Theoretical structure and surface energy of the reconstructed {01.2} form of calcite (CaCO3) crystal

Two different reconstructions of the (01.2) face (Ca or CO(3) terminated) of calcite (CaCO(3)) were studied: (i) R1 reconstruction: the outermost layer is based on the [010] x 1/3[211] rectangular mesh, which is symmetrical with respect to the c glide plane of the crystal, thus fulfilling the 2D symmetry of the face and (ii) R2 reconstruction: the outermost layer is based on a 1/6[42 (1) over bar] x 1/6[2 (2) over bar1] lozenge shaped mesh that does not respect the 2D symmetry of the face. The (0 1.2)(R1)(Ca), (0 1.2)(R1)(CO3), (01.2)(R2)(Ca) and (01.2)(R2)(CO3) slabs geometry optimizations of calcite (CaCO(3)) were performed either at DFT level or by using empirical potentials; the results obtained with these two different Calculation methodologies are in good agreement. With respect to their arrangement in the bulk, the CO(3) groups of the outermost layer are significantly rotated about the crystallographic a-axis and about the normal to the 01.2 plane; further, the thickness of the outermost layer is significantly lower than that of the underneath ones. The surfaces energies (gamma) at 0 K, for relaxed and unrelaxed (01.2)(R1)(Ca), (01.2)(R1)(CO3), (01.2)(R2)(Ca) and (01.2)(R2)(CO3) faces, were determined either at DFT level or by using empirical potentials. Independently of the method of calculation employed, the stability order of the relaxed faces is (01.2)(R1)(CO3) < (01.2)(R2)(Ca) < (01.2)(R2)(CO3) < (01.2)(R1)(Ca). Concerning the unrelaxed faces, whose energies evaluated by using empirical potentials only, the stability order is instead (01.2)(R1)(Ca) < (01.2)(R2)(Ca) < (01.2)(R1)(CO3) < (01.2)(R2)(CO3); such different ordering shows the importance of geometry relaxation in the calculation of the surface energy. The values of the relaxed surface energies are gamma(CO3)((01.2)R1) approximate to 750, gamma(Ca)((0.12)R2) approximate to 950, gamma(CO3)((01.2)R2) approximate to 980 and gamma(Ca)((01.2)R1) approximate to 1050 erg/cm(2). (C) 2008 Elsevier B.V. All rights reserved.