T h e rare e a r t h p h o s p h a t e s are i m p o r t a n t in the g e o c h e m i s t r y of the rare earths, since two of the main r a r e earth minerals are binary o r t h o p h o s p h a t e s . T h e m o s t a b u n d a n t of these, m o n a z i t e , has the a p p r o x i m a t e composition (Ce, La)PO4, while x e n o t i m e is p r i m a r i l y an yttrium p h o s p h a t e . Several o t h e r rare earth minerals containing p h o s p h a t e have b e e n found in the earth ( G m e l i n , 1984). In addition, the i m p o r t a n t p h o s p h o r u s sources apatite a n d p h o s p h o r i t e contain up to 1% of rare earths.
Besides these a n h y d r o u s o r t h o p h o s p h a t e s m a n y o t h e r binary rare earth phos- phates are k n o w n and p r e p a r e d synthetically. Study of the p h a s e equilibria of the system L a 2 0 3 - P 2 O s , f o r e x a m p l e , has shown six i n t e r m e d i a t e c o m p o u n d s having m o l a r ratios of 3:1, 7 : 3 , 1:1, 1:2, 1:3, and 1 : 5 (fig. 1) ( P a r k and Kreidler, 1984). T h e great n u m b e r of c o m p o u n d s is due to the versatile chemistry of elemental phosphorus. In p e n t a v a l e n t state, for e x a m p l e , it not only f o r m s o r t h o p h o s p h a t e and h y d r o g e n p h o s p h a t e ion's but also p o l y p h o s p h a t e s , of which
2 - - 3 - - 4 - - 3 -
the most c o m m o n for the rare earths are P207 , P309 , P4012, and P5014 ( G i e s b r e c h t and Perrier, 1960; Jaulmes, 1969; B e u c h e r , 1970). Several represen- tatives of rare earth h y d r o g e n p h o s p h a t e s , phosphites, and h y p o p h o s p h i t e s h a v e b e e n p r e p a r e d ( I o n o v et al., 1973a,b; C h u d i n o v a and Balagina, 1979).
T e r n a r y rare earth p h o s p h a t e s are f o r m e d with alkali and alkaline earth e l e m e n t s and in these c o m p o u n d s , too, the composition of the p h o s p h a t e anion can vary ( H o n g , 1975a; H o n g and Chinn, 1976; Koizumi, 1976). F r o m the
94 L. NIINISTO and M. LESKELA
ILl
I,- ,<
n- i¢1 a.
5 I¢1 I -
1 5 0 0
1 4 0 0
1 3 0 0
120¢
110¢
1 0 0 0 -
9 0 0
8 0 0 -
7 0 0
L a 2 0 3 4- L a 3 P O 7
9 3 5 ± 5
La 2 03 4- 0 L a 3 P O 7
I
I~ L a 3 P O 7 4- La r P3018
La r P3018 4- L a P O 4
0 L a 3 P O 7
L a 7 P 3 0 1 8
I I i I
I La PO 4
+
L i q . I 1 2 3 5 + 5
L a PO 4 + La ( P 0 3 ) 3
1 0 5 0 ¢ 5 - -
L a ( P 0 3 ) 3
+ _
La Ps014
L i q u i d '~
I
| 1 0 9 5 ¢ 5 -
\
\
\
L~ Ps014
i i I I
L a 2 0 3 1 0 2 0 3 0 4 0 9 0 P205
7 5 5 ± 5 La2P!013
5 0 6 0 7 0 8 0
MOLE PERCENT OF I)205
Fig. 1. A phase diagram for the system LazO3-P205 (Park and Kreidler, 1984).
quaternary systems the compounds M I I R ( P O 4 ) 6 ( O , F)2 , having the apatite struc- ture, are most common (Ito, 1968; Mathew et al., 1979).
A review on the crystal chemistry of rare earth phosphates was recently published by Palkina (1982).
The rare earth phosphates activated with trivalent lanthanides can be used as luminescent materials and neodymium-containing polyphosphates reportedly make efficient laser materials (Palilla and Tomkus, 1969; Albrand et all, 1974;
Koizumi, 1976b).
1.2. Rare earth orthophosphates
Rare earth orthophosphates, in addition to being found in nature as the minerals monazite, (La, Ce) PO4, and xenotime, (Y, Er) PO4, have been synthe-
INORGANIC COMPLEX COMPOUNDS II 95 sized in the laboratory by many methods. The phosphate anion precipitates the rare earth ions from solution. The phosphate source for the precipitation is usually phosphoric acid, but alkali phosphates and hydrogen phosphates can also be used (Schwarz, 1963f; Petushkova et al., 1969a,b; Ropp, 1970; Awazu and Matsunaya, 1974; Tsuhako et al., 1979). The precipitate contains half a molecule of water in the case of the lighter rare earths, and two water molecules in the case of the heavier rare earths (Mooney, 1948; Hikichi, 1978). Rare earth orthophos- phate hydrates having 1, 5/3, 2.5, and 3 water molecules have also been reported (Hezell and Ross, 1967; Ropp, 1968a; Petushkova et al., 1969a; Tsagareishvili et al., 1972).
Anhydrous rare earth orthophosphates have been prepared by a fusion method (Duboin, 1888) and by chemical reactions (Mooney, 1950; Schwarz, 1963f).
Single crystals have been obtained by hydrothermal crystallization from a mixture of the rare earth hydroxide and phosphoric acid (Anthony, 1957; Carron et al., 1958) and by flux growth from lead phosphate (Feigelson, 1964; Smith and Wanklyn, 1974). The rare earth phosphate phosphors are usually made by a fusion reaction between the rare earth oxide, carbonate, nitrate hydroxide, etc., and (NH4)zHPO 4 at a temperature of 900-1100°C (Smith, 1967; Palilla and Tomkus, 1969).
The orthophosphates of the cerium subgroup have been found to possess the monoclinic monazite structure (Mooney, 1948; Mooney-Slater, 1962; Schwarz, 1963f), in which the irregularly nine-coordinated rare earth atoms are linked together by isolated, distorted phosphate tetrahedra (fig. 2) (Krstanovic, 1965;
Beall et al., 1981). Crystal-structure determinations of numerous samples of monazite having slightly different compositions have been carried out (Parrish, 1939; Finney and Rao, 1967; Ghouse, 1968; Haapala et al., 1969). The space group of monazite has been reported to be P21/n but determinations have also been done with a P 2 J a setting (Ueda, 1967; Jaulmes, 1972).
Crystallization of orthophosphates of the cerium subgroup from solution at low temperatures produces hexagonal RPO4-0.5H20 crystals (Mooney, 1948;
Hikichi, 1978). The structure is stable at low temperatures but changes to monazite structure upon heating (Kuznetsov et al., 1969). Characteristic of this structure are the large tunnels running along the c-axis (fig. 3). Originally, the hexagonal phase was known only for the hemihydrate and it was suggested that water was needed to stabilize the structure (Mooney, 1950; Mooney-Slater, 1962). However, Vlasse et al. (1982) found no evidence that the presence of water was necessary.
The rare earth phosphates in the middle of the series, containing several water molecules, have also been reported to be hexagonal (Hezel and Ross, 1967;
Kuznetsov et al., 1969). In contrast, dysprosium orthophosphate containing 1.5 molecules of water has a unique, orthorhombic structure (Hikichi, 1978).
The phosphates of the yttrium subgroup have the tetragonal zircon structure, which is isomorphic with the corresponding orthovanadates, arsenates, and chromates (Schwarz, 1963f). The crystal structure of xenotime has been deter-
96 L. NIINISTO and M. L E S K E L A
,o 1 ,2 ,3 I ,s 6A.,
( ~ ° ( ~ ~° ©.
Fig. 2. A projection of the CePO 4 structure on the basal plane. The numbers give the z-coordinate in /~ngstr6ms (Mooney, 1950).
j \ - -
Fig. 3. The environment of Ce 3+ in the hexagonal CePO 4 - ½H20 structure projected on the ab-plane (Palkina, 1982).
I N O R G A N I C COMPLEX C O M P O U N D S II 97
---.--.-4b
P
Fig. 4. A stereoview of the polyhedra in the tetragonal RPO 4 structure (MiUigan et al., 1983).
mined by several groups (fig. 4) (Vegard, 1927; Durif and Forrat, 1957; Lohmul- ler et al., 1973; Milligan et al., 1983).
When crystallized from solution, the phosphates of the heavier rare earths contain two water molecules. The dihydrate is found in nature, as the minerals weinschenkite and churchite (Strunz, 1942; Claringsbull and Hey, 1953), which have monoclinic structure. According to Hezel and Ross (1967), the heavier rare earths also have a tetragonal trihydrate with a structure similar to that of zircon where the water molecules are uncoordinated.
Scandium orthophosphate contains two water molecules and is isomorphic with the corresponding iron and aluminum compounds having an orthorhombic crystal lattice (Komissarova et al., 1965a; Eshchenko et al., 1979).
When the rare earth phosphates are precipitated with sodium phosphate, not only binary and ternary phosphates but also hydroxide phosphates can be obtained (Tananaev and Vasileva, 1963). The formula of these compounds may be as complicated as PrI0(OH)3(PO4)9.26H20 (Petushkova et al., 1969b).
Except for scandium, very few hydrogenphosphates are known for the rare earth elements. The complex formation of Ce 3÷ and Ce 4÷ with the H2PO 4 has been verified by Lebedev and Kulyako (1978). In solid state, dihydrogenphos- phates, R(H2PO4)3, with tetragonal structure have been prepared for Sm, Eu, and Gd (Butuzova et al., 1982). Trihydrogen orthophosphates, with formula H3R(PO4) 2 . H 2 0 , having orthorhombic structure, have been prepared and studied by Melnikov et al. (1982). Hexagonal Sc(H2PO4) 3 can be prepared from
S c 2 0 3 and large excess of H 3 P O 4 o r by thermal decomposition of S c ( P O 3 ) 3
(Melnikov and Komissarova, 1969). This hexagonal structure has recently been solved by Smolin et al. (1982) (fig. 5). In solution, other ligands can bond along with hydrogenphosphate to the rare earth ion and a mixed ligand complex is formed (Nazarenko and Polyektov, 1979). Such an interesting compound is
98 L. NIINISTI) and M. LESKELfi,
0 (H H (~1
lz)(
o ( z.) £;~ /
0 (o)
(9
j
/ '
/
/
Fig. 5. A perspective view of the Sc(H2PO4) 3 structure on the xy-plane Smolin et al., 1982).
known for Ce 4+ in the solid state as well as in solution. The structure of (CePO4)2(HPO4)I_x(SO4) x .5HzO contains tunnels oriented along the b-axis (fig. 6) (Bartl, 1976), and other features which allow it to act as an ion exchanger (K6nig and Psotta, 1978a,b).
1.3. Condensed rare earth phosphates
The condensed phosphates contain more than one phosphorus atom in a molecule but with the structure always built UP by P O 4 tetrahedra. Whereas in orthophosphates the phosphate tetrahedra are isolated, in condensed phosphates they share corners (but never edges or faces). The larger the O / P ratio in the formula is, the more broken the tetrahedra chains are, and the smaller the O / P ratio is, the more cross-linked the chains are (Hong, 1974b). The condensed phosphates can be divided into three groups: branched or ultraphosphates [PnO3n_l] (n-2)-, cyclic or metaphosphates
[PnO3n] n-,
and chain or polyphosphates [PnO3n+l] (n+2) . In addition, there is a group of oligophosphates which contain separate anions: pyrophosphate, tripolyphosphate, etc., but compounds of the rare earths with these anions have been little studied (Palkina, 1978). TheINORGANIC COMPLEX COMPOUNDS II
7. 9
99
Fig. 6. A perspective view of the structure of (CePO4)2(HPO4/SOn).5H20 on the ac-plane. The projection shows the structural framework with 8- and 4-coordination and the porosity with the O-O distances of 6.1 ~ (Bartl, 1976).
t e m p e r a t u r e d e p e n d e n c e of the f o r m a t i o n of the different p h o s p h a t e s is shown for cerium in fig. 7.
1 . 3 . 1 .
RPsO14
T h e rare e a r t h u l t r a p h o s p h a t e s - c o m p o u n d s containing b r a n c h e d p h o s p h a t e anions and also called p e n t a p h o s p h a t e s (RPsO14) - c a n be synthesized b o t h as p o w d e r and as single crystals f r o m a solution of r a r e earth oxide in phosphoric acid allowed to cool f r o m 600-700°C to r o o m t e m p e r a t u r e . T h e p h o s p h a t e anion m u s t be in great excess with respect to the cation in the solution ( P / R = 2 0 - 4 0 ) (Jaulmes, 1969; H o n g , 1974a; C h u d i n o v a et al., 1977a). RP5014 can also b e p r e p a r e d via a solid state reaction of R z O 3 with ( N H 4 ) 2 H P O 4 ; different structure
100
800
700
6O0
o
500
400
300
L. NIINIST() and M. L E S K E L A
I f
-& #
oA oA c ~
-oA o ~ OA
-o Q~ O&
-O O OZ~
I I 1
I 2 3
,~o • • • • •
c:~ A0 A0 • • • •
OA ~ Aa ~ ~ ~ •
A A ZX A A /% ~.
/X ZX A /% A ~, ~,
A A A A A ~, ~,
, /; , f , r J T
4 5 10 20 30 40 50
A t o m i c r a t i o ( P / C e )
Fig. 7. The formation of cerium phosphates at different temperatures and atomic ratios. A: CePO4;
O: CeP2OT; [S]: Ce(PO3)3; A: Ce(PO3)4; 0 : CEP5014. (Tsuhako et al., 1979.)
types are obtained at different temperatures (Sungur et al., 1983). Since N d P s O 1 4
crystals are promising laser materials, there have been attempts to grow them as fibers. Success has been achieved in a controlled gas atmosphere at 450°C (Tofield et al., 1975). The pulling of
RPsO14
crystals from a melt is not possible because the compound decomposes before melting.All ultraphosphates contain P O 4 tetrahedra linked by three vertices to neigh- boring tetrahedra (fig. 8). The further coordination of the phosphate groups varies, giving rise to three different crystal structures: monoclinic I ( L a . . . Tb), monoclinic II ( D y . . . Lu, Y), and orthorhombic ( D y . . - Er, Y) (Beucher, 1970;