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

Abstract

In order to study the structure and dynamics of calcium–lead hydroxyapatites, Ca10−xPbx(PO4)6(OH)2, the Raman spectra of Pb10(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 Pb10(PO4)6(OH)2 occurs in two steps at 420 and 510 K. The hexagonal structure was not evidenced in the lead–calcium compound and only the first transition was observed near 420 K. At room temperature, OH− ions in pure lead hydroxyapatite (monoclinic form) are fully ordered within c‐axis columns. An increase in temperature leads to a progressive disordering of these anions, resulting in the stabilisation of the hexagonal phase. Ca4Pb6(PO4)6(OH)2 has a disordered structure at room temperature compared with pure lead hydroxyapatite. The two broad ν OH− bands observed at 3567 and 3582 cm−1 confirm this interpretation. The existence of hydrogen bonds between the OH− and the nearest PO43− is also confirmed with Raman spectroscopy. Copyright © 2001 John Wiley & Sons, Ltd.

Hydrothermal Synthesis and Crystallographic Study of Sr-Pb Hydroxyapatite Solid Solutions

“…4 In the apatite structure, the Ca ions occupy two types of nonequivalent sites: the M 1 sites are at the fourfold symmetry4 f position and the M 2 sites are at the sixfold symmetry 6 h position. 5 The apatite structure has a high flexibility, so that many metal ions can incorporate in the structure. It is interesting to investigate the site of metal ions which incorporate in the apatite structure.…”

Crystallographic study of lead-substituted hydroxyapatite synthesized by high-temperature mixing method under hydrothermal conditions

“…Calcium hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 , designated as Ca-HA], is a primary constituent of vertebral animal's hard tissues. The synthetic Ca-HA has attracted our attention for its utility in the fields of bioceramics, catalysts, adsorbents, and so on [1].…”

“…In the apatite structure, the Ca ions occupy two types of nonequivalent sites: the M (1) sites are at the fourfold symmetry 4(f) position and the M(2) sites are at the sixfold symmetry 6(h) position [2]. The apatite structure has a high flexibility, so many metal ions can be incorporated into the structure.…”

Synthesis and crystallographic study of Pb–Sr hydroxyapatite solid solutions by high temperature mixing method under hydrothermal conditions

“…In the apatite structure, the Ca ions occupy two types of nonequivalent sites: the M(1) sites are at the fourfold symmetry4(f) position and the M(2) sites are at the sixfold symmetry 6(h) position [5]. The apatite structure has a high flexibility, so many metal ions can be incorporated into the structure.…”