The Effect of Carboxylates on the Mg Content of Calcites that Transform from ACC

AbstractIn some skeletal-forming and sedimentary environments, calcite and other CaCO3 polymorphs are produced from an amorphous calcium carbonate (ACC) phase. This experimental study determined the Mg content of calcites that grew in the presence of ACC with/without carboxylated biomolecules. The Mg content of the initial ACC obeys a simple fractionation for all conditions except in the presence of oxydiacetate. For all solution compositions, this ACC transforms into calcite crystallites that contain zero to 38 mole %MgCO3, without evidence of secondary polymorphs (Mg/Ca = 0-15). Mg is in the calcite structure within the resolution of the XRD method. The biomolecules slow the ACC to calcite transformation in proportion to their selectivity for Ca over Mg. Citrate, tartarate, and oxydiacetate increase the amount of Mg in both ACC and the resulting calcite. When the Mg2+/Ca2+ ratios of initial solutions are less than ∼5-8, the Mg content of the ACC and calcite products are similar

Random Field Models for Relaxor Ferroelectric Behavior

Heat bath Monte Carlo simulations have been used to study a four-state clock model with a type of random field on simple cubic lattices. The model has the standard nonrandom two-spin exchange term with coupling energy $J$ and a random field which consists of adding an energy $D$ to one of the four spin states, chosen randomly at each site. This Ashkin-Teller-like model does not separate; the two random-field Ising model components are coupled. When $D / J = 3$, the ground states of the model remain fully aligned. When $D / J \ge 4$, a different type of ground state is found, in which the occupation of two of the four spin states is close to 50%, and the other two are nearly absent. This means that one of the Ising components is almost completely ordered, while the other one has only short-range correlations. A large peak in the structure factor $S (k)$ appears at small $k$ for temperatures well above the transition to long-range order, and the appearance of this peak is associated with slow, "glassy" dynamics. The phase transition into the state where one Ising component is long-range ordered appears to be first order, but the latent heat is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev

Investigation of strain birefringence and wavefront distortion in 001 plates of KD sub 2 PO sub 4

When 001 plates of KD{sub 2}PO{sub 4} (KD*P) are used in Pockels cells, strain induced refractive index variations result in beam depolarization and transmitted wavefront distortion. The depolarization is determined by the induced birefringence while the wavefront distortion is controlled by the average index shift. Here we show that the birefringence is determined by the shear stress in the xy-plane of the crystal while the average index shift depends only on the normal stresses. Furthermore, for depolarization losses of 0.1 to 1.0% and wavefront distortion of 0.1 to 1.0{lambda}, the critical range of stress is 10{sup 5} to 10{sup 6} Pa. We also present measured depolarization loss and wavefront distortion profiles for 5, 16 and 27cm, 95% deuterated, KD*P crystals. Using the analysis described above we show that the maximum internal stresses in the crystals are within the critical range, but that the area averaged stresses are substantially lower. We find that crystals from different locations along the length of a boule have similar strain birefringence and wavefront distortion profiles indicating that the growth conditions which generate the internal strain persist throughout much of the growth history of the boule. Finally, we discuss potential sources of strain in KD*P. 8 refs., 3 figs

Hydroxide films on mica form charge-stabilized microphases that circumvent nucleation barriers

Crystal nucleation is facilitated by transient, nanoscale fluctuations that are extraordinarily difficult to observe. Here, we use high-speed atomic force microscopy to directly observe the growth of an aluminum hydroxide film from an aqueous solution and characterize the dynamically fluctuating nanostructures that precede its formation. Nanoscale cluster distributions and fluctuation dynamics show many similarities to the predictions of classical nucleation theory, but the cluster energy landscape deviates from classical expectations. Kinetic Monte Carlo simulations show that these deviations can arise from electrostatic interactions between the clusters and the underlying substrate, which drive microphase separation to create a nanostructured surface phase. This phase can evolve seamlessly from a low-coverage state of fluctuating clusters into a high-coverage nanostructured network, allowing the film to grow without having to overcome classical nucleation barriers

The thermodynamics of calcite nucleation at organic interfaces: Classical vs. non-classical pathways

Nucleation in the natural world often occurs in the presence of organic interfaces. In mineralized tissues, a range of macromolecular matrices are found in contact with inorganic phases and are believed to direct mineral formation. In geochemical settings, mineral surfaces, which are often covered with organic or biological films, surround the volume within which nucleation occurs. In the classical picture of nucleation, the presence of such interfaces is expected to have a profound effect on nucleation rates, simply because they can reduce the interfacial free energy, which controls the height of the thermodynamic barrier to nucleation of the solid phase. However, the recent discovery of a nearly monodisperse population of calcium carbonate clusters—so called pre-nucleation clusters—and the many observations of amorphous precursor phases have called into question the applicability of classical descriptions. Here we use in situ observations of nucleation on organothiol self-assembled monolayers (SAMs) to explore the energetics and pathways of calcite nucleation at organic interfaces. We find that carboxyl SAM-directed nucleation is described well in purely classical terms through a reduction in the thermodynamic barrier due to decreased interfacial free energy. Moreover, the differences in nucleation kinetics on odd and even chain-length carboxyl SAMs are attributable to relative differences in these energies. These differences arise from varying degrees of SAM order related to oxygen-oxygen interactions between SAM headgroups. In addition, amorphous particles formed prior to or during crystal nucleation do not grow and are not observed to act as precursors to the crystalline phase. Instead, calcite appears to nucleate independently. These results imply that the recently proposed model of calcite formation as a non-classical process, one which proceeds via aggregation of stable pre-nucleation clusters that form an amorphous precursor from which the crystalline phase emerges, is not applicable to template-directed nucleation on carboxyl SAMs and does not provide a universal description of calcite formation

X-ray diffraction studies of potassium dihydrogen phosphate (KDP) crystal surfaces

We have studied the surface atomic structure of KDP crystals using X-ray scattering. These crystals were grown from an aqueous solution and we have done measurements both ex situ and in situ. The ex situ measurements were performed in vacuum or in air. In order to be able to do in situ measurements, we designed and built a crystal growth chamber which is compatible with X-ray diffraction experiments. The atomic arrangement of the two naturally existing faces of KDP has been determined. Preliminary results are presented of measurements performed during growth. Furthermore, the influence of metal impurities on the atomic structure of the growing interface is examined