Anisotropic Negative Thermal Expansion in high-pressure phases of ScF3

Abstract

Negative thermal expansion (NTE) is a counterintuitive phenomenon, in which materials undergo contraction as they are heated. ScF3, a well-known NTE material has been reported to show NTE coefficients up-to 1000K. At ambient conditions, ScF3 crystallizes in a cubic symmetry (Pm¯3m space group) same as that in the ReO3-type structures. Crystal-structure predictions (CSP) show that at P=1 GPa, a phase transition occurs in cubic ScF3 to form the rhombohedral phase (R¯3C space-group). Quasi-harmonic approximation (QHA) calculations at high pressure condition show that this new phase can show anisotropic NTE coefficients. The stability of this phase persists until 4 GPa. Beyond 4 GPa, the rhombohedral phase further undergoes a phase-transition into an orthorhombic-phase (Immm space group) having a non-corner-shared polyhedron network. This phase exhibits NTE along only one crystallographic axis, while the other two axes show no NTE response. On further increasing the pressure to 6 GPa, a trigonal-prismatic arrangement of ScF3 is obtained (R32 space group) which shows a reasonably better NTE than the previous phase due to the corner shared framework and remains stable till 9 GPa. All the phases show mechanical stability. Ab-initio molecular dynamics (AIMD) simulations show that both the cubic and the rhombohedral phases show bond-length elongation as well as deviation in dihedral angle confirming their NTE.