  Using ab initio evolutionary structure prediction method in conjunction with
density functional theory, we performed a systematic investigation on the
structural transition of elemental scandium under pressure up to 250 GPa. Our
prediction successfully reproduced several allotropes which have been reported
in the previous literature, including the Sc-I, Sc-II and Sc-V. Moreover, we
observed a series of energetically degenerate and geometrically similar
structures at 110-195 GPa, which are likely to explain the unsolved phases III
and IV reported by Akahama [Phys. Rev. Lett.,94, 19, 195503, (2005)]. A
detailed comparison on powder X-ray diffraction pattern (PXRD) suggested that
the Ccca-20 phase may account for the observed Sc-III, while Sc-IV is likely to
be explained by a mixture of multiple energetically competing structures. We
also used the candidate Sc-III structure as the model system to explore its
superconducting behavior under pressures between 80-130 GPa. The predicted
superconducting transition temperature Tc values are in satisfactory agreement
with previous experimental results.
