Structural properties of the amorphous phases produced by heating crystalline MgHPO₄·3H₂O

The crystalline phosphate MgHPO₄·3H₂O (newberyite) undergoes an unusual crystalline-to-amorphous transition when heated. The structural alterations associated with this transition have been investigated using high-performance liquid chromatography (HPLC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The dehydration of newberyite on heating results in amorphous XRD patterns that remain essentially unchanged over the interval between 150 and 600°C. Over the same temperature interval, however, HPLC results reveal a dramatic continuous evolution in the distribution of phosphate chains of corner-linked PO₄ tetrahedra leading to the formation of chains up to 13 PO₄ tetrahedra in length. Above 600°C, crystalline Mg₂P₂O₇ is formed. At each annealing temperature in the interval from 150 to 600°C, the measured distribution of phosphate anions is in quantitative agreement with theory. During the dehydration-induced crystalline-to-amorphous transition, the original faceted crystal shape is preserved even though the crystals lose up to 36% of their original weight. High-pressure DSC experiments in which the waters of hydration were retained in the specimens during heating resulted in the formation of a unique new crystalline phosphate phase that contained equal amounts of orthophosphate and pyrophosphate anions. The results of HPLC, DSC, and XRD measurements on CaHPO₄·2H₂O, SrHPO₄, and BaHPO₄ are also reported.