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The fabrication conditions of a high-pressure synthesis using a BN crucible have been established for the high-T c (T c 414 K) and the low-T c (T c o12 K) phases of the yttrium sesquicarbide (Y 2 C 3 ). The single-phase sample was synthesized from an arc-melted precursor. In contrast, Y 2 C 3 samples made from a powder precursor were not single-phase, although the reproducibilities of the high-T c phase and the highest value of T c were higher.The refinement results of neutron powder diffraction data for the high-and low-T c phases indicate a small difference between the two phases in the interatomic distance of the C-C dimer, while the lattice parameter was constant for all samples. The values of dT c /dP were measured for the high-T c phases (T c E15 K) and low-T c phases (T c E11 K) by a clamp cell technique detecting the AC susceptibility of the sample. The T c of Y 2 C 3 systematically changes with applied hydrostatic pressure for both phases. However, the trends for the dT c /dPs are opposite: the value for the high-T c phase is positive, and that for the low-T c phase is negative. From these results, we concluded that the difference between the high-T c and low-T c phases in our Y 2 C 3 samples did not originate in a simple contraction of the unit cell, but in the difference in the interatomic distances of the C-C dimers. r 2006 Published by NIMS and Elsevier Ltd.

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