Laboratory parallel-beam transmission X-ray powder diffraction investigation of the thermal behavior of nitratine NaNO3: Spontaneous strain and structure evolution

摘要

Present work provides in-situ structural data at a fine temperature scale from RT to the melting point of nitratine, NaNO3. From the analysis of log e33 versus log t plots, it is possible to prove that an univocal indication on the R3?c (low temperature, LT) → R3?m (high temperature, HT) transition mechanism cannot be obtained because of the relevant role played by the arbitrary assumptions required for defining the c0 dependence from temperature of the HT phase. This is due to the occurrence of excess thermal expansion for the HT phase. A significantly better fit for an Ising-spin structural model over a non-Ising rigid-body one has been obtained for the LT phase. Moreover, the Ising model led to a smooth variation of the oxygen site x fractional coordinate throughout the transition. The structure of the HT polymorph has been successfully refined considering an oxygen site at x, 0, 1/2, with 50% occupancy. Such model was the only acceptable one from the crystal chemical point of view as the alternative model (oxygen site at x, y, z with 25% occupancy) led to unrealistically aplanar NO-3 groups.