Crystallization Of Metastable Phase Research Articles

Nucleation of metastable aragonite CaCO3 in seawater

Predicting the conditions in which a compound adopts a metastable structure when it crystallizes out of solution is an unsolved and fundamental problem in materials synthesis, and one which, if understood and harnessed, could enable the rational design of synthesis pathways toward or away from metastable structures. Crystallization of metastable phases is particularly accessible via low-temperature solution-based routes, such as chimie douce and hydrothermal synthesis, but although the chemistry of the solution plays a crucial role in governing which polymorph forms, how it does so is poorly understood. Here, we demonstrate an ab initio technique to quantify thermodynamic parameters of surfaces and bulks in equilibrium with an aqueous environment, enabling the calculation of nucleation barriers of competing polymorphs as a function of solution chemistry, thereby predicting the solution conditions governing polymorph selection. We apply this approach to resolve the long-standing "calcite-aragonite problem"--the observation that calcium carbonate precipitates as the metastable aragonite polymorph in marine environments, rather than the stable phase calcite--which is of tremendous relevance to biomineralization, carbon sequestration, paleogeochemistry, and the vulnerability of marine life to ocean acidification. We identify a direct relationship between the calcite surface energy and solution Mg:Ca [corrected] ion concentrations, showing that the calcite nucleation barrier surpasses that of metastable aragonite in solutions with Mg:Ca ratios consistent with modern seawater, allowing aragonite to dominate the kinetics of nucleation. Our ability to quantify how solution parameters distinguish between polymorphs marks an important step toward the ab initio prediction of materials synthesis pathways in solution.

Magnetostrictions and transformation process in Fe 73.5Cu 1Ta 2Nb 1Si 13.5B 9

Finemet-based alloy with substitution of 2 at.% of niobium by tantallum is studied from the viewpoint of transition to (nano)crystalline state and its influence on magnetostriction properties. The measurements have been performed on samples annealed isothermally at different temperatures for 1 h, annealing temperatures ranging from room temperature up to 750°C. After annealing, the magnetostriction has been measured at room temperature using the capacitance method. The results of saturation magnetostriction measurements λ S, volume magnetostriction ω and force magnetostriction ∂ω ∂H and their evolution with annealing reflect the transformation process. Saturation magnetostriction λ S changes slightly (by 10%) in the course of relaxation and crystallization of metastable phases. Formation of borides in the final phases of transformation decreases strongly the value of λ S to 1/12 of the value for the as-quenched sample. Force magnetostriction ∂ω ∂H reflects the process of volume changes in the course of crystalline phase formation accompanied by changes of internal stresses which influence the domain structure of the material. X-ray and transmission electron microscopy (TEM) investigations identify the type of crystalline phases formed at selected annealing steps of the transformation process.