Journal of Structural Chemistry, VoL 36 No. 3, 1995

C L A S S I F I C A T I O N O F HEAVY M E T A L F L U O R I D E S .

F L U O R I D E S W I T H H I G H W A T E R C O N T E N T S

S. V. Borisov, N. A. Bliznyuk, N. V. Podberezskaya, and E. S. Kuklina

UDC 548.0

Five fluoride structures differing in the character of linkage of cationic po[yhedra and in relative water contents are considered. The cationic sublattice is four-layered (ABAC) in K2Cu(ZrFa)z.6H20,

face-centered (F-type, ABC) in CuZrF6.4H20, primitive cubic (P-O~pe) in Cuz(ZrFT) 2.16H20, hexagonal

one-layered (AA) in Cu2ZrF 8.12H20, and hexagonal two-layered (AB) in Zr2Fs'6H20. Some structures

have considerably distorted symmetries of cationic sublattices compared to anhydrous structures because of the anisotropy of cation surroundings in the firsg second, and third spheres, but all characteristic close-packed cation planes of the corresponding structural types are retained. In structures with cation-cation distances of --3.5/I, dimeric cations are arranged as single fragments for planes with dh/r 3.5 -,'[ or as separate fragments for planes with smaller dhld.

In our previous papers [1-4], we have carried out crystal-chemical analysis of the most representative types of fluorides with heavy cations of Zr, TR, Pb, U, Th, etc. As with other classes of compounds, fluorides with large amounts of water molecules form a separate group. When arrangement of heavy atoms (cations) is regarded as the main structure-forming factor, as is done in our analysis, the principal role of water molecules is to "dilute" or rarefy the cation matrix; as a result of such rarefaction, the mean minimal cation-cation contacts and the volumes per cation (Vc) increase. In a rarefied matrix, carious are generally separated by two anion spheres instead of one sphere. Therefore one would expect smaller cation deflections from the planes with high dhk/, i.e., linearization of the cation matrix

Consequently, a rarefied matrix provides better possibilities for meeting the stereochemical requirements of each cation, and makes each coordination polyhedron relatively independent of its neighbors. Table 1 includes data on the densest cation planes of five characteristic structures with different types of arrangement of Zr complexes and other cations. The structures are given in the order of increasing ratio of the number of anions to that of cations (N a / No); the oxygen atoms of water molecules are considered as anions, because all of them enter the coordination spheres of cations. The

first structure, KxCu(ZrF6) 2- 6H20, is constructed from [ZrzF12] 4- dimers, [Cu(H20)6] 2+ octahedral complexes, and

K + ions. With Zr cations in a dimer taken individually, the density of cation packing in a plane is relatively low (Table 1). Ass,ruing the center of the dimer to be one "atom" (Zr-Zr 3.2/k), we obtain densities that are close to unity. Analysis of the arrangement of such fragments (K, Cu, Zr2) in a cell shows that this is four-layered close packing ABAC... to a good approximation. An ideal example of this type of packing is the cation matrix of NH4Er3F10 [10]. In this structure, the basal plane is the xz coordinate plane, and the "strong" 040 plane indicates that the complex is a four-layered structure. Our previous report did not aim at describing the cation matrix of this structure as a new structural type, i.e., as a complex of close-packed planes with definite mutual orientations. We will do this now, bearing in mind that four-layered close packing is not a rare occurrence. An almost ideal cation subcell with four atoms is obtained from the cell of NH4Er3F10 (P63mc, a = 8.10, c = 13.34, z = 4) via the (1./200/01/20/001) transformation matrix. The subcell gives the following set of close-packed planes: (011) (3.39 A) 0.43, (004) (3.335 A) 0.98, (012) (3.104 A) 0.74, (013) (2.75 ,~,) 0.43, (110) (2.02/~) 0.98 (the indices are given for the cationic sublattice; dhk / values are given in

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Institute of Crystallography, Russian Academy of Sciences. Translated from Zhumal Stmktumoi IOzimii, Vol. 36, No. 3, pp. 481-487, May-June, 1995. Original article submitted July 11, 1994.

(X)22-4766/95/3603-0439512.50 o1995 Plenum Publishing Corporation 439

Formula, ref.

K2Cu(ZrF6)2 - 6H20 [5]

CuZrF6"4H20 [6]

Cu3(ZrF7) 2 - 16H20 [7]

Cu2ZrF8-12H20 [8]

Z, r2F 8 -6H20 [9]

Fedorov

N,, Ar e

779

2 78

3.6

/'21/c

421

2 105.3 5.0

P21/c 1146

2

114

6.0

C2] r

1.585

4

132

6 2/3

P I

262 1

131.2 7.0

TABLE 1. Cr I

i hk/

120

I22 013

~02 ~o4 040

02O

I l l

111 002 022

I31 200

~11

022

~2 222

002

020

]11

~13 O22

311

~02

011 I01 Ill 101 OO2 1~0 110

rstal Data for Huorides with Water

3.85 ~.61

3.69 I 0.86

3.65 058 I 3.30 0.90 ]

2.75 0.62 250 - /

[ 5.02 1.00

4.49 1.00

3.72 1.00

3.69 1.00 2.97 1.00

178 1.00

2`77 1.00

4.30 0.83

3.71 0.98

3.05 1.00

2.99

5.17

4.81

4.63

3.62

3.52

3.30

2.74

2`64

5.04

4.83 0.96 4.75 0.81 3.62 0.99 3.61 0.91 3.46 0.98 3.14 0.85

Type of cation net

Square-rhombic

36-distorted

36-distorted

44-distorted

36-distorted 3 6 44 (1 --Zr, II -- Cu) 36-distorted

44 (1 -Z t ' , II - Cu)

44-distorted 4 4 two sorts

I 36, 11 36 with a vacancy

I 36, II 36 with a vacancy

36 two sorts

44-distorted

44..distorted

36-distorted 36-distorted See Fig. 3a

36- distorted

44-distorted 44_distorted See Fig. 3b

Distorted square-rhombic

Square-rhombic 36 almost regular Deltoid

Deltoid

Cationic sublattice parameters, type of

s t r u c t u r e

ac = 6.46; be -- 6.63;

c c = 9.98; y = 114.3

a = f l = 9 0 o ABAC type

The parameters coincide with those of the unit cell

F - ~ (ABCO

ar = a/2, br = b/2; c e = c/2,

P-type with ordered vacancies

ac ~ be =, cc ,~ 5.25

crc =~ fie ~" 90~ Ye ffi 1200

Hexagonal

One-layered AA type

The parameters coincide with those of the unit cell, AB type

parentheses; the third value for each plane is the density of cation packing; Pc). If we restrict ourselves to the three "strongest" families of planes and take into account the hexagonal symmetry of the structure, then we obtain that the {110) planes form the side faces of the hexagonal prism, and the {004} planes form its basal planes; the {012} planes, which comprise a hexagonal bipyramid, give an independent system of intersection points inside the unit cell that are close to, but not coincident with, the centering atoms.

In K2Cu[ZrF6]2-6H20 , the cationic subcell is transformed to the pseudohexagonal type via the (001/2/100/010)

tran.~formation matrix so that all of the "strong" planes of the A B A C structural type are present: (120), (I22)-~(012)h; (Y.02), (7.04)-~(110)h; (040)-,(004)h. The deviations of the parameters of the cationic sublattice from the ideal parameters are primarily explained by the dumbbell form of the Zr 2 cation. Figure la, b shows ideal and real cation nets in the

440

u b

o -~-s o " d _ k _ _ _ o _ _ _ _ . - o : _ \ _ _ - - 9 ,'

- - - -o, o / \/ ; \; o

o o o o o o o 5 .

Fig. 1. Characteristic atomic net in the (012) plane of the ABAC structural type. a) 36+ 3342 + 44 (1:2:1) net for the ideal structure; b) cation net in the (I22) plane of K2Cu(ZrF6)2; a tick indicates the center of a Zr-Zr dimer; orientation of the dimer causes elongation of one side in the square net. The unit cell is marked with solid, and the net with dashed, lines; translations of the planar cell are given as vector sums of initial translations.

(012)h plane, which may be regarded as typical cation nets in the given structural type for planes with relatively

large dh/d.

The abundance of close-packed cation planes in CuZrF6-4H20 is explained by the fact that the cations are

arranged in the cell by the F-lattice law, as judged by the indices of the same parity (Table 1). In the ideal lattice, the unit cell should be cubic, but in this structure it is distorted due to the action of water as an electrically neutral filler.

The [ZrF6] 2- and [Cu(H20)nF2] 2+ complexes are linked by common F-vertices into a straight chain running in the

[101] direction. Since neither the water molecules nor the terminal F atoms in the ZrF 6 octahedron need to be coordinated to the second cation, there are no forces drawing together pairs of anion layers along the b axis, and the axis is elongated. The ac face, which should be square in the ideal unit cell, is deformed (co.ntracted) by the diagonal chain of complexes (Fig. 2). Nevertheless, the F-complex of close-packed cation planes is retained, despite the anlsotropy of intercation distances [3].

The structure of Cu3(ZrF7)2" 16H20 is pseudocubic according to lattice parameters and is constructed from

[Zr2F14] 6- dlmers (two eight-vertex polyhedra with a common edge, Zr-Zr = 3.66/~), an [Cu2(I-I20)10] 4+ dimer (two

octahedra with a common edge, Cu--Cu = 358 ~), and an isolated [Cu(H20)6] 2+ octahedron" Packing densities were

calculated both for the individual distribution of cations and for dimers regarded as one fragment with dlmer center coordinates, in which case all packing densities increase to about unity. The dimer centers and the single Cu oetahedron form a defective NaCI structure: after one half of the a period, the coordinate planes fdled by two types of cations following the NaC1 pattern alternate with the planes having only one half of the atoms (for example, at the centers of side faces). Thus the cation matrix packing is of primitive pseudocubic type with ordered defects. In Table 1,

�9 |

Cu

0

QZr

O Fig. 2. Cation net in the ac face of the F-cell of CuZrF6"4H20. The dashed line indicates the direction of the Zr-Cu octahedral chain, which distorts the square net of the ideal structure (by shortening the Zr-Cu contact along the chain compared to other intercation distances).

441

characteristics of cation nets are given for the cases where the Zr atoms are taken independently within dimers and two copper atoms are replaced by one atom. As d/at/ decreases, the ordering effect is observed for the individual

cations, rather than for the dimer centers, since dhk/ = 3.5/~, is a sufficient distance for resolution of the dimer center

and the atom (-1.75/~.). The next structure in the table, CuZrFs-12H20, is an ideal example of a purely complex structure in which

the [ZrFs] 4- eight-vertex polyhedra and the [Cu(H20)6] 2+ octahedra have no common anions, i.e., the cations are

separated by two anion spheres and are therefore more than 5 ~ apart. The wealth of close-packed cation planes (the first members of thi.g series are given in the table) testify to the high degree of spatial ordering of the complexes. It is worthwhile to note the characteristic feature of the F-lattice: all indices of strong planes are of the same parity. Closer inspection reveals that cation packing is one-layered hexagonal, and the corresponding cationic subceU is recommended for use in further description. The conversion matrix is (1/300/-1/61/20/1/601/2); the parameters are: a c -- b c -- 5 .3 /~ c c -- 5.2/~; a ~ ff -- 90", y ~- 120 ~ Earlier we encountered thi~ type of cation packing in Ag, PdZr2Fll [11] but did not

characterize it as a structural type, i.e., as a complex of close-packed planes. The sequence of planes indicated in Table 1 is transformed according to hexagonal subcell indices to the following sequence: (002)-*(001)n; (020)--(010)h; ( 'J11)-~10)h; ('J13)'-'(I11)n; (022)-~(011)n; (311)"(101)n; ~31)-~'('I20) h. In the case of the ideal cell with trigonal (or hexagonal) symmetry, cation nets of four families should be analyzed: {001}, having the regular 36 net, {100}, having the regular square net, {101}, and {110}. The two latter families have characteristic geometries. In the {101} family, the net may be conventionally called pseudohexagonal; it is obtained when the distances between the rows of one of the directions of the ideal hexagonal net increase, so that the rhomb is transformed into a parallelogram in which one side and the short diagonal are longer by a factor of d-~, and the second diagonal equals the doubled side of the initial rhomb (Fig. 3a). This type of cation net is observed in the ('J13), (022), and (311) planes of Cu2ZrFs'12H20. The

{110}h family is represented by the ('J31) and ('602) planes, in which the cation net of the ideal matrix shottld be rectangular, with the ratio of sides l :v~ (Fig. 3b). Three planes of the {100} family and the (001) plane, i.e., the coordinate planes of the hexagonal ceil, may be taken as an independent complex of planes characteristic of the given structural type. The real cell of Cu2ZrF 2" 12H80 is centered 12 times by cations; the centering also applies to the

F-cell, which accounts for the same parity of indices of all "strong" planes. In the last structure, Zr2Fs(H20)6 , the two eight-vertex polyhedra of Zr are linked by a common F -F edge

into a dimer with Zr -Zr = 3.63/~.. The Zr-Zr distances between the dlmers may be 5.8 ~ or longer, and such znlsotropy explains the low symmetry of packing. Nevertheless, in the 002 plane we have a good basal trigonal net of cations packed by the law of hexagonal close packing (AB or HCP type [12]). The character of chemically induced distortions (dimerization of the cationic polyhedra) becomes conspicuous if we compare cation nets in (110) planes that are differently oriented relative to the Zr 2 dimer (Fig. 4a, b). The {101} family, which is characteristic of hexagonal two-layered close packing, is represented in Table 1 by the first four planes. One of these planes, in which the

o '

Zr Cu

a

Zr' Cu

�9

I I I I I

I12(a+3c)

Q

O

Fig. 3. Characteristic atomic nets for hexagonal one-layered packing (AA type), a) Cation net in the (022) = (101)h plane for CuZrFs.4H20 (distorted hexagonal) with indications of sides expressed via the minimal side t;, b) a f r a ~ e n t of the 44 net in the (-602) = (ll0)h plane for the same structure; translations of the planar net are a+3c and b.

442

~7

t ~Q 1 o

2 " c \ \ ~ \ \ \

. I \ / ,v,:. / ~ 0 ~ " ~ / / I /

~ - a , ~ ' ~ ~

- , O / \ " 0

O ~ . ~

/ c / f " ~ ~ / / Y ~ "x .

\ ] ...-.-" \ / t . t - ' -

" o / o

O

c

Fig. 4. Characteristic cation nets of Zr2Fs(H20)6. a) Almost ideal, "dihedral" (according to [12]) Zr net for the case where the Zr 2 dimer atoms lie in the neighboring (110) plane_s; b) net of the same type, but with the dimer atoms lying in the plane of the net, (120);.c) square-rhombic net in the (101) plane.

square-rhombic net is pronounced, is shown in Fig. 4c. As in the previous situations, we can conclude that the set of close-packed planes is fully retained, despite significant deformation of the hexagonal cell (this is evident from the unit cell parameters of the Zr2F8(H206) 6 structure) and the absence of the 6 or 3 axes.

S,mming up, in these structures, as in other types of compounds, we observe a competition between two factors: the geometrical factor, which shows a tendency toward close but symmetric packing, and the "chemical" factor, which reflects atomic coordination determined by stoichiometry and valences, the character of links between coordination polyhedra, etc. As we have found earlier for anhydrous structures, the "chemical" factor additionally manifests itself in anisotropy of the cationic surroundings of cations. Due to the anlsotropy, the cation--c.at.ion distances vary within 10-15% of the average value, and the ideal geometry of structural types is distorted. In the structure of fluorides containing water, the anisotropy of the cationic surroundings increases, because in some directions the cationic polyhedra are linked by common anions, whereas in others they are separated by two anionic layers. As a consequence," the cationic framework is distorted further. When the cation-cation distances are almost doubled with respect to the distances within the dlmers, the latter are ordered as single fragments (for planes with high dh/d). The ordering of

heavy atoms or entire fraornents is most pronounced in the structure of Cu2ZrF 8-121-120, where Zr and Cu complexes

are independent of each other and are close to sphere in form. This structure has the most regular, i.e., almost ideal, cationic sublattice.

The general conclusion is that every structure may be assigned with confidence to a certain structural type, by which, in the ideal case, we mean a set of close-packed and symmetrically arranged atomic planes with characteristic regular cation nets, etc. The exact symmetry of mutual disposition of planes may be lost, as in the structures considered above, but the set of planes that ceased to be identical is retained. Therefore, in agreement with our concept of crystalline state [13], the structural type of cation matrix is the most important feature that serves to classify the given structure.

443

This work was fulfilled with financial support from the Russian Fund for Basic Research (Project 93-03-5991).

.

2. 3. 4. 5. 6. 7. 8. 9.

10.

i i . 12. 13.

REFERENCES

S. V. Borisov et al., Zh. StrulcL Khim., 34, No. 5, 116-125 (1993). N. A. Bllrnyuk et al., ibid., 126-132. N. A. Bllrnyuk, S. V. Borisov, and E. S. Kuldina, ibkL, 35, No. 1, 71-80 (1994). S. V. Borisov, N. A. Bliznyuk, and E. S. Kuldina, ibid., 35, No. 3, 3-10 (1994). J. Fischer and R. Weiss, Acta Crystallogr., B29, 1958-1962 (1973). J. Fischer and R. Weiss, t7~id., 1955-1957. J. Fischer and R. Weiss, ibid., 1963-1967. J. Fischer, R. Elchinder, and R. Weiss, ibid., 1967-1971. F. Gabela, B. Kojic-Prodic, M. Sljukic, and Z. Ruzik-Toros, ib/d., 1133, 3733-3736 (1977). N. V. Podberezskaya, I. A. Baidina, S. V. Borisov, and N. V. Belov, Zh. Stmkt. Khim., 17, No. 1, 14%152 (1976). B. G. Mueller, Z. Anorg. Allgem. Chem., B553, 205-211 (1987). S. V. Borisov, Zh. Stnda. 1Odin., 27, No. 3, 164-166 (1986). S. V. Borisov, ibid., 33, No. 6, 123-130 (1992).

Translated by L. Smolina

444