- Email: [email protected]
FT-Raman and high-pressure infrared spectroscopic studies of dicalcium phosphate dihydrate (CaHPO4· 2H2O) and anhydrous dicalcium phosphate (CaHPO4)
Spectrochimica Acta Part A 55 (1999) 2801 – 2809 www.elsevier.nl/locate/saa
FT-Raman and high-pressure infrared spectroscopic studies of dicalcium phosphate dihydrate (CaHPO4·2H2O) and anhydrous dicalcium phosphate (CaHPO4) Jingwei Xu, Ian S. Butler, Denis F.R. Gilson * Department of Chemistry, McGill Uni6ersity, 801 Sherbrooke St., W., Montreal, Que´bec H3A 2K6, Canada Received 22 February 1999; accepted 5 March 1999
Abstract The FT-Raman spectra and the pressure dependence of the infrared spectra of the hydrated and anhydrous forms of dicalcium phosphate, CaHPO4 · 2H2O 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 CaHPO4·2H2O 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. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Calcium phosphates; Brushite; Monetite; Vibrational spectra; High pressure
1. Introduction The properties of the anhydrous and hydrated salts dicalcium phosphate, CaHPO4 and CaHPO4·2H2O, have been studied for many years as their mineral forms, monetite and brushite. As members of the series of calcium phosphates, they have considerable biological importance with respect to the biomineralization processes in bones and teeth, and find practical uses in dental cements and restorative materials. More recently, they have been investigated as protonic conduc* Corresponding author. Tel.: +1-514-398699; fax: +1514-3983797. E-mail address: [email protected] (D.F.R. Gilson)
tors [1,2]. The structures have been examined by both X-ray [3–9] and neutron diffraction [10–12] and by solid-state nmr spectroscopy [3,13,14] and the ambient pressure vibrational spectra have also received considerable attention [2,3,15–21]. Previously, we have examined the vibrational spectra of two other members of the calcium phosphate series, hydroxyapatite, Ca10(PO4)6(OH)2, and monocalcium monophosphate, Ca(H2PO4)2·H2O, as a function of pressure [22,23] in order to study the influence of pressure on the hydrogen bonding [24,25]. Both of these compounds exhibit pressure-induced phase transitions. In the present work, we extend these pressure studies to brushite and monetite.
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 0 9 0 - 6
2802
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2. Experimental Dicalcium phosphate dihydrate and anhydrate were both obtained from Aldrich Chemicals and were used without further purification. The infrared spectra were recorded on a Bruker IFS48 FT spectrometer, at 1 cm − 1 resolution, equipped with a liquid-nitrogen cooled MCT-D326 detector. The band positions were calibrated against the internal He – Ne laser. Raman spectra were obtained at 4 cm − 1 resolution using a Bruker IFS88 spectrometer coupled to an FRA 106 FTRaman module using the 1064.1 nm line of an air-cooled Nd3 + :YAG laser for excitation. The samples were contained either in a glass capillary or compacted into solid sampling cups. The maximum laser power at the sample was 320 mW. High-pressure measurements were made with a diamond anvil cell (DAC, High Pressure Diamond Optics, Tucson, AZ). The anvil was placed on the translation stage of a Bruker 8590 microscope and aligned with a Sony color video system coupled to the infrared spectrometer. The antisymmetric N – O stretching mode of the nitrate ion diluted in a sodium bromide matrix was used as an internal calibrant [26].
3. Results and discussion The infrared and Raman spectra of dicalcium phosphate dihydrate (brushite) are shown in Fig. 1. The wavenumbers, Table 1, are in general agreement with those reported in the literature [2,17,20,21] but with some minor differences; in the IR spectrum some bands have a 1 –4 cm − 1 variation and a peak at 1075 cm − 1 reported in the literature [3] was not observed in this work but appeared, under pressure, as a shoulder. The variations in the Raman spectrum occur mainly in the lattice mode region. An obvious feature in the O–H stretching region of the infrared spectrum is the occurrence of two intense doublets: one with components at 3547 and 3492 cm − 1 and the other with components at 3295 and 3171 cm − 1. The two doublets have quite different shapes as the high-wavenumber doublet consists of sharp bands but the low-wavenumber doublet is much
broader. The appearance of these two doublets is attributed to the two types of water molecules existing in the unit cell. The structure of brushite has been described as either a non-centrosymmetric, monoclinic space group Ia (Cs4) with Z= 4, or as the centrosymmetric space group Ia/2. Infrared and Raman measurements support the former, since, if the structure had a center of symmetry, then the mutual exclusion rule should apply but the strong P–O stretching mode (n1) at 988 cm − 1 in the IR spectrum is also observed at 988 cm − 1 and is the most intense peak in the Raman spectrum. The correlation between the two doublets and the structure is still subject to controversy. Lecomte and co-workers [18] have attributed the high-wavenumber pair to the ‘bound’ and the low-wavenumber doublet to the ‘free’ water molecules. On the other hand, Petrov et al. [16] believe that the high-wavenumber lines are due to a loosely bound water molecule and the lowwavenumber doublet to direct bonds to calcium atoms. Tortet and co-workers [2] have simply assigned the vibrations on the basis of the oxygen–oxygen distances, i.e. that a shorter O···O distance indicates a stronger hydrogen bond and lower vibrational frequency. The pressure dependences of the O–H stretching modes in the infrared spectrum are shown in Fig. 2. There are breaks in the wavenumber versus pressure plot at about 21 kbar indicating that there is a pressureinduced structural change. The O–H stretching frequencies of the water molecules at 3547, 3492, 3295 and 3171 cm − 1 all decrease with increasing pressure. The normal trend is for vibrational frequencies to increase with applied pressure and the different behavior observed for vibrations involving hydrogen bonds has been explained previously [24,25]. For a weakly hydrogen bonded O–H stretch, dn/dP is always negative, with higher absolute values; dn/dP is also negative but with a lower absolute value or nearly zero for medium hydrogen-bonding, and can be positive for a very strong hydrogen-bond. The water molecule designated W(1) forms two linear hydrogen bonds with the oxygen atoms of the phosphate group, with OW1···OP distances of 2.76 and 2.78 A, . The second molecule, W(2), forms two bent hydrogen bonds with the oxygen of the phosphate ion, at an
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
OW2···OP distance of 2.83 A, , and to W(1) with an unusually long OW2···OW1 distance of 3.09 A, . The designations OP, OW1 and OW2 represent the oxygens of the phosphate and of W(1) and W(2), respectively. Comparison of the phosphate and water O···O distances with the dn/dP values shows that the phosphate O – H stretch occurring
2803
at 2964 cm − 1 with a OP···OP distance 2.68 A, has a positive dn/dP value of 1.10 cm − 1 kbar − 1. The low-wavenumber doublet with dn/dP values, − 0.02 and − 0.93 cm − 1 kbar − 1, should correspond to OW1···OP distances of 2.76 and 2.78 A, , and the high-wavenumber doublet, with dn/dP values of − 0.62 and − 1.70 cm − 1 kbar − 1, relate to the
Fig. 1. Infrared spectrum (lower) and FT-Raman spectrum (upper) of dicalcium phosphate dihydrate.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2804
Table 1 Observed infrared and Raman frequencies (in cm−1) for dicalcium phosphate dihydrate, CaHPO4·2H2O, (brushite)a IR
Raman
3547 3492 3295 3171 2964 2386 2270 2128 1720 1651 1218 1141
s s s s sh w vw vw sh s s s
1066 1006 998 875 795 665 578 530
s sh s s s m s s
Assignments
O–H Stretching of water
high-wavenumber pair; the more intense peak at 3171 cm − 1 decreases with increasing pressure and ultimately became weaker than the peak at 3295 cm − 1. The effect of pressure on the 3547– 3492 cm − 1 doublet seems to be less than that for the 3495–3171 cm − 1 pair; under high pressure the 3547 cm − 1 peak still keeps the same peak shape but 3495/3171 cm − 1 doublet does not. The two
O–H Stretching 2×1218= 2436 1218+1066= 2284 2×1066= 2132 2×875= 1750 H2O Bending O–H In-plane bending 1119 w 1083 m 1060 m 998 vs 880 m 784 w, br 588 525 414 383 323 280 209 179 144 112
m m m m w w m w m w
PO Stretching
P–O(H) Stretching H2O Libration
PO Bending H2O Translation PO Bending
Lattice vibration
a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; br, broad; sh, shoulder.
longer OW2···OP and OW2···OW1 distances of 2.83 and 3.09 A, , respectively. The relative intensity changes for the O–H stretching modes are shown in Fig. 3. At ambient pressure, the 3492 cm − 1 peak has a greater intensity than that of the 3547 cm − 1 peak. With increasing pressure, the intensity of the 3492 cm − 1 peak starts to decrease at about 10 kbar, becomes weaker than that of the 3547 cm − 1 peak at 14 kbar, and then becomes a shoulder at 20 kbar. The change involved in the low-wavenumber doublet occurs as the two peaks approach each other with increasing pressure and become almost indistinguishable at 20 kbar. The intensity changes of this doublet are similar to those of the
Fig. 2. Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate in the nOH region. (a) – (d); nOH stretching modes of water; (e) nOH stretching mode of the hydrogen-bonded phosphate.
Fig. 3. Infrared spectra of dicalcium phosphate dihydrate of O – H stretching region at different pressures.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2805
Table 3 Observed infrared and Raman frequencies (in cm−1)and assignments for anhydrous dicalcium phosphate, Ca(HPO4) (monetite)a
Fig. 4. Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate. Table 2 Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate (brushite) Low-pressure phase
High pressure-phase
n (cm−1)
dn/dP (cm−1 kbar−1)
dn/dP (cm−1 kbar−1)
3547 3492 3295 3171 2964 1218 1141 1067 1006 998 875 795 665
−1.7 −0.62 −0.93 −0.17 1.10 0.38 0.64 0.31 0.52 0.48 0.67 0.29 4.41
−0.51 −0.96 −0.14 – – 0.21 0.50 0.27 – 0.34 0.55 0.71 –
doublets have approximately the same intensity at ambient pressure but the relative intensity of the 3295/3171 cm − 1 doublet becomes much weaker
IR
Raman
Assignments
3415 w 3216 w 2837 w 2369 m 1652 w 1404 m 1353 m 1176 sh 1132 s 1070 s 1016b w 996 m 893 m 866b m – 584 m 564 sh 531 m 473 sh 430 vw 405 m – – –
– – 2814 w 2421 w 1617 w – – – 1133 m 1095 m – 988 vs 901 s 779 m 693 sh 591 m 574 m 562 m 474 vw 420 m 395 m 274 vw 182 w 143 w
O–H Stretching – – PO–H Stretching – O–H In plane bending – PO Stretching (n %3) PO Stretching (n %3) PO Stretching (n %3%) – PO Stretching (n1) P–O(H) Stretching (n %3%%) O–H Out-of-plane bending – PO Bending (n %4) – PO Bending (n %4%%) 274+182= 456 PO Bending (n %2) PO Bending (n %2%) – Lattice vibration –
a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; br, broad; sh, shoulder. b Observed under pressure.
Fig. 5. Infrared spectra of dicalcium phosphate dihydrate at different pressures.
2806
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
Fig. 6. Infrared (lower) and FT-Raman (upper) spectra of anhydrous dicalcium phosphate.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2807
than that of 3547/3492 cm − 1 doublet under high pressure. The O – H stretching mode of the hydrogen phosphate ion, at 2964 cm − 1, increases with increasing pressure. Thus the effect of pressure on the intensities and the dn/dP values indicate that the behavior of the low-wavenumber doublet is consistent with medium strength hydrogen bonds and the high-wavenumber with much weaker interactions. The pressure dependences of the other vibrational frequencies are shown in Fig. 4, and the dn/dP values are given in Tables 2 and 3. All the Table 4 Pressure dependences of the vibrational frequencies of anhydrous calcium phosphate, Ca(HPO4) (frequencies at ambient pressure) n (cm−1)
dn/dP (cm−1 kbar−1)
Vib. Assign.
1353 1176 1132 1070 996 893 866
1.21 0.42 0.26 −0.11 0.29 0.67 0.75
In-plane O–H bending PO Stretching (n %3) PO Stretching (n %3) PO Stretching (n %3%) PO Stretching (n1) P–O(H) Stretching (n %3%%) Out-of-plane O–H bending
Fig. 7. Pressure dependences of infrared bands of anhydrous dicalcium phosphate.
Fig. 8. Infrared spectra of anhydrous dicalcium phosphate at different pressures; (a) ambient pressure; (b) 6.8 kbar; (d) 9.4 kbar; (d) 16.2 kbar.
modes have positive dn/dP values and show a break at 219 1 kbar where the phase transition is particularly notable for the changes in slope for the peaks at 875, 795, and 665 cm − 1 for the P–O(H) stretch, out-of-plane P–O–H bend, and water librational modes, respectively. Clearly, the hydrogen bonding of the phosphate ion and the water molecules are intimately involved in the phase transition. In addition to the pressure-induced wavenumber change for the phosphate vibrations, another variation involves the intensities of the two shoulders at 1124 and 1006 cm − 1, which decrease with increasing pressure and almost disappear at higher pressures. The dn/dP values for the phosphate group vibrations (except for the PO–H stretch) range from 0.3 to 1.0 cm − 1 kbar − 1 and are within the normal range for the pressure-induced band shifts. The pressure-induced wavenumber changes for the PO34 − anion are, therefore, normal and so this group is not involved in either new chemical bonding or new intra- or intermolecular interactions. Anhydrous dicalcium phosphate (monetite), CaHPO4, has two phases: P1at room temperature and P1 at lower temperature [4,5,7–9,11,12]. There is a reversible order/disorder phase transition involving the positions of the hydrogen atoms only [11]. Vibrational spectroscopic investigations have been reported by several authors [2,15,16,21]. The infrared and Raman spectra of
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2808
anhydrous dicalcium phosphate are shown in Fig. 5. Three kinds of linear hydrogen bonding exist in the monetite crystal with O···O distances of 2.459, 2.560 and 2.658 A, , and the hydrogen bonding must be very strong. Due broadness of the lines and the interference of the diamond cell, it was difficult to observe the pressure dependences of the O–H stretching vibrations. The pressure dependences of the infrared wavenumbers of the remaining vibrations of monetite are shown in Fig. 6, and the dn/dP values are given in Table 4. There were no obvious changes in the infrared spectra, Figs. 7 and 8, or breaks or changes in the slopes but, since the transition occurs close to room temperature, it is possible that the observations were made at pressures that had already induced the transition. The results of the high-pressure vibrational spectroscopic studies of the series of calcium phosphates show that the compounds containing either water of crystallization or the hydroxyl ion in hydroxyapatite [22], monocalcium phosphate monhydrate [23] and dicalcium phosphate dihydrate, undergo a phase transition under high pressure. The transitions occur at about 20 kbar and involve both the phosphate and the water molecules in the crystal. The dn/dP values of the O–H stretching and O – H bending modes are generally much different in the low-pressure phase from those in the high-pressure phase. Therefore
the transitions may be caused by movement of the water molecules or hydroxyl groups under pressure. The effects of pressure on the vibrations of the phosphate ions are quite similar, even though the phosphate ions are in different environments, Tables 2, 4 and 5. The dn/dP values of the symmetric P–O stretching modes (n1) of the PO34 − phosphate ion are 0.41 cm − 1 kbar − 1 in hydroxyapatite and 0.42 cm − 1 kbar − 1 in fluoroapatite, respectively. For the same n1 mode of the HPO24 − ion the values are 0.48 cm − 1 kbar − 1 (0.34 cm − 1 kbar − 1 in high-pressure phase) in dicalcium phosphate dihydrate and 0.29 cm − 1 kbar − 1 in anhydrous dicalcium phosphate. The dn/dP values of another P–O stretching mode (n3) are: 0.58 cm − 1 kbar − 1 in hydroxyapatite and 0.56 cm − 1 kbar − 1 in fluoroapatite for the PO34 − ion; 0.64 cm − 1 kbar − 1 (0.50 cm − 1 kbar − 1 in the high-pressure phase) in dicalcium phosphate dihydrate but is somewhat lower at 0.29 cm − 1 kbar − 1 for the HPO24 − ion in anhydrous dicalcium phosphate. This similarity may not be maintained when the phase transition is involved; note the variation of the dn/dP values for hydroxyapatite in the high-pressure phase. Some vibrational modes, such as in monetite at 1070 cm − 1 and the PO2 symmetric stretching mode of monocalcium phosphate monohydrate at 1092 cm − 1, have negative dn/dP values, presumably these are involved in hydrogen bonding.
Table 5 Phosphate ion frequencies (at ambient pressure) and their pressure dependences (dn/dP, cm−1 kbar−1) and assignments for the phosphate ions in calcium phosphates dn/dP n (cm−1) Ca(H2PO4) · H2O 1158 1092 962 Ca10(PO4)6(OH)2 1089 1033 963 Ca10(PO4)6F2 (No phase transition) 1095 964
Low-pressure phase
High-pressure phase
1.11 0.48
0.23 −0.74 0.35
0.58 0.46 0.41
0.06 0.06 0.08
n3 (PO3− 4 ) n3 (PO3− 4 ) n1 (PO3− 4 )
0.56 0.42
– –
n3 (PO3− 4 ) n1 (PO3− 4 )
PO2 Asym stretch PO2 Sym stretch P(OH)2 Asym stretch
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
Acknowledgements This work was supported by grants from NSERC (Canada) and FCAR (Que´bec).
References [1] L. Tortet, J.R. Gavarri, G. Nihoul, A.J. Dianoux, Solid State Ionics 97 (1997) 253. [2] L. Tortet, J.R. Gavarri, G. Nihoul, A.J. Dianoux, J. Solid State Chem. 132 (1997) 6. [3] D.W. Jones, D.W.J. Cruickshank, Z. Kristallogr. 116 (1961) 101. [4] D.W. Jones, J.A.S. Smith, J. Chem. Soc. (1962) 1414. [5] G. MacLennan, C.A. Beevers, Acta Cryst. 8 (1956) 579. [6] C.A. Beevers, Acta Cryst. 11 (1958) 273. [7] N.A. Curry, W.A. Denne, D.W. Jones, Bull. Soc. Chim. Fr. (1968) 1748. [8] W.A. Denne, D.W. Jones, J. Cryst. Mol. Struct. 1 (1971) 347. [9] B. Dickens, J.S. Bowen, W.E. Brown, Acta Cryst. B28 (1972) 797. [10] N.A. Curry, D.W. Jones, J. Chem. Soc. (A), (1971) 3275. [11] M. Catti, G. Ferraris, A. Filhol, Acta Cryst. B33 (1977) 1223.
.
2809
[12] M. Catti, G. Ferraris, A. Filhol, Acta Cryst. B36 (1980) 254. [13] D.W. Jones, J.A.S. Smith, Trans. Faraday Soc. 56 (1960) 638. [14] K. Schaumburg, E. Shabanova, P. Eichler, Magn. Reson. Chem. 33 893. [15] R. Blinc, D. Hadzi, Spectrochim. Acta 16 (1960) 853. [16] I. Petrov, B. Soptrajanov, N. Fuson, J.R. Lawson, Spectrochim. Acta 23A (1967) 2637. [17] E.E. Berry, C.B. Baddiel, Spectrochim. Acta 23A (1967) 2089. [18] J. Lecomte, A. Boulle, M. Lang-Dupont, Comp. Rend. 241 (1955) 1927. [19] F. Casciani, R.A. Condratie, Sr. Spectrosc. Lett. 12 (1979) 699. [20] M. Lang-Dupont, Bull. Soc. Chim. France, (1959) 1897. [21] G.R. Sauer, W.B. Zunic, J.R. Durig, R.E. Wuthier, Calcif. Tissue Int. 54 (1994) 414. [22] J. Xu, D.F.R. Gilson, I.S. Butler, I. Stangel, J. Biomed. Mater. Res. 30 (1996) 239. [23] J. Xu, D.F.R. Gilson, I.S. Butler, Spectrochim. Acta A54 (1998) 1869. [24] S.H. Moon, H.G. Drickhamer, J. Chem. Phys. 61 (1974) 48. [25] S.D. Hamman, M. Linton, Austr. J. Chem. 28 (1975) 2567. [26] D.D. Klug, E.J. Whalley, Rev. Sci. Instr. 54 (1983) 1205.
FT-Raman and high-pressure infrared spectroscopic studies of dicalcium phosphate dihydrate (CaHPO4·2H2O) and anhydrous dicalcium phosphate (CaHPO4) Jingwei Xu, Ian S. Butler, Denis F.R. Gilson * Department of Chemistry, McGill Uni6ersity, 801 Sherbrooke St., W., Montreal, Que´bec H3A 2K6, Canada Received 22 February 1999; accepted 5 March 1999
Abstract The FT-Raman spectra and the pressure dependence of the infrared spectra of the hydrated and anhydrous forms of dicalcium phosphate, CaHPO4 · 2H2O 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 CaHPO4·2H2O 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. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Calcium phosphates; Brushite; Monetite; Vibrational spectra; High pressure
1. Introduction The properties of the anhydrous and hydrated salts dicalcium phosphate, CaHPO4 and CaHPO4·2H2O, have been studied for many years as their mineral forms, monetite and brushite. As members of the series of calcium phosphates, they have considerable biological importance with respect to the biomineralization processes in bones and teeth, and find practical uses in dental cements and restorative materials. More recently, they have been investigated as protonic conduc* Corresponding author. Tel.: +1-514-398699; fax: +1514-3983797. E-mail address: [email protected] (D.F.R. Gilson)
tors [1,2]. The structures have been examined by both X-ray [3–9] and neutron diffraction [10–12] and by solid-state nmr spectroscopy [3,13,14] and the ambient pressure vibrational spectra have also received considerable attention [2,3,15–21]. Previously, we have examined the vibrational spectra of two other members of the calcium phosphate series, hydroxyapatite, Ca10(PO4)6(OH)2, and monocalcium monophosphate, Ca(H2PO4)2·H2O, as a function of pressure [22,23] in order to study the influence of pressure on the hydrogen bonding [24,25]. Both of these compounds exhibit pressure-induced phase transitions. In the present work, we extend these pressure studies to brushite and monetite.
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 0 9 0 - 6
2802
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2. Experimental Dicalcium phosphate dihydrate and anhydrate were both obtained from Aldrich Chemicals and were used without further purification. The infrared spectra were recorded on a Bruker IFS48 FT spectrometer, at 1 cm − 1 resolution, equipped with a liquid-nitrogen cooled MCT-D326 detector. The band positions were calibrated against the internal He – Ne laser. Raman spectra were obtained at 4 cm − 1 resolution using a Bruker IFS88 spectrometer coupled to an FRA 106 FTRaman module using the 1064.1 nm line of an air-cooled Nd3 + :YAG laser for excitation. The samples were contained either in a glass capillary or compacted into solid sampling cups. The maximum laser power at the sample was 320 mW. High-pressure measurements were made with a diamond anvil cell (DAC, High Pressure Diamond Optics, Tucson, AZ). The anvil was placed on the translation stage of a Bruker 8590 microscope and aligned with a Sony color video system coupled to the infrared spectrometer. The antisymmetric N – O stretching mode of the nitrate ion diluted in a sodium bromide matrix was used as an internal calibrant [26].
3. Results and discussion The infrared and Raman spectra of dicalcium phosphate dihydrate (brushite) are shown in Fig. 1. The wavenumbers, Table 1, are in general agreement with those reported in the literature [2,17,20,21] but with some minor differences; in the IR spectrum some bands have a 1 –4 cm − 1 variation and a peak at 1075 cm − 1 reported in the literature [3] was not observed in this work but appeared, under pressure, as a shoulder. The variations in the Raman spectrum occur mainly in the lattice mode region. An obvious feature in the O–H stretching region of the infrared spectrum is the occurrence of two intense doublets: one with components at 3547 and 3492 cm − 1 and the other with components at 3295 and 3171 cm − 1. The two doublets have quite different shapes as the high-wavenumber doublet consists of sharp bands but the low-wavenumber doublet is much
broader. The appearance of these two doublets is attributed to the two types of water molecules existing in the unit cell. The structure of brushite has been described as either a non-centrosymmetric, monoclinic space group Ia (Cs4) with Z= 4, or as the centrosymmetric space group Ia/2. Infrared and Raman measurements support the former, since, if the structure had a center of symmetry, then the mutual exclusion rule should apply but the strong P–O stretching mode (n1) at 988 cm − 1 in the IR spectrum is also observed at 988 cm − 1 and is the most intense peak in the Raman spectrum. The correlation between the two doublets and the structure is still subject to controversy. Lecomte and co-workers [18] have attributed the high-wavenumber pair to the ‘bound’ and the low-wavenumber doublet to the ‘free’ water molecules. On the other hand, Petrov et al. [16] believe that the high-wavenumber lines are due to a loosely bound water molecule and the lowwavenumber doublet to direct bonds to calcium atoms. Tortet and co-workers [2] have simply assigned the vibrations on the basis of the oxygen–oxygen distances, i.e. that a shorter O···O distance indicates a stronger hydrogen bond and lower vibrational frequency. The pressure dependences of the O–H stretching modes in the infrared spectrum are shown in Fig. 2. There are breaks in the wavenumber versus pressure plot at about 21 kbar indicating that there is a pressureinduced structural change. The O–H stretching frequencies of the water molecules at 3547, 3492, 3295 and 3171 cm − 1 all decrease with increasing pressure. The normal trend is for vibrational frequencies to increase with applied pressure and the different behavior observed for vibrations involving hydrogen bonds has been explained previously [24,25]. For a weakly hydrogen bonded O–H stretch, dn/dP is always negative, with higher absolute values; dn/dP is also negative but with a lower absolute value or nearly zero for medium hydrogen-bonding, and can be positive for a very strong hydrogen-bond. The water molecule designated W(1) forms two linear hydrogen bonds with the oxygen atoms of the phosphate group, with OW1···OP distances of 2.76 and 2.78 A, . The second molecule, W(2), forms two bent hydrogen bonds with the oxygen of the phosphate ion, at an
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
OW2···OP distance of 2.83 A, , and to W(1) with an unusually long OW2···OW1 distance of 3.09 A, . The designations OP, OW1 and OW2 represent the oxygens of the phosphate and of W(1) and W(2), respectively. Comparison of the phosphate and water O···O distances with the dn/dP values shows that the phosphate O – H stretch occurring
2803
at 2964 cm − 1 with a OP···OP distance 2.68 A, has a positive dn/dP value of 1.10 cm − 1 kbar − 1. The low-wavenumber doublet with dn/dP values, − 0.02 and − 0.93 cm − 1 kbar − 1, should correspond to OW1···OP distances of 2.76 and 2.78 A, , and the high-wavenumber doublet, with dn/dP values of − 0.62 and − 1.70 cm − 1 kbar − 1, relate to the
Fig. 1. Infrared spectrum (lower) and FT-Raman spectrum (upper) of dicalcium phosphate dihydrate.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2804
Table 1 Observed infrared and Raman frequencies (in cm−1) for dicalcium phosphate dihydrate, CaHPO4·2H2O, (brushite)a IR
Raman
3547 3492 3295 3171 2964 2386 2270 2128 1720 1651 1218 1141
s s s s sh w vw vw sh s s s
1066 1006 998 875 795 665 578 530
s sh s s s m s s
Assignments
O–H Stretching of water
high-wavenumber pair; the more intense peak at 3171 cm − 1 decreases with increasing pressure and ultimately became weaker than the peak at 3295 cm − 1. The effect of pressure on the 3547– 3492 cm − 1 doublet seems to be less than that for the 3495–3171 cm − 1 pair; under high pressure the 3547 cm − 1 peak still keeps the same peak shape but 3495/3171 cm − 1 doublet does not. The two
O–H Stretching 2×1218= 2436 1218+1066= 2284 2×1066= 2132 2×875= 1750 H2O Bending O–H In-plane bending 1119 w 1083 m 1060 m 998 vs 880 m 784 w, br 588 525 414 383 323 280 209 179 144 112
m m m m w w m w m w
PO Stretching
P–O(H) Stretching H2O Libration
PO Bending H2O Translation PO Bending
Lattice vibration
a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; br, broad; sh, shoulder.
longer OW2···OP and OW2···OW1 distances of 2.83 and 3.09 A, , respectively. The relative intensity changes for the O–H stretching modes are shown in Fig. 3. At ambient pressure, the 3492 cm − 1 peak has a greater intensity than that of the 3547 cm − 1 peak. With increasing pressure, the intensity of the 3492 cm − 1 peak starts to decrease at about 10 kbar, becomes weaker than that of the 3547 cm − 1 peak at 14 kbar, and then becomes a shoulder at 20 kbar. The change involved in the low-wavenumber doublet occurs as the two peaks approach each other with increasing pressure and become almost indistinguishable at 20 kbar. The intensity changes of this doublet are similar to those of the
Fig. 2. Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate in the nOH region. (a) – (d); nOH stretching modes of water; (e) nOH stretching mode of the hydrogen-bonded phosphate.
Fig. 3. Infrared spectra of dicalcium phosphate dihydrate of O – H stretching region at different pressures.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2805
Table 3 Observed infrared and Raman frequencies (in cm−1)and assignments for anhydrous dicalcium phosphate, Ca(HPO4) (monetite)a
Fig. 4. Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate. Table 2 Pressure dependences of the observed infrared bands of dicalcium phosphate dihydrate (brushite) Low-pressure phase
High pressure-phase
n (cm−1)
dn/dP (cm−1 kbar−1)
dn/dP (cm−1 kbar−1)
3547 3492 3295 3171 2964 1218 1141 1067 1006 998 875 795 665
−1.7 −0.62 −0.93 −0.17 1.10 0.38 0.64 0.31 0.52 0.48 0.67 0.29 4.41
−0.51 −0.96 −0.14 – – 0.21 0.50 0.27 – 0.34 0.55 0.71 –
doublets have approximately the same intensity at ambient pressure but the relative intensity of the 3295/3171 cm − 1 doublet becomes much weaker
IR
Raman
Assignments
3415 w 3216 w 2837 w 2369 m 1652 w 1404 m 1353 m 1176 sh 1132 s 1070 s 1016b w 996 m 893 m 866b m – 584 m 564 sh 531 m 473 sh 430 vw 405 m – – –
– – 2814 w 2421 w 1617 w – – – 1133 m 1095 m – 988 vs 901 s 779 m 693 sh 591 m 574 m 562 m 474 vw 420 m 395 m 274 vw 182 w 143 w
O–H Stretching – – PO–H Stretching – O–H In plane bending – PO Stretching (n %3) PO Stretching (n %3) PO Stretching (n %3%) – PO Stretching (n1) P–O(H) Stretching (n %3%%) O–H Out-of-plane bending – PO Bending (n %4) – PO Bending (n %4%%) 274+182= 456 PO Bending (n %2) PO Bending (n %2%) – Lattice vibration –
a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; br, broad; sh, shoulder. b Observed under pressure.
Fig. 5. Infrared spectra of dicalcium phosphate dihydrate at different pressures.
2806
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
Fig. 6. Infrared (lower) and FT-Raman (upper) spectra of anhydrous dicalcium phosphate.
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2807
than that of 3547/3492 cm − 1 doublet under high pressure. The O – H stretching mode of the hydrogen phosphate ion, at 2964 cm − 1, increases with increasing pressure. Thus the effect of pressure on the intensities and the dn/dP values indicate that the behavior of the low-wavenumber doublet is consistent with medium strength hydrogen bonds and the high-wavenumber with much weaker interactions. The pressure dependences of the other vibrational frequencies are shown in Fig. 4, and the dn/dP values are given in Tables 2 and 3. All the Table 4 Pressure dependences of the vibrational frequencies of anhydrous calcium phosphate, Ca(HPO4) (frequencies at ambient pressure) n (cm−1)
dn/dP (cm−1 kbar−1)
Vib. Assign.
1353 1176 1132 1070 996 893 866
1.21 0.42 0.26 −0.11 0.29 0.67 0.75
In-plane O–H bending PO Stretching (n %3) PO Stretching (n %3) PO Stretching (n %3%) PO Stretching (n1) P–O(H) Stretching (n %3%%) Out-of-plane O–H bending
Fig. 7. Pressure dependences of infrared bands of anhydrous dicalcium phosphate.
Fig. 8. Infrared spectra of anhydrous dicalcium phosphate at different pressures; (a) ambient pressure; (b) 6.8 kbar; (d) 9.4 kbar; (d) 16.2 kbar.
modes have positive dn/dP values and show a break at 219 1 kbar where the phase transition is particularly notable for the changes in slope for the peaks at 875, 795, and 665 cm − 1 for the P–O(H) stretch, out-of-plane P–O–H bend, and water librational modes, respectively. Clearly, the hydrogen bonding of the phosphate ion and the water molecules are intimately involved in the phase transition. In addition to the pressure-induced wavenumber change for the phosphate vibrations, another variation involves the intensities of the two shoulders at 1124 and 1006 cm − 1, which decrease with increasing pressure and almost disappear at higher pressures. The dn/dP values for the phosphate group vibrations (except for the PO–H stretch) range from 0.3 to 1.0 cm − 1 kbar − 1 and are within the normal range for the pressure-induced band shifts. The pressure-induced wavenumber changes for the PO34 − anion are, therefore, normal and so this group is not involved in either new chemical bonding or new intra- or intermolecular interactions. Anhydrous dicalcium phosphate (monetite), CaHPO4, has two phases: P1at room temperature and P1 at lower temperature [4,5,7–9,11,12]. There is a reversible order/disorder phase transition involving the positions of the hydrogen atoms only [11]. Vibrational spectroscopic investigations have been reported by several authors [2,15,16,21]. The infrared and Raman spectra of
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
2808
anhydrous dicalcium phosphate are shown in Fig. 5. Three kinds of linear hydrogen bonding exist in the monetite crystal with O···O distances of 2.459, 2.560 and 2.658 A, , and the hydrogen bonding must be very strong. Due broadness of the lines and the interference of the diamond cell, it was difficult to observe the pressure dependences of the O–H stretching vibrations. The pressure dependences of the infrared wavenumbers of the remaining vibrations of monetite are shown in Fig. 6, and the dn/dP values are given in Table 4. There were no obvious changes in the infrared spectra, Figs. 7 and 8, or breaks or changes in the slopes but, since the transition occurs close to room temperature, it is possible that the observations were made at pressures that had already induced the transition. The results of the high-pressure vibrational spectroscopic studies of the series of calcium phosphates show that the compounds containing either water of crystallization or the hydroxyl ion in hydroxyapatite [22], monocalcium phosphate monhydrate [23] and dicalcium phosphate dihydrate, undergo a phase transition under high pressure. The transitions occur at about 20 kbar and involve both the phosphate and the water molecules in the crystal. The dn/dP values of the O–H stretching and O – H bending modes are generally much different in the low-pressure phase from those in the high-pressure phase. Therefore
the transitions may be caused by movement of the water molecules or hydroxyl groups under pressure. The effects of pressure on the vibrations of the phosphate ions are quite similar, even though the phosphate ions are in different environments, Tables 2, 4 and 5. The dn/dP values of the symmetric P–O stretching modes (n1) of the PO34 − phosphate ion are 0.41 cm − 1 kbar − 1 in hydroxyapatite and 0.42 cm − 1 kbar − 1 in fluoroapatite, respectively. For the same n1 mode of the HPO24 − ion the values are 0.48 cm − 1 kbar − 1 (0.34 cm − 1 kbar − 1 in high-pressure phase) in dicalcium phosphate dihydrate and 0.29 cm − 1 kbar − 1 in anhydrous dicalcium phosphate. The dn/dP values of another P–O stretching mode (n3) are: 0.58 cm − 1 kbar − 1 in hydroxyapatite and 0.56 cm − 1 kbar − 1 in fluoroapatite for the PO34 − ion; 0.64 cm − 1 kbar − 1 (0.50 cm − 1 kbar − 1 in the high-pressure phase) in dicalcium phosphate dihydrate but is somewhat lower at 0.29 cm − 1 kbar − 1 for the HPO24 − ion in anhydrous dicalcium phosphate. This similarity may not be maintained when the phase transition is involved; note the variation of the dn/dP values for hydroxyapatite in the high-pressure phase. Some vibrational modes, such as in monetite at 1070 cm − 1 and the PO2 symmetric stretching mode of monocalcium phosphate monohydrate at 1092 cm − 1, have negative dn/dP values, presumably these are involved in hydrogen bonding.
Table 5 Phosphate ion frequencies (at ambient pressure) and their pressure dependences (dn/dP, cm−1 kbar−1) and assignments for the phosphate ions in calcium phosphates dn/dP n (cm−1) Ca(H2PO4) · H2O 1158 1092 962 Ca10(PO4)6(OH)2 1089 1033 963 Ca10(PO4)6F2 (No phase transition) 1095 964
Low-pressure phase
High-pressure phase
1.11 0.48
0.23 −0.74 0.35
0.58 0.46 0.41
0.06 0.06 0.08
n3 (PO3− 4 ) n3 (PO3− 4 ) n1 (PO3− 4 )
0.56 0.42
– –
n3 (PO3− 4 ) n1 (PO3− 4 )
PO2 Asym stretch PO2 Sym stretch P(OH)2 Asym stretch
J. Xu et al. / Spectrochimica Acta Part A 55 (1999) 2801–2809
Acknowledgements This work was supported by grants from NSERC (Canada) and FCAR (Que´bec).
References [1] L. Tortet, J.R. Gavarri, G. Nihoul, A.J. Dianoux, Solid State Ionics 97 (1997) 253. [2] L. Tortet, J.R. Gavarri, G. Nihoul, A.J. Dianoux, J. Solid State Chem. 132 (1997) 6. [3] D.W. Jones, D.W.J. Cruickshank, Z. Kristallogr. 116 (1961) 101. [4] D.W. Jones, J.A.S. Smith, J. Chem. Soc. (1962) 1414. [5] G. MacLennan, C.A. Beevers, Acta Cryst. 8 (1956) 579. [6] C.A. Beevers, Acta Cryst. 11 (1958) 273. [7] N.A. Curry, W.A. Denne, D.W. Jones, Bull. Soc. Chim. Fr. (1968) 1748. [8] W.A. Denne, D.W. Jones, J. Cryst. Mol. Struct. 1 (1971) 347. [9] B. Dickens, J.S. Bowen, W.E. Brown, Acta Cryst. B28 (1972) 797. [10] N.A. Curry, D.W. Jones, J. Chem. Soc. (A), (1971) 3275. [11] M. Catti, G. Ferraris, A. Filhol, Acta Cryst. B33 (1977) 1223.
.
2809
[12] M. Catti, G. Ferraris, A. Filhol, Acta Cryst. B36 (1980) 254. [13] D.W. Jones, J.A.S. Smith, Trans. Faraday Soc. 56 (1960) 638. [14] K. Schaumburg, E. Shabanova, P. Eichler, Magn. Reson. Chem. 33 893. [15] R. Blinc, D. Hadzi, Spectrochim. Acta 16 (1960) 853. [16] I. Petrov, B. Soptrajanov, N. Fuson, J.R. Lawson, Spectrochim. Acta 23A (1967) 2637. [17] E.E. Berry, C.B. Baddiel, Spectrochim. Acta 23A (1967) 2089. [18] J. Lecomte, A. Boulle, M. Lang-Dupont, Comp. Rend. 241 (1955) 1927. [19] F. Casciani, R.A. Condratie, Sr. Spectrosc. Lett. 12 (1979) 699. [20] M. Lang-Dupont, Bull. Soc. Chim. France, (1959) 1897. [21] G.R. Sauer, W.B. Zunic, J.R. Durig, R.E. Wuthier, Calcif. Tissue Int. 54 (1994) 414. [22] J. Xu, D.F.R. Gilson, I.S. Butler, I. Stangel, J. Biomed. Mater. Res. 30 (1996) 239. [23] J. Xu, D.F.R. Gilson, I.S. Butler, Spectrochim. Acta A54 (1998) 1869. [24] S.H. Moon, H.G. Drickhamer, J. Chem. Phys. 61 (1974) 48. [25] S.D. Hamman, M. Linton, Austr. J. Chem. 28 (1975) 2567. [26] D.D. Klug, E.J. Whalley, Rev. Sci. Instr. 54 (1983) 1205.