Pressure-induced Phase Transition Of Y₂C₃: A Density Functional Theory Study

In the past decades, the superconductors have attracted great attentionin both scientific and material fields. Furthermore, the development of relatedtheoretical and experimental researches has greatly broadened our sight on it.Even the Tc in hydride was expected to may exceed100K under highpressure. The yttrium sesquicarbide （Y2Y₂C₃3） was determined as a conventionalBY₂C₃S material with Tc up to18K. In1969, Krupka et al. synthesized Y2Y₂C₃3under extreme conditions （1.5-2.5GPa,1200-1500℃）, and it crystallize in abody centered cubic Pu2Y₂C₃3-type structure with eight formula units. We call itas α phase below. Its Wyckoff positions are16c （0.05,0.05,0.05） and24d（0.2821,0,0.25） for Y and Y₂C₃, respectively. They also measured the Tc was atthe range of6.0K to11.5K, even reached17K after doping. Followingresearches reported that Y2Y₂C₃3with higher Tc value （18K） was synthesizedsuccessfully at higher pressure. In fact, different synthesis conditions lead theparameters be a little discrepancy, which will influence stronglysuperconductivity. It is well known that high pressure can vary atomic packingand electronic structure in crystal, even different structure may behave novelphenomenon. If we acquire the accurate crystal structure under high pressure,it will be helpful for us to make clear other properties. On the other hand,because of the instability of Y2Y₂C₃3in air and the difficulty in synthesizingsingle-phase sample, it is not easy to make further study in experiment.In this paper, we mainly focus on the structure of Y2Y₂C₃3under highpressure and employ our own developed particle swarm optimization （PSO） technique on its crystal structure prediction,and have got the followinginnovative results.1. We report a new orthorhombic phase （Fmmm, Z=8） under high pressurepredicted by first principles calculations. The β-Y₂C₃energetically surpassesthe experimental phase for stability above31.9GPa. The phase transition isidentified as first-order in nature with volume contractions of6.5%. Duringthe reconstruction, carbon dimers break, and they form benzene likedhexatomic ring.2. The dynamical stability of the β-Y₂C₃has been checked by the phononcalculations at40and100GPa. The absence of any imaginary phononfrequency in the whole Brillouin Zone confirmed the dynamical stability ofthe new structure.3. β-Y₂C₃is easier to compress along a axis than along other axes, indicatingthe anisotropic property of orthorhombic structure.4. At ambient pressure, the carbon dimer in β-Y₂C₃with length of1.338istypical double bond. According to our results, as there is only a little decreaseof about0.01up to30GPa, the dimers is quite stable. With pressureincreasing, the dimers are broken, and the structure reconstruct.5. β-Y₂C₃shows a very strong metallicity, as the DOS at Fermi level, yttriumis a transition metal with4d electron, and its contribution to total DOS ismuch more than the others. Y-4d and C-2p play a major role in bodingbehavior and electronic properties, as it were.6. We roughly estimated that the superconducting transition temperature of β-Y₂C₃is23.5K. It confirms the β-Y₂C₃is a superconducting material at highpressure.