Ytterbium compounds are compounds formed by the lanthanide element ytterbium (Yb). Ytterbium commonly occurs in the +3 oxidation state, with the 4f electronic configuration, but unlike most lanthanides it also forms a substantial range of stable +2 compounds. Yb has the closed-shell 4f configuration and is particularly prominent in the dihalides, monochalcogenides, hydrides and organometallic compounds. Intermediate and mixed valence between the +2 and +3 states also occurs in numerous borides, carbides, silicides, pnictides and intermetallic compounds, giving ytterbium chemistry an unusually strong connection with Kondo, heavy fermion and other strongly correlated electronic phenomena.
Ytterbium forms binary compounds with most nonmetals, including oxides, sulfides, selenides, tellurides, halides, hydrides, borides, carbides, silicides and pnictides. The trivalent sesquioxide has the cubic bixbyite structure, whereas divalent YbO and the monochalcogenides YbS, YbSe and YbTe adopt rock-salt-related structures. The hydride system is unusual among the lanthanides: contains predominantly divalent ytterbium, while the hydrogen-richer phase is mixed-valent and a conventional stoichiometric has not been obtained even at high pressure. Ytterbium also forms divalent and trivalent coordination and organometallic compounds, including reducing reagents such as and divalent ytterbocenes.
Several ytterbium compounds have important physical or technological properties. is a Kondo insulator with unusual metallic surface states, while intermetallic compounds such as , and exhibit heavy-fermion, quantum-critical or valence-transition behaviour. Yb is widely used as a dopant in solid-state and fiber laser materials, particularly in ytterbium-doped silica and Yb:YAG, and as a sensitizer in upconversion materials. Ytterbium(III) fluoride is used as a radiopaque filler in dental materials, while ytterbium halides such as and are used as reducing agents and Lewis-acid catalysts in organic synthesis.
Properties of ytterbium compounds
Selected crystallographic and physical properties of ytterbium compounds are listed below. Lattice parameters refer to the reported crystalline modification and are given in nanometres.
Formula Appearance Crystal system Space group No. a (nm) b (nm) c (nm) Density (g/cm3) Ref. White Cubic Ia 206 1.043 — — 9.17 Transparent yellow Cubic Fmm 225 0.5598 — — — — Monoclinic C2/m 12 0.6729 1.1614 0.6312 — — Cubic Pmm 221 0.4144 — — 5.45 Black Cubic Fmm 225 0.7460 — — 4.84 — Tetragonal I41/amd 141 0.694 — 0.606 — YbS Black Cubic Fmm 225 0.5693 — — 6.68 — Trigonal R 148 0.69718 — 1.91037 — — Orthorhombic Pnma 62 0.5900 0.3576 0.6776 — — Tetragonal I4/mmm 139 0.3639 — 0.6110 — Grey Orthorhombic Pnma 62 0.72633 1.47806 0.77034 7.851 YbP — Cubic Fmm 225 0.5542 — — — — Cubic Pmm 221 0.4213 — — — — Tetragonal I4/mmm 139 0.4007 — 0.9858 —
Halides
Ytterbium forms halides in both the +2 and +3 oxidation states. The four trihalides , , and contain Yb, whereas the corresponding dihalides contain genuine Yb ions with the closed-shell 4f electronic configuration. This comparatively stable divalent state places ytterbium alongside europium and samarium among the lanthanides for which a complete series of conventional ionic dihalides is accessible.
The dihalides are reducing compounds and are readily oxidized to the corresponding trihalides. They can also disproportionate at elevated temperatures:
3 YbX2 → 2 YbX3 + Yb
where X is F, Cl, Br or I. Reduction of the heavier trihalides with ytterbium or other reducing metals provides convenient routes to the lower halides. For example, can be prepared by reducing with elemental ytterbium, whereas pure has been prepared by reacting ytterbium metal with followed by removal of zinc-containing impurities.
Ytterbium(II) fluoride, , crystallizes in the cubic fluorite structure, space group Fmm, analogous to the fluorides of alkaline-earth metals. Single-crystal studies of slightly non-stoichiometric confirm the fluorite framework and show that the divalent fluoride can accommodate limited oxidation of Yb to Yb accompanied by additional fluoride ions. adopts an orthorhombic structure related to the type at ambient conditions, and and form a continuous series of mixed crystals retaining this basic structural arrangement.
Ytterbium(II) iodide, , is particularly important as a soluble reducing reagent. It can be prepared under anhydrous conditions by reaction of ytterbium metal with 1,2-diiodoethane in tetrahydrofuran, and solutions of reduce a range of organic functional groups and promote coupling reactions of allylic and benzylic halides. Solid is polymorphic; layered structures related to the type are known, while a cubic modification has also been reported.
The trihalides show the structural changes characteristic of the late lanthanides. crystallizes in the orthorhombic YF3-type structure. Ytterbium(III) chloride, , has a layered monoclinic structure, space group C2/m, in which Yb ions are octahedrally coordinated by chloride and form nearly regular two-dimensional honeycomb layers.
The heavier trihalides and instead adopt the layered BiI3 structure type, with rhombohedral space group R. In both compounds, edge-sharing YbX6 octahedra generate two-dimensional honeycomb nets of Yb ions. Their weakly connected halide layers have led to considerable study of , and as low-dimensional magnetic materials.
Several ytterbium halides are also useful synthetic reagents. Ytterbium(III) chloride is a Lewis acid and has been used in catalytic carbon–carbon bond-forming reactions, including decarboxylative aldol reactions and the allylation of aldehydes with allyltrimethylsilane.
Chalcogenides
Oxides
The principal oxide of ytterbium is ytterbium(III) oxide, , in which ytterbium is trivalent. At room temperature it crystallizes in the cubic C-type rare-earth sesquioxide structure (bixbyite type), space group Ia. The structure can be regarded as an ordered-defect derivative of the fluorite structure, with one quarter of the anion sites vacant; the ytterbium atoms occupy two crystallographically distinct sites and are six-coordinate.Wells A. F. (1984) Structural Inorganic Chemistry, 5th edition, Oxford Science Publications, . High-temperature synchrotron diffraction shows that retains the cubic bixbyite structure up to its melting point, without the high-temperature structural transitions observed for some of the lighter lanthanide sesquioxides.
Ytterbium also forms the divalent monoxide ytterbium(II) oxide, YbO. It adopts the cubic rock salt structure, in contrast to the trivalent sesquioxide, and can be obtained by reduction of with ytterbium metal. YbO is considerably less stable than and is readily oxidized; thin films of the metastable monoxide have also been prepared by molecular-beam deposition under ytterbium-rich conditions.
Other chalcogenides
Ytterbium forms binary sulfides, selenides and tellurides in both divalent and trivalent compositions. The monochalcogenides YbS, YbSe and YbTe contain predominantly Yb with its filled 4f shell and adopt the cubic rock salt structure at ambient pressure. Their divalent character distinguishes them from most corresponding late-lanthanide monochalcogenides, in which the metal is normally trivalent.
Ytterbium(II) sulfide, YbS, is a black semiconducting solid. In addition to YbS, the ytterbium–sulfur system contains several sulfur-richer phases, including and . Magnetic measurements indicate predominantly trivalent ytterbium in the sesquisulfide , whereas the monosulfide retains the characteristic divalent state. Ytterbium sesquisulfide is polymorphic. One crystalline modification, T-, has the cubic bixbyite-type structure, space group Ia, with two crystallographically distinct Yb sites in distorted octahedral sulfur environments.
The corresponding sesquiselenide, , contains trivalent ytterbium and crystallizes in the orthorhombic space group Fddd, adopting the structure type. Its structure is an ordered-defect derivative of the rock salt structure and consists of edge-sharing, slightly distorted octahedra.
The electronic properties of the divalent monochalcogenides are strongly pressure-dependent. High-pressure X-ray diffraction of YbS and YbSe showed anomalous compression associated with a continuous increase of the ytterbium valence from Yb toward Yb. YbTe similarly undergoes a pressure-induced electronic transition associated with transfer of an ytterbium 4f electron into the conduction states.
Hydrides
Ytterbium forms binary hydrides over a relatively narrow composition range compared with most of the lanthanides. This behaviour arises from the unusual stability of divalent ytterbium with its filled 4f shell. Whereas most lanthanides readily absorb sufficient hydrogen to form trihydrides, further oxidation of Yb toward Yb is considerably less favourable.
The hydrogen-poor compound is ytterbium dihydride, , in which ytterbium is predominantly divalent. Unlike the fluorite-type dihydrides formed by most trivalent lanthanides, adopts an orthorhombic CaH2-type structure at ambient pressure. High-pressure diffraction studies show that it undergoes a structural transition at about 16 GPa to a phase in which the ytterbium atoms form a hexagonal close-packed arrangement.
Higher hydrogen contents can be obtained under increased hydrogen pressure and temperature. The best-characterized higher hydride has composition , equivalently . Its composition can be represented formally as , indicating the presence of both divalent and trivalent ytterbium. crystallizes at lower pressure in the trigonal space group P1m. The ytterbium atoms form a close-packed arrangement, while hydrogen occupies all tetrahedral interstices and two-thirds of the octahedral interstices.
Compression causes further structural changes without producing a conventional stoichiometric trihydride. The trigonal higher-hydride phase transforms between approximately 13 and 18 GPa to a tetragonal I4/m phase and, above about 27 GPa, to a higher-symmetry I4/mmm structure. Experiments in hydrogen, helium and neon pressure media showed broadly similar phase sequences, although samples compressed in hydrogen had slightly larger unit-cell volumes, consistent with limited additional hydrogen uptake.
Even under hydrogen pressures approaching 75 GPa, complete conversion to was not observed, and no ytterbium polyhydride was detected. The highest compositions remained close to , although slightly hydrogen-richer phases may occur under some conditions. The mixed-valent higher hydride remains semiconducting above 50 GPa, but its activation energy for electrical conduction decreases strongly with pressure, indicating that metallization may occur at still greater compression.
Borides
Ytterbium forms several binary borides with extended covalently bonded boron frameworks. As in other rare-earth borides, the structure changes substantially with increasing boron content. Particularly well-studied ytterbium borides include , and , several of which exhibit intermediate-valence or strongly correlated electronic behaviour.
Ytterbium tetraboride, , crystallizes in the tetragonal structure type, space group P4/mbm. Its structure contains octahedra linked by additional boron atoms and gives a two-dimensional arrangement of ytterbium sites related to the Shastry–Sutherland lattice. Resonant X-ray emission measurements show that ytterbium is intermediate-valent rather than purely divalent or trivalent: its average valence increases from about +2.61 at 12 K to +2.67 at 300 K. This intermediate valence has been associated with strong hybridization between the ytterbium 4f states and conduction electrons and helps suppress long-range magnetic ordering.