxalates.
Single crystals of hydrated erbium oxalates have also been grown by allowing aqueous solutions of erbium(III) chloride and potassium oxalate to diffuse slowly through gel media.
Properties Hydrates and crystal structures Erbium(III) oxalate is unusual among the lanthanide oxalates because erbium lies near the boundary between two hydrate structure families. For lighter lanthanides, compositions of the form Ln2(C2O4)3(H2O)6·4H2O are common, whereas heavier lanthanides more often form Ln2(C2O4)3(H2O)4·2H2O. Both structure types are reported for erbium.
The decahydrate, Er2(C2O4)3(H2O)6·4H2O, crystallizes in the monoclinic space group P21/c. Its lattice parameters are a = 11.3590 Å, b = 9.6162 Å, c = 9.9401 Å and β = 118.723°, with four formula units per unit cell.
Its structure consists of parallel honeycomb-like molecular layers formed by erbium ions linked through oxalate groups.
A six-water form, Er2(C2O4)3(H2O)4·2H2O, also occurs and belongs to the heavier-lanthanide oxalate structure family.
An octadecahydrate, Er2(C2O4)3(H2O)6·12H2O, has also been structurally characterized. It crystallizes in the trigonal space group R, with a = 30.8692 Å and c = 7.2307 Å, Z = 12.
The octadecahydrate forms a three-dimensional open framework containing channels of approximately 12 Å diameter. Water molecules of crystallization occupy these channels. Erbium is nine-coordinate in this phase, bonded to six oxalate oxygen atoms and three coordinated water molecules in a distorted tricapped trigonal-prismatic environment.
Thermal decomposition Erbium(III) oxalate hydrates dehydrate stepwise on heating. A detailed thermal study of Er2(C2O4)3·6H2O found progressive dehydration between about 90 and 370 °C.
The hexahydrate initially loses water to form lower hydrates, including trihydrate and dihydrate phases. Anhydrous erbium oxalate is then formed but is thermally unstable.
Further heating produces carbonate-containing intermediates. An erbium oxycarbonate, Er2O2CO3, is formed around 450 °C and decomposes near 600 °C to crystalline Er2O3. Water vapour, carbon monoxide and carbon dioxide are evolved during the overall decomposition.
Earlier work on rare-earth oxalates likewise found that hydrated erbium oxalate ultimately decomposes to erbium oxide on heating.
Complex oxalates Erbium forms a number of complex oxalates in addition to the simple sesquioxalate. Reaction with alkali-metal oxalates can give compounds such as KEr(C2O4)2·4H2O.
This potassium erbium oxalate crystallizes in the tetragonal space group I41/amd, with a = 11.4145 Å and c = 8.893 Å. Its structure contains square channels running parallel to the c axis.
Acidic oxalate complexes have also been reported, including H[Er(C2O4)2]·6H2O.
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