Lattice parameter of sodium close to the melting point

We report high precision lattice parameter measurements (Δa/a < 10^?6) of high purity sodium single crystals close to the melting transition. Data were taken by a neutron-double crystal-back-scattering device. Just below the melting point a decrease in the lattice parameter is observed. This effect is discussed in terms of the defect structure and the limit of stability in the crystalline solid state.