A càrrec de Marco BRUNO (Dto. di Scienze Mineral. e Petrol., Università degli Studi di Torino, Itàlia)
Organitzat conjuntament CSIC-UB

Data: 10/11/2010
Hora: 12:00 h
Lloc: Sala d'actes de l'Institut Jaume Almera

Descripció:
The real surface profile of a (hkl) face rarely coincides with the ideal hkl lattice plane. As a matter of fact, the hkl plane is a geometrical abstraction of the crystal structure, while to generate the corresponding surface profile, one has to consider the face character: flat (F), stepped (S) or kinked (K), according to the Hartman-Perdok theory, along with its interactions with the mother phase. According to the Tasker's rules, three types of surfaces in ionic or partly ionic crystal can be identified. Type I consists of neutral planes with both anions and cations. Type II consists of charged planes arranged symmetrically so that there is no dipole moment perpendicular to the unit cell. The type III surface is charged and there is a perpendicular dipole moment. These surfaces have infinite surface energies (or very large surface energies for finite crystals) and produce a polarising electric field in the bulk. An electrostatic argument therefore indicates that such s

The characterization of the surfaces is done by determining their (relaxed) structures and surface free energies by means of empirical (force field) and ab initio (DFT level, B3LYP Hamiltonian) calculations, and by considering the 2D slab mode. The slice energies (SE), and the end chain energies (ECE) of the different Periodic Bound Chains (PBCs) running in the dhkl slice permitted by the extinction rules, are also calculated. The ab intio calculations were performed by using the CRYSTAL06 package, whereas the empirical calculations were done with the GULP code.

A detailed discussion on free energies is done. Indeed, the surface free energy is a fundamental thermodynamical quantity needed to determine the equilibrium morphology of a crystal, according to the Gibbs-Wulff theorem. The definitions of unrelaxed and relaxed surface energies will be given, and the different methodologies to calculate them will be explained. Furthermore, even the definitions of SE and ECE will be given, as well as and the calculation strategies employed to determine such quantities. Finally, examples of Type I and III surfaces will be discussed. In particular, the relaxed structures and the surface energies of the {10.4}, {01.2}, {00.1} forms of calcite (CaCO3), and the {10.4} and {00.1} forms of nitratine (NaNO3) will be discussed.