Abstract
As a metastable phase, vaterite is involved in the first step of crystallization of several carbonate-forming systems including the two stable polymorphs calcite and aragonite. Its complete structural determination would consequently shed important light to understand scaling formation and biomineralization processes. While vaterite’s hexagonal substructure (a0 ~ 4.1 Å and c0 ~ 8.5 Å) and the organization of the carbonate groups within a single layer is known, conflicting interpretations regarding the stacking sequence remain and preclude the complete understanding of the structure. To resolve the ambiguities, we performed precession electron diffraction tomography (PEDT) to collect single crystal data from 100 K to the ambient temperature. The structure was solved ab initio and described over all the temperature range using a unified modulated structure model in the superspace group C12/c1(α0γ)00 with a = a0 = 4.086(3) Å, b = sqrt{{bf{3}}}a0 = 7.089(9) Å, c = c0 = 8.439(9) Å, α = β = γ = 90° and q = tfrac{{bf{2}}}{{bf{3}}}a* + γc*. At 100 K the model presents a pure 4-layer stacking sequence with γ = tfrac{{bf{1}}}{{bf{2}}} whereas at the ambient temperature, ordered stacking faults are introduced leading to γ < tfrac{{bf{1}}}{{bf{2}}}. The model was refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement.
Highlights
The model was at refined against precession electron diffraction tomography (PEDT) data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement
In the light of new single crystal data collected on more ordered crystals at low temperature by precession electron diffraction tomography (PEDT) with low dose condition, we show that an accurate description of vaterite exists using the superspace formalism to index the superstructure reflections
In order to know if the observed deviation in γ is significant, we plotted the absolute difference between the commensurate value 0.5 and the value obtained for each measurement at different temperatures (Fig. 4b)
Summary
The model was at refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement. Among the three crystallized anhydrous polymorphs of CaCO3, vaterite is known to be the least stable form under natural conditions. The crystallite sizes of vaterite synthesized in controlled conditions do not exceed 60 nm[11] Another limitation comes from the dual nature of the structure that shows both ordered and disordered features. The pseudocell of Kamhi is hexagonal with two layers, space group P63/mmc, with a0 = 4.13 Å, c0 = 8.49 Å, Z = 2 and it has CO3 groups disordered between three orientations around threefold axes In his original paper Kamhi reported eight weak diffraction peaks that could not be indexed www.nature.com/scientificreports/. The single layer was used in monoclinic settings as a basic brick for many of the proposed structural models of vaterite that have been interpreted within the order-disorder (OD)
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