Monoclinic to hexagonal phase transition and hydroxyl motion in calcium-lead hydroxyapatites studied by Roman spectroscopy

In order to study the structure and dynamics of calcium-lead hydroxyapatite s, Ca10-xPbx(PO4)(6)(OH)(2), the Raman spectra of Pb-10(PO4)(6)(OH)(2) and Ca4Pb6(PO4)(6)(OH)(2) are presented between 300 and 650 K in the 4000-10 cm (-1) wavenumber range. For the two compounds, structural phase transitions are evidenced. The reversible structural transformation from monoclinic to hexagonal observed for Pb-10(PO4)(6)(OH)(2) occurs in two steps at 420 and 510 K. The hexagonal structure was not evidenced in the lead-calcium compou nd and only the first transition was observed near 420 K. At room temperatu re, OH- ions in pure lead hydroxyapatite (monoclinic form) are fully ordere d within c-axis columns. An increase in temperature leads to a progressive disordering of these anions, resulting in the stabilisation of the hexagona l phase. Ca4Pb6(PO4)(6)(OH)(2) has a disordered structure at room temperatu re compared with pure lead hydroxyapatite. The two broad nu OH- bands obser ved at 3567 and 3582 cm(-1) confirm this interpretation. The existence of h ydrogen bonds between the OH- and the nearest PO43- is also confirmed with Raman spectroscopy. Copyright (C) 2001 John Wiley & Sons, Ltd.