Experimental and theoretical aspects of the growth and equilibrium morphology of NaNO_3

摘要

Sodium nitrate (mineral nitratine, NaNO_3, space group R $ 3 $c at room temperature) is a material used in a great variety of fields, such as the molten salt technology. It is also of very interest from a crystallographic point of view, being iso-structural with calcite (CaCO _3). We performed several growth experiments in aqueous solution, near-equilibrium, with the aim of finding the ideal conditions for investigating both face-by-face growth kinetics and surface characterization, as well as the influence of K~+ and Li~+ ions on the growth morphology. Single {10.4} rhombohedral crystals have been systematically grown from pure aqueous solution, while {00.1} truncated rhombohedra were obtained in the presence of K~+ and Li~+ ions in the mother solution. When determining the theoretical equilibrium morphology of NaNO_3 crystals, the {00.1} and {10.4} surfaces were studied by using the 2D-slab model and relaxed and athermal surface energies (surfaces energies at T = 0 K) were evaluated at DFT level (B3LYP Hamiltonian) by using the CRYSTAL06 package. The {00.1} dipolar surfaces were reconstructed by the octopolar Hartman-Lacmann's model, already proposed for NaCl-like lattices. This type of reconstruction is the only one which respects the bulk symmetry of the crystal. The octopolar Na+ terminated {00.1} form resulted to be more stable than the NO_3- terminated one. However, the {00.1} form does not enter the theoretical athermal equilibrium shape of nitratine crystals, being λ_((10.4)Na)=160, λ_((00.1)Na)= 695 and λ_((00.1)NO3)=1535 erg/cm~2. Finally, a simple 2D-epitaxy model is proposed to explain the appearance of NaNO_3{00.1} truncated rhombohedra in the presence of K~+ and Li~+ ions.