Phase evolution during heat treatment of amorphous calcium phosphate derived from fast nitrate synthesis

The phase evolution in amorphous calcium phosphate (ACP, with a Ca/P ratio of 1 : 1), derived from the fast nitrate synthesis using different conditions, was studied in temperature range 20-980?C. ACP crystallized within 600-700?C and the phase composition depended on the synthesis duration. It was firstly revealed that for an extremely short synthesis (1min) two metastable phases ??-CPP and ??-TCP of the high-temperature calcium pyrophosphate ?-CPP and tricalcium phosphate ?-TCP were crystallized. For a longer synthesis (5min), ??- CPP and minor ?-CPP crystallized. The metastable phases gradually transformed to stable polymorphs ?-CPP and ?-TCP above 800?C, and a biphasic mixture ?-CPP/?-TCP or ?-CPP formed at 980?C. The crystallization of the metastable phases was attributed to the Ostwald step rule. A mechanism for the formation of TCP (Ca/P = 1.5) from ACP (Ca/P = 1) was proposed. The prepared powders of ?-CPP/?-TCP, ?-CPP or initial ACP were fine-grained and would have enhanced sinterability. Contribution to the densification was demonstrated due to the thermal transformation of the metastable polymorphs into stable phases having higher densities.

On the crystallization kinetics of highly soluble salts

For the spontaneous crystallization of highly soluble salts, a sufficiently high concentration of certain ionic species (complexes) or clusters has to be created in the solution, so that their grouping could yield a suitable crystal nucleus in a reasonably short time. The lowest critical supersaturation needed for nucleation and the highest rate of crystallization are displayed by those salts whose complexes in the solution have analogues in the crystal structure of the crystallizing salt, i.e., when the structure and the composition of the complexes enable their incorporation into the crystal lattice of the crystallizing salt with minimum changes. From this point of view a crystallochemical nucleation mechanism for explaining the Ostwald step rule is advanced. Concerning the rate of crystallization this concept was confirmed by studies on the system Na+, Mg2+/Cl–, SO42–//H2O and by parallel Raman spectroscopic studies of the microstructure of these solutions. It was established that upon increasing the concentration of Mg2+ ions and respective lowering of the water activity in the solution, the variety of the SO42– complexes increases. A direct correlation was found between the presence of various SO42– associations and the rate of crystallization of the corresponding salts in these systems.

Synthesis and stability of wroewolfeite, Cu4SO4(OH)6·2H2O

Titration of aqueous copper(II) sulfate solutions with aqueous NaOH at temperatures ranging from 0 to 25 °C results in a complex Ostwald step rule cascade of basic copper sulfate phases. At 25 °C, the thermodynamically stable phase is brochantite [Cu4SO4(OH)6], but posnjakite [Cu4SO4(OH)6·H2O] is formed first. At lower temperatures, but above 0 °C, wroewolfeite [Cu4SO4(OH)6·2H2O] forms first. If left in contact with the reaction solution, wroewolfeite is converted to posnjakite and brochantite in turn. However, at 0 °C, synthetic wroewolfeite is stable for periods longer than a week, even in contact with the reaction solution, and a stability constant could be determined for its formation. For the reaction below, lg K = -16.3(1) at 0 °C and I = 0, as determined by solution methods. 0.25Cu4SO4(OH)6·2H2O(s,wroewolfeite) = Cu2+(aq) + 0.25SO42-(aq) + 1.5OH-(aq) + 0.5H2O(l) Stability relations between minerals of stoichiometry Cu4SO4(OH)6·2H2O (n = 0, 1, 2) are discussed. High concentrations of Mg2+ ions (1 M) prevent the isolation of wroewolfeite at any temperature down to 0 °C.