Solid Solutions of Lindbergite–Glushinskite Series: Synthesis, Ionic Substitutions, Phase Transformation and Crystal Morphology

To clarify the crystal chemical features of natural and synthetic oxalates Me2+(C2O4)∙2H2O (Me2+ = Fe, Mn, Mg, Zn), including minerals of the humboldtine group, solid solutions of lindbergite Mn(C2O4)∙2H2O–glushinskite Mg(C2O4)∙2H2O were precipitated under various conditions, close to those characteristic of mineralization in biofilms: at the stoichiometric ratios ((Mn + Mg)/C2O4 = 1) and non-stochiometric ratios ((Mn + Mg)/C2O4 Fddd), while lindbergite has a monoclinic α-modification (sp. gr. C2/c). Mg ions incorporate lindbergite in much higher quantities than Mn ions incorporate glushinskite; moreover, Mn glushinskites are characterized by violations of long-range order in their crystal structure. Lindbergite–glushinskite transition occurs abruptly and can be classified as a first-order isodimorphic transition. The Me2+/C2O4 ratio and the presence of citric acid in the solution affect the isomorphic capacity of lindbergite and glushinskite, the width of the transition and the equilibrium Mg/Mn ratio. The transition is accompanied by continuous morphological changes in crystals and crystal intergrowths. Given the obtained results, it is necessary to take into account in biotechnologies aimed at the bioremediation/bioleaching of metals from media containing mixtures of cations (Mg, Mn, Fe, Zn)

Crystal Chemistry of the Copper Oxalate Biomineral Moolooite: The First Single-Crystal X-ray Diffraction Studies and Thermal Behavior

Moolooite, Cu(C2O4)·nH2O, is a typical biomineral which forms due to Cu-bearing minerals coming into contact with oxalic acid sources such as bird guano deposits or lichens, and no single crystals of moolooite of either natural or synthetic origin have been found yet. This paper reports, for the first time, on the preparation of single crystals of a synthetic analog of the copper-oxalate biomineral moolooite, and on the refinement of its crystal structure from the single-crystal X-ray diffraction (SCXRD) data. Along with the structural model, the SCXRD experiment showed the significant contribution of diffuse scattering to the overall diffraction data, which comes from the nanostructural disorder caused by stacking faults of Cu oxalate chains as they lengthen. This type of disorder should result in the chains breaking, at which point the H2O molecules may be arranged. The amount of water in the studied samples did not exceed 0.15 H2O molecules per formula unit. Apparently, the mechanism of incorporation of H2O molecules governs the absence of good-quality single crystals in nature and a lack of them in synthetic experiments: the more H2O content in the structure, the stronger the disorder will be. A description of the crystal structure indicates that the ideal structure of the Cu oxalate biomineral moolooite should not contain H2O molecules and should be described by the Cu(C2O4) formula. However, it was shown that natural and synthetic moolooite crystals contain a significant portion of “structural” water, which cannot be ignored. Considering the substantially variable amount of water, which can be incorporated into the crystal structure, the formula Cu(C2O4)·nH2O for moolooite is justified

Ti-Modified Hydroxyapatites: Synthesis, Crystal Chemistry, and Photocatalytic Activity

The structural nature of photocatalytic properties of hydroxyapatites (HAp) synthesized from Ti-containing media (of Ti-modified hydroxyapatites) needs clarification. We synthesized hydroxyapatites from Ti-containing water solutions under various conditions and studied the received powder precipitations (before and after calcination at a temperature of 700 °C for 6 h) by a wide set of methods: powder X-ray diffraction (PXRD); vibrational, energy-dispersive X-ray, X-ray photoelectron, and diffuse reflectance spectroscopy; scanning electron microscopy, and gas phase photocatalytic activity tests. Analyzing the variations of unit cell parameters, we have proved that titanium is able to incorporate into the hydroxyapatite lattice in amounts of up to ∼10 wt % depending on synthesis conditions. At low concentrations (Ti/Ca ≤ 0.16), Ti4+ ions incorporate predominantly to the P site of hydroxyapatite structure. At higher concentrations (0.16 ≤ Ti/Ca ≤ 0.28), additional Ti4+ ions incorporate Ca sites. Besides, a significant share of titanium forms predominantly amorphous impurity phases, which transform into crystalline TiO2 (anatase and rutile) upon calcination and are often undetectable by PXRD. The presence of crystalline titanium oxides (anatase and rutile) has a crucial effect on photocatalytic activity of calcined Ti-modified hydroxyapatites. Synthesized HAp/TiO2 compositions are biocompatible heterostructured materials with photocatalytic activity comparable to that of commercial photocatalysts and could be recommended for medical–biological applications