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 CO3 terminated) of calcite (CaCO3) were studied: (i) R1 reconstruction: the outermost layer is based on the [0 1 0] × 1/3[2 1 1] 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 [ 4 2 1 ¯ ] × 1 / 6 [ 2 2 ¯ 1 ] lozenge shaped mesh that does not respect the 2D symmetry of the face. 01.2 ) R 1 Ca , ( 01.2 ) R 1 CO 3 , ( 01.2 ) R 2 Ca and ( 01.2 ) R 2 CO 3 slabs geometry optimizations of calcite (CaCO3) 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 CO3 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. rfaces energies (γ) at 0 K, for relaxed and unrelaxed ( 01.2 ) R 1 Ca , ( 01.2 ) R 1 CO 3 , ( 01.2 ) R 2 Ca and ( 01.2 ) R 2 CO 3 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 ) R 1 CO 3 < ( 01.2 ) R 2 Ca < ( 01.2 ) R 2 CO 3 < ( 01.2 ) R 1 Ca . Concerning the unrelaxed faces, whose energies were evaluated by using empirical potentials only, the stability order is instead ( 01.2 ) R 1 Ca < ( 01.2 ) R 2 Ca < ( 01.2 ) R 1 CO 3 < ( 01.2 ) R 2 CO 3 ; such different ordering shows the importance of geometry relaxation in the calculation of the surface energy. The values of the relaxed surface energies are γ ( 01.2 ) R 1 CO 3 ≈ 750 , γ ( 01.2 ) R 2 Ca ≈ 950 , γ ( 01.2 ) R 2 CO 3 ≈ 980 and γ ( 01.2 ) R 1 Ca ≈ 1050 erg/cm2.