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 La 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 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 form; LaH10 is a high-temperature superconductor with a transition temperature near 250 K under megabar pressures.
Properties of lanthanum compounds
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- White Trigonal Pm1 164 0.3937 — 0.6130 6.6 White Trigonal Pc1 165 0.7185 — 0.7351 5.94 White Hexagonal P63/m 176 0.7478 — 0.4375 3.85 — Hexagonal P63/m 176 0.7970 — 0.4512 5.07 — Orthorhombic Cmcm 63 0.4425 1.3979 1.0048 5.55 Black Cubic Fmm 225 0.5667 — — — — Tetragonal P4/mbm 127 0.7324 — 0.4181 — — Cubic Pmm 221 0.4157 — — 4.72 — Tetragonal I4/mmm 139 0.3934 — 0.6572 — — Cubic I3d 220 0.8817 — — 6.08 LaN — Cubic Fmm 225 0.5300 — — 6.82 White Hexagonal P63/m 176 0.6547 — 0.3854 — — Monoclinic P21/n 14 — — — — — Hexagonal P6/mmm 191 0.502 — 0.398 —
Chalcogenides
Oxides
The principal oxide of lanthanum is lanthanum(III) oxide, , 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, , 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, , 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 I3d. 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 (0 ≤ x ≤ 1/3). Its electrical properties depend strongly on the concentration of lanthanum vacancies: contains an excess conduction electron and is metallic, whereas the vacancy-rich end member 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 adopt the Th3P4-type structure, with the lanthanum-vacancy concentration controlling the number of conduction electrons. Increasing x from compositions near toward 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 , , and . 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 or related salts instead tends to give oxyhalides such as .
Lanthanum fluoride, , is sparingly soluble in water and crystallizes in the trigonal tysonite structure, space group Pc1. 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. and both crystallize in the hexagonal structure type, space group P63/m, in which lanthanum is nine-coordinate in a tricapped trigonal-prismatic environment. instead adopts the orthorhombic 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 with lithium produces black crystals of and . has a layered 2H-MoS2-type structure with space group P63/mmc, whereas is monoclinic and isostructural with .
The iodide system contains an even wider range of reduced phases. is a metallic compound in which lanthanum remains essentially trivalent; it is commonly represented as with an additional electron occupying a delocalized conduction band rather than as a conventional La salt. It crystallizes in a tetragonal -type structure and has metallic electrical conductivity.
A further iodine-rich phase, , crystallizes in the monoclinic space group P21/m and is isostructural with . Still stronger reduction gives lanthanum monoiodide, LaI. It can be prepared by reducing or 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 and LaCl, together with a new high-pressure polymorph of . LaCl adopts an orthorhombic TlI-type structure, while has a tetragonal structure related to the anti- arrangement.
Hydrides
Lanthanum reacts readily and exothermically with hydrogen to form non-stoichiometric hydrides. The hydrogen-poor phase is based on lanthanum dihydride, , 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. 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 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 – found several composition ranges showing low-temperature cubic-to-tetragonal distortions and hydrogen ordering. Stoichiometric is conventionally described using a cubic structure related to the type, although experimental and theoretical studies indicate that lower-symmetry distortions can occur.
The unusual metallic state of 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 . In the predicted and experimentally supported cubic structure, space group Fmm, each lanthanum atom is enclosed by a cage of 32 hydrogen atoms.
is superconducting at very high pressure. Electrical-resistance measurements on samples with the cubic Fmm 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 () and lanthanum hexaboride (). Phase-equilibrium studies of the La–B system found to have only a narrow homogeneity range and to decompose peritectically at about 1800 °C, whereas 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.
crystallizes in the tetragonal 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 octahedra and chains of boron atoms, giving a strongly covalently bonded three-dimensional framework. LaB4 is metallic, hard and brittle.
is the better-known lanthanum boride. It crystallizes in the cubic CaB6 structure type, space group Pmm, with a lattice parameter of about 0.4157 nm. The structure consists of a simple cubic array of 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