Synthetic alpha - and beta -Hopeite, two polymorphs of zinc phosphate tetrahydrates (ZPT) have been synthesized by hydrothermal crystallization from aqueous solution at 20 degrees C and 90 degrees C respectively. Aside from their sub-title crystallographic differences originating from a unique hydrogen bonding pattern, their thermodynamic interrelation has been thoroughfully investigated by means of X-Ray diffraction (XRD) and differential scanning calorimetry (DSC), combined with thermogravimetry (TGA-MS). Using a new heterogeneous step-reaction approach, the kinetics of dehydration of the two forms of ZPT was studied and their corresponding transition temperature determined. Low temperature DRIFT, FT-Raman and (1)H, (31)P MAS-NMR reveal an oriented distortion of the zinc phosphate tetrahedra, due to a characteristic hydrogen bonding pattern and in accordance with the molecular tetrahedral linkage scheme of the phosphate groups. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were also obtained and used to underline the resulting variations of chemical reactivity in zinc phosphates.
The compounds alpha- and beta-hopeite have been synthesised by hydrothermal crystallisation from aqueous solution at 90 degrees C and 20 degrees C, respectively. The crystal structures of these polymorphic forms of zinc phosphate tetrahydrate (ZPT), Zn(3)(PO(4))(2).4 H(2)O, have been resolved. Single-crystal analysis proves that the main difference between the alpha and beta forms of ZPT is caused by the difference in orientation of one of the water molecules in the ZnO(6) octahedral network, indicating two different hydrogen-bonding patterns. A previously unknown hopeite, Zn(3)(HPO(4))(3).3 H(2)O (ZHPT), has been isolated and analysed. This helps to achieve a better understanding of the mechanism of formation of zinc phosphate compounds. Unambiguous identification of each phase is established by analysis of their unique thermal behaviour and thermodynamic interrelationship.
Surface chemical reactivity of two modifications of synthetic zinc phosphate tetrahydrate (alpha - and beta -form of Hopeite, alpha -,beta -ZPT) has been studied by selective chemical and e-beam etching in presence of diluted phosphoric acid and ammonia by Scanning Electron Microscopy (SEM) and microelectrophoresis (zeta potential measurements) in correlation with the corresponding bulk properties and crystal size distributions. The subtitle crystallographic differences between alpha -and beta -ZPT originating from a unique hydrogen bonding pattern, induce drastic variations of both surface potential and surface charge. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were used to underline this resulting variation of chemical reactivity in zinc phosphates. In-situ monitoring of the transformation of Brushite in Hydroxyapatite is also reported.
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