H. Dabringhaus

Studies of the growth and dissolution kinetics of the CaCO3 polymorphs calcite and aragonite II. The influence of divalent cation additives on the growth and dissolution rates

The influence of the divalent cations Fe2+, Cu2+, Zn2+, Mg2+, Sr2+, and Ba2+ on the growth and dissolution rates of calcite and aragonite has been studied. In most cases the results can be interpreted by a reversible adsorption of the impurity ions at kinks sites according to the Langmuir-Volmer model. Ions of the transition metals show a stronger inhibition than earth alkaline ions. Mg2+ and Fe2+ have no influence on growth and dissolution of aragonite. Further, Cu2+ and Zn2+ have a stronger effect on calcite, while Sr2+ and Ba2+ are more effective on aragonite. Results are used to show consequences for the polymorphic precipitation of CaCO3.

Studies of the growth and dissolution kinetics of the CaCO3 polymorphs calcite and aragonite I. Growth and dissolution rates in water

Abstract Growth and dissolution rates of calcite and aragonite in water were measured by the pH-stat method as functions of the super- and undersaturation from ( S − 1) = −0.6 to 3, at temperatures T = 20–70°C, and for different calcium and carbonate concentrations. For calcite from fits of power laws r = p¦S − 1¦ n to the experimental growth and dissolution rates almost linear laws are obtained. From the high activation energy of the prefactor p it follows that for moderate deviations from saturation growth and dissolution are determined by processes at the surface and not by diffusion from the bulk of the solution. Measurements with constant (Ca 2+ )(HCO − 3 ) ion product and different (Ca 2+ ):(HCO − 3 ) ratios show that the growth and dissolution rates are independent of the individual concentrations but depend solely on the ion product. The results are interpreted by a new two-step growth model. The first, chemical step involves a formation of CaCO 3 molecules in the adsorption layer of the crystals possibly via CaHCO + 3 ion pairs, the second, surface kinetic one an incorporation of the formed CaCO 3 molecules into the crystal lattice by surface and step diffusion to kink sites. While the surface kinetics is rate determining at low deviations from equilibrium, the formation and decomposition of CaCO 3 dominate growth and dissolution at large super- and undersaturations. The same growth model can be applied to aragonite. The interpretation of the experimental results is, however, complicated because of the occurrence of different crystallographic faces, which allows only a description of the measured rates as overall rates. Results for growth may be interpreted by different power laws for different faces or by two-dimensional nucleation. For dissolution an influence of diffusion in the bulk of the solution is observed.

Untersuchung der kondensation und verdampfung von alkalihalogenid-kristallen mit molekularstrahlmethoden: I. Der kondensationskoeffizient von KCl als funktion der überund Untersättigung

Abstract A method has been developed for studying condensation and evaporation processes, replacing the vapour phase by a known molecular flux onto the surface. The flux leaving the surface is measured by a surface ionisation detector. Measurements were performed with KCl (100) surfaces with saturation ratios S ranging from 0 to 77 and at temperatures between 325 and 425 °C. The condensation coefficient α( S ) exhibits a flat minimum at S = 1 and two bends. Below resp. above these critical ratios relaxation phenomena are observed. From the temperature dependence they can be interpreted as two-dimensional nucleation or hole nucleation.

Untersuchung der kondensation und verdampfung von alkalihalogenid-kristallen mit molekularstrahlmethoden: II. Relaxationseffekte auf der (100)-oberfläche von KCl

Abstract Relaxation effects are discussed, which were observed while investigating condensation and evaporation of KCl (100) (cf. part I). A striking phenomenon was found when the molecular flux onto the crystal for saturation was suppressed suddenly by a shutter. After this change a relaxation of the free evaporation flux and the condensation coefficient passed as a damped vibration, which can be interpreted by a process of hole nucleation. The period of the vibration was found nearly equal to the time needed for evaporation of one monolayer.

Untersuchung der kondensation und verdampfung von alkalihalogenid- kristallen mit molekularstrahlmethoden: III. Elektronenmikroskopische untersuchung periodischer verdampfungsvorgänge auf der (100)-oberfläche von KCI☆

Abstract In previous papers we reported on a molecular beam method to investigate evaporation and condensation processes of crystals. A sudden change from saturation state to conditions of free evaporation resulted in a periodic variation of the vaporization flux of the (100) surface of KCl. We assumed this phenomenon to be caused by periodic hole nucleation and growth of holes in the outermost atom layer of the crystal. The present work confirms this interpretation by a combination of the previous procedure and an electron microscopic method (gold decoration). Sizes and densities of the holes have been determined as a function of the time of free evaporation.

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. I. The system MgF2LiF(0 0 1)

Epitaxial growth of CaF2 on the (0 0 1) surface of LiF is studied for crystal temperatures between 573 and 773 K and for molecular beam fluxes of 1 × 1012–1 × 1013 CaF2 molecules per cm2 and s. For T = 573 K layer growth with the orientation CaF2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 1 0] is found. The layer growth mode is interpreted by a restricted mobility of CaF2 molecules at this low temperature. LEED studies suggest a rearrangement of the (0 0 1) plane of CaF2 leading to the formation of nm-scaled fourfold pyramids with (1 1 1) facets or, alternatively, an ab initio formation of such pyramids accompanied by a fast coalescence to a closed layer. For temperatures T ⩾ 673 K growth of islands occurs. With increasing temperature the orientation changes to CaF2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 0 0]. This latter orientation may be attributed to an increasing mixing of Li+ and Ca2+ ions, enforcing in this way a continuation of the cation arrangement of the underlying LiF crystal into the growing CaF2 islands. As for MgF2LiF(0 0 1), for the present system growth of LiF crystallites — presumably on already grown CaF2 layers — is found. The epitaxial growth of these crystallites with the (1 1 1) surface parallel to the original (0 0 1) surface of the LiF crystal is interpreted by a space filling arrangement of F− anions above Ca2+ ions.

Investigation of condensation and evaporation of alkali halide crystals by molecular beam methods: IX. Measurements with “static” molecular beams on the (100) surface of LiF

Abstract Measurements of the desorption fluxes from (100) surfaces of LiF for constant impinging molecular flux yield the condensation coefficients of monomers, dimers, and trimers as functions of the crystal temperature and the saturation ratio. When the dimer/monomer ratio of the impinging flux is varied by means of a “double oven”, the desorption fluxes of the monomers, dimers, and trimers prove to be linear functions of the impinging fluxes of the corresponding species. Therefore, the different condensation coefficients of the monomers, dimers, and trimers are not dependent on binary collisions of molecules on the surface but only on their different diffusion lengths. Oscillations of the desorption fluxes, which were observed when a smoothed surface was suddenly exposed to high supersaturation, are due to recurring two-dimensional nucleation. From the course of these “nucleation oscillations” it follows that — under the condition of high supersaturation — the condensation kinetics of the (100) surface of LiF is dominated by processes on the terrace and not at the steps.

Investigation of condensation and evaporation of alkali halide crystals by molecular beam methods: VIII. Molecular beam pulse experiments with lithium flouride

Abstract Experiments with molecular beam pulses allow the determination of the relaxation times of monomeric, dimeric, and trimeric molecules of lithium fluoride on (100) surfaces of LiF as functions of the crystal temperature T and of the molecular flux j on impinging onto the surface. At high T and low j on the relaxation times increase exponentially with 1/ T . The corresponding activation energies are 0.79, 0.77, and 1.14 eV for the monomers, dimers, and trimers. At lower T and higher j on they become independent of T but increase with decreasing j on . The experiments were performed with different lithium isotopes: 6 Li for the impinging molecular beam and 7 Li for the crystal. Measurements with dimers and trimers of differing isotopic composition show that the measured relaxation times are independent of the isotope exchange on the surface. Therefore, the Li exchange times are immeasurably small (

Investigation of condensation and evaporation of alkali halide crystals by molecular beam methods: X. Discussion of the condensation and evaporation kinetics of lithium fluoride

Abstract Dynamic and static experiments for the study of the condensation and evaporation kinetics of LiF, Li 2 F 2 , and Li 3 F 3 molecules on the (100) surface of LiF are discussed on the basis of the “BCF” model. From this model it follows that the effective residence times τ i eff are connected to the desorption times τ i and to the condensation coefficients by τ i eff = τ i (1- α i ). A rough correspondence between theory and experiments has been achieved. The concentrations of the different molecular species on the terrace, and surface diffusion coefficients and mean diffusion distances are estimated. The lithium isotope exchange is interpreted by the reaction of admolecules with vacancies in the surface.

Experimental study of the adsorption of lithium fluoride on the (111) surface of CaF21

The interaction of molecular beams of the lithium fluoride monomers LiF and dimers Li 2 F 2 with the (111) surface of CaF 2 crystals has been studied by the method of desorption transients for crystal temperatures between T=777 and 985 K and for total lithium fluoride fluxes j on = I × 10 10 -1 × 10 14 cm -2 s - 1 . The studies show that the monomers are adsorbed at the monatomic steps on the CaF 2 surface. The steps have a limited capacity of the order of 10 12 cm -2 for an adsorption direct at them; further monomers are adsorbed in their immediate neighborhood beside already adsorbed monomers. The adsorption times derived from transient desorption fluxes result as τ 0 (s)=1.3 (+9.5, -1.15) × 10 -15 exp[(2.70±0.17) eV/kT] for an adsorption direct at the steps and as τ 1 (s)=7.6 (+30, -6.0) ×10 -15 exp[(2.44±0.11) eV./kT] beside them. For the dimers, only a portion α d , 0.5<α d (T)<0.85, of the impinging dimer flux reaches the steps, while the remainder desorbs quickly. From α d (T), the ratio of the mean diffusion length x d s on the terrace to the mean step distance, λ, is determined as x s /λ=(2.9±0.9) × 10 -2 exp[(0.5 ±0.05) eV/2kT]. With λ 8 × 10 -6 cm as derived from atom force microscopic studies jump distances for the dimers in the order of the lattice constant of CaF 2 are obtained corresponding to a surface diffusion model with jumps between neighboring lattice sites. Dimers at the steps are completely dissociated into monomers. For larger monomer concentrations at the steps also a formation of dimers and their subsequent desorption is observed. By a comparison of experimental desorption transients with a simple theoretical model, the above adsorption times as well as a constant k (cm s -1 )=8.0(+42, -6.8) exp[-(1.63±0.16) eV/kT describing the formation and desorption of the dimers are determined.