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

Abstract

AbstractSodium nitrate (mineral nitratine, NaNO3, space group R $ \bar 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 (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)