Carbon Diffusion in Lanthanum Erbium Carbide Stabilized in the Cubic Fluorite Structure

Abstract

Lanthanum erbium carbide, La0.5Er0.5C2, a salt-like carbide with a cubic fluorite phase structure, has been produced from 13C, allowing carbon diffusion rate to be determined using 12C. Carbon in salt-like carbides exhibits significant ionicity, and a high carbon diffusion rate would enable a new class of high temperature fuel cells based on carbon-ion transport. The complete lack of carbon diffusion data for salt-like carbides is the motivation for this work. The carbon diffusion rate in La0.5Er0.5C2 has now been determined to be 2.0 ± 0.8 × 10−13 cm2/s at 850 °C, increasing to 1.8 ± 0.8 × 10−11 cm2/s at 1150 °C, with an activation energy of about 95 kJ/mole. These diffusion rates are too low for a carbon-ion fuel cell, but a number of other salt-like carbides exist. Be2C, in particular, is a salt-like carbide with an antifluorite structure, and should have higher carbon-ion diffusion than cubic La0.5Er0.5C2 due to the unoccupied octahedral sites in the antifluorite structure, but Be2C presents special difficulties due to the toxic nature of its hydrolysis products.