Thermal Instability in Synthetic Hydroxyapatites (13)M. Eisenstadt, V. S.Rao, and G. M. Rothberg, J. Chem. Phys., 30, 604 (1959). (14)D. R. Stuii and H. Prophet, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 37 (1971). (15)R. C. Miller and P. Kusch, J. Chem. Phys., 25,860 (1956). (16)P. A. Akishin, L. N. Gorokhov, and L. N. Sidorov, Zhur. fiz. Khim., 33, 2822 (1959). (17)A. W. Searcyand D. A. Schulz, J. Chem. Phys., 38, 772(1963). (18)K. D. Carlson, P. W. Gilles, and R. J. Thorn, J. Chem. Phys.. 38, 2725 (1963). (19)S.J. Weyand P. G. Wahlbeck, J. Chem. Phys., 57,2932(1972). (20)F. F. Coleman and A. Egerton, Phil. Trans. A,, 234, 177 (1934). (21)T. D. Sandryand F. D. Stevenson, J. Chem. Phys., 53, 151 (1970). (22)H. G. Wiedemann, Chem. lng. Tech., 36, 1105 (1964). (23)An. N. Nesmeyanov, "Vapour Pressure of the Element", Publishing House of the USSR Academy of Sciences, Moscow, 1961. (24)P. C. Marx, E. T. Chang, and N. A. Gokcen, High Temp. Sci., 2, 140 (1970). (25)S. K. Tarby and V. S. Robinson, 111, Trans. Met. SOC.AIM€, 242, 719 ( 1968). (26)J. Bohdansky and H. E. J. Schins, J. Phys. Chem., 71,215 (1967). (27) H. E. J. Schins, R. W. M. van Wijk, and B. Dorpema, Z. MetalNtd., 330
2017
(1 971). (28)I. Ansara and E. Bonnier, Conf. lnt. Metali. Beryllium, [ Commun.], 3rd, 1965, 17 (1966). (29)E. W. R . Steacie and F. M. G. Johnson, Proc. R. SOC.(London), 112, 542 (1926). (30)0 . Knacke and I. N. Stranski. Frog. Met. Phys., 6, 181 (1956). (31)J. P. Hirth and G. M. Pound, Prog. Mater. Sci., 11, 1 (1963). (32)M. Volmer and I. Estermann, Z.Phys., 7, 13 (1921). (33)K. Neumann and K. Schmoll, 2.Phys., 2,215 (1954). (34) R. B. Holden, R. Speiser, and H. L. Johnson, J. Am. Chem. SOC., 70, 3897 (1948). (35)R. P. Burns, A. L. Jason, and M. G. Inghram, J. Chem. Phys., 40, 2739 (1964). (36)J. G. Davy and G. A. Somorjai, J. Chem. Phys., 55,3624(1971). (37)D. L. Howlett, J. E. Lester, and G. A. Somorjai, J. Phys. Chem., 75, 4049 (1971). (38)E. M. Mortensen and H. Eyring, J. Phys. Chem., 64,846 (1960). (39)R. S.Bradley and P. Volans, Proc. R. SOC.,Ser. A, 217,508 (1953). (40)R. P. Burns. J. Chem. Phys., 44,3307 (1966). (41)W. Lu, M. S. Jhon, T. Ree, and H. Eyring, J. Chem. Phys., 46, 1075 (1967). (42) For a summary see A. R. Ubbelohde, J. Chem. Phys.. 61,58 (1964).
Thermal Instability in Synthetic Hydroxyapatites H. Catherine W. Skinner,* Department of Surgery, Yale University,New Haven, Connecticut 065 10
J. Steven Kitfelberger, Xerox Corporation, 114 Webster, New York 14580
and Ralph A. Beebe Department of Chemistry, Amherst College, Amherst, Massachusetts 0 1002 (Received September 18, 1974: Revised Manuscript Received June 19, 1975)
Hydrothermally synthesized hydroxyapatites, Ca10(P04)6(OH)2,were subjected to pyrolysis at atmospheric pressure and under vacuum and lo-' Torr, H2O pressure). A t high vacuum evolved gases were observed with a mass spectrometer as the sample temperature was raised 10'/min from room temperature to 800'. A dehydration peak was observed at 705'. All treated samples exhibited an absence of ir bands assigned to (OH) stretching and librational modes indicating a loss of (OH) from the structure. X-Ray analysis gave broadened diffraction maxima for samples pyrolyzed under vacuum conditions. In addition, maxima from a second phase appeared, which were identified as p-Ca3(P04)2. Dehydroxylated apatite is considered an intermediate in the reaction Calo(P04)6(OH)2 2Ca3(PO4)2 Ca4P209 HzO. 4
Hydroxyapatite, Calo(P04)6(OH)z (HA), is considered the mineral model for the inorganic phase of bones and teeth. There has been and is great interest in the substance per se as well as in the mineral-biological tissue reactions. Generalized chemical and physical properties and the crystal structures of apatites have been known for a long time.1-3 Equilibrium phase diagrams for the CaO-P205H2O system have contributed toward our understanding of the reactions and idiocyncracies of the pure phase HA.4,5 HA is the stable calcium phosphate phase over large portions of the diagram but the HA-fluid phase field has a restricted compositional range, H2O as well as CaO/P205 (Figure 1field 5). We report here on investigations into the stability of well-crystallized stoichiometric HA synthesized by hydrothermal techniques. Two samples of HA were synthesized at 450' and 22,000 lb/in.2 H2O pressure in standard hydrothermal equipment
+
+
from different bulk starting compositions? Sample SM2 had Ca/P = 1.5, H2O 75 wt %; SM1 had C a p = 1.5, 50% H20. SM2 plots in field 5 (Figure 1) and SM1 in field 4. The samples were held at temperature and pressure for 70 days, sufficient time to assure attainment of the equilibrium. The samples produced well-crystallized HA with sharp X-ray diffraction patterns (Figure 2), and differ only in having been synthesized in different phase fields. SM2 contained crystalline HA plus fluid; SM1 contained HA plus P-Ca~P207plus fluid. The HA phase of these samples (hand-picked in the case of SM1) was subjected to pyrolysis at atmospheric pressure, and under vacuum at two partial pressures of HzO (measured at and Torr). At the high vacuum, we employed a thermal desorption spectrometer6 equipped to raise the sample temperature at a linear rate of 10°/min. The spectrometer included a quadrupole mass spectromeThe Journal of Physical Chemistry, Vol. 79, No. 19, 1975
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H. C. W. Skinner, J. S. Kttelberger. and R. A. Beebe
1:705'C-A
I
I 160 200 300 400 500 600 760 860 TEMPERATURE
(TI
Flgure 3. Temperature programed dehydration spectrum for SM2 hydroxyapatite (upper curve). The lower curve shows the water evc-
luiion in the apparatus with no hydroxyapatite present.
Figwe 1. Equilibrium phase diagram for CaO-P&sH& at 500'. 30.000 Win.* H20 pressure showing bulk composition of samples SM1. SM2.
A
C
D
E
F
G
Portions of Guinier X-ray powder diffractionphotographs from &,approximately 3.5 A at bonom to 1.67 A at top: (A) lowtemperature precipitated "apatite": (E) SM2 hydrothermally synthesized hydroxyapatite before pyrolysis: (C) SM2 pyrolyzed in a muffle furnace lo 850': (D) SM2 pyrolyzed under vacuum (-lo-' Torr) to 850°: (E) SMl pyrolyzed under vacuum Torr) Io 850': (F) SM1 hydrothermally synthesized hydroxyapatite before pyrolysis: (G) SM1 pyrolyzed in a muffle furnace to 850'. Calculated unit cell parameters (in A) for SM2 samples: (E) 9.429(3).6.886 (3)V = 530.0 Figure 2.
A3:
(C) 9.421 (1). 6.896(1) V = 530.1 A3: (D) 9.459(2).6.915 (2)V
= 535.8 A3.
ter for continuous monitoring of gaseous decomposition products. After initial pump-down to 5 X lo-" Torr a t rmm temperature, each sample was heated to 85O0 a t 10°/min. The samples were held a t 850° for 15 min and then cooled to room temperature under vacuum over a period of approximately 2 hr. The partial pressure of water varied from 5 to 10 X Torr over the approximately 3 hr total experimental time. During the period of linear temperature rise, discrete peaks recorded in a PH* vs. time trace indicated discrete hydration processes.? For both rhs Jamal Of mysicalchemirhy. Vd. 79. No. 19. 1975
SM1 and SM2 a peak occurs in the dehydration spectrum between 675 and 800'. The thermal dehydration plot for SM2 (Figure 3) illustrates the results with the peak maximum a t 705'. We attrihute the low-temperature peak to desorption from the apatit.e sample surface, and the hightemperature peak to dehydroxylation. At the conclusion of each experiment the samples were transferred with minimum exposure to the atmosphere into capped vials. Pyrolysis a t atmospheric pressure (3 hr a t 1000°)was made in a standard muffle furnace. All pyrolyzed samples were submitted to X-ray and infrared examination. Infrared spectra of all pyrolyzed samples (in KBr pellets) differed from the starting material in not showing absorption hands a t 3570 and 633 cm-', hands which have been unambiguously assigned to (OH) stretching and librational modes in HA. This result implies a loss of (OH) from that site in the structure. PO4 bands in the 500-600- and 10W lZO-cm-' regions were somewhat broadened. These features are well known.8 A curious feature was the appearance, a t 1950 and 2020 cm-', of a pair of very sharp bands. These bands increase in intensity and sharpness with decreasing partial pressure of water during pyrolysis. Fowlerg has identified several low-intensity bands in this region as overtones and combinations of orthophosphate stretching modes. Our samples before pyrolysis also showed low-intensity bands in this region. The sharpness of the 1950and 2020-cm-' bands after pyrolysis suggests that they are characteristic of pyrolysis products. Further work leading to definite assignment of this pair of hands is in progress; contamination due to organic constituents has been ruled out. Muffle furnace pyrolyzed SMl and SM2 samples exhihited sharp, single-phase, apatite diffraction patterns (Figure 2C,G). X-Ray diffraction patterns of the high-vacuum pyrolyzed SM2 show markedly broadened maxima (Figure 2D). Somewhat broadened maxima were also noted in the low-vacuum pyrolyzed sample. SM1 did not exhibit line broadening under either of these conditions. However, diffraction maxima, in addition to those of HA, appeared in both high- and low-vacuum pyrolyzed SMI samples. The maxima were used to identify the second phase as tricalcium phosphate (&Cas(PO&). A maximum belonging to tricalcium phosphate is barely discernible on the broadened SM2 pattern in the same position. (See arrow between Figure 2D and E.) The original hydrothermal SM1 and SM2 samples grown in different phase fields a t the same temperature and pressure have discrete unit cell parameters.'O The parameters of the HA phase calculated from diffraction data up to 80°
Thermal Instability in Synthetic Hydroxyapatites
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28 show minor differences (Figure 2, legend). After vacuum pyrolysis the parameters, and therefore the unit cell volume, increase, suggesting crystal structure alteration. The fact that HA is sensitive to partial pressures of H2Q is not unexpected. In a truly anhydrous system HA should not appear.ll The reaction is
-
Ca10(P04)6(OH)2
2Ca3(PO& f Ca4P209
+ H2012
The breakdown of HA and the appearance of Ca3(PO4)2 has been docutnented on our samples. Ca4PzQ9 has not been identified, possibly because the three most, intense diffraction maxima of Ca4P209 overlap those of HA. However, it is also true that major crystal structural changes involving the Po4 group would he required to nucleate and form Ca4P209. The major fraction of the vacuum-pyrolyzed material remains “apatitic” with increased unit cell volume. Increased unit cell volume is anticipated as both Ca3(PQ4)2and Ca4P209 have unit cell volumes greater than HA: Ca3(PQ4)2= 3495 A3, Ca4P209 = 795 A3, HA = 529 A3. The formation of anhydrous calcium phosphates from HA involves not only dehydroxylation but major structural transformations. After atmospheric pyrolysis ( 1000’ for 3 hr) HA dehydroxylates (loas of 3570 633-cm-l ir hands) but no other structural changes are indicated. After 72 hr a t 1000a x i 0 further structural changes were noted. Atmospheric p(H2O) is sufficient to maintain apatite structure elements even after prolonged heating a t 1000’. However, after vacuum pyrolysis (3 hr to 850’) both dehydroxylation and structural changes were indicated (broadened diffraction maxima). The effect of vacuum is to decrease the stability of HA accelerating the breakdown reaction. The dehydroxylated apatite phase is probably a metastable intermediate in the breakdown of HA and might be called “oxyapatite”. The term oxyapatite has been in the
+
literature for a long timela designating apatites which do not exhibit OH bands in ir but have the apatite structure. The dehydroxylated apatite phase is probably analagous to the phase y-Ca2PzO7 in the reaction 2CaHP04 Ca2P207 H20
-
+
-y-CazP207was shown14 to be metastable relative to the pCazP2O7 modification. The reaction is extremely sluggish due to the necessary conversion of PO4 to P2O7 groups. The kinetics of this15 reaction and the HA conversion must depend on p(H20) as well as temperature. Acknowledgment. We wish to acknowledge grant support from the National Institutes of Health for both the hydrothermal synthesis work (M.C. W. Skinner, DE 2716) and the dehydration studies (R. A. Beebe, DE 2819).
References and Nates (1) H. C. Hodge, M. LeFevre, and W. F. Bale, lnd. Eng. Chem., Anal. Ed., I O , 156 (1938). (2) S. B. Hendrlcks, M. D.Jefferson, and V. M. Mosley, 2. Anorg. Chem., 81, 352 (1932). (3) $1. R. VanWazer, “Phosphorus and Its Compounds”, Vol. I, Wiley-lnterscience, New York, N.Y., 1966, p 513. (4) H. C. W. Skinner, Amer. J. Sci., 273, 545 (1973). (5) H. J. Bassett, J. Chem. SOC.,601, 2929 (1958). (6) C. W. Anderson, R. A. Beebe, and J. S. Kittelberger, J. Phys. Chem., 78, 1631 (1974). (7) R. J. Cetanovic and Y. Amenomiya, Catal. Rev., 6, 21 (1972). (8) C. B. Baddiel and E. E. Berry, Spectrochim. Acta, 22, 1407 (1966). (9) B, Fowler, lnorg. Chem., 13, 194 (1974). (10) H. C. W, Skinner, Appl. Spectrasc.. 22, No. 5, 414 (1968). Calculatlons have been programmed to discriminate and exclude phases other than apatite. (11) G. Tromel and H. Moller, 2. Chem., 206, 227 (1932); J. H. Welch and W. Gutt, J. Chem. Soc., 4442 (1961). (12) P. V, Riboud, Bull. Sac. Chem. Fr., 1701 (1968). (13) A. F. Rogers, J. Sci., 33, 475 (1912); J. Ito, Amer. Min.. 53, 890 (1968). (14) H. C. W. Skinner, Mat. Res. Bull., 5, 437 (1970). (15) N. W. Wikholm, R . A. Reebe, and J. S.Kittelberger, J. Phys. Chem., 79, 853 (1975).
The Journal of Physical Chemistry, Vol. 79, No. 19, 1975
