YbPsOtq

100

800

700

6O0

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;

I N O R G A N I C C O M P L E X C O M P O U N D S I I 101

b r-

S

0 e

Fig. 9. A perspective, view along the a-axis showing four phosphate ribbons linked by neodymium in the structure of NdPsO14 (Albrand et al., 1974).

Hong and Pierce, 1974). All the structures contain eight-membered conjugate rings (fig. 9). The monoclinic I structure and the orthorhombic structure differ only in the orientation of the middle tetrahedra, which form common sheets of conjugate rings (Durif, 1971; Tranqui et al., 1972; Hong, 1974c). In the monoclinic II structure each PO 4 ring is not linked with two rings as in the other structures but with four rings, and thus eight- and twenty-membered rings are formed (fig. 10) (Bagieu et al., 1973; Hong and Pierce, 1974; Palkina, 1978). In all rare earth ultraphosphates the cations are eight-coordinated. The isolated polyhedra can best be described as bicapped trigonal prisms. Albrand et al.

(1974), however, propose a square antiprismatic coordination for neodymium in NdPsO14.

Recently, a new type of ultraphosphate has been found for CEP5014 (Rzaigui et al., 1984; Rzaigui and Ariguip, 1985). Its structure is triclinic and the atomic arrangement can be described as P10028 sheets spreading in the (110) planes. The internal structure of these sheets is mainly a linkage of P12036 rings in which 40%

of the phosphorus atoms are branching. Cerium atoms have an eight-fold coordination and the CeO 8 polyhedra have no common oxygen atoms.

According to Schulz et al. (1974), NdPsO14 has an orthorhombic high-temperature form which can be derived from the monoclinic structure. Otherwise, the ultraphosphates decompose to metaphosphates at high temperatures (900- 1000°C) (Chudinova et al., 1975; Chudinova and Balagina, 1979).

102 L. NIINISTO and M. LESKEL.~

~ Q

Fig. 10. An ab-projection of one phosphate layer in YbPsO14 (Hong and Pierce, 1974).

1.3.2. RP309 and R(P03) 3

The chemical formula of the rare earth metaphosphates has been presented as RP309 and R(PO3) 3. The correct name for the first formula would be cyclotriphosphate and for the second catena-polyphosphate. Most of the metaphosphates described in the literature are catena-polyphosphates, but this is not obvious from either the formulas or the names generally used.

Rare earth catena-polyphosphates (metaphosphates) can be prepared by the following methods: (a) heating of rare earth oxides with phosphoric acid within a specific temperature range (Chudinova et al., 1975, 1977a; Chudinova and Balagina, 1979); (b) roasting of a mixture of R 2 0 3 and N H a H 2 P O 4 (Chudinova et al., 1978a,b); (c) thermal dehydration of RP30 9 • n H 2 0 precipitated from aqueous solution (Birke and Kempe, 1973a,b); (d) decomposition of ultraphosphate RPsO14 above 900°C with release of P205 (Bagieu-Beucher and Tranqui, 1970);

(e) thermal condensation of dihydrogentriphosphates, RH2P3019 (Melnikov et al., 1981b); and (f) crystallizing from a solution of R203, H3PO 4, NaF, and P205 at high temperature (Hong, 1974b). In the preparation of catena-polyphosphate from a mixture of R203 and H 3 P O 4 the P/R ratio should be 2-3. The product is not always pure but may contain ortho- and ultraphosphates (Tsuhako et al., 1979).

INORGANIC COMPLEX COMPOUNDS II 103 In the series of rare earth metaphosphates (polyphosphates), the larger cations form an orthorhombic structure containing helical chains of corner-sharing PO 4 tetrahedra. The PO 8 dodecahedra share edges in a zigzag fashion (fig. 11) (Hong, 1974a). The polyphosphates of the smaller rare earths are monoclinic and the phosphate tetrahedra form chains characteristic of metaphosphates. The rare earth atoms lie at the centers of the isolated oxygen octahedra (fig. 12) (Hong, 1974b).

The smaller rare earth cations form compounds with a composition R(PO3) 3 corresponding to metaptiosphate, but according to their crystal structure the correct formula is U4(P4012)3 (Bagieu-Beucher, 1976; Smolin et al., 1978). Thus the anions are cyclic polyphosphates. Their structure is cubic, containing eight- membered phosphate rings linked together by RO 6 octahedra (fig. 13) (Mezent- seva et al., 1977; Bagieu-Beucher and Guitel, 1978; Chudinova, 1979).

The dehydration of scandium hydrogenphosphate, Sc(H2PO4)3, leads to the formation of Sc(PO3) 3. This material decomposes above 1200°C to normal orthophosphate (Melnikov and Komissarova, 1969; Komissarova et al., 1971a;

Khrameeva et al., 1971). The monoclinic structure of Sc(PO3) 3 consists of infinite chains of PO 4 tetrahedra, linked together by octahedrally coordinated scandium atoms (fig. 14) (Domanskii et al., 1982).

q?

1 " " ~ / ' P 20 1 (

Fig. 11. An ab-projection of the NdP309 structure showing how the PO 4 tetrahedra are connected by Nd atoms (Hong, 1974@

104 L. NIINIST0 AND M. LESKELA

o ~ o o ~ o

0~'- ~ ~ 0,-! ~'~

ii \ "~

~ o ~ .~

8~ o ~

O ~

0 . ~ '~ ~ ~.

~.~, .\.. ~ /;~~., ^{° ~i}

0 . ~ 0 . o 0 ~

O~

^{C ~}

o~ ~

0.o~0 0 -0 G

0>" ~ 0~- ~

INORGANIC COMPLEX COMPOUNDS II 105

Fig. 13. A projection of the cubic 5c4(P3012)3 structure on the ab-plane showing the anion rings and regular ScO 6 octahedra (Bagieu-Beucher and Guitel, 1978).

Besides anhydrous compounds R(PO3)3, hydrates of cyclotri-, cyclotetra-, cyclohexa- and cyclooctaphosphates of rare earth elements have been obtained from aqueous solutions by an exchange reaction with the corresponding sodium phosphate. The precipitation of cyclotriphosphate, having 3 to 5 water molecules, with Na3P30 9 has been reported by Birke and Kempe (1973a,b) and Serra and Giesbrecht (1968). The cerium cyclotriphosphate trihydrate, CeP30 9 - 3 H 2 0 , which is isomorphic with the corresponding La and Pr compounds, has been studied by X-ray diffraction analysis. The hexagonal structure contains fiat P30~- rings and Ce has a tricapped trigonal prismatic coordination to six oxygens of the phosphate rings and to the three oxygens of the water molecules (Bagieu-Beucher et al., 1971).

Compounds having the general formula of metaphosphates but containing hexa- or octaphosphate as anion have been studied by Chudinova et al. (1978c) and Lazarevski et al. (1982). These compounds, precipitated with Na6P6Ols or NasP8024 , may also contain crystal water. The structure is unknown; only the X-ray diffraction powder pattern and IR spectra have been recorded. Europium

106 L. N I I N I S T 0 and M. L E S K E L A

~ o

~ 0

,-k

¢',,I

: ,-..,

,-&

.e

I..i

I N O R G A N I C COMPLEX COMPOUNDS II 107

and lanthanum form hydrated polyphosphates having the same O/P ratio as metaphosphates, and for these Ezhova et al. (1978a,b) have investigated the formation, thermal behavior, and IR spectra.

1.3.3. Other condensed phosphates

Except for the catena-polyphosphates, the binary condensed phosphates or polyphosphates have been investigated much less thoroughly than the ternary polyphosphates containing alkali as an additional cation. The crystal structures of the rare earth binary polyphosphates are largely unknown. Trivalent rare earth ions react with sodium triphosphate in solution to form insoluble compounds where the molar ratio between R 3+ and P3051o is 1:1 and 5:3 (Giesbrecht and Audrieht, 1958; Giesbrecht, 1960; Giesbrecht and Melardi, 1963). In the solid state, these compounds contain more than twenty water molecules (Petushkova et al., 1971). The rare earth polyphosphates also form a complex in solution where the r a t i o R3+/P3OSlo is 1:2 (Rodicheva, 1981). The separated solid state 1:1 and 5:3 compounds dissolve in excess of sodium triphosphate.

Tetravalent cerium behaves exceptionally upon heating with H 3 P O 4. With low P/Ce ratio (1-3) it forms diphosphate, CeP207, and with a higher proportion of phosphorus it forms catena-polyphosphate, Ce(PO3) 4. Cerium may also reduce to trivalent in this system, with the formation of Ce(PO3) 3 at lower temperatures and the ultraphosphate CEP5014 at higher temperatures (Tsuhako et al., 1979).

Whereas cerium diphosphate can be prepared from phosphoric acid, the other rare earth diphosphates require the diphosphate anion as starting material (Tananaev et al., 1967; Ukrainskaya et al., 1971). The formula of the diphosphate of the trivalent rare earths is R4(P2OT) 3 (Chudinova et al., 1967; Kuznetsov and Vasileva, 1967). In excess, the alkali diphosphates used in the preparation of the diphosphate lead to the formation of double diphosphate. An alternative preparation method for scandium diphosphate (Muck and Petrfl, 1971) is the decomposition of scandium hydrogen phosphite, Sc2(HPO3)3, at 380-500°C. The crystal structures of the rare earth diphosphates are unknown. Petrfi and Muck (1971) have indexed the powder of scandium diphosphate as tetragonal with the axes a = 6.60 and c = 14.02 A.

In solid-state studies of R203-2P20 5 mixtures, a phase with the composition R2P4013 has been found. The X-ray diffraction powder pattern and IR spectrum of this compound have been reported but the detailed structure is unknown (Park and Kreidel, 1984; Agrawal and White, 1985).

In the system R203-H3PO4, hydrogen diphosphate and dihydrogen triphosphate appear as anions forming complexes with R ~+, the first anion only with the heavier lanthanides however (Chudinova et al., 1977a,c, 1978d). For the dihydrogen triphosphates a single crystal structure determination has been carried out for YbH2P3010. The monoclinic structure consists of s h o r t pO34 - chains, the P30~o ions are symmetric, and the isolated ytterbium atoms have octahedral coordination (fig. 15) (Palkina et al., 1979).

108

o(~)

0(5)

ot3)

Dalam dokumen Edited by: Karl A. Gschneidner, Jr. and LeRoy Eyring ISBN: 978-0-444-87045-2 (Halaman 106-114)

400