Origin of the difference between the high and low-Tc phases in the yttrium sesquicarbide system

The refinement results of neutron powder diffraction data for the high- and low-Tc 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 dTc/dP were measured for the high-Tc phases (Tc≈15 K) and low-Tc phases (Tc≈11 K) by a clamp cell technique detecting the AC susceptibility of the sample. The Tc of Y2C3 systematically changes with applied hydrostatic pressure for both phases. However, the trends for the dTc/dPs are opposite: the value for the high-Tc phase is positive, and that for the low-Tc phase is negative. From these results, we concluded that the difference between the high-Tc and low-Tc phases in our Y2C3 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.