Lanthanide compounds are compounds formed by the 15 elements classed as lanthanides. The lanthanides are generally trivalent, although some, such as cerium and europium, are capable of forming compounds in other oxidation states.
Hydrides
Chemical elementLaCePrNdPmSmEuGdTbDyHoErTmYbLu Atomic number575859606162636465666768697071Metal lattice (RT)dhcp fcc dhcp dhcp dhcp r bcc hcp hcp hcp hcphcp hcp hcp hcpDihydrideLaH2+xCeH2+xPrH2+x NdH2+x SmH2+x EuH2 o "salt like"GdH2+x TbH2+x DyH2+x HoH2+x ErH2+x TmH2+x YbH2+x o, fcc "salt like" LuH2+xStructureCaF2CaF2CaF2CaF2CaF2CaF2*PbCl2 CaF2CaF2CaF2CaF2CaF2CaF2 CaF2metal sub latticefccfccfccfccfccfccofccfccfccfccfccfcco fccfccTrihydrideLaH3−xCeH3−x PrH3−xNdH3−x SmH3−x EuH3−x GdH3−xTbH3−x DyH3−x HoH3−xErH3−x TmH3−x LuH3−xmetal sub latticefccfccfcchcphcphcpfcchcphcphcphcphcphcphcphcpTrihydride properties transparent insulators (color where recorded)red bronze to grey PrH3−x fccNdH3−x hcp golden greenish EuH3−x fcc GdH3−x hcp TbH3−x hcpDyH3−x hcp HoH3−x hcpErH3−x hcp TmH3−x hcp LuH3−x hcp
Lanthanide metals react exothermically with hydrogen to form LnH2, dihydrides. With the exception of Eu and Yb, which resemble the Ba and Ca hydrides (non-conducting, transparent salt-like compounds), they form black pyrophoric, conducting compounds where the metal sub-lattice is face centred cubic and the H atoms occupy tetrahedral sites. Further hydrogenation produces a trihydride which is non-stoichiometric, non-conducting, more salt like. The formation of trihydride is associated with and increase in 8–10% volume and this is linked to greater localization of charge on the hydrogen atoms which become more anionic (H− hydride anion) in character.
Hydroxides
All of the lanthanides form hydroxides, Ln(OH)3. With the exception of lutetium(III) hydroxide, which has a cubic structure, they have the hexagonal UCl3 structure. The hydroxides can be precipitated from solutions of LnIII. They can also be formed by the reaction of the sesquioxide, Ln2O3, with water, but although this reaction is thermodynamically favorable it is kinetically slow for the heavier members of the series. Fajans' rules indicate that the smaller Ln3+ ions will be more polarizing and their salts correspondingly less ionic. The hydroxides of the heavier lanthanides become less basic, for example Yb(OH)3 and Lu(OH)3 are still basic hydroxides but will dissolve in hot concentrated NaOH.
Halides
+ Lanthanide halidesHolleman, p. 1942Chemical elementLaCePrNdPmSmEuGdTbDyHoErTmYbLu Atomic number575859606162636465666768697071Tetrafluoride CeF4PrF4NdF4 TbF4DyF4 Color m.p. °C white decwhite dec white dec Structure C.N.UF4 8UF4 8 UF4 8 TrifluorideLaF3CeF3PrF3NdF3PmF3SmF3EuF3GdF3TbF3DyF3HoF3ErF3TmF3YbF3LuF3Color m.p. °Cwhite 1493 white 1430green 1395violet 1374green 1399white 1306white 1276white 1231white 1172green 1154pink 1143pink 1140white 1158white 1157white 1182Structure C.N. LaF3 9 LaF3 9 LaF3 9 LaF3 9 LaF3 9 YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8TrichlorideLaCl3CeCl3PrCl3NdCl3PmCl3SmCl3EuCl3GdCl3TbCl3DyCl3HoCl3ErCl3TmCl3YbCl3LuCl3Color m.p. °Cwhite 858white 817green 786mauve 758green 786yellow 682yellow decwhite 602white 582white 647yellow 720violet 776yellow 824white 865white 925Structure C.N.UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9PuBr3 8 PuBr3 8YCl3 6YCl3 6YCl3 6YCl3 6YCl3 6TribromideLaBr3CeBr3PrBr3NdBr3PmBr3SmBr3EuBr3GdBr3TbBr3DyBr3HoBr3ErBr3TmBr3YbBr3LuBr3Color m.p. °Cwhite 783white 733green 691violet 682green 693yellow 640grey decwhite 770white 828white 879yellow 919violet 923white 954white decwhite 1025Structure C.N.UCl3 9UCl3 9UCl3 9PuBr3 8PuBr3 8PuBr3 8PuBr3 8 6 6 6 6 6 6 6 6TriiodideLaI3CeI3PrI3NdI3PmI3SmI3EuI3GdI3TbI3DyI3HoI3ErI3TmI3YbI3LuI3Color m.p. °C yellow 766green 738green 784green 737orange 850 dec. yellow 925 957 green 978yellow 994violet 1015yellow 1021white decbrown 1050Structure C.N.PuBr3 8PuBr3 8PuBr3 8PuBr3 8 BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6Difluoride SmF2EuF2 TmF2YbF2Color m.p. °C purple 1417yellow 1416 grey Structure C.N. CaF2 8CaF2 8 CaF2 8Dichloride NdCl2 SmCl2EuCl2 DyCl2 TmCl2YbCl2Color m.p. °C green 841 brown 859white 731 black dec. green 718green 720Structure C.N. PbCl2 9 PbCl2 9PbCl2 9 SrBr2 SrI2 7SrI2 7 Dibromide NdBr2 SmBr2EuBr2 DyBr2 TmBr2YbBr2Color m.p. °C green 725 brown 669white 731 black green yellow 673Structure C.N. PbCl2 9 SrBr2 8SrBr2 8 SrI2 7 SrI2 7SrI2 7 DiiodideLaI2 metallicCeI2 metallic PrI2 metallic NdI2 high pressure metallic SmI2EuI2 GdI2 metallic DyI2 TmI2YbI2Color m.p. °C bronze 808bronze 758violet 562 green 520green 580bronze 831 purple 721 black 756yellow 780LuStructure C.N.CuTi2 8CuTi2 8CuTi2 8 SrBr2 8 CuTi2 8 EuI2 7EuI2 7 2H-MoS2 6 CdI2 6 CdI2 6 Ln7I12La7I12 Pr7I12 Tb7I12 SesquichlorideLa2Cl3 Gd2Cl3 Tb2Cl3 Er2Cl3 Tm2Cl3 Lu2Cl3Structure Gd2Cl3 Gd2Cl3 Sesquibromide Gd2Br3 Tb2Br3 Structure Gd2Cl3 Gd2Cl3 Monoiodide LaI Structure NiAs type
Tetrahalides
Of the lanthanide tetrahalides, only the fluorides of cerium, praseodymium and terbium are well characterised.
Neodymium(IV) fluoride and dysprosium(IV) fluoride are also known under matrix conditions.
Trihalides
All lanthanides form trihalides: trifluorides, chlorides, tribromides, and triiodide. They are all dense, high melting solids. The bonding is described as highly ionic. The fluorides are only slightly soluble in water but the others are readily soluble in water. The chlorides, bromides, and iodides are susceptible to hydrolysis, giving lanthanide oxyhalides.
Dihalides Some of the dihalides are conducting while the rest are insulators. The conducting forms can be considered as LnIII electride compounds where the electron is delocalised into a conduction band, Ln3+ (X−)2(e−). All of the diiodides have relatively short metal-metal separations. The CuTi2 structure of the lanthanum, cerium and praseodymium diiodides along with HP-NdI2 contain 44 nets of metal and iodine atoms with short metal-metal bonds (393-386 La-Pr). these compounds should be considered to be two-dimensional metals (two-dimensional in the same way that graphite is). The salt-like dihalides include those of Eu, Dy, Tm, and Yb. The formation of a relatively stable +2 oxidation state for Eu and Yb is usually explained by the stability (exchange energy) of half filled (f7) and fully filled f14. GdI2 possesses the layered MoS2 structure, is ferromagnetic and exhibits colossal magnetoresistance.
Lower halides The sesquihalides Ln2X3 and the Ln7I12 compounds listed in the table contain metal clusters, discrete Ln6I12 clusters in Ln7I12 and condensed clusters forming chains in the sesquihalides. Scandium forms a similar cluster compound with chlorine, Sc7Cl12 Unlike many transition metal clusters these lanthanide clusters do not have strong metal-metal interactions and this is due to the low number of valence electrons involved, but instead are stabilised by the surrounding halogen atoms.
LaI is the only known monohalide. Prepared from the reaction of LaI3 and La metal, it has a NiAs type structure and can be formulated La3+ (I−)(e−)2.
Oxides
Monoxides
Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure. EuO is ferromagnetic at low temperatures, and is a semiconductor with possible applications in spintronics.Nasirpouri, Farzad and Nogaret, Alain (eds.) (2011) Nanomagnetism and Spintronics: Fabrication, Materials, Characterization and Applications. World Scientific. A mixed EuII/EuIII oxide Eu3O4 can be produced by reducing Eu2O3 in a stream of hydrogen. Neodymium and samarium also form monoxides, but these are shiny conducting solids, although the existence of samarium monoxide is considered dubious.
Sesquioxides
LaCePrNdPmSmEuGdTbDyHoErTmYbLu
All of the lanthanides form sesquioxides, Ln2O3. The lighter (larger) lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M2O3" structure. All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates.Adachi, G.; Imanaka, Nobuhito and Kang, Zhen Chuan (eds.) (2006) Binary Rare Earth Oxides. Springer. They dissolve in acids to form salts.
Dioxides
Lanthanide dioxides, LnO2, are only formed by Ce, Pr and Tb.
Other oxides
Praseodymium and terbium also form intermediate-valence oxides of varying stoichiometry. The most stable compound of praseodymium at room temperature is Pr6O11 and the most stable of compound of terbium at room temperature is Tb4O7. Cerium can also form intermediate-valence oxides such as Ce6O11 and Ce4O7.
Chalcogenides
All of the lanthanides form Ln2Q3 (Q= S, Se, Te). The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu2S3) sulfidizing the oxide (Ln2O3) with H2S. The sesquisulfides, Ln2S3 generally lose sulfur when heated and can form a range of compositions between Ln2S3 and Ln3S4. The sesquisulfides are insulators but some of the Ln3S4 are metallic conductors (e.g. Ce3S4) formulated (Ln3+)3 (S2−)4 (e−), while others (e.g. Eu3S4 and Sm3S4) are semiconductors. Structurally the sesquisulfides adopt structures that vary according to the size of the Ln metal. The lighter and larger lanthanides favoring 7-coordinate metal atoms, the heaviest and smallest lanthanides (Yb and Lu) favoring 6 coordination and the rest structures with a mixture of 6 and 7 coordination.
Polymorphism is common amongst the sesquisulfides. The colors of the sesquisulfides vary metal to metal and depend on the polymorphic form. The colors of the γ-sesquisulfides are La2S3, white/yellow; Ce2S3, dark red; Pr2S3, green; Nd2S3, light green; Gd2S3, sand; Tb2S3, light yellow and Dy2S3, orange. The shade of γ-Ce2S3 can be varied by doping with Na or Ca with hues ranging from dark red to yellow, and Ce2S3 based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.
All of the lanthanides form monochalcogenides, LnQ, (Q= S, Se, Te). The majority of the monochalcogenides are conducting, indicating a formulation LnIIIQ2−(e-) where the electron is in conduction bands. The exceptions are SmQ, EuQ and YbQ which are semiconductors or insulators but exhibit a pressure induced transition to a conducting state. Compounds LnQ2 are known but these do not contain LnIV but are LnIII compounds containing polychalcogenide anions.Holleman, p. 1944.
Oxysulfides Ln2O2S are well known, they all have the same structure with 7-coordinate Ln atoms, and 3 sulfur and 4 oxygen atoms as near neighbours.Liu, Guokui and Jacquier, Bernard (eds) (2006) Spectroscopic Properties of Rare Earths in Optical Materials, Springer Doping these with other lanthanide elements produces phosphors. As an example, gadolinium oxysulfide, Gd2O2S doped with Tb3+ produces visible photons when irradiated with high energy X-rays and is used as a scintillator in flat panel detectors. When mischmetal, an alloy of lanthanide metals, is added to molten steel to remove oxygen and sulfur, stable oxysulfides are produced that form an immiscible solid.
Pnictides
Nitrides
All of the lanthanides form a mononitride, LnN, with the rock salt structure. The mononitrides have attracted interest because of their unusual physical properties. SmN and EuN are reported as being "half metals". NdN, GdN, TbN and DyN are ferromagnetic, SmN is antiferromagnetic. Applications in the field of spintronics are being investigated. CeN is unusual as it is a metallic conductor, contrasting with the other nitrides also with the other cerium pnictides. A simple description is Ce4+N3− (e–) but the interatomic distances are a better match for the trivalent state rather than for the tetravalent state. A number of different explanations have been offered.Dronskowski, R. (2005) Computational Chemistry of Solid State Materials: A Guide for Materials Scientists, Chemists, Physicists and Others, Wiley,