Lanthanide compounds


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 number575859606162636465666768697071
Metal lattice dhcpfccdhcpdhcpdhcprbcchcphcphcphcphcphcphcphcp
DihydrideLaH2+xCeH2+xPrH2+xNdH2+xSmH2+xEuH2 o
"salt like"		GdH2+xTbH2+xDyH2+xHoH2+xErH2+xTmH2+xYbH2+x o, fcc
"salt like"		LuH2+x
StructureCaF2CaF2CaF2CaF2CaF2CaF2*PbCl2CaF2CaF2CaF2CaF2CaF2CaF2CaF2
metal sub latticefccfccfccfccfccfccofccfccfccfccfccfcco ''fccfcc
TrihydrideLaH3−xCeH3−xPrH3−xNdH3−xSmH3−xEuH3−xGdH3−xTbH3−xDyH3−xHoH3−xErH3−xTmH3−xLuH3−x
metal sub latticefccfccfcchcphcphcpfcchcphcphcphcphcphcphcphcp
Trihydride properties
transparent insulators
redbronze to greyPrH3−x fccNdH3−x hcpgolden greenishEuH3−x fccGdH3−x hcpTbH3−x hcpDyH3−x hcpHoH3−x hcpErH3−x hcpTmH3−x hcpLuH3−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,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 in character.

Hydroxides

All of the lanthanides form hydroxides, Ln3. 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 Lu3 are still basic hydroxides but will dissolve in hot concentrated NaOH.

Halides

Chemical elementLaCePrNdPmSmEuGdTbDyHoErTmYbLu
Atomic number575859606162636465666768697071
TetrafluorideCeF4PrF4NdF4TbF4DyF4
Color m.p. °Cwhite decwhite decwhite dec
Structure C.N.UF4 8UF4 8UF4 8-
TrifluorideLaF3CeF3PrF3NdF3PmF3SmF3EuF3GdF3TbF3DyF3HoF3ErF3TmF3YbF3LuF3
Color m.p. °Cwhite 1493white 1430green 1395violet 1374green 1399white 1306white 1276white 1231white 1172green 1154pink 1143pink 1140white 1158white 1157white 1182
Structure C.N.LaF3 9LaF3 9LaF3 9LaF3 9LaF3 9YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8YF3 8
TrichlorideLaCl3CeCl3PrCl3NdCl3PmCl3SmCl3EuCl3GdCl3TbCl3DyCl3HoCl3ErCl3TmCl3YbCl3LuCl3
Color m.p. °Cwhite 858white 817green 786mauve 758green 786yellow 682yellow decwhite 602white 582white 647yellow 720violet 776yellow 824white 865white 925
Structure C.N.UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9UCl3 9PuBr3 8PuBr3 8YCl3 6YCl3 6YCl3 6YCl3 6YCl3 6
TribromideLaBr3CeBr3PrBr3NdBr3PmBr3SmBr3EuBr3GdBr3TbBr3DyBr3HoBr3ErBr3TmBr3YbBr3LuBr3
Color m.p. °Cwhite 783white 733green 691violet 682green 693yellow 640grey decwhite 770white 828white 879yellow 919violet 923white 954white decwhite 1025
Structure C.N.UCl3 9UCl3 9UCl3 9PuBr3 8PuBr3 8PuBr3 8PuBr3 866666666
TriiodideLaI3CeI3PrI3NdI3PmI3SmI3EuI3GdI3TbI3DyI3HoI3ErI3TmI3YbI3LuI3
Color m.p. °Cyellow 766green 738green 784green 737orange 850dec.yellow 925957green 978yellow 994violet 1015yellow 1021white decbrown 1050
Structure C.N.PuBr3 8PuBr3 8PuBr3 8PuBr3 8BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6BiI3 6
DifluorideSmF2EuF2TmF2YbF2
Color m.p. °Cpurple 1417yellow 1416grey
Structure C.N.CaF2 8CaF2 8CaF2 8
DichlorideNdCl2SmCl2EuCl2DyCl2TmCl2YbCl2
Color m.p. °Cgreen 841brown 859white 731black dec.green 718green 720
Structure C.N.PbCl2 9PbCl2 9PbCl2 9SrBr2SrI2 7SrI2 7
DibromideNdBr2SmBr2EuBr2DyBr2TmBr2YbBr2
Color m.p. °Cgreen 725brown 669white 731blackgreenyellow 673
Structure C.N.PbCl2 9SrBr2 8SrBr2 8SrI2 7SrI2 7SrI2 7
DiiodideLaI2
metallic		CeI2
metallic		PrI2
metallic		NdI2
high pressure metallic		SmI2EuI2GdI2
metallic		DyI2TmI2YbI2
Color m.p. °Cbronze 808bronze 758violet 562green 520green 580bronze 831purple 721black 756yellow 780Lu
Structure C.N.CuTi2 8CuTi2 8CuTi2 8SrBr2 8
CuTi2 8		EuI2 7EuI2 72H-MoS2 6CdI2 6CdI2 6
Ln7I12La7I12Pr7I12Tb7I12
SesquichlorideLa2Cl3Gd2Cl3Tb2Cl3Er2Cl3Tm2Cl3Lu2Cl3
StructureGd2Cl3Gd2Cl3
SesquibromideGd2Br3Tb2Br3
StructureGd2Cl3Gd2Cl3
MonoiodideLaI
StructureNiAs 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 of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. They are all high melting and predominantly ionic in nature. The fluorides are only slightly soluble in water and are not sensitive to air, and this contrasts with the other halides which are air sensitive, readily soluble in water and react at high temperature to form oxohalides.
The trihalides were important as pure metal can be prepared from them. In the gas phase the trihalides are planar or approximately planar, the lighter lanthanides have a lower % of dimers, the heavier lanthanides a higher proportion. The dimers have a similar structure to Al2Cl6.

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+ 2. 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. these compounds should be considered to be two-dimensional metals. 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 of half filled 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+ 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. 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 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. 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. The sesquisulfides can be produced by reaction of the elements or sulfidizing the oxide 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 formulated 3 4, while others 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 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,. The majority of the monochalcogenides are conducting, indicating a formulation LnIIIQ2− 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.
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.
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− 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.
The nitrides can be prepared by the reaction of lanthanum metals with nitrogen. Some nitride is produced along with the oxide, when lanthanum metals are ignited in air. Alternative methods of synthesis are a high temperature reaction of lanthanide metals with ammonia or the decomposition of lanthanide amides, Ln3. Achieving pure stoichiometric compounds, and crystals with low defect density has proved difficult. The lanthanide nitrides are sensitive to air and hydrolyse producing ammonia.

Other pnictides

The other pnictides phosphorus, arsenic, antimony and bismuth also react with the lanthanide metals to form monopnictides, LnQ, where Q = P, As, Sb or Bi. Additionally a range of other compounds can be produced with varying stoichiometries, such as LnP2, LnP5, LnP7, Ln3As, Ln5As3 and LnAs2.

Carbides

Carbides of varying stoichiometries are known for the lanthanides. Non-stoichiometry is common. All of the lanthanides form LnC2 and Ln2C3 which both contain C2 units. The dicarbides with exception of EuC2, are metallic conductors with the calcium carbide structure and can be formulated as Ln3+C22−. The C-C bond length is longer than that in CaC2, which contains the C22− anion, indicating that the antibonding orbitals of the C22− anion are involved in the conduction band. These dicarbides hydrolyse to form hydrogen and a mixture of hydrocarbons. EuC2 and to a lesser extent YbC2 hydrolyse differently producing a higher percentage of acetylene.
The sesquicarbides, Ln2C3 can be formulated as Ln43. These compounds adopt the Pu2C3 structure which has been described as having C22− anions in bisphenoid holes formed by eight near Ln neighbours. The lengthening of the C-C bond is less marked in the sesquicarbides than in the dicarbides, with the exception of Ce2C3.
Other carbon rich stoichiometries are known for some lanthanides. Ln3C4 containing C, C2 and C3 units; Ln4C7 contain C atoms and C3 units and Ln4C5 containing C and C2 units.
Metal rich carbides contain interstitial C atoms and no C2 or C3 units. These are Ln4C3 ; Ln2C and Ln3C.

Borides

Diborides

Diborides, LnB2, have been reported for Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. All have the same, AlB2, structure containing a graphitic layer of boron atoms. Low temperature ferromagnetic transitions for Tb, Dy, Ho and Er. TmB2 is ferromagnetic at 7.2 K.

Tetraborides

Tetraborides, LnB4, have been reported for all of the lanthanides except EuB4, all have the same UB4 structure. The structure has a boron sub-lattice consists of chains of octahedral B6 clusters linked by boron atoms. The unit cell decreases in size successively from LaB4 to LuB4. The tetraborides of the lighter lanthanides melt with decomposition to LnB6. Attempts to make EuB4 have failed. The LnB4 are good conductors and typically antiferromagnetic.

Hexaborides

Hexaborides, LnB6, have been reported for all of the lanthanides. They all have the CaB6 structure, containing B6 clusters. They are non-stoichiometric due to cation defects. The hexaborides of the lighter lanthanides melt without decomposition, EuB6 decomposes to boron and metal and the heavier lanthanides decompose to LnB4 with exception of YbB6 which decomposes forming YbB12. The stability has in part been correlated to differences in volatility between the lanthanide metals. In EuB6 and YbB6 the metals have an oxidation state of +2 whereas in the rest of the lanthanide hexaborides it is +3. This rationalises the differences in conductivity, the extra electrons in the LnIII hexaborides entering conduction bands. EuB6 is a semiconductor and the rest are good conductors. LaB6 and CeB6 are thermionic emitters, used, for example, in scanning electron microscopes.

Dodecaborides

Lanthanide dodecaborides, LnB12, are formed by the heavier smaller lanthanides from Gd to Lu. With the exception YbB12, the dodecaborides are all metallic compounds. They all have the UB12 structure containing a 3 dimensional framework of cubooctahedral B12 clusters.

Higher borides

The higher boride LnB66 is known for all lanthanide metals. The composition is approximate as the compounds are non-stoichiometric. They all have similar complex structure with over 1600 atoms in the unit cell. The boron cubic sub lattice contains super icosahedra made up of a central B12 icosahedra surrounded by 12 others, B1212. Other complex higher borides LnB50 and LnB25 are known and these contain boron icosahedra in the boron framework.

Organolanthanide compounds

Lanthanide-carbon σ bonds are well known; however as the 4f electrons have a low probability of existing at the outer region of the atom there is little effective orbital overlap, resulting in bonds with significant ionic character. As such organo-lanthanide compounds exhibit carbanion-like behavior, unlike the behavior in transition metal organometallic compounds. Because of their large size, lanthanides tend to form more stable organometallic derivatives with bulky ligands to give compounds such as Ln. Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide compounds are also known, such as Cp*2Eu.