Negative thermal expansion and local lattice distortion in the (Sc1-xTix)F3 and related solid solutions

Negative thermal expansion (NTE) is unusual but important property for control of thermal expansion. In the present study, the chemical modification is utilized to engineer controllable thermal expansion in cubic NTE ScF3. A broad window of the coefficient of thermal expansion (CTE = -1.51 ~ -3.4*10^-6 K-1, 300-800 K) has been achieved in Sc1-xMxF3 (M = Ti, Al and Ga). The long-range crystallographic structure of (Sc1-xTix)F3 adheres to the cubic Pm-3m symmetry according to the analysis of high-energy synchrotron X-ray powder diffraction. Pair distribution function (PDF) analysis of synchrotron X-ray total scattering was performed to investigate the local lattice distortion. It was found that the weakness of NTE has a close correlation with the local lattice distortion. Based on the coupled rotation model, it is presumably that this local distortion might dampen the transverse vibration of F atoms and thus reduce NTE. The present work provides a possible reference for the design of controllable NTE in open framework solids.