Thermal expansion and Grüneisen parameters of a polyoxymethylene crystal

The linear thermal expansion coefficient α of polyoxymethylene has been measured parallel (∥) and normal (⊥) to the symmetry axis, from 2 to 32 K, 55 to 90 K, and at 283 K. Below 100 K, α∥ is small and negative, while α⊥ is relatively large and positive. The heat capacity has been measured from 2 to 18 K and values have been calculated for the principal Grüneisen parameters γ, γ⊥, and γ∥. The first two are nearly constant at ∼+1.3 from 3 to 80 K, while γ∥ has a value of ∼−1.1 at helium temperatures but becomes positive at higher temperatures.