CaB2C2:: Reinvestigation of a semiconducting boride carbide with a layered structure and an interesting boron/carbon ordering scheme

Calcium diboride dicarbide, CaB2C2, was synthesized as a crystalline powder and investigated by electron energy loss spectroscopy, X-ray powder diffractometry, conductivity measurements, and LMTO band structure calculations. A new structure model was derived, and the crystal structure was refined by Rietveld methods in the tetragonal space group 14/mcm (No. 140, a = 537.33(1) pm and c = 741.55(2) pm, Z = 4). The boron and carbon atoms are well ordered within layers consisting of four- and eight-membered rings. A convincing coloring scheme is proven by the detection of a superstructure reflection. An earlier assignment of the compound into the LaB2C2 structure family (space group P (4) over bar 2c or P4(2)/mmc, respectively) has been shown to be incorrect. LMTO band structure calculations suggest semiconducting behavior for CaB2C2, which has been confirmed by conductivity measurements.