17-
0 O ' 0
e
0 O I 0
27-
0 0
75
O O
o o 0
o 0 0 0
-ZE I
50 25 0
mole % GIJ. Z {~[Itl'} 3
Fig. 73. A phase equilibrium diagram of the C s 2 S O 4 - G d 2 ( 8 0 4 ) 3 system. (I): L + 0£-Cs2504; (II):
~-Cs2SO 4 -t- Cs3Gd (904)3; (III): I3-Cs2SO 4 + Cs3Gd(SO4)3; (IV): Cs3Gd(SO4) 3 + ct-Gd2(SO4)3;
(V): Cs3Gd(SO4) 3 q- a-CsGd(SO4)2; (VI): C s a G d ( S O 4 ) 3 -I- [~-CsGd(SO4)2, (VII): Cs3Gd(SO4) 3 h-
"y-CsGd(SO4)2; (VIII): ot'CsGd(SO4)2+c~-Gd/(SO4)3; (IX): ot-CsGd(SO4)2+~-Gdz(SO4)3; (X):
~-Csad(SO4) 2 + [~-Gd2(804)3; (XI): "/-CsGd(SO4) 2 + [~-Gd2(804)3; (XlI): g + C s s G d ( S O 4 ) 3 (Samartsev et al., 1977).
(fig. 74). In the La compound the coordination polyhedron is more irregular (La. • • O range: 2.48-2.80 A) and an analysis of it has not been attempted. In both structures the Cs ions are situated between the layers (fig. 75) and have a large number of short contacts to sulfate oxygens.
A comparison of the lattice parameters and the space group of CsSc(SO4) 2 with the corresponding values of KAI(SO4) 2 (Manoli et al., 1970) suggests that the compounds are isostructural and thus Sc 3÷ is octahedrally coordinated, forming sulfate bridged layers. According to Korytnaya et al. (1983) the high-temperature form of e~-CsSc(SO4) 2 probably has a similar structure.
The tetrahydrates of C s R ( S O 4 ) 2 ( R = L a . . . Lu, Y) are likewise frequently studied, and unit cell and IR data are both available (Bukovec et al., 1977).
Complete structural determinations have been carried out (table 8) and confirm the isostructurality with the corresponding NH 4 (Eriksson et al., 1974) and Rb (Iskhakova et al., 1975) compounds. It is interesting that the smaller ionic size of Lu causes the coordination of sulfate groups to change slightly relative to the Pr compounds (Bukovec et al., 1979b). The unit cell size and space group are not
d r ~
d r ~
t ~ o5
_ ~ ~ ~ - ~~ - ~ - ~ _ ~ - ~
~ , ~ .~ .~ .~ .~
. • ~ ~ _ ~ _ ~ s
V V V ~ ~
0 0 0 0
• ° ,
= d
©
0
a ~ d
189
190 L. NIINIST0 and M. L E S K E L A
04
flq
a ~ PR
o
132
I]4
03
Fig. 74. The square antiprismatic coordination around Pr 3+ in the structure of CsPr(SO4) 2 (Bukovec et al., 1978).
q
0(4)
0(3) 0 ( 6 ) ~ 0 ( 5 ) 0
Fig. 75. A perspective view of the unit cell of CsLa(SO4) 2 showing the layer-like structure (Bukovec et al., 1980a).
affected (table 8) but, evidently due to steric reasons, the S(2) sulfate group in CsLu(SO4) 2 • 4 H 2 0 is rotated, placing the oxygen atom 0 ( 8 ) away from the inner coordination sphere of Lu and resulting in a lower coordination number (8 versus 9 for Pr). A parallel case has been reported earlier for the structures of (NO)z[R(NO3)5] (R = Sc, Y), see chapter 56, section 6.4.1, of Volume 8 of this Handbook.
The thermal behavior of the C s R ( S O 4 ) 2 • 4 H 2 0 series has been studied in detail by Bukovec et al. (1979a, 1980b) and others (Iskhakova et al., 1973; Storozhen- ko, 1983b). Dehydration experiments indicate that stable monohydrates are formed for P r . . . G d around 100°C (Bukovec et al., 1979a). The dehydration mechanism is explained in terms of the crystal structure, where one of the water molecules is not coordinated to the lanthanide but held in the structure by
INORGANIC COMPLEX COMPOUNDS II 191 hydrogen bonds (Bukovec at al., 1979b). The mechanism of the dehydration may be compared with that of the corresponding Rb compounds, e.g., Sm (Erfimetsfi and Niinist6, 1971) and Dy (Zaitseva et al., 1972), where the monohydrate also appears as a stable intermediate, although the mechanism is strongly dependent on experimental conditions (Eriksson et al., 1974). In view of the suggested relationship between structure and dehydration mechanism (Bukovec et al., 1979b), it would be interesting to study, using the high-temperature X-ray diffraction method, whether or not the.dehydration intermediate CsR(SO4) 2
• H 2 0 has the same structure as the well characterized RbR(SO4)2 • H 2 0 phase.
A comparison of the available X-ray powder patterns of R b H o ( S O 4 ) 2 . H 2 0 (Sarukhanyan et al., 1982) and CsNd(SO4)2-H20 (Bukovec et al., 1979a) does not seem to give immediate support to isostructurality but different structure types and polymorphism may exist in both series.
Of the other hydrates of the 1:1 compounds, CsSc(SO4)2.12H20 appears particularly interesting because it is the only known rare earth sulfate having the alum structure (Bashkov et al., 1972). Its thermal stability is low and it has been prepared only at 0°C. The indexed powder pattern indicates that the unit cell parameter a = 12.51A and the volume V = 1 9 5 7 4 3. All other scandium com- pounds with Cs are anhydrous, viz. CsScSO 4 and Cs3Sc(SO4) 3 (Komissarova et al., 1970b).
Like the CsR(SO4) 2 series, the Cs3R(SO4) 3 series comprises several structure types (table 10), in addition, the Sc compound probably has a monoclinic structure of its own (Komissarova et al., 1970b). Only the structure of Cs3Yb- (SO4) 3 is known from single-crystal studies (Samartsev et al., 1980a). In this compound, Yb has the unusually (table 9) low coordination number of six, and the Y b - O distances are short, ranging from 2.14 to 2.23 A. The structure consists of infinite chains of [Yb(SO4)3]3n n- parallel to the z-axis (fig. 76), with Cs atoms located between the chains, each with nine neighbors at a range of 2.97-3.64 4 . 4.4.6. Ammonium compounds
Ammonium compounds are among the most frequently studied sulfatometal- lates of the rare earths (Pascal, 1959), even though their chemistry is limited to lower temperatures than that of alkali compounds. The NH4R(SO4)2-4HzO series has been used in fractional crystallization processes for separation of the lighter and heavier rare earth elements and even of individual elements within groups. The solubility diagrams have been presented for several R2SO 4 - (NH4)2SO4-H20 systems (e.g., Ce3+; Schr6der et al., 1938).
Besides the 1 : 1 compound tetrahydrates, the anhydrous phase (Sarukhanyan et al., 1984c) and the intermediate hydrates are known; the latter are of low stability, however. Of other compositions, the (NH4)sR(SO4) 4 (R = L a . . . Pr) (Iskhakova et al., 1981b; Niinist6 et al., 1980a) and ( N H g ) 6 R 4 ( S O 4 ) 9 " 2 H 2 0 (R = T b - . . Er) series (Iskhakova et al., 1985) have been structurally character-
192 L. NIINISTO and M. LESKELA
, y
Fig. 76. A projection of part of the structure of C s 3 Y b ( S O 4 ) 3 o n the xy-plane (Samartsev et al., 1980a).
ized. Scandium forms at least two anhydrous phases, NH4Sc(SO4) 2 and (NH4)3Sc(SO4) 3 (Erfimetsfi and Haukka, 1966; Bashkov et al., 1972), of which NH4Sc(SO4) 2 is isostructural with the Rb and Cs compounds (Couchot et al., 1971) and (NH4)3Sc(SO4) 3 appears to have a different structure (Erfimetsfi and Haukka, 1966). The existence of an ammonium scandium alum has been claimed, but the compound has not been isolated (Purkaystha and Dutta, 1963).
The NH4R(SO4) 2 - 4 H 2 0 ( R = L a . . . T b ) series forms isostructural crystals (Eriksson et al., 1974; table 8). It is not certain how far the isostructural character extends among the heavier rare earths, since the published powder patterns for the D y . . . Ho region are not in agreement. Thus, Iskhakova et al. (1975) have reported that the Ho compound is isostructural with Tb, while Belousova et al.
(1970) have published X R D patterns for Dy and Y which are similar to each other but different from those for the L a . . . Tb compounds. A crystal structure analysis of N H 4 U ( S O 4 ) 2 . 4 H 2 0 has revealed that it is isostructural with the corresponding rare earth series (Bullock et al., 1980). NH4Sm(SO4)2 • 4 H 2 0 was among the first "double" sulfates studied by single-crystal X-ray diffraction techniques (Niinist6, 1973; Eriksson et al., 1974). The samarium ion is nine- coordinated by six sulfate oxygens and three water molecules; the coordination polyhedron is an intermediate one and can equally well be described as a monocapped square antiprism or a tricapped trigonal prism (fig. 77). The sulfato groups join the SmO 9 polyhedra into chains with the ammonium ions and
"noncoordinated" water molecules located between them (fig. 78).
I N O R G A N I C COMPLEX COMPOUNDS II 193
a
k
I
b
Fig. 77. The coordination around Sm 3÷ in the structure of NH4Sm(SO4)2" 4H20. (a) monocapped square antiprism; (b) tricapped trigonal prism. (Niinist6, 1973; Eriksson et al., 1974.)
Fig. 78. A perspective view of the structure of NH4Sm(SO4) 2 - 4H20 showing the unit cell packing (Eriksson et al., 1974).