Negative thermal expansion of a crystal does not imply that every local bond shortens. In a neutron total-scattering study of cubic scandium trifluoride, $\mathrm{ScF_3}$, Wendt et al. (2019) found that the nearest-neighbor Sc–F pair-distribution peak shifted to slightly larger separation on heating while the average crystal structure contracted. A separate neutron-scattering and reverse Monte Carlo study by Dove et al. (2020) also found positive thermal expansion of the mean Sc–F bond distance together with a decreasing lattice parameter.

These are different quantities: a local interatomic separation and the periodic spacing of the average crystal structure. Transverse fluorine motion in Sc–F–Sc linkages provides a qualitative tension-effect explanation for their opposite trends. The observation concerns cubic $\mathrm{ScF_3}$ in its negative-expansion temperature regime. The two studies disagree about the detailed motion model, so the bond/lattice distinction should be separated from any claim that the microscopic mechanism is settled.

Sources: Wendt et al. (2019); Dove et al. (2020).