Lanthanum compounds

Chalcogenides

Oxides

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.

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

Other chalcogenides

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

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. 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.

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. This tunable carrier concentration has led to extensive study of cubic lanthanum sulfides as high-temperature thermoelectric materials.

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.

Chalcogen-rich phases can contain direct chalcogen–chalcogen bonding rather than being simple ionic salts. 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. 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. Still more chalcogen-rich compounds, including LaSe₂ and LaTe₃, are also known.

Halides

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. Direct heating of hydrated LaCl₃ or related salts instead tends to give oxyhalides such as LaOCl.

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. Under pressure, LaF₃ undergoes a reconstructive structural transition at approximately 20 GPa.

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. LaI₃ instead adopts the orthorhombic PuBr₃ structure type, with eight-coordinate lanthanum centres arranged in layers.

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.

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. It crystallizes in a tetragonal MoSi₂-type structure and has metallic electrical conductivity.

A further iodine-rich phase, La₂I₅, crystallizes in the monoclinic space group P2₁/m and is isostructural with Pr₂I₅. 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.

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.

Hydrides

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. 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.

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. The LaH₂–LaH₃ system is not structurally simple: X-ray studies over LaH_1.93–LaH_2.90 found several composition ranges showing low-temperature cubic-to-tetragonal distortions and hydrogen ordering. 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.

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.

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.

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. Subsequent calculations incorporating quantum nuclear motion support a highly symmetric cubic structure over much of the experimentally investigated pressure range.

Borides

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. Rare-earth borides are characterized by covalently bonded boron frameworks whose structures change markedly with boron content.

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. 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.

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. LaB₆ is metallic and combines a high melting temperature, low vapour pressure and chemical stability with a comparatively low electronic work function.

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. 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.

Carbides and silicides

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. Both can be prepared by high-temperature reaction of lanthanum metal with carbon.

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. It is metallic and becomes superconducting below about 1.8 K, with properties consistent with weak-coupling BCS superconductivity.

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. La₂C₃ is superconducting, and carefully prepared carbon-deficient samples show transition temperatures around 13.2–13.3 K. The cubic structure remains stable to at least 30 GPa at room temperature.

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. La₅Si₃ adopts the tetragonal Cr₅B₃ structure type, La₃Si₂ the U₃Si₂ type, La₅Si₄ the Zr₅Si₄ type and LaSi the orthorhombic FeB type.

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. LaSi and the α-GdSi₂-type disilicide melt congruently at about 1620 and 1730 °C respectively, whereas the other intermediate silicides form peritectically.

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.

Pnictides

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.

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. 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.

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. 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.

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. Similar extreme magnetoresistance occurs in LaBi.

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. 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.

Other inorganic compounds

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.

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.

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.

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. An octahydrate is also known; it crystallizes in the monoclinic space group Pn with two formula units per unit cell.

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. 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.

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. In monazite-type LaPO₄, lanthanum is coordinated by nine oxygen atoms in a distorted polyhedron.

Coordination compounds

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⁺.

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.

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. The composition and coordination geometry of lanthanum complexes can therefore depend strongly on the solvent and crystallization conditions.

Organolanthanum compounds

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.

Cyclopentadienyl complexes

The classical cyclopentadienyl compound La(C₅H₅)₃ can be prepared by salt metathesis of anhydrous LaCl₃ with sodium cyclopentadienide in tetrahydrofuran. 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.

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.

Alkyl and hydride complexes

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.

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. Lanthanum alkyl and hydride complexes are also used as reactive intermediates and catalysts in carbon–carbon bond-forming and polymerization chemistry.

Alloys and intermetallic compounds

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.

Lanthanum–nickel compounds

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.

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. 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.

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.

Lanthanum–cobalt compounds

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

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.

Iron-rich intermetallics

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. 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.

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.

Applications

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.

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. 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. Their use for very-low-background measurements is complicated by intrinsic radioactivity, principally from naturally occurring ¹³⁸La.

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. 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.

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. Systematic reviews have found it effective in lowering serum phosphate, with a lower risk of hypercalcaemia than calcium-containing phosphate binders.

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. Lanthanum(III) oxide itself has also been extensively investigated as a basic catalyst, including for the oxidative coupling of methane to ethane and ethylene.

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.

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.