36 powder artificial mixtures and 2 usual samples with a bimodal crystallite size distribution have been prepared and examined by an X-ray diffraction method. We estimated the capability of the Rietveld refinement at a “double-phase model” mode and the correctness of the results. The homogeneity and the crystallite sizes of pure phases used in the preparation of the mixtures were checked by TEM. In most cases, Rietveld refinement at a “double-phase model” mode provided reliable information about both the crystallite size and percentage of constituents. Reliability of XRD results was higher for the powder mixtures with a larger difference in crystallite sizes. It was found that the shape of the crystallites can have a significant impact on the analysis results, and for columnar crystals, the employed method did not always provide correct results. When size distribution of the material deviates from a bimodal one, the correctness of the result decreases. Such an investigation was performed for the first time.

Calcium carbonate (CaCO₃) is one of the most abundant substances on earth and has a large array of industrial applications. Considerable research has been conducted in an effort to synthesize calcium carbonate microparticles with controllable and specific morphologies and sizes. CaCO₃ produced by a precipitation reaction of calcium nitrate and sodium carbonate solution was found to have high polymorphism and batch to batch variability. In this study, we investigated the polymorphism of the precipitated material and analyzed the chemical composition, particle morphology, and crystalline state revealing that the presence of silicon atoms in the precipitant is a key factor effecting particle shape and crystal state. An elemental analysis of single particles within a polymorphic sample, using energy-dispersive X-ray spectroscopy (EDS) conjugated microscopy, showed that only spherical particles, but not irregular shaped one, contained traces of silicon atoms. In agreement, silicon-containing additives lead to homogenous, amorphous nanosphere particles, verified by X-ray powder diffraction (XRD). Our findings provide important insights into the mechanism of calcium carbonate synthesis, as well as introducing a method to control the precipitants at the micro-scale for many diverse applications.