Erbium compounds are compounds containing the chemical element erbium (Er). As with most of the later lanthanides, the chemistry of erbium is dominated by the +3 oxidation state, corresponding to the Er ion with a 4f electron configuration. Erbium(III) compounds are generally highly ionic, and many hydrated salts and coordination compounds are pink or rose-coloured owing to characteristic f–f electronic transitions. Lower oxidation states are much less common, although +2, +1 and formally 0 oxidation states have been reported in specialized inorganic and organometallic systems.Yttrium and all lanthanides except Ce and Pm have been observed in the oxidation state 0 in bis(1,3,5-tri-t-butylbenzene) complexes, see and
A wide range of binary erbium compounds are known. These include erbium(III) oxide and other chalcogenides, the four trihalides , , and , hydrides, borides, carbides, silicides and pnictides. Many of these compounds display magnetic ordering at low temperatures as a result of the localized 4f electrons of Er. Erbium also forms numerous hydrated oxoacid salts and coordination compounds, as well as organometallic complexes containing cyclopentadienyl, alkyl and aryl ligands.
The characteristic optical transitions of Er, particularly those in the near-infrared region, make erbium compounds important in optical and photonic materials. Erbium-containing fluorides, oxides and glasses are widely studied for infrared emission and upconversion luminescence, while erbium silicides and nitrides have also attracted interest for electronic, magnetic and semiconductor applications.
Properties of erbium compounds
Formula Appearance Crystal system Space group No. a (nm) b (nm) c (nm) Density(g/cm) pale pink cubic Ia 206 1.05376 8.69 pink orthorhombic Pnma 62 0.6320 0.6822 0.4385 7.81 violet monoclinic C2/m 12 0.6804 1.17456 0.63187 3.79Calculated as molar mass divided by the room-temperature molar volume: 273.61 g/mol ÷ 72.11 cm3/mol. The molar volume is given in Table 1, p. 9294, of violet trigonal R 148 0.7005 0.7005 1.889 4.93Calculated as molar mass divided by the room-temperature molar volume: 406.97 g/mol ÷ 82.56 cm3/mol. The molar volume is given in Table 1, p. 9540, of cubic Fmm 225 0.51287 tetragonal P4/mbm 127 0.70705 0.399710 tetragonal I4/mmm 139 0.3622 0.6106 ErN cubic Fmm 225 0.4839 10.63 ErP cubic Fmm 225 0.5595 ErAs cubic Fmm 225 0.5732 ErSb cubic Fmm 225 0.6106 ErBi cubic Fmm 225 0.6206 orthorhombic Fddd 70 0.8085 1.1346 2.4140 orthorhombic Fddd 70 1.2134 0.8579 2.5737
Chalcogenides
Oxides
Erbium(III) oxide (), also known as erbia, is the principal oxide of erbium. It was first isolated in impure form by Carl Gustaf Mosander in 1843, and was obtained in pure form in 1905 by Georges Urbain and Charles James. At ambient conditions it adopts the cubic bixbyite structure characteristic of many rare-earth sesquioxides, in which Er occupies two crystallographically distinct six-coordinate sites. Other polymorphs of can be produced under appropriate conditions.
Erbium(III) oxide can be prepared by burning erbium metal in oxygen, and is also produced by thermal decomposition of several erbium salts. It is a pale pink solid that is insoluble in water but dissolves in mineral acids.
Other chalcogenides
Several sulfides of erbium are known. Erbium(III) sulfide, , occurs in several structural modifications. The δ modification is monoclinic, with space group P2/m, and is isostructural with δ-. Its structure contains erbium atoms in several different sulfur coordination environments. A second modification, F-, is monoclinic and can be obtained by chemical vapour transport in the presence of niobium and chlorine. In this structure the erbium atoms have coordination numbers of six, seven and eight. The monosulfide ErS is also known and adopts the sodium chloride structure. Nanocrystals of ErS have been observed to form from under electron-beam irradiation.
The erbium–selenium system contains several binary phases, including ErSe, and . ErSe adopts the cubic sodium chloride structure. crystallizes in an orthorhombic structure of the type, space group Fddd, which may be regarded as a vacancy-ordered derivative of the sodium chloride structure. In this structure both crystallographically independent erbium sites are approximately octahedrally coordinated by selenium. has more than one structural modification and undergoes a phase transition on heating. Intermediate compositions also occur between and more erbium-rich phases.
Erbium(III) telluride, , can be prepared by direct reaction of erbium with tellurium or by chemical vapour transport using erbium(III) chloride as the transport agent. It crystallizes in the orthorhombic structure type, space group Fddd. It is a semiconductor with a measured band gap of 0.77(5) eV and is paramagnetic between 5 and 300 K.
Halides
Erbium forms the four trihalides , , and . They can be obtained by direct reaction of erbium metal with the corresponding halogen.
Erbium(III) fluoride is a pink solid. It can be prepared by precipitation from an aqueous erbium(III) salt with a fluoride source; reaction of erbium(III) nitrate with ammonium fluoride, for example, has been used to prepare micro- and nanostructures. At ambient pressure it crystallizes in the orthorhombic β- structure, space group Pnma, in which the erbium atoms are nine-coordinate. At approximately 9.8 GPa it undergoes a reversible transformation to a trigonal -type structure. exhibits characteristic Er luminescence and has consequently been studied in infrared and upconversion optical materials.
Erbium(III) chloride, , is a violet solid. Anhydrous can be prepared from erbium(III) oxide by the ammonium chloride route. Heating the oxide with ammonium chloride first produces an ammonium chloridoerbium intermediate, which decomposes on further heating under vacuum to give the anhydrous chloride. Anhydrous adopts the monoclinic structure type.
The hexahydrate is also monoclinic. In its crystal structure the erbium centre is eight-coordinate, with six water molecules and two chloride ions forming the first coordination sphere, while a further chloride ion remains outside it.
Erbium(III) bromide and erbium(III) iodide are likewise violet trihalides. Unlike , which has an -type arrangement, and crystallize in closely related layered structures of the type. In these compounds the Er ions form two-dimensional honeycomb layers separated by halide ions.
The three heavier trihalides also display related low-temperature magnetic behaviour. develops three-dimensional antiferromagnetic order below about 350 mK, whereas and show long-range two-dimensional magnetic order between approximately 280 and 400 mK, followed by only short-range three-dimensional correlations at lower temperatures. These differences arise from the different stacking of the erbium honeycomb layers in their crystal structures.
Hydrides
Erbium forms two principal binary hydrides, erbium dihydride () and erbium trihydride (). At low hydrogen concentrations, hydrogen forms a solid solution in metallic erbium while retaining the hexagonal close-packed structure of the metal. With increasing hydrogen content, a cubic dihydride phase appears, followed at still higher concentrations by the trihydride phase.
adopts the cubic calcium fluoride structure, in which the erbium atoms form a face-centered cubic sublattice and hydrogen occupies the tetrahedral interstitial sites. It is metallic, and its electronic structure differs substantially from that of elemental erbium because hybridized erbium–hydrogen bands occur below the Fermi level.
At higher hydrogen concentrations the cubic dihydride coexists with a hydrogen-richer phase until an H/Er ratio of about 2.7, above which the trihydride phase predominates. has a hexagonal erbium sublattice and adopts a structure related to the type, with hydrogen occupying several kinds of interstitial site. The transition from to is accompanied by a metal–insulator transition, a characteristic feature of several rare-earth hydride systems.
Under pressure, erbium trihydride undergoes a transformation from its ambient-pressure hexagonal structure to a cubic phase. On heating, hydrogen-rich decomposes through progressively hydrogen-poorer phases, passing through the dihydride region before returning to metallic erbium.
Borides
Several borides of erbium are known, including , , and the boron-rich compound . The existence and stability of erbium hexaboride, , have been questioned because of difficulties reproducing earlier syntheses. However, in 2012, single-crystalline nanowires were reported among the products of palladium-nanoparticle-assisted chemical vapour deposition, together with nanoplatelets. Transmission electron microscopy and electron diffraction identified cubic and tetragonal structures for the nanowires and nanoplatelets, respectively.
Erbium diboride () crystallizes in a hexagonal structure. Single crystals have been grown from molten erbium-rich flux at temperatures around 1750 °C. It is a metallic magnetic compound and exhibits easy-plane ferromagnetic ordering at low temperatures.
Erbium tetraboride () adopts the tetragonal structure type, space group P4/mbm. Like other rare-earth tetraborides, its structure contains a two-dimensional network of rare-earth atoms interspersed with boron units. is metallic and orders antiferromagnetically below approximately 15–16 K. It is strongly magnetically anisotropic, with the easy axis parallel to the crystallographic c axis; increasing magnetic field produces successive magnetic transitions before a field-polarized state is reached.
Erbium dodecaboride () crystallizes in the cubic structure type, space group Fmm. In this structure the erbium atoms form an fcc sublattice within a three-dimensional boron framework built from clusters. It is a metallic antiferromagnet with a Néel temperature of about 6.6–6.7 K and exhibits a complex magnetic phase diagram and strong anisotropy in its magnetotransport properties.
The boron-rich compound crystallizes in the cubic space group Fmc, with a lattice parameter of a = 2.3408(3) nm. Its boron framework contains icosahedra grouped into larger supericosahedral units, with erbium atoms located in cavities within the framework. Electrical resistivity measurements between 300 and 1000 K are consistent with variable-range hopping conduction. The Seebeck coefficient is approximately 700 μV/K near room temperature and decreases markedly with increasing temperature.
Carbides and silicides
Erbium forms several binary compounds with carbon and silicon. The best-characterized carbide is erbium dicarbide, , while the erbium–silicon system contains a substantially larger number of stable phases.
Erbium dicarbide () crystallizes at room temperature in a tetragonal structure, space group I4/mmm. Its structure contains discrete units, and neutron diffraction measurements give a C–C distance of approximately 1.275 Å, consistent with substantial multiple-bond character. can be prepared by arc melting the constituent elements. Carbon-coated nanoparticles containing have also been produced by arc discharge and have been investigated for their magnetocaloric and microwave-absorption behaviour.
The erbium–silicon system contains at least five established binary silicides: , , ErSi1-x, ErSi and the silicon-deficient disilicide ErSi2-δ. ErSi2-δ is dimorphous, and the various silicides exhibit metallic electrical conductivity.