Aplicación de la microscopía de fuerza atómica al estudio de la cristalización de soluciones sólidas a partir de soluciones acuosas

Abstract

Traditionally, theoretical and experimental studies carried out in crystal growth field have been focused on the crystallisation of solids with a fixed composition. However, in nature, most crystals show a wide compositional variability (solid solution). Solid composition changes during growth will affect both physical and chemical properties of the crystal. As crystallisation is a phenomenon that occurs at the crystal-medium interface, those changes will be reflected on the microtopographic evolution of crystal surface. Currently, the use of Atomic Force Microscopy is allowing to study the characteristics of crystal surfaces and their changes at a molecular scale during growth. In this work, we describe some phenomena that occur on crystal surfaces when the growing phase is a solid solution. The complexity of such phenomena both makes very difficult their interpretation and indicates that there are many differences between the crystal growth of solid solutions and substances of fixed composition. For all the solid solutions studied in this paper, calcite is one of the end-members. The divalent cations substituting Ca2+ are Cd2+, Mn2+, Sr2+, and Ba2+. The increasing difference in ionic radii determines the thermodynamic characteristics of the respective solid solutions and, therefore, their behaviour: 1) In the system CdCO3-CaCO3-H2O, as Cd2+ and Ca2+ have very similar sizes, the growth of CdCO3 on a calcite substrate occurs without restrictions, with almost perfect structural fitting. 2) In the system MnCO3-CaCO3-H2O, the growth of MnxCa1-xCO3 layers on a calcite substrate inhibits the development of successive layers, leading to the reproduction of the original microtopography. This phenomenon has d{101-4} been referred as 'template effect'. 3) In the system BaCO3-CaCO3-H2O, the great difference between Ba2+ and Ca2+ sizes determines the selective incorporation of Ba in certain steps containing bigger and opener growth kinks. The incorporation of Ba substituting Ca promotes the change of calcite cell parameters. The thickening of d{101-4} can be clearly observed in AFM images. 4) Finally, in the system SrCO3-CaCO3-H2O, several phenomena have been observed: the 'template effect', the selective incorporation of Sr in certain steps, and a solvent-mediated transformation. This transformation consists in the simultaneous dissolution of newly-formed SrxCa1-xCO3 (rhombohedral) layers and the growth of three-dimensional nuclei of SrxCa1-xCO3 (orthorhombic) on calcite surface.