FT-Raman and high-pressure infrared spectroscopic studies of dicalcium phosphate dihydrate (CaHPO4•2H2O) and anhydrous dicalcium phosphate (CaHPO4)

The FT-Raman spectra and the pressure dependence of the infrared spectra of the hydrated and anhydrous forms of dicalcium phosphate, CaHPO4. 2H(2)O and CaHPO4, have been studied. The hydrated salt exhibits a phase transition at 21 kbar (1.0 kbar = 0.1 Gpa) but no high pressure transition was observed for anhydrous dicalcium phosphate. The O-H stretching frequencies of the water molecules in CaHPO4. 2H(2)O all showed negative pressure dependences and correlate with the O ... O distances. The PO-H stretch increased with increasing pressure, indicating a strong hydrogen bond. The frequencies associated with the phosphate ion showed a normal pressure dependence. (C) 1999 Elsevier Science B.V. All rights reserved.