ScF3 is a material with the ReO3 structure which shows negative thermal expansion. This experiment will use implanted muons to create an F-mu-F state in this structure and then follow this spectroscopically to measure the change in the muon fluorine bond length, and F-mu-F bond angle, as a function of temperature. Our experimental data will be complemented by detailed "DFT+\mu" calculations carried out by fixing the lattice constant to different values to simulate the effect of temperature.This experiment could open up the use of the muon more widely in investigating local structural (or even quantum) effects in materials with unusual thermal expansion properties.