76 Fluorination of Phosphoapatites Possible Alterations of Their Structure G. M O N T E L , G. BONEL, J. C. H E U G H E B A E R T , M. V I G N O L E S , and M. H A M A D Laboratoire de Physico-Chimie des Solides et des Hautes Températures, INP, E R A N° 263, CNRS, 38, Rue des 36 Ponts. 31400 Toulouse, France G. B A C Q U E T
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Laboratoire de Physique des Solides, L A CNRS, Université Paul Sabatier, 118, Route de Narbonne. 31062 Toulouse Cédex, France
As soon as chemists become interested in natural apatites, the problem with fluorine appears. When phosphate ores are treated the fluorine must be removed. On the other hand it is sometimes necessary to fluorinate calcium phosphates in the mineral part of calcified tissues. Obviously, in industry and medecine it is neces sary to know the mechanism of phosphate fluorination as well as the structure and the properties of products obtained. The structure of fluorapatite Ca (PO ) F was described in the 30's owing to the existence of well-defined single crystals. It is crystallized in the hexagonal system (space group Ρ 6 / ) and it is characterized by the presence of channels, crossing the crystal from one end to the other. The fluoride ions (two for each unit-cell) are localized along the axis of the channel. The methods of synthesis of fluorapatite have been widely dis cussed (1). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in aqueous solution at room temperature, or through a solid state re action at 800°C. It can also be prepared by the action of β-tricalcium phosphate on calcium fluoride at about 800°C. Its solubi lity and thermal stability have already been established. While much is known about fluorapatite, many questions still exist con cerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapa tite by a solid state reaction between calcium fluoride and apatitic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 10
4 6
2
3 m
Mechanism of formation of fluorapatite from apatitic tricalcium phosphate and calcium fluoride in the solid state. We s t u d i e d ( 2 ) t h e mechanism o f f o r m a t i o n o f f l u o r a p a t i t e f r o m a p a t i t i c t r i c a l c i u m p h o s p h a t e and c a l c i u m f l u o r i d e i n t h e s o l i d s t a t e . O v e r a l l , t h e c h e m i c a l r e a c t i o n may be w r i t t e n : Ca (HP0 )(P0 ) (OH) + CaF 9
4
4
5
2
^
C a
( P 1 0
)
F
H
°4 6 2 * 2°
0097-6156/81/0171-O367$05.00/0 © 1981 American Chemical Society Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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368
PHOSPHORUS CHEMISTRY
As t h e r e a c t i o n o c c u r s e n t i r e l y i n t h e a p a t i t i c p h a s e , i t c a n be n e i t h e r a n e x c h a n g e r e a c t i o n , n o r a n a d d i t i o n r e a c t i o n . Our o b s e r v a t i o n s o n t h i s r e a c t i o n p o i n t o u t t h a t t h e mechanism i s v e r y com p l e x . The f o l l o w i n g r e a c t i o n s o c c u r s u c c e s s i v e l y w i t h o v e r l a p p i n g temperature : ^_ (a) From 110 t o 550°C, t h e d e h y d r a t i o n o f HPO^ ions of a p a t i t e i s o b s e r v e d g i v i n g r i s e t o ?2®7^~ i o n s i n s o l i d s o l u t i o n i n apatite. (b) From 400 t o 760°C, an e x c h a n g e r e a c t i o n b e t w e e n t h e OH ions o f a p a t i t e and t h e F~ i o n s o f c a l c i u m f l u o r i d e o c c u r s . T h i s phenomenon l e a d s t o a n i n c r e a s e i n t h e c r i s t a l l i n i t y , w i t h o u t any v a r i a t i o n i n t h e v a l u e o f t h e u n i t - c e l l p a r a m e t e r a_. ( c ) From a b o u t 600°C t o 900°C, t h e r e m a i n i n g c a l c i u m f l u o r i d e a d d s to w i t h t h e f l u o r i n a t e d a p a t i t e . T h i s r e a c t i o n i s a c c o m p a n i e d by a d e c r e a s e o f t h e v a l u e o f t h e u n i t - c e l l p a r a m e t e r _a. A calcium d e f i c i e n t f l u o r a p a t i t e i s then observed. (d) From 600 t o 1000°C, t h e P 0 " * i o n s i n t h e a p a t i t e l a t t i c e r e a c t w i t h t h e c a l c i u m o x i d e p r e v i o u s l y formed d u r i n g the ex change r e a c t i o n , l e a d i n g t o t h e f l u o r a p a t i t e . However, f r o m 200° t o 500°C, a s m a l l q u a n t i t y o f ?2°7 ions react w i t h t h e f l u o r i d e i o n s p r o d u c i n g an u n i d e n t i f i e d f l u o r i n a t e d phos p h a t e s p e c i e s . The f l u o r a p a t i t e o b t a i n e d by t h i s m e t h o d , t h o u g h very c l o s e to the i d e a l f l u o r a p a t i t e , i s not e x a c t l y stoichiome tric. 4
2
7
A p a t i t e s w i t h l a r g e amounts o f f l u o r i d e i o n s : t h e B - t y p e c a r b o n a ted f l u o r a p a t i t e . LEHR and Mc CLELLAN (3) d e m o n s t r a t e d numerous n a t u r a l a p a t i t e s and a c o r r e l a t i o n b e t w e e n t h e amount o f f l u o r i d e i o n s and t h a t o f t h e c a r b o n a t e i o n s . T h i s l e d them t o p r o p o s e t h a t Ρ 0 ^ ~ i o n s c a n be r e p l a c e d by € 0 3 ^ " i o n s a s s o c i a t e d w i t h F~ i o n s . S u c h a h y p o t h e s i s c o u l d e x p l a i n t h e a b n o r m a l l y h i g h amount o f f l u o r i d e i n some FRANCOLITES. However t h i s t y p e o f s u b s t i t u t i o n was n o t p r o v e d by t h e a u t h o r s . We s t u d i e d some s y n t h e t i c a p a t i t e s where f l u o r i d e and c a r b o n a t e i o n s were s i m u l t a n e o u s l y i n t r o d u c e d . Samples o f B - t y p e c a r b o n a t e d f l u o r a p a t i t e ( 0 0 3 ^ " s u b s t i t u t i n g PO4 ) were o b t a i n e d a s a powder f r o m a n a q u e o u s medium r i c h i n f l u o r i d e i o n s and a l s o a n aqueous medium p o o r i n f l u o r i d e i o n s . C h e m i c a l a n a l y s i s o f t h e p r o d u c t s p r e p a r e d i n t h e medium r i c h i n f l u o r i d e i o n s showed t h e p r e s e n c e o f more t h a n two f l u o r i d e i o n s p e r u n i t c e l l ( T a b l e I ) and a c o r r e l a t i o n b e t w e e n t h e amounts o f c a r b o n a t e and f l u o r i d e i o n s . _ , Sample
1
2
:
3
:
4
:
5
:
6
9,74
9,72
9,78
9,72
9,84
9,87
ca ;_ nPO/ 4 nC0 "
4,03
4,20
4,75
4,87
5,30
5,54
1,97
1 ,80
1 ,25
1,13
0,70
0,46
η -
3,46
3,30
2,60
2,59
2,36
2,20
n
2
0
3
ρ
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
76.
MONTEL ET A L .
Fluorination
of
369
Phosphoapatites
T h e s e r e s u l t s a g r e e w i t h t h e h y p o t h e s i s o f LEHR a n d Me CLELLAN ^_ t h a t c a r b o n a t e i o n s a s s o c i a t e d w i t h F i o n s s u b s t i t u t e s f o r PO^ Such a p a t i t e s m i g h t h a v e t h e f o r m u l a : Ca
(PO,). ( C O F ) F_ 10 4 6-x 3 χ 2
i r i
Q
However a l a c k o f c a l c i u m i s o b s e r v e d a n d t h e c o r r e l a t i o n b e t w e e n the amounts o f f l u o r i d e i o n s a n d c a r b o n a t e i o n s i s n o t r i g o r o u s . T h i s i s due t o t h e e x i s t e n c e o f a second type o f s u b s t i t u t i o n as o r i g i n a l l y p r o p o s e d b y LABARTHE ( 4 - 5 ) . A c c o r d i n g t o t h i s a u t h o r , the s u b s t i t u t i o n o f a P O ^ i o n s b y a CO^" i o n i s a c c o m p a n i e d b y the f o r m a t i o n o f v a c a n c i e s i n a n o x y g e n s i t e and a l s o i n t h e C a ^ and F~ s i t e s o f t h e c h a n n e l w h i c h a r e t h e c l o s e s t t o t h e m i s s i n g o x y g e n . The B - t y p e c a r b o n a t e d f l u o r a p a t i t e c a n be d e s c r i b e d b y t h e formula :
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+
10-x+u
x-u
(PO,), (C0 ,D ) (C0_F) F 4 6-x 3' x - u 3 u 2-x+u Q
Q x-u
Both types o f charge compensation are c o n s i d e r e d p r e s e n t i n each c a s e , b u t t h e v a l u e o f u i s l o w when t h e a p a t i t e s a r e p r e p a r e d i n an aqueous medium p o o r i n F " ( i n w h i c h t h e s e c o n d t y p e o f l a t t i c e s u b s t i t u t i o n i s dominant) ( T a b l e I I ) and h i g h e r f o r t h e a p a t i t e s p r e p a r e d i n a n aqueous medium r i c h i n F~ i o n s ( i n w h i c h t h e f i r s t type o f s u b s t i t u t i o n dominates) (Table I ) Table I I Sample n
Ca
2 +
3
"PO. " n
2
co, -
V
3
1
2
9,61
9,46
4,90
5,05
5,65
1, 10
0,95
0,35
2,36
1 ,87
1 ,87
9,76
The X-band E.S.R. r e s u l t s (6) o b t a i n e d w i t h t h e c a r b o n a t e d a p a t i t e s s u p p o r t t h e p r o p o s e d m o d e l . I n d e e d , i n X - i r r a d i a t e d sam p l e s we have o b s e r v e d t h e r e s o n a n c e o f a d e f e c t i n w h i c h a n e l e c t r o n i s t r a p p e d b y a n o x y g e n v a c a n c y . The number o f s u c h d e f e c t s i s g r e a t e r i n t h e compounds p r e p a r e d f r o m a n aqueous medium p o o r i n F i o n s t h a n i n t h e a p a t i t e s p r e p a r e d f r o m a n aqueous medium r i c h i n f l u o r i d e i o n s . Moreover t h e r e e x i s t s a c l o s e c o r r e l a t i o n b e t w e e n t h e s p e c t r a l i n t e n s i t y and t h e number o f o x y g e n v a c a n c i e s c a l c u l a t e d f r o m t h e m o d e l we p r o p o s e . The p r o p e r t i e s o f t h i s m a t e r i a l s w i l l l e a d t o a b e t t e r i n s i g h t i n t o phosphates o f b i o l o g i c a l i n t e r e s t .
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
370
PHOSPHORUS CHEMISTRY
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Literature cited
1. WALLAEYS R. Ann. Chim., Paris, 12ème série, 1952, 7, 808-848. 2. HAMAD M. Thèse, Institut National Polytechnique de Toulouse, 1980. 3. LEHR J.R. ; Me CLELLAN G.H. Colloque International sur les Phosphates Minéraux Solides. Toulouse 16-20 mai 1967. 2, 29-44. 4. LABARTHE J.C. Thèse, Université Paul Sabatier, Toulouse, 1972. 5. LABARTHE J.C. ; BONEL G. ; MONTEL G. Ann. Chim., Paris, 1973, 8, 289- 301. 6. BACQUET G. ; VO QUANG TRUONG ; BONEL G. ; VIGNOLES M. J. Solid State Chem., 1980, 33, 189-195. RECEIVED
June 30, 1981.
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
