Polarized Raman spectra of brushite (CaHPO₄.2H₂O) crystal. Investigation of the phosphate stretching modes, study of the LOTO splitting

Abstract

Polarized Raman measurements were recorded on a monoclinic brushite (CaHPO 4 .2H 2 O) crystal in the 800-1200 cm À1 spectral range, corresponding to the P-O stretching modes. This study is a continuation of the investigation of the phosphate stretching modes observed in polarized infrared reflectance spectra of brushite crystal. In such ionic non-centrosymmetric crystals, the splitting between the transverse and longitudinal components of the optic vibrations was observed in the polarized Raman spectra recorded at several scattering geometries. A″ symmetry modes of the brushite crystal were measured. Using a simple model based on the symmetry of the PO 4 group, the Raman intensities of the stretching modes are calculated and compared with experimental bands. IntroductionDi(basic) calcium phosphate dihydrate (DCPD; CaHPO 4 .2H 2 O) is also known as the rare mineral 'brushite'. This mineral was discovered in 1865 by G. E. Moore [1] who called it 'brushite' in honor of Professor George Jarvis Brush an American mineralogist (Yale University, New Haven, Connecticut, USA). DCPD can be found [2][3][4] in pathological calcifications and some carious lesions. In bone surgery, DCPD is used as calcium phosphate cements, [5][6][7][8] and in dentistry, [9][10][11][12][13] it is added to toothpaste both for caries protection and as a polishing agent. In the dairy industry, DCPD is used as a mineral supplement as its importance as a constituent of infant's food was discovered [14] as early as 1917. In the food industry, it serves as a texturizer, bakery improver and water retention additive. [15] Other applications [16,17] have been reported including flame retardant, slow-release fertilizer, anticorrosive and passivating pigment for some ground coat paints.The crystalline structure of DCPD has been investigated by several groups. [18][19][20][21] This compound as well as other isostructural ones (gypsum and pharmacolite [22,23] ) is described by a noncentrosymmetric monoclinic space group Ia with, for brushite, the following cell parameters a = 5.812 Å; b = 15.180 Å; c = 6.239 Å; β = 116.25°and Z = 4. Large tabular thin-plate transparent crystals can be obtained.Among the vibrations of this compound, the P-O stretching modes are located in the 800-1200-cm À1 wavenumber range where no other bands are expected. In Raman scattering, as in infrared (IR) spectroscopy, these bands are very intense and well separated; moreover, the spectra exhibit a strong molecular character associated with the internal modes of PO 4 group. [24] Finally, in several calcium orthophosphate compounds, changes in the structure strongly affect these modes, for example, calcium substitution by other cations. [25][26][27] That is why, the study of these bands is especially interesting, and we focused our work on these modes.Four P-O stretching, vibrations are combined in the phosphate tetrahedron to give four modes, numbered [28] Q 1 to Q 4 in increasing wavenumber order (with the pseudo-symmetry [29] C 3v ). Considering the space group Ia and the presenc...

Comparison of the Morphological and Structural Characteristic of Bioresorbable and Biocompatible Hydroxyapatite-Loaded Biopolymer Composites

“…The Raman spectra of CaP1 shows the most intense band of ν 1 PO 4 vibration, which is shifted towards a higher wavenumber (986 cm −1 ), and smaller bands appear at 1130, 1082, 1089, and 880 cm −1 . These bands are typical of brushite (CaHPO 4 •2H 2 O) [46]. For the CaP2 sample, the IR spectrum indicated Ca pyrophosphate phase, and its Raman spectrum is quite different from those of hydroxyapatites.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Lattice dynamics has been simulated and polarization dependencies of lines intensities have been analyzed to interpret experimental Raman spectra and to attribute lines to the spectra of monoclinic and orthorhombic phases . Quillard and co‐workers have measured the polarized Raman spectra of brushite (CaHPO 4 .2H 2 O) crystal in order to investigate the phosphate stretching modes and study of the longitudinal and transverse optical phonon splitting . Wang et al reported the temperature‐dependent Raman spectra of microstructure present in dipotassium molybdate crystals and their melts.…”

“…[137] Quillard and co-workers have measured the polarized Raman spectra of brushite (CaHPO 4 .2H 2 O) crystal in order to investigate the phosphate stretching modes and study of the longitudinal and transverse optical phonon splitting. [138] Wang et al reported the temperature-dependent Raman spectra of microstructure present in dipotassium molybdate crystals and their melts. The mechanisms of the microstructural evolution of K 2 Mo n O 3n + 1 (n = 1, 2, 3) crystals while being melted were schematically illustrated.…”