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.

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 O2 &minus;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 O2 &minus; 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).