STRUCTURAL-PROPERTIES OF THE AMORPHOUS PHASES PRODUCED BY HEATING CRYSTALLINE MGHPO4.3H2O

The crystalline phosphate MgHPO4.3H2O (newberyite) undergoes an unusua l crystalline-to-amorphous transition when heated. The structural alte rations associated with this transition have been investigated using h igh-performance liquid chromatography (HPLC), differential scanning ca lorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffracti on (XRD). The dehydration of newberyite on heating results in amorphou s XRD patterns that remain essentially unchanged over the interval bet ween 150 and 600-degrees-C. Over the same temperature interval, howeve r, HPLC results reveal a dramatic continuous evolution in the distribu tion of phosphate chains of comer-linked PO4 tetrahedra leading to the formation of chains up to 13 PO4 tetrahedra in length. Above 600-degr ees-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 anion s. The results of HPLC, DSC, and XRD measurements on CaHPO4.2H2O, SrHP O4, and BaHPO4 are also reported.