Lanthanum oxide


Lanthanum oxide, also known as lanthana, chemical formula La2O3, is an inorganic compound containing the rare earth element lanthanum and oxygen. It is used in some ferroelectric materials, as a component of optical materials, and is a feedstock for certain catalysts, among other uses.

Properties

Lanthanum oxide is an 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. La2O3 is hygroscopic; under atmosphere, it absorbs moisture over time and converts to lanthanum hydroxide. Lanthanum oxide has p-type semiconducting properties and a band gap of approximately 5.8 eV.. Its average room temperature resistivity is 10 kΩ·cm, which decreases with an increase in temperature. La2O3 has the lowest lattice energy of the rare earth oxides, with very high dielectric constant, ε = 27.

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−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. On the other hand, at high temperatures lanthanum oxide converts to a C-M2O3 cubic crystal structure. The La3+ ion is surrounded by six O2− ions in a hexagonal configuration.

Elements obtained from lanthana

Several elements were discovered as a consequence of lengthy analysis and decomposition of the ore gadolinite. As the ore was progressively analysed, the residue was first given the label ceria, then lanthana, and subsequently yttria, erbia, and terbia. In order of date discovered, the list of elements includes cerium, lanthanum, erbium, terbium, yttrium, ytterbium, holmium, thulium, scandium, praseodymium, neodymium and dysprosium. Several of these new elements were either discovered or isolated by Carl Gustaf Mosander in the 1830s and 1840s.

Synthesis

Lanthanum oxide can be crystallized in several polymorphs.
To produce hexagonal La2O3, a 0.1 M solution of LaCl3 is sprayed onto a preheated substrate, usually made of metal chalcogenides. The process can be viewed as occurring in two steps – hydrolysis followed by dehydration:
An alternative route to obtaining hexagonal La2O3 involves precipitation of nominal La3 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:
Other routes include:

Reactions

Lanthanum oxide is used as an additive to develop certain ferroelectric materials, such as La-doped Bi4Ti3O12.
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.
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. 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 and 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 piezoelectric and thermoelectric materials. Automobile exhaust-gas converters contain La2O3. La2O3 is also used in X-ray imaging intensifying screens, phosphors as well as dielectric and conductive ceramics. Gives off bright glow.
La2O3 has been examined for the oxidative coupling of methane.
La2O3 films can be deposited by many different methods, including chemical vapor disposition, atomic layer deposition, thermal oxidation, sputtering, and spray pyrolysis. Depositions of these films occur in a temperature range of 250–450 °C. Polycrystalline films are formed at 350 °C.
La2O3 tungsten electrodes are replacing thoriated tungsten electrodes in Gas tungsten arc welding due to safety concerns with thorium's radioactivity.