6752
J. Phys. Chem. 1995,99, 6752-6754
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Monoclinic Hexagonal Phase Transition in Hydroxyapatite Studied by X-ray Powder Diffraction and Differential Scanning Calorimeter Techniques Hiroyuki Suda,* Masatomo Y ashima, Masato Kakihana, and Masahiro Yoshimura Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226, Japan Received: December 22, 1994@
We investigated the transition between the monoclinic (P21/b) and hexagonal (P64m) phases of hydroxyapatite, Ca5(P04)30H, both by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) techniques. Peak intensities of XRD of the low-temperature monoclinic phase were almost unchanged up to 473 K but dropped abruptly to zero above 483 K. Endo- and exothermic DSC peaks and a specific heat anomaly associated with the transition were observed. The transition enthalpy and entropy were estimated to be 630 f 25 J/mol and 1.30 f 0.05 J/molK, respectively.
Introduction The physical and chemical properties of any condensed matter largely depend on its crystal structure. Especially the knowledge about abrupt change of crystal structure, i.e. structural phase transition, is significantly important not only for pure science but for technical application under various conditions. Hydroxyapatite, Cas(PO&OH (HAP), continues to attract considerable interest, since it is the main inorganic component of bones and teeth and has biocompatibility with living bodies. HAp and its compositionally modified analogs have numerous functions and are used as catalysts, media for chromatograms, one-dimensional ionic conductors, and humidity sensors, to name a few. It should be essential to know whether HAp undergoes a structural phase transition or not, since it affects the various properties of HAP. At room temperature, HAp has monoclinic (M) symmetry and the space group P211b.l In the M form, OH ions are fully ordered within c-axis columns and the columns are ordered relative to each other.' The chlorinesubstituted apatite, Ca5(P04)3Cl (ClAp), is also in the M phase at room temperature2 and exhibits a structural phase transition to hexagonal (H) symmetry at about 473 K.3 These studies suggest the possibility of M H phase transition in HAp and led to the finding of the disappearance of birefringence in HAp at 484.7(5) K.4 The disappearance of birefringence was interpreted to be ascribed to a simple order-disorder phase transition between the low-temperatureM and high-temperature H phases similar to that found in CIAP.~ Althowh both structural and thermal studies are indispensable for the confirmation of a phase transition, neither in-situ neutron diffraction5 nor specific heat studies6-10 revealed any phase transition in HAP. The relationship between reorientation of OH ions5*'1-15 and the phase transition is far from understood. Here we investigated the M H phase transition in HAp through combined X-ray powder diffraction and differential scanning calorimeter techniques. These observations led us to confirm the phenomenon.
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Experimental Section Pure and stoichiometric HAp in the monoclinic form was prepared by a solid state reaction of stoichiometric amounts
* To whom correspondence should be addressed. Resent address: National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305, Japan. @Abstractpublished in Advance ACS Abstracts, April 1, 1995. 0022-365419512099-6752$09.00/0
(CaP = 1.67) of reagent grade CaHP04QH20(Kanto Chemical, Japan) and CaC03 (Wako Chemical, Japan). The compounds were mixed and ground in an agate mortar with methanol for 1 h and then pressed into pellets at 200 MPa. The pellets mounted on a Pt crucible were ignited at 1273 K for 10 h in vacuo, in order to remove H2O and C02 gas. Then, the pellets were crushed, ground, pressed into pellets, and calcined at 1173 K for 10 h under flowing pure Ar gas saturated with CO2-free O 23.8 " H g ) . The heating and cooling water vapor ( P H ~= rates were 10 and 5 Wmin, respectively. The latter heating cycle was repeated three times until the reaction reached completion. The purity and compositional stoichiometry of HAP were confiied by various methods. The X-ray diffraction profile and infrared spectrum revealed that the HAp had neither additional crystalline phases such as ,!?-Ca3(PO4)2nor CaO or carbonate ions. The chemical analysis showed the calcium content, 39.87 wt % (theoretical 39.84 wt %), phosphorus content, 18.37 wt % (18.53 wt %), and Ca/P molar ratio 1.68 (1.67). The lattice parameters of the HAP (a0 = 9.4187(5) A, co = 6.8805(2) A) refined by the least-squares method using an inner standard Si (NBS-640, a0 = 5.43088A) agreed well with the standard values for ideal and stoichiometric HAP (a0 = 9.4184 A, co = 6.8800 A).16 The high-temperature X-ray diffraction (XRD) experiment was conducted with a diffractometer model MXP18 (MAC Science, Co. Ltd., Tokyo, Japan) from 298 to 573 K, where the Cu K a radiation was monochromatized by curved graphite. The sample mounted on a Pt holder was heated by Pt heaters under pure N2 flow (50 mllmin). The accuracy of the sample temperature was f 3 K. Data were collected keeping the sample temperature constant within f 0 . 3 K. The XRD profile at each temperature was obtained from 20" to 55" in 28 degree using the step scan mode after verifying the temperature stabilization. Rietveld profile-fitting was performed by the program RIETAN." Thermal analysis was conducted by a differential scanning calorimeter (DSC model 3200, MAC Science, Co. Ltd., Tokyo, Japan) from 298 to 573 K with a heating and cooling rate of 1 Wmin under flowing 99.99% N2 gas. About 50 mg of the HAP was used for measurement. The sample temperature and heat of transition including the instrumental cell constant were calibrated by a least-squares fitting of the measured melting points and enthalpies of standard 99.999% lead, 99.99% indium, and 99.99% tin. The reproducibility for three runs was within 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 17, 1995 6753
Phase Transition in Hydroxyapatite
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Figure 1. Observed (dots) and fitted (solid line) X-ray powder diffraction profiles of hydroxyapatite at (a) 298, (b) 473, and (c) 483 K. The abscissa scale is in 28 degrees (Cu K a radiation). The leastsquares refinements were performed in space groups P2Jb (M) and P 6 3 h (H) for the XRD profiles obtained at 298 and 473 K, and 483 K, respectively. Correspondent Miller indices in monoclinic symmetry are shown. The position of each reflection and the difference between the observed and fitted profiles (I, - I,) at 298 K are also shown by vertical bars and a solid line, respectively. f 0 . 3 K and f l % for melting points and enthalpies, respectively. The standard deviation for the heat of transition was estimated from three runs. The specific heat capacity, Cp, of HAp was obtained by the same DSC apparatus from 298 to 573 K with a heating rate of 10 Wmin under flowing 99.99% N2 gas. The temperature dependencies of the heat capacities of the blank A1 pan and standard calcined a-Al2O3 were measured to calibrate the Cp data of HAP.
Results The XRD profile of pure HAP at 298 K was successfully fitted only in the space group P21/b, particularly for the ‘extra’ reflections at 28 = 35.7-37.0’ (Figure 1). The superstructure reflections were indexed on the basis of the monoclinic symmetry as 231,371,212,252, 151,171, which are forbidden for the hexagonal symmetry (space group P6dm). The structural parameters including atomic coordinates were identical to those reported’ within the estimated standard deviation. On heating, the superstructure reflection intensities remained almost unchanged up to 473 K. The most prominent feature in the diffraction profiles was the disappearance of the superstructure reflections at 483 K (Figures 1 and 2). The XRD profile at 483 K was fitted in the space group P63lm. After the sample was cooled to room temperature, the same intensity of the superstructure reflections as before heating was obtained. The results clearly indicate the reversible M (P21/6) H (P63h) structural phase transition between 473 and 483 K. DSC curves (Figure 3) show the endo- and exothermic peaks at onset temperatures of 480.3 and 479.7 K during heating and cooling, respectively. The onset was estimated using a straight line tangent to the DSC curve where the curve shows maximum slope with respect to temperature. No apparent thermal hysteresis was observed within the experimental error. The enthalpy and entropy of the transition estimated from the peak
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Temperature I K Figure 3. Differential scanning calorimetry curves of hydroxyapatite, showing the endo- and exothermic peaks at onset temperatures of 480.0 f 0.3 K during heating and cooling, respectively. Note that the temperature at which the heat anomaly was observed agreed well with that at which the superstructure reflection disappeared, indicating the heat anomaly associated with M H phase transition. intensity were AHaans= 630 f 25 J/mol and ASaan,= 1.30 f 0.05 Jlmol-K, respectively. Figure 4a shows the specific heat capacity, Cp, of HAP, compared with those r e p ~ r t e d . ~The ,~,~ obtained Cp data roughly lie on the previously reported ~ u r v e . ~ , ~ Of particular importance is that the heat anomaly which had not been detected can clearly be seen in Figure 4b. The temperature at which the heat anomaly was detected in the DSC curve (480 K in Figure 3) was in good agreement with the phase transition point Tc observed by XRD (473 K < Tc < 483 K). Hence, the heat anomaly is evidently ascribed to the structural M H phase transition. Up to now, no heat anomaly had been d e t e ~ t e d ~nor -~ theoretically deduced.’O One of the most probable reasons why no anomaly was detected in the heat capacity measurements6,’ is that they were done under discontinuous heating with large temperature increments of about 100 K. Another reason might be the presence of impurities in the sample. Any impurities or vacancies are considered to act as a “reversible point” at which the ordered OH ions can be reversed,18 resulting in the stabilization of the hexagonal phase. The decrease of the transition temperature of ClAp with increasing vacancy content3 supports this reversible point model. Also, the smaller unit cell parameter = 9.376(4) A of HAp used in the Cp measurement9
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6754 J. Phys. Chem., Vol. 99, No. 17, 1995
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The structural refinement via an in-situ neutron single-crystal diffraction method5 was made assuming only the timehpace averaged space group P6dm and not P21/b. Moreover, diffraction data collected only at 298, 673, and 1073 K did not reveal structural evolution in the vicinity of T,. Most of the spectroscopic s t u d i e ~ ~ J demonstrated ~-'~ continuous changes of spectra in a wide temperature range of 80-873 K. On the other hand, the superstructure reflection intensities remained unchanged up to 473 K but abruptly decreased to zero above 483 K (Figures 1 and 2). The present work also showed the heat anomaly in a narrow temperature range (Figures 3 and 4). Thus, the continuous changes in the spectroscopic studies are not directly connected with the M H phase transition but other structural changes in HAP. In summary, we demonstrated the monoclinic (P211b) hexagonal (P63h) phase transition in HAP both by in-situ XRD and DSC measurements. Clarifying the nature of the phase transition in HAp is of great importance for a theoretical understanding of various physical and chemical properties related to the configurations of OH ions and other constituents. The detailed analysis of the crystal structure in the vicinity of Tc is now in progress.
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Acknowledgment. We would like to express our thanks to Mr. T. Komata, Mr. K. Takahashi, and Mr. A. Asai (MAC Science, Co. Ltd., Tokyo, Japan) for use of X-ray diffraction and DSC apparatus. Experimental support from Ceramics Research Center in our institute is also acknowledged. References and Notes
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Temperature / K Figure 4. Specific heat capacity C p curves of hydroxyapatite: (a) a comparison with the previously reported v a l ~ e s ~ .and ' . ~ (b) a magnified C p curve in the present study, showing an anomaly associated with M H phase transition. Note that the observed peak temperature 483 K is a little higher than that in Figure 3 due to the higher heating rate of 10 Wmin (Figure 4) than 1 Wmin (Figure 3).
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compared with that of stoichiometric HAP (a= 9.4184 A)16 suggests the presence of either impurities or vacancies. This is probably the most important reason why no heat anomaly has been detected so far. In contrast, we succeeded in detecting the heat anomaly associated with the M H phase transition in the pure and stoichiometric HAP. Structural changes in HAP at elevated temperatures have been investigated by various methods such as optical microscopy,4 neutron diffra~tion,~ dielectric relaxation,' thermally stimulated current,'* proton nuclear magnetic resonance,13 and in-situ infrared s p e c t r o ~ c o p y .In-situ ~ ~ ~ ~optical ~ ~ ~ microscope observation by Van Rees et al? is consistent with the present results.
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(1) Elliott, J. C.; Mackie, P. E.; Young, R. A. Science 1973,180, 1055. (2) Mackie, P. E.; Elliott, J. C.; Young, R. A. Acta Crystallogr. 1972, B28, 1840. (3) Prener, J. S. J. Electrochem. SOC. 1967, 114, 77. (4) Van Rees, H. B.; Mengeot, M.; Kostiner, E. Mater. Res. Bull. 1973, 8, 1307. ( 5 ) Sanger, A. T.; Kuhs, W. F. 2. Kristallogr. 1992, 199, 123. (6) Eean. E. P.: Wakefield. 2. T.: Elmore. K. L. J. Am. Chem. SOC. 1950,72,2418. 17) Eean. E. P.: Wakefield. 2. T.: Elmore, K. L. J. Am. Chem. SOC. 1951,73,5579. (8) Kijima, T.; Tsutsumi, M. J. Am. Ceram. SOC. 1979, 62, 455. (9) Palkin, V. A.; Kuzina, T. A.; Orlovskii, V. P.; Ezhova, Zh. A.; Rodicheva, G. V.; Sukhanova, G. E. Russ. J. Inorg. Chem. 1991,36, 1718. (10) Tacker, R. C.; Stormer, J. C., Jr. Am. Mineral. 1989, 74, 877. (11) Arends, J.; Royce, B. S. H.; Siege], J.; Smoluchowski, R. Phys. Lett 1968, 27A, 720. (12) Hitmi, N.; LaCabanne, C.; Young, R. A. J. Phys. Chem. Solids 1986, 47, 533. (13) Tang, H.-L. L. Diss. Absr. Int. B 1974, 35 (5), 2379. (14) Cant. N. W.: Bett, J. A. S.: Wilson. G. R.: Hall, W. K. Spectrochim. Acta 1'971, 27A, 425. (15) Reisner, I.: Klee, W. E. Smctrochim. Acra 1982, 38A. 899. (16) Young, R. A.; Holcomb, D. W. Calcg Tissue Int. 1982, 34, S17. (17) Izumi, F. In The Rietveld Method; Young, R. A., Ed.; Oxford University Press Inc.: New York, 1993; pp 236. (18) Kay, M. I.; Young, R. A,; Posner, A. S . Nature 1964, 204, 1050. P943384M
