The effect of pressure on the T c value of Y2C3 compounds

We have improved the preparation technique to obtain a single-phase sample of Y2C3, and the effect of hydrostatic pressure on the superconducting transition temperature, T c, of the sample was investigated. The single-phase samples of Y2C3 were synthesized from an arc-melted precursor under high-pressure (3.5 GPa at 1400 deg. C for 30 min) using a BN crucible. The reproducibility of T c was low, and they were roughly classified into high T c phases (T c around 14 ∼ 18 K) and low T c phases (T c around 6 ∼ 12 K). The lattice parameter was constant for all samples (a ∼ 8.235 A). We measured the dependence of T c on applied hydrostatic pressure for both high T c (T c ∼ 15 K) and low T c phases (T c ∼ 11 K) in order to clarify the difference between these phases. The T cs of both phases change linearly with the applied pressure, but the gradients, dT c/dP, show opposite trends; they were positive for high T c and negative for low T c phases. The high T c phase is believed to be Y2C3 with a shorter lattice parameter than that of the low T c phase. However, our results show that a high T c is not obtained by a simple contraction of the unit cell of the low T c phase. We conclude that the differences between the high T c and low T c phases are due to a intrinsic change in the electronic state of the Y2C3 phase