Pressure Induced Rhombohedral Phase in ScF3 shows Enhanced Negative Thermal Expansion

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 show that this new phase can show NTE coefficients for temperatures as high as 4500 K due to soft rigid-unit modes (RUM). 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 shows very poor NTE. On further increasing the pressure to 6 GPa, a trigonal-prismatic arrangement of ScF3 is obtained (R32 space group) which shows a reasonably good NTE phase due to the corner shared framework and remains stable till 9 GPa. Amongst the various pressure generated phases in ScF3, the rhombohedral phase shows the highest NTE behaviour. Ab-inito molecular dynamics (AIMD) simulations show that maximum volume contraction occurs for this R-3C phase due to facile bending of the ∠Sc-F-Sc causing framework softening.