Soft-mode anisotropy in the negative thermal expansion material ReO3

We use a symmetry-motivated approach to analyze neutron pair distribution function data to investigate the character of the soft phonon modes in negative thermal expansion (NTE) material ${\mathrm{ReO}}₃$. This analysis shows that its local structure is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring-type deformations than in the related NTE compound ${\mathrm{ScF}}₃$. The lack of flexibility in ${\mathrm{ReO}}₃$ restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. These results support the idea that structural flexibility is an important factor in NTE materials. Surprisingly, our results show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the initial pressure-induced phase transition in ${\mathrm{ReO}}₃$. The result indicates that the dynamic motions associated with rigid unit modes are highly anisotropic in these systems.