Experimental and theoretical aspects of the growth and equilibrium morphology of NaNO3

Sodium nitrate (mineral nitratine, NaNO3, space group Rc 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 (CaCO3). 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 NaNO3 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 NO3– terminated one. However, the {00.1} form does not enter the theoretical athermal equilibrium shape of nitratine crystals, being γ(10.4)Na=160, γ(00.1)Na= 695 and γ(00.1)NO3=1535 erg/cm2. Finally, a simple 2D-epitaxy model is proposed to explain the appearance of NaNO3{00.1} truncated rhombohedra in the presence of K+ and Li+ ions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)