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// Workers AI · dad joke modeWhy did lanthanum compounds go to therapy? They had a lot of "element-ary" issues.

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From Wikipedia, the free encyclopedia
Lanthanum(III) acetate, a compound of lanthanum

Lanthanum compounds are compounds formed by the lanthanide metal lanthanum (La). Lanthanum occurs almost exclusively in the +3 oxidation state, and its compounds are generally strongly ionic because of the large size and electropositivity of the La3+ ion. This gives rise to extensive high-coordinate chemistry, particularly with hard donor atoms such as oxygen and halogens. Lanthanum forms numerous binary compounds, including oxides, halides, hydrides, chalcogenides, borides, carbides, silicides and pnictides, as well as a wide range of coordination, organometallic and intermetallic compounds.

Several lanthanum compounds have important physical or technological properties. Lanthanum hexaboride (LaB6) is used as a low-work-function thermionic electron emitter, while lanthanum bromide doped with cerium is a high-performance scintillator for gamma-ray detection. The intermetallic compound LaNi5 and related alloys reversibly absorb hydrogen and are used in hydrogen-storage and nickel–metal hydride battery materials. Lanthanum(III) oxide is used or investigated in catalytic and high-k dielectric materials, lanthanum fluoride is a constituent of fluoride optical glasses, and lanthanum carbonate is used medically as a phosphate binder. At very high pressures, hydrogen-rich phases such as LaH10 form; LaH10 is a high-temperature superconductor with a transition temperature near 250 K under megabar pressures.

Properties of lanthanum compounds

[edit]

Selected crystallographic properties of lanthanum 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.
A-La2O3 White Trigonal P3m1 164 0.3937 — 0.6130 6.6 [1]
LaF3 White Trigonal P3c1 165 0.7185 — 0.7351 5.94 [2]
LaCl3 White Hexagonal P63/m 176 0.7478 — 0.4375 3.85 [3]
LaBr3 — Hexagonal P63/m 176 0.7970 — 0.4512 5.07 [4]
LaI3 — Orthorhombic Cmcm 63 0.4425 1.3979 1.0048 5.55 [5]
LaH2 Black Cubic Fm3m 225 0.5667 — — — [6]
LaB4 — Tetragonal P4/mbm 127 0.7324 — 0.4181 — [7]
LaB6 — Cubic Pm3m 221 0.4157 — — 4.72 [8]
LaC2 — Tetragonal I4/mmm 139 0.3934 — 0.6572 — [9]
La2C3 — Cubic I43d 220 0.8817 — — 6.08 [9]
LaN — Cubic Fm3m 225 0.5300 — — 6.82 [10]
La(OH)3 White Hexagonal P63/m 176 0.6547 — 0.3854 — [11]
LaPO4 — Monoclinic P21/n 14 — — — — [12]
LaNi5 — Hexagonal P6/mmm 191 0.502 — 0.398 — [13]

Chalcogenides

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Oxides

[edit]
Lanthanum(III) oxide

The principal oxide of lanthanum is lanthanum(III) oxide, La2O3, a white, strongly hygroscopic solid. At ordinary temperatures it adopts the hexagonal A-type rare-earth sesquioxide structure, characteristic of the largest trivalent lanthanide ions. At high temperature it undergoes successive transformations to the hexagonal H-type and cubic X-type modifications. Calorimetric measurements place the A-to-H transition at about 2046 °C and the H-to-X transition at about 2114 °C; melting occurs at about 2301 °C.[14]

Lanthanum oxide reacts readily with water to form lanthanum hydroxide, La(OH)3, in a strongly exothermic reaction.[15] The hydroxide and oxide also absorb atmospheric carbon dioxide, ultimately forming basic carbonates and lanthanum carbonate.[16]

Other chalcogenides

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Lanthanum forms numerous binary chalcogenides with sulfur, selenium and tellurium, including monochalcogenides, sesquichalcogenides and chalcogen-rich polychalcogenides.[17] The equiatomic compounds LaS, LaSe and LaTe are known; like many early-lanthanide monochalcogenides, they have structures related to the cubic rock salt structure.[17]

Lanthanum sesquisulfide, La2S3, is polymorphic. Three principal modifications are traditionally distinguished: low-temperature α-La2S3, which is orthorhombic with space group Pnma; an intermediate tetragonal β-phase; and high-temperature γ-La2S3, which has a cation-deficient cubic Th3P4-type structure, space group I43d.[18] Single-crystal studies of γ-La2S3 confirm the defective Th3P4 structure, in which the lanthanum sites are statistically occupied. At the ideal La2S3 composition, one ninth of the available cation sites are vacant.[19][20]

The cubic sulfide phase forms part of the non-stoichiometric series La−3xS4 (0 ≤ x ≤ 1/3). Its electrical properties depend strongly on the concentration of lanthanum vacancies: La3S4 contains an excess conduction electron and is metallic, whereas the vacancy-rich end member La2S3 is semiconducting or insulating.[18] This tunable carrier concentration has led to extensive study of cubic lanthanum sulfides as high-temperature thermoelectric materials.[21]

A closely related vacancy-controlled system occurs among the tellurides. Compounds of composition La−3xTe4 adopt the Th3P4-type structure, with the lanthanum-vacancy concentration controlling the number of conduction electrons. Increasing x from compositions near La3Te4 toward La2Te3 changes the material from a heavily doped degenerate semiconductor toward a non-degenerate semiconductor.[22]

Chalcogen-rich phases can contain direct chalcogen–chalcogen bonding rather than being simple ionic salts.[17] Lanthanum disulfide, LaS2, occurs in at least two modifications; the low-temperature monoclinic α-form transforms reversibly to an orthorhombic β-form at about 750 °C.[23] Lanthanum ditelluride, LaTe2, contains distorted tellurium square nets with Te–Te bonding and crystallizes in a monoclinically distorted superstructure of the idealized anti-Fe2As arrangement.[24] Still more chalcogen-rich compounds, including LaSe2 and LaTe3, are also known.[17]

Halides

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Space-filling model of the crystal structure of lanthanum(III) bromide

Lanthanum forms the four trihalides LaF3, LaCl3, LaBr3 and LaI3. They can be prepared by direct reaction of lanthanum metal with the corresponding halogen, while high-purity anhydrous chlorides, bromides and iodides can also be obtained from hydrated salts using the corresponding ammonium halide to suppress hydrolysis during dehydration.[25][26] Direct heating of hydrated LaCl3 or related salts instead tends to give oxyhalides such as LaOCl.[16]

Lanthanum fluoride, LaF3, is sparingly soluble in water and crystallizes in the trigonal tysonite structure, space group P3c1. Its structure contains three crystallographically distinct fluoride sites and can be regarded as a close-packed lanthanum framework with fluoride ions occupying interstitial positions.[2] Under pressure, LaF3 undergoes a reconstructive structural transition at approximately 20 GPa.[27]

The heavier trihalides are hygroscopic and readily soluble in water. LaCl3 and LaBr3 both crystallize in the hexagonal UCl3 structure type, space group P63/m, in which lanthanum is nine-coordinate in a tricapped trigonal-prismatic environment.[4] LaI3 instead adopts the orthorhombic PuBr3 structure type, with eight-coordinate lanthanum centres arranged in layers.[5]

Reduction of the heavier trihalides gives several electron-rich subhalides. In the bromide system, reduction of LaBr3 with lithium produces black crystals of LaBr2 and La2Br5. LaBr2 has a layered 2H-MoS2-type structure with space group P63/mmc, whereas La2Br5 is monoclinic and isostructural with Pr2I5.[4]

The iodide system contains an even wider range of reduced phases. LaI2 is a metallic compound in which lanthanum remains essentially trivalent; it is commonly represented as La3+(I−)2(e−) with an additional electron occupying a delocalized conduction band rather than as a conventional La2+ salt.[28] It crystallizes in a tetragonal MoSi2-type structure and has metallic electrical conductivity.[29]

A further iodine-rich phase, La2I5, crystallizes in the monoclinic space group P21/m and is isostructural with Pr2I5.[30] Still stronger reduction gives lanthanum monoiodide, LaI. It can be prepared by reducing LaI2 or LaI3 with sodium metal at about 550 °C and exhibits metallic conductivity between 10 and 300 K.[31]

At very high pressures, still more reduced lanthanum chlorides have been obtained. Laser-heating lanthanum with sodium chloride at around 80 GPa produced La2Cl and LaCl, together with a new high-pressure polymorph of LaCl3. LaCl adopts an orthorhombic TlI-type structure, while La2Cl has a tetragonal structure related to the anti-LaI2 arrangement.[32]

Hydrides

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Lanthanum reacts readily and exothermically with hydrogen to form non-stoichiometric hydrides. The hydrogen-poor phase is based on lanthanum dihydride, LaH2, a black, brittle and pyrophoric solid with the cubic fluorite structure.[6] In this structure the lanthanum atoms form a face-centred cubic lattice and hydride ions occupy the tetrahedral interstices. LaH+2x can accommodate additional hydrogen in the octahedral interstices while largely retaining the metal sublattice.[33]

The electronic properties change strongly with hydrogen content. Dihydride-rich compositions are metallic conductors, whereas increasing the hydrogen concentration toward LaH3 causes the electrical conductivity to decrease by several orders of magnitude and gives semiconducting behaviour.[34] The LaH2–LaH3 system is not structurally simple: X-ray studies over LaH1.93–LaH2.90 found several composition ranges showing low-temperature cubic-to-tetragonal distortions and hydrogen ordering.[33] Stoichiometric LaH3 is conventionally described using a cubic structure related to the BiF3 type, although experimental and theoretical studies indicate that lower-symmetry distortions can occur.[35]

The unusual metallic state of LaH+2x has also been interpreted in terms of electride chemistry. Electronic-structure studies describe the fluorite-type lanthanum dihydrides as hydride-based electrides in which itinerant electrons occupy interstitial cavities surrounded by H− ions. Similar behaviour occurs in the corresponding cerium and yttrium hydrides.[36]

At megabar pressures, lanthanum forms much more hydrogen-rich compounds known as superhydrides. Heating lanthanum in excess hydrogen at pressures near 170 GPa produces a phase with a face-centred cubic lanthanum sublattice consistent with LaH10. In the predicted and experimentally supported cubic structure, space group Fm3m, each lanthanum atom is enclosed by a cage of 32 hydrogen atoms.[37]

LaH10 is superconducting at very high pressure. Electrical-resistance measurements on samples with the cubic Fm3m structure found a superconducting transition at about 250 K under pressures around 170 GPa. The assignment was supported by zero electrical resistance, suppression of the transition temperature by an applied magnetic field, and an isotope effect in the corresponding deuteride.[38] Subsequent calculations incorporating quantum nuclear motion support a highly symmetric cubic structure over much of the experimentally investigated pressure range.[39]

Borides

[edit]
CaB6-type crystal structure adopted by lanthanum hexaboride

Lanthanum forms two well-established binary borides at ambient pressure, lanthanum tetraboride (LaB4) and lanthanum hexaboride (LaB6). Phase-equilibrium studies of the La–B system found LaB4 to have only a narrow homogeneity range and to decompose peritectically at about 1800 °C, whereas LaB6 is stable over a small range of boron-rich compositions and remains stable to substantially higher temperatures.[40] Rare-earth borides are characterized by covalently bonded boron frameworks whose structures change markedly with boron content.[41]

LaB4 crystallizes in the tetragonal ThB4 structure type, space group P4/mbm, with four formula units per unit cell. Single-crystal measurements give lattice parameters of approximately a = 0.7324 nm and c = 0.4181 nm.[7] Its boron sublattice contains both B6 octahedra and chains of boron atoms, giving a strongly covalently bonded three-dimensional framework. LaB4 is metallic, hard and brittle.[42]

LaB6 is the better-known lanthanum boride. It crystallizes in the cubic CaB6 structure type, space group Pm3m, with a lattice parameter of about 0.4157 nm. The structure consists of a simple cubic array of B6 octahedra joined by B–B bonds, with lanthanum atoms occupying the large cavities between them.[8] LaB6 is metallic and combines a high melting temperature, low vapour pressure and chemical stability with a comparatively low electronic work function.[43]

The work function of LaB6 depends on crystallographic orientation and surface composition. Measurements on clean single-crystal faces generally give values in the range of about 2.4–2.9 eV, with low-index surfaces such as (100) showing particularly efficient electron emission.[44] This combination of low work function and low evaporation rate at elevated temperature makes LaB6 an important thermionic emitter, particularly as an electron source in electron microscopes and related instruments.[43]

Carbides and silicides

[edit]

Lanthanum forms two principal binary carbides, lanthanum dicarbide, LaC2, and lanthanum sesquicarbide, La2C3. The La–C phase diagram contains broad homogeneity ranges around the sesquicarbide composition, while LaC2 is stable close to the ideal 1:2 stoichiometry.[45] Both can be prepared by high-temperature reaction of lanthanum metal with carbon.[45]

At room temperature LaC2 adopts the tetragonal calcium carbide structure type, space group I4/mmm, with carbon present as C–C pairs. Neutron diffraction gave lattice parameters a ≈ 0.3934 nm and c ≈ 0.6572 nm, with a C–C distance of about 0.128 nm.[9] It is metallic and becomes superconducting below about 1.8 K, with properties consistent with weak-coupling BCS superconductivity.[46]

La2C3 crystallizes in the cubic Pu2C3 structure type, space group I43d, with eight formula units per unit cell. Its structure likewise contains C–C pairs, enclosed by distorted lanthanum coordination polyhedra.[9] La2C3 is superconducting, and carefully prepared carbon-deficient samples show transition temperatures around 13.2–13.3 K.[47] The cubic structure remains stable to at least 30 GPa at room temperature.[48]

Lanthanum also forms numerous binary silicides. The ambient-pressure La–Si phase diagram contains La5Si3, La3Si2, La5Si4, LaSi and silicon-deficient disilicide phases conventionally written LaSi−2x.[49] La5Si3 adopts the tetragonal Cr5B3 structure type, La3Si2 the U3Si2 type, La5Si4 the Zr5Si4 type and LaSi the orthorhombic FeB type.[49]

The disilicide region is structurally more complex. Two closely related phases occur near LaSi2, one with the α-GdSi2 structure type and the other with the α-ThSi2 structure type; both are appreciably silicon-deficient rather than strictly stoichiometric LaSi2.[49] LaSi and the α-GdSi2-type disilicide melt congruently at about 1620 and 1730 °C respectively, whereas the other intermediate silicides form peritectically.[49]

Much more silicon-rich lanthanum silicides can be stabilized at high pressure. At 13.5 GPa, the binary system contains superconducting LaSi5 and LaSi10, neither of which is stable under ordinary pressure–temperature conditions. LaSi5 occurs in two polymorphs; the α-form crystallizes in the monoclinic space group C2/m. Both high-pressure silicides contain extended silicon frameworks and exhibit superconductivity at low temperature.[50]

Pnictides

[edit]

Lanthanum forms the complete series of equiatomic monopnictides LaN, LaP, LaAs, LaSb and LaBi. At ambient pressure they adopt the cubic rock salt structure, space group Fm3m, with octahedral coordination of both elements.[51][52]

Lanthanum nitride, LaN, can be prepared as a polycrystalline powder by mechanochemical reaction under nitrogen. X-ray and electron diffraction confirm the rock-salt structure characteristic of the lanthanide mononitrides.[52] More nitrogen-rich lanthanum nitrides can be stabilized at high pressure. Reaction of lanthanum with supercritical nitrogen at about 30 GPa and 2000 K produced two phases assigned approximately as La2N+3x: a cubic phase related to the Mn2O3 structure and a trigonal phase with the same basic structure type as A-La2O3.[53]

LaP and LaAs are likewise rock-salt monopnictides. Their valence bands are dominated by pnictogen p states, while the lowest conduction states are primarily La 5d in character. The degree of overlap between these bands increases along the series toward the heavier pnictides, giving progressively more semimetallic behaviour.[54] LaAs is a compensated semimetal and exhibits extreme magnetoresistance at low temperature and high magnetic field, despite having no experimentally observed band inversion at ambient pressure.[55]

The heavier monopnictides LaSb and LaBi are semimetals with small, nearly compensated electron and hole pockets. LaSb displays extreme magnetoresistance: measurements on single crystals found an increase in resistivity of about one million percent at 9 T at low temperature, together with a low-temperature resistivity plateau.[56] Similar extreme magnetoresistance occurs in LaBi.[57]

The possible relation between this behaviour and electronic band topology has been extensively studied. Angle-resolved photoemission measurements show a clear inversion between La 5d and Bi 6p bands in LaBi, consistent with a topologically non-trivial semimetal, whereas LaSb lies close to the band-inversion boundary and LaAs shows no inversion at ambient pressure.[58] The occurrence of extreme magnetoresistance in topologically trivial LaAs indicates that band topology alone is not required for the effect; electron–hole compensation and carrier mobility also contribute substantially to the transport behaviour.[55]

Other inorganic compounds

[edit]
Lanthanum(III) nitrate

Lanthanum hydroxide, La(OH)3, is obtained by precipitation from aqueous La3+ solutions with hydroxide ions and is also formed rapidly when La2O3 is exposed to water. It crystallizes in the hexagonal space group P63/m, with two formula units per unit cell; each lanthanum centre is nine-coordinate in a tricapped trigonal-prismatic environment.[11]

Lanthanum nitrate is commonly isolated as the hexahydrate La(NO3)3·6H2O. Its dehydration is complex and depends on the water-vapour pressure. On further heating, nitrate-containing intermediates are formed; above about 350 °C the products include LaONO3 and La3O4NO3, followed ultimately by La2O3.[59]

Lanthanum oxalate forms the decahydrate La2(C2O4)3·10H2O. It crystallizes in the monoclinic space group P21/c, with two formula units per unit cell. Each La3+ ion is coordinated by three chelating oxalate groups and three water molecules, while additional water molecules occupy the spaces between the coordination layers.[60]

Lanthanum(III) sulfate, La2(SO4)3, forms several hydrates. The nonahydrate La2(SO4)3·9H2O crystallizes in a hexagonal structure with two crystallographically distinct lanthanum environments.[61] An octahydrate is also known; it crystallizes in the monoclinic space group Pn with two formula units per unit cell.[62]

Lanthanum carbonate occurs in several hydrated forms. Lanthanum carbonate octahydrate, La2(CO3)3·8H2O, has a layered structure in which carbonate groups link lanthanum centres and additional water molecules occupy the interlayer region.[63] A pentahydrate, La2(CO3)3·5H2O, has also been characterized; it crystallizes in the orthorhombic space group Pbca and contains related carbonate–lanthanum layers separated by a disordered interlayer region containing carbonate and water.[64]

Lanthanum phosphate, LaPO4, occurs in both hydrated and anhydrous forms. Hydrated lanthanum phosphate can adopt the hexagonal rhabdophane structure, conventionally written approximately LaPO4·0.5H2O, whereas heating above about 650 °C converts it to the monoclinic monazite structure, space group P21/n.[12] In monazite-type LaPO4, lanthanum is coordinated by nine oxygen atoms in a distorted polyhedron.[65]

Coordination compounds

[edit]
Structure of [La(acac)3(H2O)2], an eight-coordinate lanthanum(III) complex

Lanthanum(III), as the largest of the trivalent lanthanide ions, commonly forms complexes with high coordination numbers. Bonding is predominantly ionic and shows relatively little directional preference, so the coordination number and geometry are strongly influenced by ligand size, denticity and solvent. Hard oxygen donors are particularly common, although nitrogen- and mixed-donor ligands are also widely used. Coordination numbers from about 8 to 12 are well established for La3+.[66][67]

An example is the acetylacetonate complex [La(acac)3(H2O)2], in which three bidentate acetylacetonate ligands and two water molecules give an eight-coordinate LaO8 environment. X-ray crystallography shows that the coordination geometry is close to a square antiprism.[68]

The large size of La3+ also allows unusually high coordination numbers in nitrate complexes. Reaction of hydrated lanthanum nitrate with 2,2′:6′,2″-terpyridine in acetonitrile gives [La(terpy)(NO3)3(H2O)2], in which lanthanum is eleven-coordinate: the terpyridine contributes three nitrogen donors, the three nitrate groups are bidentate and two water molecules complete the coordination sphere. Under comparable conditions, the smaller lanthanides form analogous complexes with lower coordination numbers.[66] The composition and coordination geometry of lanthanum complexes can therefore depend strongly on the solvent and crystallization conditions.[67]

Organolanthanum compounds

[edit]

Organolanthanum compounds contain strongly polarized La–C bonds and are known with cyclopentadienyl, alkyl and other carbon-based ligands. Lanthanum normally remains in the +3 oxidation state in these compounds. As in organometallic compounds of the other lanthanides, the bonding has a substantial ionic component, although multihapto ligands such as cyclopentadienyl groups can form stable complexes.[69]

Cyclopentadienyl complexes

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The classical cyclopentadienyl compound La(C5H5)3 can be prepared by salt metathesis of anhydrous LaCl3 with sodium cyclopentadienide in tetrahydrofuran.[70] In the solid state, base-free La(C5H5)3 is not a simple discrete tris(cyclopentadienyl) molecule, but forms a polymeric structure. The cyclopentadienyl ligands exhibit several different modes of interaction with neighbouring lanthanum centres in addition to conventional η5 coordination.[71]

Bulkier substituted cyclopentadienyl ligands can instead stabilize soluble molecular complexes and leave space for additional ligands bound directly to lanthanum. Bis(pentamethylcyclopentadienyl) lanthanum halides and alkyl derivatives were among the early examples of such compounds.[72]

Alkyl and hydride complexes

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Well-defined La–C σ-bonded complexes can be stabilized by bulky ligand environments. For example, substituted lanthanocene alkyl complexes of the form [η−51,−3(Me3C)2C5H3]2La(CH2C6H−4o−NMe2) have been isolated by salt metathesis with the corresponding organolithium reagent. The polarized La–C bond is reactive toward small molecules: sulfur and selenium give thiolate and selenolate derivatives, while nitriles, carbodiimides and isothiocyanates undergo insertion reactions.[73]

Molecular lanthanum hydrides are likewise accessible when sufficiently bulky ligands prevent aggregation or decomposition. Hydrogenolysis of a pentaarylcyclopentadienyl-supported lanthanum dibenzyl complex gives the dinuclear hydride [(Cp(Ar5))La(−H)]2, in which hydride ligands bridge the two lanthanum centres.[74] Lanthanum alkyl and hydride complexes are also used as reactive intermediates and catalysts in carbon–carbon bond-forming and polymerization chemistry.[75]

Alloys and intermetallic compounds

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Lanthanum forms numerous intermetallic compounds with transition metals and other metallic elements. Well-characterized families include compounds of the La–Ni and La–Co systems, together with ternary iron-rich phases related to the NaZn13 structure.[13][76][77]

Lanthanum–nickel compounds

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Lanthanum-nickel alloys, particularly LaNi5, have been extensively studied because LaNi5 reacts reversibly with hydrogen. LaNi5 crystallizes in the hexagonal CaCu5 structure type, space group P6/mmm, with lanthanum occupying the 1a site and nickel occupying two crystallographically distinct sites, 2c and 3g. Typical lattice parameters are approximately a = 0.502 nm and c = 0.398 nm.[13]

The structure contains several interstitial sites that can accommodate hydrogen without destroying the underlying metal lattice. Near room temperature, LaNi5 can reversibly absorb approximately six to seven hydrogen atoms per formula unit, producing hydride compositions conventionally written near LaNi5H6.[13][78] Hydrogen occupies both tetrahedral and octahedral interstices, causing substantial expansion of the unit cell. The reaction is reversible at pressures close to ambient under suitable conditions, making LaNi5 and substituted AB5-type alloys important hydrogen-storage materials.[13]

Substitution of nickel by elements such as cobalt, manganese, aluminium or copper modifies the lattice dimensions and the thermodynamics of hydride formation. Such substituted LaNi5-type alloys have been widely used as hydrogen-storage electrodes in nickel–metal hydride batteries.[13]

Lanthanum–cobalt compounds

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The La–Co system contains several binary intermetallic phases. Reported compounds include La3Co, La2Co3, La2Co7, LaCo5 and LaCo13, together with additional intermediate compositions.[76] LaCo5 adopts the same hexagonal CaCu5 structure type as LaNi5, while LaCo13 crystallizes in the cubic NaZn13 structure type.[76]

The structures of several rare-earth–cobalt phases can be related through ordered replacement of rare-earth atoms by pairs of cobalt atoms. This produces families of structures at compositions between the RCo5 and cobalt-richer limits, including the 2:7 and 1:13 stoichiometries.[76]

Iron-rich intermetallics

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A binary compound of ideal composition LaFe13 is not stable at ambient pressure. Partial replacement of iron by silicon or aluminium, however, stabilizes compounds such as LaFe−13xSix and LaFe−13xAlx with the cubic NaZn13 structure type, space group Fm3c.[77] In these structures lanthanum occupies the 8a position, while iron and the substituting element occupy the transition-metal sublattice; one iron site is surrounded by an icosahedron of twelve neighbouring transition-metal atoms.[77]

The magnetic properties of LaFe−13xSix depend strongly on composition. Silicon-rich compositions can undergo first-order magnetostructural transitions accompanied by large changes in magnetic entropy, and related La(Fe,Si)13 materials have consequently been studied for magnetocaloric refrigeration.[77][79]

Applications

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Lanthanum hexaboride cathodes used as thermionic electron emitters

Lanthanum hexaboride is widely used as a thermionic electron emitter. Its low work function, high melting temperature and low evaporation rate allow LaB6 cathodes to operate at lower temperatures and with longer lifetimes than conventional tungsten emitters. Single-crystal and polycrystalline LaB6 cathodes are used as high-brightness electron sources in electron microscopes, electron-beam lithography systems and other electron-optical instruments.[43]

Cerium-doped lanthanum bromide, LaBr3:Ce, is a high-performance scintillator for X-ray and gamma-ray detection. It combines a high light yield of more than 60,000 photons per MeV with a decay time of about 16 ns and an energy resolution of approximately 3% at 662 keV.[80] LaBr3:Ce detectors are used in gamma-ray spectroscopy and have been applied in nuclear and particle physics, medical imaging, security screening and geophysical measurements.[80] Their use for very-low-background measurements is complicated by intrinsic radioactivity, principally from naturally occurring 138La.[81]

LaNi5 and related AB5 intermetallic compounds are used for reversible hydrogen storage. Their relatively rapid hydrogen absorption and desorption, good reversibility and operation near ambient temperature have also led to electrochemical applications.[13] Substituted LaNi5-type alloys form the negative-electrode material in many nickel–metal hydride batteries, while related materials have been investigated for hydrogen compression, purification and heat-management systems.[13]

Lanthanum carbonate is used medicinally as a non-calcium, non-aluminium phosphate binder. In the gastrointestinal tract, La3+ released from the salt binds dietary phosphate to form poorly soluble lanthanum phosphate, reducing phosphate absorption. It is used to control hyperphosphatemia in patients with chronic kidney disease, particularly those receiving dialysis.[82] Systematic reviews have found it effective in lowering serum phosphate, with a lower risk of hypercalcaemia than calcium-containing phosphate binders.[83]

Lanthanum compounds are also used in heterogeneous catalysis. Rare-earth-modified Y-type zeolites are important components of fluid catalytic cracking catalysts used in petroleum refining; incorporation of lanthanum and related rare-earth ions increases the hydrothermal stability and Brønsted acidity of the zeolite and can improve its resistance to deactivation.[84] Lanthanum(III) oxide itself has also been extensively investigated as a basic catalyst, including for the oxidative coupling of methane to ethane and ethylene.[85][86]

The high dielectric constant of La2O3 has led to extensive study of lanthanum-based oxides as high-k dielectric materials for metal–oxide–semiconductor devices. Their practical use is complicated by the hygroscopicity of La2O3, oxygen-related defects and interfacial reactions, and approaches including nitrogen incorporation, alloying with other high-k oxides and post-deposition treatment have been investigated to improve device performance.[87]

Lanthanum fluoride is a constituent of fluoride optical glasses. In particular, the widely used ZBLAN composition contains ZrF4, BaF2, LaF3, AlF3 and NaF. Its broad transparency range and relatively low phonon energy make ZBLAN suitable for mid-infrared optical fibres and as a host for rare-earth-doped fibre lasers and amplifiers.[88]

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See also

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References

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  1. ↑ Gouteron, J.; Michel, D.; Lejus, A. M.; Zarembowitch, J. (1981). "Raman spectra of lanthanide sesquioxide single crystals: Correlation between A and B-type structures". Journal of Solid State Chemistry. 38 (3): 288–296. Bibcode:1981JSSCh..38..288G. doi:10.1016/0022-4596(81)90058-X.
  2. 1 2 Zalkin, Allan; Templeton, David H. (1985). "Refinement of the trigonal crystal structure of lanthanum trifluoride with neutron diffraction data". Acta Crystallographica Section B. 41 (2): 91–93. Bibcode:1985AcCrB..41...91Z. doi:10.1107/S0108768185001689.
  3. ↑ Morosin, B. (1968). "Crystal Structures of Anhydrous Rare-Earth Chlorides". The Journal of Chemical Physics. 49 (7): 3007–3012. Bibcode:1968JChPh..49.3007M. doi:10.1063/1.1670543.
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