  Time-of-flight neutron powder diffraction and specific heat measurements were
used to study the nature of thermal expansion in rhenium trioxide, an
electrically conducting oxide with cubic symmetry. The temperature evolution of
the lattice parameters show that ReO3 can exhibit negative thermal expansion at
low temperatures and that the transition from negative to positive thermal
expansion depends on sample preparation; the single crystal sample demonstrated
the highest transition temperature, 300 K, and largest negative value for the
coefficient of thermal expansion, alpha = -1.1(1)x 10^-6 K^-1. For the oxygen
atoms, the atomic displacement parameters are strongly anisotropic even at 15
K, indicative of a large contribution of static disorder to the displacement
parameters. Further inspection of the temperature evolution of the oxygen
displacement parameters for different samples reveals that the static disorder
contribution is greater for the samples with diminished NTE behavior. In
addition, specific heat measurements show that ReO3 lacks the low energy
Einstein-type modes seen in other negative thermal expansion oxides such as
ZrW2O8.
