Lanthanum(III) oxide

Lanthanum(III) oxide is La₂O₃, a chemical compound containing the rare earth element lanthanum and oxygen. It is used to develop ferroelelectric material, and in optical materials. Production is on laboratory scale, mostly.

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Properties

La₂O₃ has largest band gap of the rare earth oxides at 4.3 eV, while also having the lowest lattice energy, with very high dielectric constant, ε = 27 pF/m. La₂O₃ is widely used in industry as well as in the research laboratory. Lanthanum oxide is an amorphous, odorless, white solid that is insoluble in water, but soluble in dilute acid. Depending on the pH of the compound, different crystal structures can be obtained. Lanthanum oxide has p-type semi-conducting properties because its resistivity decreases with an increase in temperature, average room temperature resistivity is 10³ Ω cm.

Structure

At low temperatures, La₂O₃ has an A-M₂O₃ hexagonal crystal structure. The La³⁺ metal atoms are surrounded by a 7 coordinate group of O^2-atoms, the oxygen ions are in an octahedral shape around the metal atom and there is one oxygen ion above one of the octahedral faces (Wells 546). On the other hand, at high temperatures the Lanthanum oxide converts to a C-M₂O₃ cubic crystal structure. The La³⁺ ion is surrounded by a 6 coordinate group of O^2- ions.

Synthesis

Different crystalline forms of lanthanum oxide have been prepared.

To produce hexagonal La₂O₃, a 0.1 M solution of LaCl₃ is sprayed onto a preheated substrate, usually made of metal chalcogenides (Kale 3007). The process can be viewed as occurring in two steps - hydrolysis followed by dehydration:

2 LaCl₃ + 3 H₂O → La(OH)₃ + 3 HCl

2 La(OH)₃ + heat → La₂O₃ + 3 H₂O

An alternative route to hexagonal La₂O₃ involves precipitation of nominally La(OH)₃ from aqueous solution using a combination of 2.5% NH₃ and the surfactant sodium dodecyl sulfate followed by heating and stirring for 24 hours at 80 °C:

2LaCl₃+ 3 H₂O + 3 NH₃ → La(OH)₃ + 3 NH₄Cl

LaCl₃.3H₂O → La₂O₃

Other routes include:

2La₂S₃ + 3CO₂ → 2La₂O₃ + 3CS₂

2La₂(SO₄)₃ + read heat → 2La₂O₃ + 6SO₃

Reactions

Lanthanum oxide is used to develop ferroelectric materials, such as La-doped Bi₄Ti₃O₁₂ (BLT). Lanthanum oxide is used in optical materials, often the optical glasses are doped with La₂O₃ to improve the glass’ refractive index, chemical durability, and mechanical strength.

3B₂O₃ + La₂O₃ → 3La₂O₃•B₂O₃

When this 1:3 reaction is mixed into a glass composite, the high molecular weight of the lanthanum causes an increase of the homogeneous mixture of the melt which leads to a lower melting point (Vinogradova 1). The addition of the La₂O₃ to the glass melt leads to a higher glass transition temperature from 658 °C to 679 °C. The addition also leads to a higher density, microhardness, and refractive index of the glass.

Uses & Applications

La₂O₃ is used to make optical glasses, to which this oxide confers increased density, refractive index, and hardness. Together with oxides of tungsten, tantalum, and thorium, La₂O₃ improves the resistance of the glass to attack by alkali. La₂O₃ is an ingredient for the manufacture of devices for piezoelectricity, galvanothermy, and thermoelectricity material. Automobile exhaust-gas converters contain La₂O₃ (Cao 408). La₂O₃ is also used in X-Ray imaging intensifying screens, phophors as well as dielectric and conductive ceramics.

La₂O₃ has been examined for the oxidative coupling of methane (Maoilova 15770).

La₂O₃ films can be deposited by many different methods, including: chemical vapor disposition, thermal oxidation, sputtering, and spray pyrolysis. Depositions of these films occur in a temperature range of 523–723 K (Kale 3007). Polycrystalline films are formed at 623 K (Kale 3008).

Sources

• Maoilova, O. V. “Surface Acidity and Basicity of La2O3, LaOCl, and LaCl3 Characterized by IR Spectroscopy, TPD, and DFT Calculations.” Journal of Physical Chemistry. 180 (1980):15770-16781.
• Bedoya, C. “MOCVD of Lanthanum Oxides from La(tmhd)3 and La(tmod)3 Precursors: A Thermal and Kinetic Investigation.” Chemical Vapor Deposition. 12 (2006): 46-53.
• Vinogradova, L. N. “Glass Transition and Crystallization of Glasses Based on Rare-Earth Borates.” Glass Physics and Chemistry. 30 (2004): 1-5.
• Kale, S.S. “Characterizations of spray-deposited lanthanum oxide (La2O3) thin films.” Materials Letters. 59 (2005): 3007-3009.
• Imanaka, Nobuhito. “Preparation of the cubic-type La2O3 phase by thermal decomposition of LaI3.” Journal of Solid State Chemistry. 178 (2005): 395-398.
• Cao, Jieming. “Controllable syntheses of hexagonal and lamellar mesostructured lanthanum oxide.” Materials Letters. 59 (2005): 408-411.
• Wyckoff, R. W.G. Crystal Structures: Inorganic Compounds RXn, RnMX2, RnMX3. New York: Interscience Publishers (1963).
• “Lanthanum oxide.” The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biological. 11th ed. 1989.
• Veldurthy, B. “Magnesium-Lanthanum Mixed Metal Oxide: A Strong Solid Base for the Michael Addition Reaction.” Adv. Synth. Catal. 347 (2005): 767-771.
• Wells, A.F. Structural Inorganic Chemistry. Oxford: Clarendon Press, 1984.
• "Lanthanum oxide." Encyclopedia of Chemical Reactions. 1951.
• R and S data source.

References