Quantum-Mechanical and Thermodynamical Study on the (110) and Reconstructed (111) Faces of NaCl Crystals

The (110) and R1 reconstructed (111) face (Na- or Cl-terminated) of halite (NaCl) were studied; the R1 reconstruction was performed by removing 50% of ions in the outermost layer of the face. The structures of the (110), (111)R1 Na and (111)R1 Cl surfaces were determined by means of ab initio quantum mechanical calculations (density functional theory, DFT). The (111) R1 surfaces show higher surface relaxation with respect to the (110) surface. The surface energies (γ) at T = 0 K for relaxed and unrelaxed (110) and (111) R1 faces were determined at DFT level. The values of the surface energy for the relaxed faces are: γ(110) = 330, γ(111)R1 Na = 520 and γ(111)R1 Cl = 530 erg/cm2; therefore, the stability order of relaxed surfaces reads (110) < (111)R1 Na < (111)R1 Cl. For the unrelaxed faces the surface energies result to be higher: γ(110) = 387, γ(111)R1 Na = 825 and γ(111)R1 Cl = 769 erg/cm2; the stability order of the unrelaxed surfaces is (110) < (111)R1 Cl < (111)R1 Na. To check if the (111) R1 faces can belong to the equilibrium morphology of the crystal/vapor system, the relaxed surface energies at T > 0 K were calculated by considering both the vibrational motion of atoms and the surface configurational entropy. From these calculations it resulted that the (111)R1 Na and (111)R1 Cl faces cannot belong to the equilibrium morphology. Furthermore, it was also demonstrated that at room temperature the {110} and {111} forms cannot belong to the equilibrium shape of the NaCl crystal grown in pure aqueous solution. At equilibrium, the NaCl crystals can only show the {100} form.