Lattice parameters and cation distribution of solid solutions of calcium and lead hydroxyapatite

Lattice parameters and cation distribution of solid solutions of

calcium and lead hydroxyapatite

Citation for published version (APA):

Verbeeck, R. M. H., Lassuyt, C. J., Heijligers, H. J. M., Driessens, F. C. M., & Vrolijk, J. W. G. A. (1981). Lattice

parameters and cation distribution of solid solutions of calcium and lead hydroxyapatite. Calcified Tissue

International, 33(3), 243-247. https://doi.org/10.1007/BF02409444

DOI:

10.1007/BF02409444

Document status and date:

Published: 01/01/1981

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Calcif. Tissue Int. 33, 243-247 (1981)

Calcified Tissue

International

L a t t i c e P a r a m e t e r s a n d C a t i o n D i s t r i b u t i o n o f S o l i d S o l u t i o n s

o f C a l c i u m a n d L e a d H y d r o x y a p a t i t e

R. M. H. V e r b e e c k , ' C. J. Lassuyt, 1 H. J. M. Heijligers, 2 F. C. M. Driessens, 3 and J. W. G. A. Vrolijk 2

1Laboratory for Analytical Chemistry, University of Gent, J. Plateaustraat 22, B-9000 Gent, Belgium; 2Laboratory for Physical

Chemistry, University of Technology, Eindhoven; and :qnstitute of Dental Materials Science, Catholic University, P.O. Box 9101,

6500 HB Nijmegen, The Netherlands

S u m m a r y . Solid solutions o f calcium h y d r o x y a p a -

tite (CaOHA) and lead h y d r o x y a p a t i t e (PbOHA) of

the formula Ca,0.x Pbx (PO4)6 (OH)2 were prepared

by coprecipitation followed by heating at 800~ in a

stream o f CO2-free w a t e r v a p o r o f 1 atm. The sam-

ples were apatitic in the range 0 < x < 6 and con-

tained lead phosphates as a second phase at higher

Pb/Ca ratios. Lattice p a r a m e t e r s and cation dis-

tribution o f the apatitic samples were determined by

X-ray diffraction. T h e lattice p a r a m e t e r s varied

linearly with x in the range considered, whereas all

Pb 2+ were located in the sixfold position for cations.

T h e r e was a miscibility gap in the apatite series o f

solid solutions in the range 1 < x < 4, whereas

apatites in the range 6 < x < 10 were not stable

under the conditions o f preparation. It is concluded

that apatites in the range 4 < x < 6 represent a

minimum in the free energy of solid solutions be-

t w e e n C a O H A and P b O H A .

K e y words: Calcium hydroxyapatite - - L e a d hy-

droxyapatite - - Cation distribution - - Lattice param-

eters - - Solid solutions.

E x p o s u r e to lead in food and the e n v i r o n m e n t in

general or in industry in particular can cause anemia

[1], renal insufficiency [2, 3], and e n c e p h a l o p a t h y

[4], the latter resulting in disturbed behavior [5].

L e a d is retained in liver, kidneys, and blood, and

especially in bone, teeth, and brain [6]. Infants have

a relatively high retention as c o m p a r e d to adults [7].

Indications have b e e n found that lead interferes

with the metabolism o f calcium [ 8 - 1 6 ] , magnesium

[17], strontium [9], c o p p e r [1, 18], iron [18, 19], zinc

S e n d offprint r e q u e s t s to

F. C. M. Driessens at the above

address.

[20], phosphorus [8, 15], and sulfur [21, 22]. The

disappearance c u r v e s for lead excretion in blood,

plasma, hematic cells, and some o t h e r soft tissues

can be e x p r e s s e d as sums o f exponential functions.

In contrast, lead is r e m o v e d from bone tissue at a

constant and e x t r e m e l y slow rate [23]. F o r this rea-

son time integrals o f body burdens o f lead may well

be diagnosed by determination o f the lead c o n t e n t in

bones and teeth [ 2 3 - 3 1 ] , whereas m o m e n t a r y b o d y

b u r d e n s o f lead s h o u l d be d e r i v e d r a t h e r f r o m

analyses of biochemical factors [32-34]. When lead

ions are injected in the form o f lead acetate, they

not only interact with the calcified tissues [35, 36],

but they also induce ectopic calcification [37, 38].

The strong capacity o f lead ions to induce calcifica-

tion is clearly illustrated by a case study reporting

arthritis o f the hip s e c o n d a r y to r e t a i n e d bullet

fragments which c o r r o d e d w h e n in c o n t a c t with

synovial fluid [39].

L i t t l e is k n o w n a b o u t t h e p h y s i c o c h e m i c a l

background of the interaction o f lead ions with cal-

cified tissues. It might be a s s u m e d that lead ions are

incorporated in the apatite phase of the minerals in

these tissues, as the existence of lead apatite has

been established. H o w e v e r , Kato and Ogura [38]

have shown that the first lead-containing mineral in

the ectopic calcifications f o r m e d after injection o f

lead acetate in the rat is lead p y r o p h o s p h a t e . T h a t

the intensity o f the characteristic X-ray diffraction

peaks diminishes with time [38] might indicate that

this p y r o p h o s p h a t e dissolves slowly w h e r e b y the

dissolved ions react as yet with the apatitic mineral

present in the calcification. In o r d e r to gain more

insight into these processes, more basic physico-

chemical studies should be carried out.

Some investigators have p r e p a r e d solid solutions

o f calcium h y d r o x y a p a t i t e (CaOHA) and lead hy-

droxyapatite (PbOHA). According to Mtiller [40],

Narasaraju et al. [41], and Rao [42], the lattice pa-

rameters of these solid solutions b o t h v a r y linearly

244 R . M . H . Verbeeck r al.: Solid Solutions o f Ca and Pb H y d r o x y a p a t i t e

with c o m p o s i t i o n b e t w e e n t h o s e o f the pure end

m e m b e r s . On the contrary, Engel et al. [43] found

that the variation o f the c parameter with c o m p o s i -

tion deviates markedly from Vegard's law. This w a s

attributed to the preference o f Pb z+ for the sixfold

position o f the cation sublattices o f apatite. Unfor-

tunately, Engel et al. [43] determined the cation

distribution o n l y in o n e sample. In order to c h e c k

this h y p o t h e s i s and as a start for a s y s t e m a t i c

p h y s i c o c h e m i c a l study o n the interaction o f lead

ions with C a O H A , the present investigation w a s

undertaken. In this study the lattice parameters and

the cation distribution o f solid solutions o f C a O H A

and P b O H A were determined in the m o s t relevant

c o m p o s i t i o n range.

T h e o r e t i c a l C o n s i d e r a t i o n s

In the apatite structure t w o sublattices o c c u r for the

cation. Position I is fourfold and its W y c k h o f f nota-

tion is f, w h e r e a s position hi is sixfold and is de-

noted by h. Therefore, if the c h e m i c a l formula o f a

solid solution o f lead and calcium hydroxyapatite is

given by

Ca,0_•215

0 < x < 10

(1)

its structural f o r m u l a can be written as

Ca4.~xPb~x(Ca6_(,_~)xPbc,_~)x) (PO4)6(OH)2

(2)

At the temperature o f preparation, the equilibrium

Ca(l) + Pb(II) ~ Ca(II) + Pb(I)

(3)

m a y be r e a c h e d [44]. A distribution coefficient K

can then be defined as

[6 - (1 - a ) x ] [ a x ]

K = [4 - a x ] [(1 - a ) x ]

(4)

If the Pb and Ca ions h a v e no preference for either

the sixfold or the fourfold position, their distribu-

tion o v e r the c o r r e s p o n d i n g sublattices will be at

r a n d o m w h e r e b y K = 1. H o w e v e r , a preference o f

Pb for the sixfold position II and h e n c e a preference

o f Ca for the fourfold position I, as suggested by

Engel et al. [43], w o u l d result in K < < 1. In that

c a s e the cation distribution is m o s t accurately de-

termined on samples with x ~< 6. A c o n s t a n t value

for K throughout the c o m p o s i t i o n range 0 < x < 10

w o u l d m e a n that the solid solutions are ideal in the

t h e r m o d y n a m i c s e n s e [44]. In that case K is identi-

cal to the equilibrium c o n s t a n t for equation (3).

B o t h P b O H A [45, 46] and C a O H A [47] h a v e the

space group P63/m. The structure is hexagonal. Lit-

erature values for the lattice parameters a and c o f

P b O H A are s u m m a r i z e d in Table I. F o r t h o s e o f

C a O H A , see [48].

M a t e r i a l s a n d M e t h o d s

Pure C a O H A and solid solutions o f C a O H A and P b O H A were prepared according to the m e t h o d described by Wright [51] for the preparation of solid solutions o f P b O H A and strontium hy-

droxyapatite. The apatites were precipitated from a boiling aque- o u s solution o f piperidine at pH 12 by the simultm~eous and slow addition o f a solution containing the appropriate a m o u n t s of

Ca(NOa)2 and Pb(NO~)., and a H~PO4 solution. T h e p H of the

latter was adjusted to p H 12 by the addition of piperidine. Re- agent grade c h e m i c a l s were used t h r o u g h o u t . F u r t h e r details on

the method of preparation can be found elsewhere [51]. Finally, the s a m p l e s were p r e s s e d into bars u n d e r 2 x 107 N m -2 and

heated at 800~ in a s t r e a m o f COx-free w a t e r v a p o r o f l atm. After 4 h the s a m p l e s were q u e n c h e d in air, c r u s h e d , and pow- dered in an agate ball mill. The calcium and lead c o n t e n t s of the

single-phase s a m p l e s were d e t e r m i n e d by atomic a b s o r p t i o n s p e c t r o m e t r y and are s u m m a r i z e d in Table 2.

X-ray diffraction was carried out in the Philips Guinier XDC- 700 camera. T h e c a m e r a c o n s t a n t was d e t e r m i n e d with c~-AlzO3 as an internal s t a n d a r d . CuK~, radiation was used for an expo- sure time of about 8 h. T h e films were d e v e l o p e d in the usual way. D e n s i t o g r a m s were recorded on the L i n / L o g D e n s i t o m e t e r DD2 (Kipp) having logarithmic sensitivity.

The d e n s i t o g r a m s w e r e analyzed for the o c c u r r e n c e of second p h a s e s by c o m p a r i n g the d values o f e v e n t u a l extra peaks with those of k n o w n c o m p o u n d s in the q u a t e r n a r y s y s t e m CaO-PbO-

P~O~-H._,O. The cell p a r a m e t e r s a and c o f the apatite p h a s e were d e t e r m i n e d by m e a s u r i n g the position of as m a n y apatite peaks

as possible (n). A l e a s t - s q u a r e s calculation on these positions

T a b l e 1. Literature v a l u e s for the lattice parameters of pure

lead h y d r o x y a p a t i t e S u b s t a n c e a c R e f e r e n c e P b O H A 9.34 6.87 N a r a s a r a j u et al. [41] 9.86 7.22 Rao [42] 9 . 8 4 - 9 . 8 8 7 . 4 0 - 7 . 4 2 B h a t n a g a r [50] 9.89 7.28 Mtdler [40] 9.868 7.430 Wright [51] 9.877 7.429 Engel [49] 9.879 7.434 Engel [46] 9.878 7.432 N e g a s and Roth [52] 9.877 7.247 Blakeslee and Condrate [53] Preparations of the above s u b s t a n c e s have been different and

m a y have affected the results

T a b l e 2. C h e m i c a l c o m p o s i t i o n and substitution degree x of the

C a O H A - P b O H A solid solutions x~a~,,,, wt % Pb wt % C a x~r, 1 15.2 28.8 0.93 2 29.6 24.8 1.88 3 40.5 20.6 2.76 4 47.5 15.2 3.77 5 56.1 11.3 4.90 6 60.2 8.09 5.90

R. M. H. Verbeeck et al.: Solid Solutions o f C a and Pb H y d r o x y a p a t i t e 245

T a b l e 3. P h a s e c o m p o s i t i o n and lattice p a r a m e t e r s o f the apatite p h a s e in samples prepared in this study

x,~,~ Second p h a s e a c N " 0 Absent 9,416 6.885 16 1 A b s e n t 9,442 6.910 30 2 A b s e n t 9.48 ~ 6.94 ~' 20 3 A b s e n t 9,54 a 7.01 ~ 10 4 A b s e n t 9,572 7.014 27 5 A b s e n t 9.625 7.051 31 6 Pb.P~O,3(trace) 9.661 7.099 24

Nearly all peaks were very broad or e v e n double

t, N u m b e r o f reflections u s e d to calculate the lattice p a r a m e t e r s

r a m e t e r s a and c o f the apatite phase in the other

s a m p l e s p r e p a r e d in this w o r k are s u m m a r i z e d in

T a b l e 3. At xtheo~ = 6 a trace o f s e c o n d phase was

o b s e r v e d which could be identified by c o m p a r i s o n

with data o f B r i x n e r and Foris [54]. S a m p l e s at xtheo~

values o f 2 and 3 s h o w e d b r o a d and e v e n double

p e a k s suggesting the o c c u r r e n c e o f a miscibility gap

of the apatites in the range 1 < Xtheo r < 4. H o w e v e r ,

w h e n calculated f r o m the position o f the m o s t sharp

r e f l e c t i o n s , the e s t i m a t e d lattice p a r a m e t e r s o f

these samples as given in Table 3 fit r e a s o n a b l y well

within the rest o f the values. The latter vary within

e x p e r i m e n t a l e r r o r linearly with x according to

T a b l e 4. M e a n v a l u e s a n d c o r r e s p o n d i n g v a r i a n c e s for the cation distribution p a r a m e t e r a and distribution coefficient K of the C a O H A - P b O H A solid solutions

1 0.152 0.0135 0.279 0.0579

2 0.153 0.0056 0.230 0.0236

4 0.091 0.0058 0.084 0.0077

5 0.075 0.0034 0.041 0.0023

6 0.055 0.0032 0.013 0,0004

produced the best fitting values for a and c. In t h o s e i n s t a n c e s where n > 25, the a c c u r a c y is e s t i m a t e d to be better than 0.003 and 0.002 A for a and c, respectively.

In the single-phase apatitic samples the cation distributions were d e t e r m i n e d from the relative intensities o f the suitable re- flections in diffractograms obtained without addition o f the inter- nal standard. Peak a r e a w a s taken as a m e a s u r e for peak inten- sity. The c o m b i n a t i o n o f the film sensitivity and the logarithmic sensitivity o f the d e n s i t o m e t e r gives an overall linear relationship between peak and reflection intensity.

As standardization of all steps in the intensity m e a s u r e m e n t s is difficult, intensity ratios o f pairs o f reflections were used to de- termine the cation distribution. T h e s e pairs were c h o s e n so that a certain preference o f Pb ions for a certain sublattice would have a n opposite effect on their intensity and so that the absorption correction and the t e m p e r a t u r e factor could be neglected. For e a c h x value and for e a c h of the 12 c h o s e n pairs o f reflections, theoretical intensity ratios were calculated for values of a rang- ing from m i n i m u m to m a x i m u m . L o r e n t z polarization, multiplic- ity, and structure factor were taken into account. For each cho- sen pair o f reflections the value o f the distribution p a r a m e t e r a c o r r e s p o n d i n g to the m e a s u r e d intensity ratio was then ob- tained by c o m p a r i s o n with t h e s e theoretical intensity ratios. In this w a y 12 v a l u e s for a a n d K were o b t a i n e d for e a c h sample at a given x.

Results

Since the s a m p l e s with xt~eo~ > 6 w e r e found to

contain fairly large a m o u n t s o f certain lead phos-

phates as a second phase, they were not e x a m i n e d

further. The phase c o m p o s i t i o n and the lattice pa-

a = (9.410 _+ 0.005) + (0.043 ___ 0.001)x

with o-a = 0.006

(5)

and

c = (6.880 _ 0.005) + (0.036 +_ 0.001)x

with o-c = 0.006

(6)

w h e r e o- r e p r e s e n t s the standard deviation o f esti-

mate.

In Table 4, the cation distribution p a r a m e t e r a

and the distribution coefficient K are s u m m a r i z e d .

E a c h value r e p r e s e n t s the m e a n o f 12 determina-

tions c o r r e s p o n d i n g to 12 selected intensity ratios.

T h e r e s p e c t i v e v a r i a n c e s o-2~ and cr2K are also given.

F r o m the table it is seen that within the e x p e r i m e n -

tal error all Pb ions o c c u p y the sixfold position II.

T h e fact that two apatite p h a s e s are p r o b a b l y pres-

ent at x = 2 and 3 m e a n s that K and a for these

s a m p l e s in T a b l e 4 are a sort of a v e r a g e for these

t w o apatite phases, as the intensities o f the X-ray

diffraction p e a k s are m o n o t o n o u s functions o f o~

and K.

D i s c u s s i o n

T h e cell p a r a m e t e r s for o u r heat-treated s a m p l e s

both vary linearly with x within the limits of ex-

perimental error. T h e a p a r a m e t e r at x = 10 as ex-

trapolated f r o m Eq. (5) agrees fairly well with liter-

ature values c o m m o n l y r e p o r t e d for P b O H A (Table

1). H o w e v e r , t h e r e is c o n s i d e r a b l e d i s a g r e e m e n t

b e t w e e n the e x t r a p o l a t e d value for c at x = 10 [Eq.

(6)] and the c p a r a m e t e r of P b O H A (Table 1). Such

deviation f r o m V e g a r d ' s law indicates that K [Eq.

(4)] differs m a r k e d l y f r o m unity. This is c o r r o b o -

rated by the results in Table 4 showing the strong

p r e f e r e n c e o f the Pb ions for the sixfold position II

in the apatite lattice arid confirming the opinion o f

Engel et al. [43]. On this basis no ideal b e h a v i o r c a n

be e x p e c t e d for these solid solutions [44]. The pref-

e r e n c e of Pb ~+ ions for the sixfold position II m a y

246 R . M . H . Verbeeck et al.: Solid Solutions of Ca and Pb Hydroxyapatite b e r e l a t e d to t h e f a c t t h a t t h e r e is s o m e w h a t m o r e s p a c e in p o s i t i o n II t h a n in p o s i t i o n I [57]. T h e v a l u e s f o r t h e l a t t i c e p a r a m e t e r s o f t h e P b O H A - C a O H A s o l i d s o l u t i o n s i n T a b l e 3 a r e g e n - e r a l l y 0.03 A l o w e r t h a n t h o s e r e p o r t e d b y E n g e l et al. [43]. A c a l c u l a t i o n b a s e d o n t h e r e s u l t s o f E n g e l [46] s h o w s t h a t t h e d e h y d r a t i o n d e g r e e o f p u r e P b O H A at 800~ a n d f o r a p a r t i a l w a t e r v a p o r p r e s - s u r e o f 760 m m H g a m o u n t s to 3% m a x i m u m . S i n c e t h e d e h y d r a t i o n t e n d e n c y o f P b O H A - C a O H A s o l i d s o l u t i o n s d e c r e a s e s w i t h i n c r e a s i n g c a l c i u m c o n t e n t [43], it is u n l i k e l y t h a t t h e l o w e r a a n d c v a l u e s in this s t u d y a r e d u e to a p a r t i a l d e h y d r a t i o n o f t h e s a m p l e s . T h e d i s c r e p a n c y m o s t p r o b a b l y is r e l a t e d t o a d i f f e r e n t c r y s t a l - c h e m i c a l c o n s t i t u t i o n o f h e a t - t r e a t e d a n d h y d r o t h e r m a l l y t r e a t e d s o l i d s as is s h o w n f o r C a O H A [53, 55]. T h e f a c t t h a t f o r Xtheo r v a l u e s o f 2 a n d 3 t h e r e w a s a d o u b l i n g o f m o s t l i n e s in t h e X - r a y d i f f r a c t i o n p a t t e r n is i n t e r p r e t e d as t h e o c c u r r e n c e o f a m i s c i - b i l i t y g a p in t h e r a n g e 1 < Xth~or < 4. T h e f o r m a t i o n o f a s u p e r s t r u c t u r e a r o u n d x = 3 is n o t l i k e l y as it w o u l d h a v e r e s u l t e d in e x t r a l i n e s r a t h e r t h a n in a d o u b l i n g o f lines. M o r e o v e r , s u c h e x t r a l i n e s w o u l d h a v e a p p e a r e d o n l y in a n a r r o w c o m p o s i t i o n a l r a n g e a r o u n d x = 3. A m i s c i b i l i t y g a p is n o t u n c o m m o n in s e r i e s o f a p a t i t e s w i t h i s o m o r p h o u s s u b s t i t u t i o n [56] a n d m e a n s t h a t t h e r e is a m a x i m u m in t h e f r e e e n e r g y o f t h e a p a t i t e s w i t h i n t h a t r a n g e . F u r t h e r , t h e f a c t t h a t o u r p r e p a r a t i o n s in t h e r a n g e 6 < Xtheo r < 10 c o n t a i n e d fairly l a r g e a m o u n t s o f c e r t a i n l e a d p h o s p h a t e s as a s e c o n d p h a s e is an i n d i c a t i o n t h a t a p a t i t e s in t h a t r a n g e a r e n o t v e r y s t a b l e e i t h e r . T h e r e f o r e , w e c o n c l u d e t h a t a p a t i t e s in t h e r a n g e 4 < Xtheo r < 6 r e p r e s e n t a m i n i m u m in t h e f r e e e n e r g y b e t w e e n C a O H A a n d P b O H A . H e n c e o n e s h o u l d e x p e c t t h a t i n c o r p o r a t i o n o f Pb 2+ i o n s in b o n e a n d t e e t h r e s u l t s in t h e f o r m a t i o n o f a s e p a r a t e ( P b , C a ) O H A p h a s e s o m e w h e r e in t h e r a n g e 4 < Xtheo r < 6, at l e a s t as f a r as t h e m i n e r a l s in b o n e a n d t e e t h c a n b e c o m p a r e d w i t h C a O H A . In t h e f o l l o w - ing s t u d y w e will c a r r y o u t s o l u b i l i t y d e t e r m i n a t i o n s o n t h e a p a t i t e s p r e p a r e d in this s t u d y in o r d e r to i n v e s t i g a t e w h e t h e r this h y p o t h e s i s is c o r r e c t f o r in v i v o c o n d i t i o n s o r w h e t h e r o t h e r , e v e n t u a l l y h y - d r a t e d s o l i d p h a s e s ( e . g . , P b H P O 4 ) i n t e r f e r e in t h e c o n t r o l o f t h e s o l u b i l i t y b e h a v i o r . R e f e r e n c e s

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