Interaction of lanthanum with boron and carbon: phase diagram and structural chemistry

摘要

The isothermal section of the La-B-C phase diagram at 1,270 K has been investigated by means of X-ray, neutron powder diffraction, microstructure and EPMA analyses. Eight ternary compounds were found, and for six of them, the crystal structures have been established. The phase with the structure type of La5B2C6 has a broad homogeneity range described by the formula La5(BC)_x (5.6 ≤ x ≤8.8). The lanthanum sesquicarbide La2C3 exhibits an extended solid solution in the ternary domain La2C_(3-x)B_x (x = 0.4). The boron substitution of carbon leads to the decrease of the superconducting temperature from 13.4 K for La2C3 to 10.0 K for La2C_(2.8)B_(0.2) and from 5.6 K for La2C_(2.7) to 4.1 K for La2C_(2.6)B_(0.4). The compositions of two new compounds ～La4B3C_(12) and ～La4B5C_(18) were found via WDX analysis.