ABSTRACT: Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO3) is often used as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO3 nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid-liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO3 in dilute aqueous solutions. We propose that a dense liquid phase (containing 4-7 H2O per CaCO3 unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO3 in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca2+ + z CO32- ? z CaCO3 The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.
Project description:Calcium carbonate (CaCO<sub>3</sub>) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. However, its phase relations above 3?GPa and 1000?K are controversial. Here we report a reversible temperature-induced aragonite-amorphization transition in CaCO<sub>3</sub> at 3.9-7.5?GPa and temperature above 1000?K. Amorphous CaCO<sub>3</sub> shares a similar structure as liquid CaCO<sub>3</sub> but with much larger C-O and Ca-Ca bond lengths, indicating a lower density and a mechanism of lattice collapse for the temperature-induced amorphous phase. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO<sub>3</sub> melting curve overturn at about 6?GPa. Amorphous CaCO<sub>3</sub> is stable at subduction zone conditions and could aid the recycling of carbon to the surface.
Project description:Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH<sub>2</sub>PO<sub>4</sub> (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.
Project description:We report an experimental approach for high-resolution real-time monitoring of transiently formed species occurring during the onset of precipitation of ionic solids from solution. This is made possible by real-time nuclear magnetic resonance (NMR) monitoring using dissolution dynamic nuclear polarization (D-DNP) to amplify signals of functional intermediates and is supported by turbidimetry, cryogenic electron microscopy, and solid-state NMR measurements. D-DNP can provide drastic signal improvements in NMR signal amplitudes, permitting dramatic reductions in acquisition times and thereby enabling us to probe fast interaction kinetics such as those underlying formation of prenucleation species (PNS) that precede solid-liquid phase separation. This experimental strategy allows for investigation of the formation of calcium phosphate (CaP)-based minerals by <sup>31</sup>P NMR-a process of substantial industrial, geological, and biological interest. Thus far, many aspects of the mechanisms of CaP nucleation remain unclear due to the absence of experimental methods capable of accessing such processes on sufficiently short time scales. The approach reported here aims to address this by an improved characterization of the initial steps of CaP precipitation, permitting detection of PNS by NMR and determination of their formation rates, exchange dynamics, and sizes. Using D-DNP monitoring, we find that under our conditions (i) in the first 2 s after preparation of oversaturated calcium phosphate solutions, PNS with a hydrodynamic radius of <i>R</i><sub>h</sub> ? 1 nm is formed and (ii) following this rapid initial formation, the entire crystallization processes proceed on considerably longer time scales, requiring >20 s to form the final crystal phase.
Project description:Hydroxyapatite (HAP) participates in vertebral bone and tooth formation by a nonclassical hitherto unknown nucleation mechanism, in which amorphous precursors form and transform during long induction periods. Elucidation of the mechanism by which amorphous precursors assemble and transform is essential to understanding how hard tissues form in vivo and will advance the design and fabrication of new biomaterials. The combination of conductance and potentiometric techniques to monitor Ca-P mineral formation has given new insight into the mechanism of nucleation. Differences detected in the dehydration rates of calcium and phosphate ions indicate the formation of nonequilibrium calcium-deficient clusters. The aggregation of these clusters forms a calcium-deficient amorphous phase I [Ca-(HPO<sub>4</sub>)<sub>1+<i>x</i></sub> ·<i>n</i>H<sub>2</sub>O]<sup>2<i>x</i>-</sup>) early in the induction period, which slowly transforms to amorphous phase II [Ca-(HPO<sub>4</sub>)·<i>m</i>H<sub>2</sub>O] by dehydration. Precritical nuclei form within amorphous phase II later in the induction period, leading to mineral formation.
Project description:The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to monitor the early stages of the crystallization of ibuprofen as a model system for small organic molecules. Ibuprofen undergoes liquid-liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid-liquid miscibility gap were analyzed and confirmed. An increase in viscosity sustains the kinetic stability of the dense liquid intermediate. Since the distances between ibuprofen molecules within the dense liquid phase are similar to those in the crystal forms, this dense liquid phase is identified as a precursor phase in the nucleation of ibuprofen, in which densification is followed by generation of structural order. This discovery may make it possible to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments.
Project description:Bacteria mineralization is a promising biotechnological approach to apply in biomaterials development. In this investigation, we demonstrate that Bacillus subtilis 168 induces and influences CaCO<sub>3</sub> composites precipitation. Crystals were formed in calcium-carbon non-coupled (glycerol?+?CaCl<sub>2</sub>, GLY; or glucose?+?CaCl<sub>2</sub>, GLC) and coupled (calcium lactate, LAC; or calcium acetate, ACE) agar-sources, only maintaining the same Ca<sup>2+</sup> concentration. The mineralized colonies showed variations in morphology, size, and crystallinity form properties. The crystals presented spherulitic growth in all conditions, and botryoidal shapes in GLC one. Birefringence and diffraction patterns confirmed that all biogenic carbonate crystals (BCC) were organized as calcite. The CaCO<sub>3</sub> in BCC was organized as calcite, amorphous calcium carbon (ACC) and organic matter (OM) of biofilm; all of them with relative abundance related to bacteria growth condition. BCC-GLY presented greatest OM composition, while BCC-ACE highest CaCO<sub>3</sub> content. Nucleation mechanism and OM content impacted in BCC crystallinity.
Project description:Although Al(III) hydrolysis, condensation, and nucleation play pivotal roles in the synthesis of Al-based compounds and determine their chemical behavior, we still lack experimental evidence regarding the chemistry of nucleation from solution. Here, by combining advanced titration assays, high-resolution transmission electron microscopy (HR-TEM), and 27Al-nuclear magnetic resonance spectroscopy, we show that highly dynamic solute prenucleation clusters (PNCs) are fundamental precursors of nanosolid formation. Chemical changes from olation to oxolation bridging within PNCs rely on the formation of tetrahedral AlO4 in solution and trigger phase separation at low driving force (supersaturation). This does not include the formation of Keggin-Al13 ions, at least during the earliest stages. The PNC pathway of the formation of Al(III) (oxy)(hydr)oxides offers new possibilities toward the development of strategies for controlling the entire crystallization process.
Project description:Otoliths, calcium carbonate (CaCO<sub>3</sub>) ear bones, are among the most commonly used age and growth structures of fishes. Most fish otoliths are comprised of the most dense CaCO<sub>3</sub> polymorph, aragonite. Sturgeon otoliths, in contrast, have been characterized as the rare and structurally enigmatic polymorph, vaterite-a metastable polymorph of CaCO<sub>3</sub>. Vaterite is an important material ranging from biomedical to personal care applications although its crystal structure is highly debated. We characterized the structure of Lake Sturgeon otoliths using thermal analysis and neutron powder diffraction, which is used non-destructively. We confirmed that while Lake Sturgeon otoliths are primarily composed of vaterite, they also contain the denser CaCO<sub>3</sub> polymorph, calcite. For the vaterite fraction, neutron diffraction data provide enhanced discrimination of the carbonate group compared to x-ray diffraction data, owing to the different relative neutron scattering lengths, and thus offer the opportunity to uniquely test the more than one dozen crystal structural models that have been proposed for vaterite. Of those, space group P6<sub>5</sub>22 model, a?=?7.1443(4)Å, c?=?25.350(4)Å, V?=?1121.5(2)Å<sup>3</sup> provides the best fit to the neutron powder diffraction data, and allows for a structure refinement using rigid carbonate groups.
Project description:There has been a growing interest in applying CsPbX<sub>3</sub> (X = Cl, Br, I) nanocrystals (NCs) for optoelectronic application. However, research on doping of this new class of promising NCs with optically active and/or magnetic transition metal ions is still limited. Here we report a facile room temperature method for Mn<sup>2+</sup> doping into CsPbCl<sub>3</sub> NCs. By addition of a small amount of concentrated HCl acid to a clear solution containing Mn<sup>2+</sup>, Cs<sup>+</sup>, and Pb<sup>2+</sup> precursors, Mn<sup>2+</sup>-doped CsPbCl<sub>3</sub> NCs with strong orange luminescence of Mn<sup>2+</sup> at ?600 nm are obtained. Mn<sup>2+</sup>-doped CsPbCl<sub>3</sub> NCs show the characteristic cubic phase structure very similar to the undoped counterpart, indicating that the nucleation and growth mechanism are not significantly modified for the doping concentrations realized (0.1 at. % - 2.1 at. %). To enhance the Mn<sup>2+</sup> emission intensity and to improve the stability of the doped NCs, isocrystalline shell growth was applied. Growth of an undoped CsPbCl<sub>3</sub> shell greatly enhanced the emission intensity of Mn<sup>2+</sup> and resulted in lengthening the radiative lifetime of the Mn<sup>2+</sup> emission to 1.4 ms. The core-shell NCs also show superior thermal stability and no thermal degradation up to at least 110 °C, which is important in applications.
Project description:Super-ionic solids, which exhibit ion mobilities as high as those in liquids or molten salts, have been employed as solid-state electrolytes in batteries, improved thermoelectrics and fast-ion conductors in super-capacitors and fuel cells. Fast-ion transport in many of these solids is supported by a disordered, 'liquid-like' sub-lattice of cations mobile within a rigid anionic sub-lattice, often achieved at high temperatures or pressures via a phase transition. Here we show that ultrasmall clusters of copper selenide exhibit a disordered cationic sub-lattice under ambient conditions unlike larger nanocrystals, where Cu<sup>+</sup> ions and vacancies form an ordered super-structure similar to the bulk solid. The clusters exhibit an unusual cationic sub-lattice arrangement wherein octahedral sites, which serve as bridges for cation migration, are stabilized by compressive strain. The room-temperature liquid-like nature of the Cu<sup>+</sup> sub-lattice combined with the actively tunable plasmonic properties of the Cu<sub>2</sub>Se clusters make them suitable as fast electro-optic switches.