The ideal commensurate value of Sc and the superconducting phase under high pressure

Abstract

The host–guest structure of scandium is described as being built of two penetrating substructures with the incommensurate periods of the channels along the c axis. We present the ideal commensurate value of 4/3 in Sc-II using ab initio calculations. We reveal that the 3cH and 4cG structures do interpenetrate and combine to the commensurate value of 4/3 of Sc-II at a pressure of 70 GPa. Ab initio molecular dynamics confirms the stability of the commensurate value 4/3 of the host–guest structure at 300 K and 72 GPa. The pressure-induced structural phase transitions in scandium under high pressure up to 200 GPa are investigated. We use ab initio random structure searching to predict the crystal structure of Sc-III: it is the tetragonal structure with space group P41212. Our calculations show that superconductivity arises in the P41212 structure. This high pressure structure is not only a superconducting phase but also has been reported for the first time in this group of elements.