Erbium carbide is an inorganic compound of erbium and carbon with the formula ErC2. At room temperature it crystallizes in the tetragonal calcium carbide structure type and contains C2 units characteristic of rare-earth dicarbides.
Preparation Erbium carbide can be prepared by arc melting stoichiometric amounts of metallic erbium and carbon:
Er + 2 C → ErC2
Arc melting was used to prepare phase-pure ErC2 for neutron-diffraction studies.
Thin films of ErC2 have also been grown directly on diamond. Reactive deposition of erbium onto hydrogen-terminated diamond at about 750 °C under a low pressure of methane gives an epitaxial erbium dicarbide layer.
Properties Crystal structure At room temperature, ErC2 crystallizes in the tetragonal crystal system, space group I4/mmm (No. 139), with the CaC2 structure type.
Neutron powder diffraction gives the lattice parameters a = 3.622(1) Å, c = 6.106(1) Å, with two formula units per unit cell.
The carbon atoms occur as discrete C2 units. Their C–C bond length is 1.275(3) Å, close to that of an acetylide C≡C bond. Mild hydrolysis of ErC2 also supports the presence of acetylide-like carbon pairs.
High-temperature phase Like many rare-earth dicarbides, ErC2 undergoes a tetragonal-to-cubic structural transition on heating.
Thermal and electrical measurements place the transition at approximately 1100–1200 °C, depending on experimental method.
The high-temperature form has a cubic fluorite-related structure in space group Fmm. In this phase the orientation of the C2 units becomes dynamically or statistically disordered.
Electronic properties ErC2 is metallic and has a comparatively low work function. Values around 2.9–3.0 eV have been reported for bulk or carbide-covered surfaces.
Its low work function and thermal stability have led to investigation of ErC2 as an electrode material in electronic devices.
Thin films and applications Epitaxial ErC2 films have been grown on boron-doped diamond (100). The ErC2(100) plane lies parallel to the diamond (100) surface, with the carbide lattice rotated by 45° relative to the diamond substrate.
ErC2 has also been deposited by atomic layer deposition on high-k dielectric materials. Films deposited on HfO2 showed an effective work function as low as about 3.9 eV and remained stable after high-temperature annealing.
The ALD-grown material was investigated as a possible low-work-function metal electrode for n-channel MOSFET technology.
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