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-degrees-C. Over the same temperature interval, however, HPLC results reveal a dramatic continuous evolution in the distribution of phosphate chains of comer-linked PO4 tetrahedra leading to the formation of chains up to 13 PO4 tetrahedra in length. Above 600-degrees-C, crystalline Mg2P2O7 is formed. At each annealing temperature in the interval from 150 to 600-degrees-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.