Measurement of the lattice parameter of sodium by neutron-back-scattering

We report high precision results (δa/a ≈ 3 x 10^-6) of the lattice parameter of high purity sodium (99.99%) in the temperature region from 32°C up to the melting point. Data of the crystal quality are also given: the half width of the angular mosaic spread amounts to about 4'. Preliminary results on vacancy formation energy and entropy, using literature data of volume expansion, are 0.35 ± 0.02 eV and (3.7 ± 0.5) k_B resp. for a single vacancy.