Lanthanum carbide

Lanthanum carbide ( La C₂) is a chemical compound. It is being studied in relation to the manufacture of certain types of superconductors and nanotubes.

Preparation

LaC₂ can be prepared by reacting lanthanum oxide, La₂O₃, with carbon in an electric furnace, or by melting pellets of the elements in an arc furnace.

Properties

LaC₂ reacts with water to form ethyne, C₂H₂ and a mixture of complex hydrocarbons.
LaC₂ is a metallic conductor, in contrast to CaC₂ which is an insulator.
The crystal structure of LaC₂ shows that it contains C₂ units with a C-C bond length of 130.3 pm, which is longer than the C-C bond length in calcium carbide, 119.2 pm, which is close to that of ethyne. The structure of LaC₂ can be described as La³⁺C₂²⁻(e-) where the electron enters the conduction band and antibonding orbitals on the C₂ anion, increasing the bond length. This is analogous to the bonding present in the nitridoborate, CaNiBN.

Lanthanum carbide in carbon nano structures

A method for making macroscopic quantities of C₆₀ and the confirmation of the hollow, cagelike structures was published in 1990 by Kratschmer and co-workers. This was followed by the publication of methods for higher fullerenes (C₇₀ and higher). In 1993, scientists discovered how to make a compound which is not as susceptible to moisture and air. They made containers to hold buckminsterfullerenes, or buckyballs; therefore they nicknamed the containers ‘buckyjars’.Wellesley Web Page
A few US patents were issued to universities in the mid-1990s; experiments with manufacturing techniques have continued at universities around the globe, including India, Japan, and Sweden.

Lanthanum atoms caged in fullerenes

In La@C₇₂, the lanthanum appears to stabilize the C₇₂ carbon cage. A 1998 study by Stevenson et al. verified the presence of La@C₇₂ as well as La₂@C₇₂, but empty-cage C₇₂ was absent, based on laser desorption mass spectrometry and UV−vis spectroscopy. A 2008 study by Lu et al. showed that La₂C₇₂ do not adhere to the isolated pentagon rule (IPR), but has two pairs of fused pentagons at each pole of the cage and that the two La atoms reside close to the two fused-pentagon pairs. This result lends additional support to the idea that the carbon cage is stabilized by the La atoms.

In addition to the properties included in the table at right, the magnetic properties of bulk amounts of La@C₈₂ (isolated from various hollow fullerenes) have been tested. Magnetization data for an isolated La@C₈₂ isomer were obtained using a SQUID magnetometer at temperatures ranging from 3 to 300 K. For La@C₈₂ the inverse susceptibility as a function of temperature was observed to follow a Curie-Weiss law. The effective magnetic moment per La@C₈₂ was found to be 0.38μ_B.

Lanthanum carbide has also shown superconductive properties when converted into a layered lanthanum carbide halide La₂C₂X₂ (X=Br,I). Investigations using high-resolution neutron powder diffraction measurements from room temperature to 1.5 Kelvin showed that it has superconductive properties at about 7.03 Kelvin for X=Br and at about 1.7 Kelvin for X=I, respectively.

References

External links

MIT Open Courseware 3.091 – Introduction to Solid State Chemistry2001 US Patent – Carbide nanomaterials.1997 US Patent – Storage of hydrogen in layered nanostructures.1996 US Patent – Metal, alloy, or metal carbide nanoparticles and a process for forming same.1995 US Patent – Magnetic metal or metal carbide nanoparticles and a process for forming same.

{{Carbides

Carbides
Lanthanum compounds
Acetylides
Electrides