Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO 2 sequestration. amorphous calcium carbonate (ACC) | calorimetry | crystallization enthalpy | sea urchin larval spicules | synthetic and biogenic ACC

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A fundamental issue that remains to be solved when approaching the nanoscale is how the size induces transformation among different polymorphic structures. Understanding the sizeinduced transformation among the different polymorphic structures is essential for widespread use of nanostructured materials in technological applications. Herein, we report water adsorption and high-temperature solution calorimetry experiments on a set of samples of single-phase monoclinic zirconia with different surface areas. Essential to the success of the study has been the use of a new ternary water-in-oil/water liquid solvothermal method that allows the preparation of monoclinic zirconia nanoparticles with a broad range of (BET) Brunauer-Emmett-Teller surface area values. Thus, the surface enthalpy for anhydrous monoclinic zirconia is reported for the first time, while that for the hydrous surface is a significant improvement over the previously reported value. Combining these data with previously published surface enthalpy for nanocrystalline tetragonal zirconia, we have calculated the stability crossovers between monoclinic and tetragonal phases to take place at a particle size of 2876 nm for hydrous zirconia and 3475 nm for anhydrous zirconia. Below these particle sizes, tetragonal hydrous and anhydrous phases of zirconia become thermodynamically stable. These results are within the margin of the theoretical estimation and confirm the importance of the presence of water vapor on the transformation of nanostructured materials.

Carbonate and chloride ions mediate an ordered stacking of metal hydroxide slabs to yield ordered layered double hydroxides (LDHs) of Zn with Al, by virtue of their ability to occupy crystallographically well-defined interlayer sites. Other anions such as ClO(4)- (T(d)), BrO(3)- (C(3v)), and NO(3)- (coordination symmetry C(2v)) whose symmetry does not match the symmetry of the interlayer sites (D(3h) or O(h)) introduce a significant number of stacking faults, leading to turbostratic disorder. SO(4)(2-) ions (coordination symmetry C(3v)) alter the long-range stacking of the metal hydroxide slabs to nucleate a different polytype. The degree of disorder is also affected by the method of synthesis. Anion-exchange reactions yield a solid with a greater degree of order if the incoming ion is a CO3(2-) or Cl-. Incoming NO(3)- ions yield an interstratified phase, whereas incoming SO(4)(2-) ions generate turbostratic disorder. Conservation or its converse, elimination, of stacking disorders during anion exchange is the net result of several competing factors such as (i) the orientation of the hydroxyl groups in the interlayer region, (ii) the symmetry of the interlayer sites, (iii) the symmetry of the incoming ion, and (iv) the configuration of the anion. These short-range interactions ultimately affect the long-range stacking order or "crystallinity" of the LDH.

A combined approach using the Rietveld technique of structure refinement and DIFFaX simulations of the powder patterns enables us to not only arrive at the complete structure of the layered double hydroxides (LDHs), but also classify and quantify the nature of structural disorder. Hydrolysis of urea dissolved in mixed-metal salt solutions containing a divalent metal (Mg(2+), Co(2+)) with Al(3+) results in the homogeneous precipitation of the corresponding LDH. The products obtained are highly crystalline enabling a complete structure determination including subsequent refinement by the Rietveld method. In contrast, the LDH of Ni(2+) with Al(3+) crystallizes with the incorporation of stacking faults. A combined Rietveld-DIFFaX approach shows that even ;crystalline' samples of this LDH incorporate up to 9% of stacking faults, which are not eliminated even at elevated temperatures (473 K). These studies have implications for the order, disorder and ;crystallinity' of layered phases in general and metal hydroxides in particular.

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