Lanthanum oxide
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Lanthanum(III) oxide, also known as lanthana, chemical formula Template:Chem2, 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 a white solid that is insoluble in water, but dissolves in acidic solutions. Template:Chem2 absorbs moisture from air, converting to lanthanum hydroxide.[2] Lanthanum oxide has p-type semiconducting properties and a band gap of approximately 5.8 eV.[3] Its average room temperature resistivity is 10 kΩ·cm, which decreases with an increase in temperature. Template:Chem2 has the lowest lattice energy of the rare earth oxides, with very high dielectric constant ε = 27.
Structure
At low temperatures, Template:Chem2 has an A-Template:Chem2 hexagonal crystal structure. The Template:Chem2 metal atoms are surrounded by a 7 coordinate group of Template:Chem2 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.[4] On the other hand, at high temperatures lanthanum oxide converts to a C-Template:Chem2 cubic crystal structure. The Template:Chem2 ion is surrounded by six Template:Chem2 ions in a hexagonal configuration.[5][6]
Synthesis
Lanthanum oxide can crystallize in at least three polymorphs.[2]
Hexagonal Template:Chem2 has been produced by spray pyrolysis of lanthanum chloride.[7]
An alternative route to obtaining hexagonal Template:Chem2 involves precipitation of nominal Template:Chem2 from aqueous solution using a combination of 2.5% Template:Chem2 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 bismuth titanate (Template:Chem2 - BLT). Lanthanum oxide is used in optical materials; often the optical glasses are doped with Template:Chem2 to improve the glass' refractive index, chemical durability, and mechanical strength.[8]
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The addition of the Template:Chem2 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.
Potential applications
Lanthanum oxide is most useful as a precursor to other lanthanum compounds.[9] Neither the oxide nor any of the derived materials enjoys substantial commercial value, unlike some of the other lanthanides. Many reports describe efforts toward practical applications of Template:Chem2, as described below.
Template:Chem2Script error: No such module "anchor". forms glasses of high density, refractive index, and hardness. Together with oxides of tungsten, tantalum, and thorium, Template:Chem2 improves the resistance of the glass to attack by alkali. Template:Chem2 is an ingredient in some piezoelectric and thermoelectric materials.
Template:Chem2 has been examined for the oxidative coupling of methane.[10]
References
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