H.J. Meyer

The influence of impurities on the growth rate of calcite

The effects of 34 different additives on the growth rate of calcite were investigated. An initial growth rate of about one crystal monolayer (3 × 10-8 cm) per minute was adjusted at a constant supersaturation which was maintained by a control circuit. Then the impurity was added step by step and the reduction of the growth rate was measured. The impurity concentration necessary to reduce the initial growth rate by a certain percentage increased in the order Fe2+, ATP, P3O5-10, P2O4-7, (PO3)6-6, Zn2+, ADP, Ce3+, Pb2+, carbamyl phosphate, Fe3+, PO3-4, Co2+, Mn2+, Be2+, β-glycerophosphate, Ni2+, Cd2+, “Tris”, phenylphosphate, chondroitine sulphate, Ba2+, citrate, AMP, Sr2+, tricarballylate, taurine, SO2-4, Mg2+ by 4 orders of magnitude. The most effective additives halved the initial growth rate in concentrations of 2 × 10-8 mol/1. For Fe2+ the halving concentration was nearly proportional to the initial rate. The mechanism of inhibition by adsorption of the impurities at growth sites (kinks) is discussed.

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.

Untersuchung der kondensation und verdampfung von alkalihalogenidkristallen mit molekularstrahlmethoden: V. zur zweidimensionalen keimbildung und lochkeimbildung☆

Zusammenfassung A method for measuring two-dimensional nucleation (KB) and hole nucleation (LKB) rates as a function of the saturation ratio S = p/p s was developed on the basis of preceding papers [2,5]. S and the length of time for nucleation are adjusted by combination of two molecular beam sources, several shutters and molecular beam detectors. Experiments with cleavage surfaces of KCl indicate that both KB and LKB are induced by impurities (e.g. by 5 ppm Ba 2+ ). Therefore on these surfaces reproducible measurements are practically impossible. A drastic reduction of KB and LKB rates is observed after vapour deposition of a mono-crystalline layer of zone-refined KCl on the (100) surface of a KCl crystal. Now KB occurs only for saturation ratios S > 7 and LKB only for S

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.

Untersuchung der kondensation und verdampfung von alkalihalogenidkristallen mit molekularstrahlmethoden: VI. Zweidimensionale keimbildung auf (100)-flächen von kaliumchlorid

Abstract The rate of two-dimensional nucleation (KB) was measured on (100) surfaces of potassium chloride at crystal temperatures T between 289 and 343°C and saturation ratios between 7 and 42. For this purpose the molecular beam method characterized in [1] was developed further by allowing the possibility to vary both the length of time for nucleation t and the saturation ratio S in one experiment and to produce large surface areas (up to 400 μm 2 ) free of steps for KB. For long t (> 10 s) the observed concentrations of two-dimensional islands (grown nuclei) approach limits which increase with both S and T . The measured rates of KB are interpreted by means of the classical nucleation theory and its modification according to Stoyanov. The specific free ledge energy obtained in this way is ϵ ≈ 7.5 × 10 -6 erg / cm . The sizes of (critical) nuclei i ∗ amount 10, 8 and 4 molecules. Experiments with higher KB rates are interpreted quantitatively by KB at surface defects (impurity ions).

Wachstumsgeschwindigkeit von calcit aus wässerigen lösungen

Abstract The linear growth rate (WG) of calcite was measured by means of a pH-stat method. The calcite used was a crystalline powder material. Supersaturation and solution concentrations were held constant by synchronous dosage of precipitation reagents (NaOH, CaCl2, NaHCO3) from three automatic burettes. In this system, the pH of the equilibrium state pHs is defined by the product of the calcium and bicarbonāte activities. The saturation ratio S = [Ca2+]⋅[CO2-3]/Kc is given by ▵pH = pH − pHs =log S (Kc = solubility product). After a “critical” ▵pH (≈0.1−0.3) is exceeded, the linear growth rate WG exhibits a considerable acceleration at constant pH, which can be explained by an increase in the number of growth steps on the crystal surfaces. For several series of experiments at 20°C and concentrations of Ca2+ and HCO-3 between 0.15 x 10-3 and 4.1 x 10-3m, ▵pH was raised up to a WG ≈ 6 x 10-8 cm/min. The WG measurements were then made with decreasing pH. WG is proportional to (S - 1) and increases with increasing the product of calcium and bicarbonate concentrations independently of the ratio [Ca2+] : [HCO-3]. The results are compared with measurements of other authors. A molecular kinetic model of calcite growth is proposed.