Structural phase transitions in CaC2.

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.