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Synthesis and single-crystal structure of Cs3Zn4O(AsO4)3 · 4H2O, an open-framework zinc arsenate
Microporous and Mesoporous Materials 39 (2000) 359±365
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Synthesis and single-crystal structure of Cs3Zn4O(AsO4)3 á 4H2O, an open-framework zinc arsenate William T.A. Harrison a,*, Mark L.F. Phillips b, Xianhui Bu c a
Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, UK Gem®re Corporation, 2440 Embarcadero Way, Palo Alto, CA 94303, USA Department of Chemistry, University of California, Santa Barbara, CA 93106, USA b
c
Received 2 February 2000; received in revised form 27 March 2000; accepted 27 March 2000
Abstract The synthesis and crystal structure of caesium zinc arsenate hydrate, Cs3 Zn4 O(AsO4 )3 á 4H2 O, are described. This open-framework zincoarsenate, which is built up from a three-dimensional network of vertex-sharing ZnO4 and AsO4 tetrahedra, shows post-dehydration thermal stability to at least 600°C. A model involving merohedral rotational twinning about [0 0 1] was required to arrive at a satisfactory single-crystal structure solution. Crystal data: c 7:9216 2 A, Cs3 Zn4 O(AsO4 )3 á 4H2 O, Mr 1165:05, tetragonal, space group P 4, (no. 81), a 11:3099 2 A, 3 V 1013:28 4 A , Z 2, R F 3:90%, Rw F 4:81% (740 re¯ections with I > 3r I, 81 parameters). Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Zincophosphate; Twinning; Pharmacosiderite; Open framework
1. Introduction Several zincophosphate/zincoarsenates (ZnPOs/ ZnAsOs) are known to crystallise as analogues of aluminosilicate (AlSiO) zeolite frameworks including the sodalite (IZA framework code: SOD) [1], zeolite-X (FAX) [2], zeolite Li-A (ABW) [3], and cancrinite (CAN) [4] types. NaZnPO4 á H2 O [5] crystallises as a novel, chiral, framework (CZP), which has not been observed for other framework compositions. Very recently, ZnPO analogues of the ``®brous'' zeolites edingtonite (EDI) [6] and thomsonite (THO) [7] have been described. * Corresponding author. Tel.: +44-1224-272-921; fax: +441224-272-921. E-mail address: [email protected] (W.T.A. Harrison).
Structurally, all these zeolite-like ZnPOs and ZnAsOs contain a 1:1 ratio of Zn:P/As and a strictly alternating array of ZnO4 and PO4 /AsO4 nodes, linked solely by Zn±O±P/As bonds. Unfortunately, most of these ZnPOs and ZnAsOs are much less stable to thermal treatment than their AlSiO analogues, and their open frameworks do not survive dehydration. We have recently reported [8] a distinctive new family of open-framework zincophosphate/ arsenate phases of stoichiometry M3 Zn4 O(XO4 )3 á nH2 O (M Na, K, Rb, Cs,. . .; X P, As; n 3:5±6). The unusual zinc-rich, ®xed 4:3 ratio Zn:X framework stoichiometry arises due to the presence of l4 -O atoms occupying the centres of novel ``OZn4 '' tetrahedral building units as well as ZnO4 and XO4 tetrahedra. Unlike the
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phases mentioned above, several of these M3 Zn4 O(XO4 )3 á nH2 O materials show post-dehydration thermal stability to 600°C. Some of them also undergo facile cationic ion-exchange [8]. In this paper, we report the full structural characterisation of Cs3 Zn4 O(AsO4 )3 á 4H2 O, which adopts a new tetragonal superstructure for this class of material. 2. Experimental 2.1. Synthesis A zinc oxide pellet was made by pressing ZnO powder (St. JosephÕs) at 2000 bar, then ®ring for 30 min at 1100°C. A 2.95 g lump of pelleted ZnO was added to a solution containing 18.00 g (60 mmol) 50% CsOH, 10.95 g (40 mmol) CsH2 AsO4 and 16 ml H2 O. This mixture was heated to 95°C for 14 days, after which the remnant of the ZnO pellet was removed and 0.66 g (6.2% yield based on Zn) of small crystals (transparent cubes with maximum linear dimension 0.05 mm) of Cs3 Zn4 O(AsO4 )3 á 4H2 O were recovered by vacuum ®ltration and washing with cold water. The ZnO pelleting process may release zinc ions into solution at a controlled rate to assist in the formation of single crystals of useable size rather than microcrystalline powder as reported previously [8]. The product appears to be completely stable when stored in dry air. 2.2. Physical characterisation X-ray powder data (Siemens D5000 automated powder diractometer, CuKa radiation, T 25(2)°C) for a well-ground k 1:54178 A, sample of Cs3 Zn4 O(AsO4 )3 á 4H2 O were in very good agreement with a simulation of the singlecrystal structure, indicating a high degree of crystallinity and phase purity. Thermogravimetric analysis (TGA) showed a weight loss of 6% over the temperature range 80± 180°C, in good accord with a dehydration reaction involving the loss of four water molecules from Cs3 Zn4 O(AsO4 )3 á 4H2 O to result in a residue of Cs3 Zn4 O(AsO4 )3 (calculated weight loss 6:2%).
No further weight change occurred up to 600°C. Upon cooling, the same sample showed negligible weight uptake over several hours, which is somewhat surprising in view of its open-framework nature. It is possible that the caesium cations occupy the 8-ring sites (see below) and thus kinetically hinder the re-adsorption of water. The powder pattern of the cooled, dehydrated sample showed some peak broadening compared to the as-prepared material, and could be modelled with c a similar tetragonal cell (a 11:256 3 A, 3 7:885 3, V 999:0 7 A ) [9]. Preliminary ion-exchange studies on Cs3 Zn4 O(AsO4 )3 á 4H2 O have been carried out, by heating a 0.5 g sample to 90°C overnight with 20 ml of 5 M solutions of various transition metal cations. On the basis of the colours of the resulting powders, at least a partial ion-exchange of transition metal for caesium has probably occurred. We are studying these products further. 2.3. Single crystal structure determination A transparent cube (0:04 0:04 0:04 mm) of Cs3 Zn4 O(AsO4 )3 á 4H2 O was mounted on a thin glass ®bre with cyanoacrylate adhesive. Roomtemperature (25(2)°C) intensity data were collected on a Siemens SMART CCD area detector diractometer (graphite-monochromated MoKa Preliminary scans inradiation, k 0:71073 A). dicated a primitive tetragonal unit cell. A hemisphere of data was collected in narrow-slice mode (x scan width 0.30°) with an exposure time of 30 s per frame. Peak integration with SAINT [10] resulted in 11 054 intensities (ÿ14 6 h 6 14; ÿ14 6 k 6 14; ÿ10 6 l 6 10) for 3:3 6 2h 6 54:0 . The ®nal cell constants were re®ned from the locations of 3891 re¯ections with 3:3 6 2h 6 54:0 (Table 1). An absorption correction (range of equivalent transmission factors: 0.767±1.000) was applied with SADABS [10] on the basis of multiple measurements of symmetry-equivalent re¯ections. Data merging resulted in 1350 unique re¯ections, of which 740 were considered observed according to the criterion I > 3r I. There were no systematic absences. Test data merges favoured Laue class 4/m RInt 0:056 over 4/mmm RInt 0:102.
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365 Table 1 Crystallographic parameters for Cs3 Zn4 O(AsO4 )3 á 4H2 O Empirical formula Formula weight Crystal system a (A) c (A) 3 ) V (A
Cs3 Zn4 As3 O17 H8 1165.05 Tetragonal 11.3099 (2) 7.9216 (2) 1013.28 (4) 2 P 4 (no. 81) 298 (2) 0.71073 3.82 150.0 11054 740 81 ÿ0:94; 1:33 3.74 4.78
Z Space group T (K) k (A) qcalc (g/cm3 ) l (cm±1 ) Total data Observed data Parameters 3 ) Min., max. Dq (e/A R(F)a Rw (F)b P P a R 100 kFo j ÿ jFc k= jFo j. P P b Rw 100 w jFo j ÿ jFc j2 = wjFo j2 1=2 .
The only reasonable direct method solution [11] was obtained in the space group P 4 (no. 81), which was assumed for the remainder of the crystal structure analysis. Least-squares re®nement with CRYSTALS [12] resulted in a chemically plausible structure, with R 10%. However, the geometrical parameters for the ZnO4 and AsO4 groupings were unrealistic and anisotropic thermal factors for the heavy atoms could not be successfully re®ned. A much better re®nement was obtained by assuming 90° rotational merohedral twinning about [0 0 1], with a transformation matrix of 0 1 0 1 0 @1 0 0A 0 0 1 relating the twin components. This type of twinning results in exact overlap of non-equivalent re¯ections from the two crystal domains, and can be analysed using well-established procedures [13]. Re®nement of their relative fractions, di , subject to the constraint d1 d2 1:00 resulted in a 0.618(6):0.322(6) domain ratio in the crystal studied here. This model (®nal R(F) 3.90%) allowed for the re®nement of anisotropic thermal factors for the non-oxygen atoms, and reasonable geometrical parameters resulted. No hydrogen atoms
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Table 2 Atomic coordinates/thermal factors for Cs3 Zn4 O(AsO4 )3 á 4H2 O Atom
x
y
z
Ueq
Cs1 Cs2 Zn1 Zn2 As1 As2 As3 O1 O2 O3 O4 O5 O6 O7 O8 O9a O10a
0.15719(18) 0 0.3590(3) ÿ0.0169(3) 0 0.5 0.2393(2) 0.5 0.2747(18) 0.2347(19) 0.4039(17) 0 0.1120(14) 0.2568(17) ÿ0.052(2) 0.436(2) 0.907(2)
0.31118(17) 0.5 0.4947(3) 0.1436(3) 0 0.5 0.2656(2) 0.5 0.643(2) 0.3798(19) 0.4351(18) 0 0.2610(16) 0.1412(18) 0.1145(19) 0.188(2) 0.388(2)
0.0204(3) 0.4859(5) 0.3511(4) 0.6428(4) 0 0 0.5346(3) 0.5 0.328(3) 0.405(3) 0.122(3) 0.5 0.647(2) 0.421(3) 0.883(3) 0.192(3) 0.124(4)
0.0281 0.0277 0.0115 0.0115 0.0114 0.0111 0.0142 0.017(9) 0.024(5) 0.023(5) 0.014(4) 0.008(8) 0.010(3) 0.019(5) 0.022(5) 0.034(6) 0.042(7)
a
O atom of water molecule.
could be located. Crystallographic data are summarised in Table 2. Supplementary data (anisotropic thermal factors and observed and calculated structure factors) are available from the authors. 3. Results Final atomic positional and thermal parameters for Cs3 Zn4 O(AsO4 )3 á 4H2 O are listed in Table 2, with the selected bond distance/angle data in Table 3. This phase is an open framework caesium zincoarsenate hydrate built up from ZnO4 and AsO4 tetrahedra fused together via Zn±O±As and Zn± O±Zn bonds. An ORTEP-3 [14] view of a fragment of Cs3 Zn4 O(AsO4 )3 á 4H2 O is shown in Fig. 1, and the complete crystal structure in Fig. 2. The two distinct zinc atoms adopt tetrahedral coordination to O atoms with geometrical pa and dav (Zn2± rameters of dav (Zn1±O) 1.97 (2) A Both zinc atoms make three O) 1.97 (2) A. Zn±O±As linkages; in addition, a fourth bond is made to a tetrahedrally coordinated O atom, resulting in OZn4 centres (Fig. 1). The three distinct As atoms form the centres of tetrahedral arsenate As1 and As2 groups (dav As3±O 1:69 2 A). have site symmetry 4. Each arsenate group forms four As±O±Zn links. The eight distinct framework
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Table 3 and angles (°) for Cs3 Zn4 OSelected bond distances (A) (AsO4 )3 á 4H2 O Cs1±O2 Cs1±O3 Cs1±O4 Cs1±O6 Cs1±O8 Cs1±O8 Cs1±O9 Cs1±O10 Cs1±O10 Cs2±O3 2 Cs2±O6 2 Cs2±O7 2 Cs2±O9 2 Cs2±O10 2 Zn1±O1 Zn1±O2 Zn1±O3 Zn1±O4 Zn2±O5 Zn2±O6 Zn2±O7 Zn2±O8 As1±O8 4 As2±O4 4 As3±O2 As3±O3 As3±O6 As3±O7 Zn1±O1±Zn1 Zn1±O1±Zn1 Zn1±O2±As3 Zn1±O3±As3 Zn1±O4±As2 Zn2±O5±Zn2 Zn2±O5±Zn2 Zn2±O6±As3 Zn2±O7±As3 Zn2±O8±As1
3.59(2) 3.26(2) 3.23(2) 3.058(19) 3.43(2) 3.06(2) 3.34(2) 3.07(3) 3.58(3) 3.05(2) 3.245(18) 3.26(2) 3.40(2) 3.30(3) 1.984(4) 1.94(2) 1.96(2) 2.00(2) 1.989(3) 1.972(17) 1.97(2) 1.97(2) 1.70(2) 1.63(2) 1.73(2) 1.65(2) 1.692(17) 1.68(2) 110.70(9) 107.05(18) 129.5(12) 129.2(13) 123.8(12) 108.88(9) 110.65(18) 129.9(11) 131.2(12) 125.6(13)
oxygen atoms in Cs3 Zn4 O(AsO4 )3 á 4H2 O divide into six Zn±O±As bridges (hav 128.2°) and the tetrahedral (to four Zn) O1 and O5 species (site symmetry 4 for these two atoms). The polyhedral connectivity in Cs3 Zn4 O(AsO4 )3 á 4H2 O results in a three-dimensional network of polyhedral 3-rings and 8-rings, as described previously [8]. This network is built up from ZnO4 , AsO4 and OZn4 tetrahedra linked by Zn±O±As and Zn±O±Zn bonds. The anionic
Fig. 1. Fragment of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure (50% thermal ellipsoids) showing the atom labelling scheme.
Fig. 2. The structure of Cs3 Zn4 O(AsO4 )3 á 4H2 O viewed down [0 0 1], with selected atoms labelled.
[Zn4 O(AsO4 )3 ]3± framework encloses a three-dimensional network of 8-ring windows propagating along [1 1 0], [1 1 0], and [0 0 1], which interconnect roughly spherical cavities (atom-to-atom dimen An analysis of the volume occupied sion 6.5 A). by the framework [Zn4 O(AsO4 )3 ]3± component of the structure with the CALC SOLV option of 3 (43% of the PLATON97 [15] indicated that 437 A unit-cell volume) is empty space, emphasising the zeolitic nature of this material. However, no alu-
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365
minosilicate or aluminophosphate molecular sieves contain tetrahedrally coordinated framework O atoms as observed here. The extra-framework caesium cations are located in the vicinity of framework 8-ring windows. They are irregularly coordinated, assuming a Cs1 maximum Cs±O contact distance of 3.6 A. (Fig. 3) is nine coordinate [dav (Cs1±O) 3.29(2) A] to six framework O atoms and three water molecule O atoms; these latter atoms are in the same cage as the Cs1 species. Fig. 4 shows that Cs2 (site to symmetry 2..) is 10 coordinate (dav 3.25(2) A) six framework O atoms and four water molecule O atoms. For Cs2, the water molecule O atoms are distributed over both the adjacent extra-framework spherical cages. Bond valence sum (BVS) calculations using the Brown formalism [16] yielded BVS Cs1 0:97 and BVS Cs2 1:10 expected values 1:00 indicating that the valence requirements of the Cs species are satis®ed by these coordination environments. The two extra-framework water molecules (O9 and O10) occupy the spherical cages in this phase, each cage contains four molecules arranged in an approximate U shape. A short O9 O10 contact suggests that a hydrogen distance of 2.78(4) A
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Fig. 4. Detail of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure showing the Cs2 coordination.
bond might be present between these two extraframework species. Several O9/O10 to framework are also present: O9 O atom distances of <3 A bonds to two Cs ; O10 to three Cs .
4. Discussion
Fig. 3. Details of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure showing the Cs1 coordination.
The open-framework caesium zincoarsenate hydrate, Cs3 Zn4 O(AsO4 )3 á 4H2 O, has been prepared as single crystals and structurally characterised by diraction methods. The title compound is con®rmed to be a modi®cation of the M3 Zn4 O(XO4 )3 á nH2 O family [8]. Here, the anionic [Zn4 O(AsO4 )3 ]3± framework is charge balanced by three caesium cations accompanied by four extra-framework water molecules. A crystallographic model involving merohedral rotational twinning was required to arrive at a convincing structure solution. Cs3 Zn4 O(AsO4 )3 á 4H2 O shows a new tetragonal superstructure for the M3 Zn4 O(XO4 )3 á nH2 O class of compounds. The most simple structural model [8] for the [Zn4 O(XO4 )3 ]3± framework component of this family can be established in the cubic space group P 43m, with a unit cell parameter
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depending on the chemical a0 7:2±7.9 A, composition. Because of tetrahedral framework distortions and guest cation ordering, real M3 Zn4 O(XO4 )3 á nH2 O frameworks show superstructures, most commonly an a 2a0 face-centred cube, space group F 43c [8]. The primitive tetragonalpsymmetry of Cs3 Zn4 O(AsO4 )3 á 4H2 O with at 2a0 and ct a0 may be correlated with the distinctive ordering pattern of the extraframework Cs1 species located near the [1 1 0] and [1 1 0] 8-ring windows arrayed normal to the (1 1 0) plane (Fig. 2). With respect to any particular cage, two Cs1 species are displaced inwards by 1.23 A from the planes of the 8-ring windows, and two are displaced outwards by the same distance. Conversely, the Cs2 species are located close to the all in the mid-points (displacement of 0.11 A, same direction) of 8-ring windows forming the [0 0 1] channels. These displacements are consistent with the ``unbalanced'' water coordination of Cs1 (Fig. 3: all three H2 O in the same cage), versus the balanced water coordination of Cs2 (Fig. 4: two water molecules either side). Based on unit-cell data derived from powder patterns, [8] the large-cation M3 Zn4 O(XO4 )3 á nH2 O phases Rb3 Zn4 O(PO4 )3 á nH2 O (n 3:5) c 7:623(2) A) and Cs3 (a 10:931(3) A, c Zn4 O(PO4 )3 á nH2 O (n 4) (a 11:074(3) A, 7:750(2) A) are probably isostructural with Cs3 Zn4 O(AsO4 )3 á 4H2 O. Conversely, in the smallcation phases, Na3 Zn4 O(PO4 )3 á 6H2 O [8] and Li3 Zn4 O(PO4 )3 á 6H2 O [17], a completely dierent cation-ordering pattern occurs with the univalent species shifting towards the side of an 8-ring window, and rhombohedral crystal symmetry (space group R3c) results. The small cations allow six water molecules to occupy each cage, compared to 3.5 or four water molecules per cage for the large cation M3 Zn4 O(XO4 )3 á nH2 O phases. Yet another symmetry modi®cation (monoclinic, with am a0 , bm 2b0 , cm c0 , and b 90°) is shown by the recently reported (H3 CNH3 )2 Zn4 O(XO4 )3 phases (X P; As) [18], in which the methylammonium cation replaces a combination of alkali-metal cation and water as the templating agent for this framework type. An interesting contrast to the title compound occurs in the pharmacosiderite-type [19] frame-
work Cs3 H(TiO)4 (SiO4 )3 á 4H2 O [20,21], in which an equivalent octahedral (TiO6 ) tetrahedral (SiO4 ) framework surrounds a similar threedimensional channel/cage system to that found in Cs3 Zn4 O(AsO4 )3 á 4H2 O. These frameworks may be regarded [22] as ``homeomorphic'' to the ReO3 structure. The extra-framework contents of these two materials are identical, although the framework of the pharmacosiderite phase requires additional charge compensation in the form of a proton attached to a framework O atom [20,21]. In Cs3 H(TiO)4 (SiO4 )3 á 4H2 O, the Cs cations are randomly disordered over pairs of sites adjacent to the inter-cage 8-ring windows in the [1 0 0], [0 1 0], and [0 0 1] directions [20,21], and primitive cubic symmetry is maintained. However, several M3 H(AO)4 (BO4 )3 á 4H2 O (A Ti Ge; B Si Ge) mixed-framework-cation synthetic pharmacosiderites [23] show rather similar superstructures to the M3 Zn4 O(XO4 )3 á nH2 O type phases which may be empirically correlated with guest cation ordering patterns. Acknowledgements We thank the National Science Foundation (Grant DMR 95-20971) for partial funding. References [1] T.E. Gier, G.D. Stucky, Nature (London) 349 (1991) 508. [2] W.T.A. Harrison, T.E. Gier, K.L. Moran, J.M. Nicol, H. Eckert, G.D. Stucky, Chem. Mater. 3 (1991) 27. [3] W.T.A. Harrison, T.E. Gier, J.M. Nicol, G.D. Stucky, J. Solid State Chem. 114 (1995) 249. [4] O.V. Yakubovich, O.V. Karimova, O.K. MelÕnikov, Crystallogr. Reports 39 (1994) 564. [5] W.T.A. Harrison, R.B. Broach, R.A. Bedard, T.E. Gier, G.D. Stucky, Chem. Mater. 8 (1996) 145. [6] R.W. Broach, R.L. Bedard, S.G. Song, J.J. Pluth, A. Bram, C. Reikle, H.-P. Weber, Chem. Mater. 11 (1999) 2076. [7] W.T.A. Harrison, H.Y. Ng, unpublished work. [8] W.T.A. Harrison, R.B. Broach, R.A. Bedard, T.E. Gier, X. Bu, G.D. Stucky, Chem. Mater. 8 (1996) 691. [9] T.J.B. Holland, S.A.T. Redfern, Mineral Mag. 61 (1997) 65. [10] SMART Software Suite. Bruker Inc., Madison, WI, USA, 1997.
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365 [11] G.M. Sheldrick, SHELXS-86 User Guide, University of G ottingen, Germany, 1986. [12] D.J. Watkin, J.R. Carruthers, P.W. Betteridge, CRYSTALS User Guide, Chemical Crystallography Laboratory, University of Oxford, UK, 1997. [13] C.S. Pratt, B.A. Coyle, J.A. Ibers, J. Chem. Soc. (1971) 2146. [14] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565. [15] A.L. Spek, Acta Crystallogr. A46 (1990) C34. [16] I.D. Brown, J. Appl. Crystallogr. 29 (1996) 479. [17] W.T.A. Harrison, T.E. Gier, G.D. Stucky, T. Vogt, unpublished work.
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[18] W.T.A. Harrison, M.L.F. Phillips, A.V. Chavez, T.M. Neno, J. Mater. Chem. 9 (1999) 3087. [19] M.J. Buerger, W.A. Dollase, I. Garaycochea-Wittke, Zeit. Kristallogr. 125 (1967) 92. [20] W.T.A. Harrison, T.E. Gier, G.D. Stucky, Zeolites 15 (1995) 408. [21] E.A. Behrens, D.A. Poojary, A. Clear®eld, Chem. Mater. 8 (1996) 1236. [22] M. Schindler, F.C. Hawthorne, W.H. Baur, Acta Crystallogr. B55 (1999) 811. [23] E.A. Behrens, D.M. Poojary, A. Clear®eld, Chem. Mater. 10 (1998) 959.
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Synthesis and single-crystal structure of Cs3Zn4O(AsO4)3 á 4H2O, an open-framework zinc arsenate William T.A. Harrison a,*, Mark L.F. Phillips b, Xianhui Bu c a
Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, UK Gem®re Corporation, 2440 Embarcadero Way, Palo Alto, CA 94303, USA Department of Chemistry, University of California, Santa Barbara, CA 93106, USA b
c
Received 2 February 2000; received in revised form 27 March 2000; accepted 27 March 2000
Abstract The synthesis and crystal structure of caesium zinc arsenate hydrate, Cs3 Zn4 O(AsO4 )3 á 4H2 O, are described. This open-framework zincoarsenate, which is built up from a three-dimensional network of vertex-sharing ZnO4 and AsO4 tetrahedra, shows post-dehydration thermal stability to at least 600°C. A model involving merohedral rotational twinning about [0 0 1] was required to arrive at a satisfactory single-crystal structure solution. Crystal data: c 7:9216 2 A, Cs3 Zn4 O(AsO4 )3 á 4H2 O, Mr 1165:05, tetragonal, space group P 4, (no. 81), a 11:3099 2 A, 3 V 1013:28 4 A , Z 2, R F 3:90%, Rw F 4:81% (740 re¯ections with I > 3r I, 81 parameters). Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Zincophosphate; Twinning; Pharmacosiderite; Open framework
1. Introduction Several zincophosphate/zincoarsenates (ZnPOs/ ZnAsOs) are known to crystallise as analogues of aluminosilicate (AlSiO) zeolite frameworks including the sodalite (IZA framework code: SOD) [1], zeolite-X (FAX) [2], zeolite Li-A (ABW) [3], and cancrinite (CAN) [4] types. NaZnPO4 á H2 O [5] crystallises as a novel, chiral, framework (CZP), which has not been observed for other framework compositions. Very recently, ZnPO analogues of the ``®brous'' zeolites edingtonite (EDI) [6] and thomsonite (THO) [7] have been described. * Corresponding author. Tel.: +44-1224-272-921; fax: +441224-272-921. E-mail address: [email protected] (W.T.A. Harrison).
Structurally, all these zeolite-like ZnPOs and ZnAsOs contain a 1:1 ratio of Zn:P/As and a strictly alternating array of ZnO4 and PO4 /AsO4 nodes, linked solely by Zn±O±P/As bonds. Unfortunately, most of these ZnPOs and ZnAsOs are much less stable to thermal treatment than their AlSiO analogues, and their open frameworks do not survive dehydration. We have recently reported [8] a distinctive new family of open-framework zincophosphate/ arsenate phases of stoichiometry M3 Zn4 O(XO4 )3 á nH2 O (M Na, K, Rb, Cs,. . .; X P, As; n 3:5±6). The unusual zinc-rich, ®xed 4:3 ratio Zn:X framework stoichiometry arises due to the presence of l4 -O atoms occupying the centres of novel ``OZn4 '' tetrahedral building units as well as ZnO4 and XO4 tetrahedra. Unlike the
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phases mentioned above, several of these M3 Zn4 O(XO4 )3 á nH2 O materials show post-dehydration thermal stability to 600°C. Some of them also undergo facile cationic ion-exchange [8]. In this paper, we report the full structural characterisation of Cs3 Zn4 O(AsO4 )3 á 4H2 O, which adopts a new tetragonal superstructure for this class of material. 2. Experimental 2.1. Synthesis A zinc oxide pellet was made by pressing ZnO powder (St. JosephÕs) at 2000 bar, then ®ring for 30 min at 1100°C. A 2.95 g lump of pelleted ZnO was added to a solution containing 18.00 g (60 mmol) 50% CsOH, 10.95 g (40 mmol) CsH2 AsO4 and 16 ml H2 O. This mixture was heated to 95°C for 14 days, after which the remnant of the ZnO pellet was removed and 0.66 g (6.2% yield based on Zn) of small crystals (transparent cubes with maximum linear dimension 0.05 mm) of Cs3 Zn4 O(AsO4 )3 á 4H2 O were recovered by vacuum ®ltration and washing with cold water. The ZnO pelleting process may release zinc ions into solution at a controlled rate to assist in the formation of single crystals of useable size rather than microcrystalline powder as reported previously [8]. The product appears to be completely stable when stored in dry air. 2.2. Physical characterisation X-ray powder data (Siemens D5000 automated powder diractometer, CuKa radiation, T 25(2)°C) for a well-ground k 1:54178 A, sample of Cs3 Zn4 O(AsO4 )3 á 4H2 O were in very good agreement with a simulation of the singlecrystal structure, indicating a high degree of crystallinity and phase purity. Thermogravimetric analysis (TGA) showed a weight loss of 6% over the temperature range 80± 180°C, in good accord with a dehydration reaction involving the loss of four water molecules from Cs3 Zn4 O(AsO4 )3 á 4H2 O to result in a residue of Cs3 Zn4 O(AsO4 )3 (calculated weight loss 6:2%).
No further weight change occurred up to 600°C. Upon cooling, the same sample showed negligible weight uptake over several hours, which is somewhat surprising in view of its open-framework nature. It is possible that the caesium cations occupy the 8-ring sites (see below) and thus kinetically hinder the re-adsorption of water. The powder pattern of the cooled, dehydrated sample showed some peak broadening compared to the as-prepared material, and could be modelled with c a similar tetragonal cell (a 11:256 3 A, 3 7:885 3, V 999:0 7 A ) [9]. Preliminary ion-exchange studies on Cs3 Zn4 O(AsO4 )3 á 4H2 O have been carried out, by heating a 0.5 g sample to 90°C overnight with 20 ml of 5 M solutions of various transition metal cations. On the basis of the colours of the resulting powders, at least a partial ion-exchange of transition metal for caesium has probably occurred. We are studying these products further. 2.3. Single crystal structure determination A transparent cube (0:04 0:04 0:04 mm) of Cs3 Zn4 O(AsO4 )3 á 4H2 O was mounted on a thin glass ®bre with cyanoacrylate adhesive. Roomtemperature (25(2)°C) intensity data were collected on a Siemens SMART CCD area detector diractometer (graphite-monochromated MoKa Preliminary scans inradiation, k 0:71073 A). dicated a primitive tetragonal unit cell. A hemisphere of data was collected in narrow-slice mode (x scan width 0.30°) with an exposure time of 30 s per frame. Peak integration with SAINT [10] resulted in 11 054 intensities (ÿ14 6 h 6 14; ÿ14 6 k 6 14; ÿ10 6 l 6 10) for 3:3 6 2h 6 54:0 . The ®nal cell constants were re®ned from the locations of 3891 re¯ections with 3:3 6 2h 6 54:0 (Table 1). An absorption correction (range of equivalent transmission factors: 0.767±1.000) was applied with SADABS [10] on the basis of multiple measurements of symmetry-equivalent re¯ections. Data merging resulted in 1350 unique re¯ections, of which 740 were considered observed according to the criterion I > 3r I. There were no systematic absences. Test data merges favoured Laue class 4/m RInt 0:056 over 4/mmm RInt 0:102.
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365 Table 1 Crystallographic parameters for Cs3 Zn4 O(AsO4 )3 á 4H2 O Empirical formula Formula weight Crystal system a (A) c (A) 3 ) V (A
Cs3 Zn4 As3 O17 H8 1165.05 Tetragonal 11.3099 (2) 7.9216 (2) 1013.28 (4) 2 P 4 (no. 81) 298 (2) 0.71073 3.82 150.0 11054 740 81 ÿ0:94; 1:33 3.74 4.78
Z Space group T (K) k (A) qcalc (g/cm3 ) l (cm±1 ) Total data Observed data Parameters 3 ) Min., max. Dq (e/A R(F)a Rw (F)b P P a R 100 kFo j ÿ jFc k= jFo j. P P b Rw 100 w jFo j ÿ jFc j2 = wjFo j2 1=2 .
The only reasonable direct method solution [11] was obtained in the space group P 4 (no. 81), which was assumed for the remainder of the crystal structure analysis. Least-squares re®nement with CRYSTALS [12] resulted in a chemically plausible structure, with R 10%. However, the geometrical parameters for the ZnO4 and AsO4 groupings were unrealistic and anisotropic thermal factors for the heavy atoms could not be successfully re®ned. A much better re®nement was obtained by assuming 90° rotational merohedral twinning about [0 0 1], with a transformation matrix of 0 1 0 1 0 @1 0 0A 0 0 1 relating the twin components. This type of twinning results in exact overlap of non-equivalent re¯ections from the two crystal domains, and can be analysed using well-established procedures [13]. Re®nement of their relative fractions, di , subject to the constraint d1 d2 1:00 resulted in a 0.618(6):0.322(6) domain ratio in the crystal studied here. This model (®nal R(F) 3.90%) allowed for the re®nement of anisotropic thermal factors for the non-oxygen atoms, and reasonable geometrical parameters resulted. No hydrogen atoms
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Table 2 Atomic coordinates/thermal factors for Cs3 Zn4 O(AsO4 )3 á 4H2 O Atom
x
y
z
Ueq
Cs1 Cs2 Zn1 Zn2 As1 As2 As3 O1 O2 O3 O4 O5 O6 O7 O8 O9a O10a
0.15719(18) 0 0.3590(3) ÿ0.0169(3) 0 0.5 0.2393(2) 0.5 0.2747(18) 0.2347(19) 0.4039(17) 0 0.1120(14) 0.2568(17) ÿ0.052(2) 0.436(2) 0.907(2)
0.31118(17) 0.5 0.4947(3) 0.1436(3) 0 0.5 0.2656(2) 0.5 0.643(2) 0.3798(19) 0.4351(18) 0 0.2610(16) 0.1412(18) 0.1145(19) 0.188(2) 0.388(2)
0.0204(3) 0.4859(5) 0.3511(4) 0.6428(4) 0 0 0.5346(3) 0.5 0.328(3) 0.405(3) 0.122(3) 0.5 0.647(2) 0.421(3) 0.883(3) 0.192(3) 0.124(4)
0.0281 0.0277 0.0115 0.0115 0.0114 0.0111 0.0142 0.017(9) 0.024(5) 0.023(5) 0.014(4) 0.008(8) 0.010(3) 0.019(5) 0.022(5) 0.034(6) 0.042(7)
a
O atom of water molecule.
could be located. Crystallographic data are summarised in Table 2. Supplementary data (anisotropic thermal factors and observed and calculated structure factors) are available from the authors. 3. Results Final atomic positional and thermal parameters for Cs3 Zn4 O(AsO4 )3 á 4H2 O are listed in Table 2, with the selected bond distance/angle data in Table 3. This phase is an open framework caesium zincoarsenate hydrate built up from ZnO4 and AsO4 tetrahedra fused together via Zn±O±As and Zn± O±Zn bonds. An ORTEP-3 [14] view of a fragment of Cs3 Zn4 O(AsO4 )3 á 4H2 O is shown in Fig. 1, and the complete crystal structure in Fig. 2. The two distinct zinc atoms adopt tetrahedral coordination to O atoms with geometrical pa and dav (Zn2± rameters of dav (Zn1±O) 1.97 (2) A Both zinc atoms make three O) 1.97 (2) A. Zn±O±As linkages; in addition, a fourth bond is made to a tetrahedrally coordinated O atom, resulting in OZn4 centres (Fig. 1). The three distinct As atoms form the centres of tetrahedral arsenate As1 and As2 groups (dav As3±O 1:69 2 A). have site symmetry 4. Each arsenate group forms four As±O±Zn links. The eight distinct framework
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Table 3 and angles (°) for Cs3 Zn4 OSelected bond distances (A) (AsO4 )3 á 4H2 O Cs1±O2 Cs1±O3 Cs1±O4 Cs1±O6 Cs1±O8 Cs1±O8 Cs1±O9 Cs1±O10 Cs1±O10 Cs2±O3 2 Cs2±O6 2 Cs2±O7 2 Cs2±O9 2 Cs2±O10 2 Zn1±O1 Zn1±O2 Zn1±O3 Zn1±O4 Zn2±O5 Zn2±O6 Zn2±O7 Zn2±O8 As1±O8 4 As2±O4 4 As3±O2 As3±O3 As3±O6 As3±O7 Zn1±O1±Zn1 Zn1±O1±Zn1 Zn1±O2±As3 Zn1±O3±As3 Zn1±O4±As2 Zn2±O5±Zn2 Zn2±O5±Zn2 Zn2±O6±As3 Zn2±O7±As3 Zn2±O8±As1
3.59(2) 3.26(2) 3.23(2) 3.058(19) 3.43(2) 3.06(2) 3.34(2) 3.07(3) 3.58(3) 3.05(2) 3.245(18) 3.26(2) 3.40(2) 3.30(3) 1.984(4) 1.94(2) 1.96(2) 2.00(2) 1.989(3) 1.972(17) 1.97(2) 1.97(2) 1.70(2) 1.63(2) 1.73(2) 1.65(2) 1.692(17) 1.68(2) 110.70(9) 107.05(18) 129.5(12) 129.2(13) 123.8(12) 108.88(9) 110.65(18) 129.9(11) 131.2(12) 125.6(13)
oxygen atoms in Cs3 Zn4 O(AsO4 )3 á 4H2 O divide into six Zn±O±As bridges (hav 128.2°) and the tetrahedral (to four Zn) O1 and O5 species (site symmetry 4 for these two atoms). The polyhedral connectivity in Cs3 Zn4 O(AsO4 )3 á 4H2 O results in a three-dimensional network of polyhedral 3-rings and 8-rings, as described previously [8]. This network is built up from ZnO4 , AsO4 and OZn4 tetrahedra linked by Zn±O±As and Zn±O±Zn bonds. The anionic
Fig. 1. Fragment of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure (50% thermal ellipsoids) showing the atom labelling scheme.
Fig. 2. The structure of Cs3 Zn4 O(AsO4 )3 á 4H2 O viewed down [0 0 1], with selected atoms labelled.
[Zn4 O(AsO4 )3 ]3± framework encloses a three-dimensional network of 8-ring windows propagating along [1 1 0], [1 1 0], and [0 0 1], which interconnect roughly spherical cavities (atom-to-atom dimen An analysis of the volume occupied sion 6.5 A). by the framework [Zn4 O(AsO4 )3 ]3± component of the structure with the CALC SOLV option of 3 (43% of the PLATON97 [15] indicated that 437 A unit-cell volume) is empty space, emphasising the zeolitic nature of this material. However, no alu-
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365
minosilicate or aluminophosphate molecular sieves contain tetrahedrally coordinated framework O atoms as observed here. The extra-framework caesium cations are located in the vicinity of framework 8-ring windows. They are irregularly coordinated, assuming a Cs1 maximum Cs±O contact distance of 3.6 A. (Fig. 3) is nine coordinate [dav (Cs1±O) 3.29(2) A] to six framework O atoms and three water molecule O atoms; these latter atoms are in the same cage as the Cs1 species. Fig. 4 shows that Cs2 (site to symmetry 2..) is 10 coordinate (dav 3.25(2) A) six framework O atoms and four water molecule O atoms. For Cs2, the water molecule O atoms are distributed over both the adjacent extra-framework spherical cages. Bond valence sum (BVS) calculations using the Brown formalism [16] yielded BVS Cs1 0:97 and BVS Cs2 1:10 expected values 1:00 indicating that the valence requirements of the Cs species are satis®ed by these coordination environments. The two extra-framework water molecules (O9 and O10) occupy the spherical cages in this phase, each cage contains four molecules arranged in an approximate U shape. A short O9 O10 contact suggests that a hydrogen distance of 2.78(4) A
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Fig. 4. Detail of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure showing the Cs2 coordination.
bond might be present between these two extraframework species. Several O9/O10 to framework are also present: O9 O atom distances of <3 A bonds to two Cs ; O10 to three Cs .
4. Discussion
Fig. 3. Details of the Cs3 Zn4 O(AsO4 )3 á 4H2 O crystal structure showing the Cs1 coordination.
The open-framework caesium zincoarsenate hydrate, Cs3 Zn4 O(AsO4 )3 á 4H2 O, has been prepared as single crystals and structurally characterised by diraction methods. The title compound is con®rmed to be a modi®cation of the M3 Zn4 O(XO4 )3 á nH2 O family [8]. Here, the anionic [Zn4 O(AsO4 )3 ]3± framework is charge balanced by three caesium cations accompanied by four extra-framework water molecules. A crystallographic model involving merohedral rotational twinning was required to arrive at a convincing structure solution. Cs3 Zn4 O(AsO4 )3 á 4H2 O shows a new tetragonal superstructure for the M3 Zn4 O(XO4 )3 á nH2 O class of compounds. The most simple structural model [8] for the [Zn4 O(XO4 )3 ]3± framework component of this family can be established in the cubic space group P 43m, with a unit cell parameter
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depending on the chemical a0 7:2±7.9 A, composition. Because of tetrahedral framework distortions and guest cation ordering, real M3 Zn4 O(XO4 )3 á nH2 O frameworks show superstructures, most commonly an a 2a0 face-centred cube, space group F 43c [8]. The primitive tetragonalpsymmetry of Cs3 Zn4 O(AsO4 )3 á 4H2 O with at 2a0 and ct a0 may be correlated with the distinctive ordering pattern of the extraframework Cs1 species located near the [1 1 0] and [1 1 0] 8-ring windows arrayed normal to the (1 1 0) plane (Fig. 2). With respect to any particular cage, two Cs1 species are displaced inwards by 1.23 A from the planes of the 8-ring windows, and two are displaced outwards by the same distance. Conversely, the Cs2 species are located close to the all in the mid-points (displacement of 0.11 A, same direction) of 8-ring windows forming the [0 0 1] channels. These displacements are consistent with the ``unbalanced'' water coordination of Cs1 (Fig. 3: all three H2 O in the same cage), versus the balanced water coordination of Cs2 (Fig. 4: two water molecules either side). Based on unit-cell data derived from powder patterns, [8] the large-cation M3 Zn4 O(XO4 )3 á nH2 O phases Rb3 Zn4 O(PO4 )3 á nH2 O (n 3:5) c 7:623(2) A) and Cs3 (a 10:931(3) A, c Zn4 O(PO4 )3 á nH2 O (n 4) (a 11:074(3) A, 7:750(2) A) are probably isostructural with Cs3 Zn4 O(AsO4 )3 á 4H2 O. Conversely, in the smallcation phases, Na3 Zn4 O(PO4 )3 á 6H2 O [8] and Li3 Zn4 O(PO4 )3 á 6H2 O [17], a completely dierent cation-ordering pattern occurs with the univalent species shifting towards the side of an 8-ring window, and rhombohedral crystal symmetry (space group R3c) results. The small cations allow six water molecules to occupy each cage, compared to 3.5 or four water molecules per cage for the large cation M3 Zn4 O(XO4 )3 á nH2 O phases. Yet another symmetry modi®cation (monoclinic, with am a0 , bm 2b0 , cm c0 , and b 90°) is shown by the recently reported (H3 CNH3 )2 Zn4 O(XO4 )3 phases (X P; As) [18], in which the methylammonium cation replaces a combination of alkali-metal cation and water as the templating agent for this framework type. An interesting contrast to the title compound occurs in the pharmacosiderite-type [19] frame-
work Cs3 H(TiO)4 (SiO4 )3 á 4H2 O [20,21], in which an equivalent octahedral (TiO6 ) tetrahedral (SiO4 ) framework surrounds a similar threedimensional channel/cage system to that found in Cs3 Zn4 O(AsO4 )3 á 4H2 O. These frameworks may be regarded [22] as ``homeomorphic'' to the ReO3 structure. The extra-framework contents of these two materials are identical, although the framework of the pharmacosiderite phase requires additional charge compensation in the form of a proton attached to a framework O atom [20,21]. In Cs3 H(TiO)4 (SiO4 )3 á 4H2 O, the Cs cations are randomly disordered over pairs of sites adjacent to the inter-cage 8-ring windows in the [1 0 0], [0 1 0], and [0 0 1] directions [20,21], and primitive cubic symmetry is maintained. However, several M3 H(AO)4 (BO4 )3 á 4H2 O (A Ti Ge; B Si Ge) mixed-framework-cation synthetic pharmacosiderites [23] show rather similar superstructures to the M3 Zn4 O(XO4 )3 á nH2 O type phases which may be empirically correlated with guest cation ordering patterns. Acknowledgements We thank the National Science Foundation (Grant DMR 95-20971) for partial funding. References [1] T.E. Gier, G.D. Stucky, Nature (London) 349 (1991) 508. [2] W.T.A. Harrison, T.E. Gier, K.L. Moran, J.M. Nicol, H. Eckert, G.D. Stucky, Chem. Mater. 3 (1991) 27. [3] W.T.A. Harrison, T.E. Gier, J.M. Nicol, G.D. Stucky, J. Solid State Chem. 114 (1995) 249. [4] O.V. Yakubovich, O.V. Karimova, O.K. MelÕnikov, Crystallogr. Reports 39 (1994) 564. [5] W.T.A. Harrison, R.B. Broach, R.A. Bedard, T.E. Gier, G.D. Stucky, Chem. Mater. 8 (1996) 145. [6] R.W. Broach, R.L. Bedard, S.G. Song, J.J. Pluth, A. Bram, C. Reikle, H.-P. Weber, Chem. Mater. 11 (1999) 2076. [7] W.T.A. Harrison, H.Y. Ng, unpublished work. [8] W.T.A. Harrison, R.B. Broach, R.A. Bedard, T.E. Gier, X. Bu, G.D. Stucky, Chem. Mater. 8 (1996) 691. [9] T.J.B. Holland, S.A.T. Redfern, Mineral Mag. 61 (1997) 65. [10] SMART Software Suite. Bruker Inc., Madison, WI, USA, 1997.
W.T.A. Harrison et al. / Microporous and Mesoporous Materials 39 (2000) 359±365 [11] G.M. Sheldrick, SHELXS-86 User Guide, University of G ottingen, Germany, 1986. [12] D.J. Watkin, J.R. Carruthers, P.W. Betteridge, CRYSTALS User Guide, Chemical Crystallography Laboratory, University of Oxford, UK, 1997. [13] C.S. Pratt, B.A. Coyle, J.A. Ibers, J. Chem. Soc. (1971) 2146. [14] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565. [15] A.L. Spek, Acta Crystallogr. A46 (1990) C34. [16] I.D. Brown, J. Appl. Crystallogr. 29 (1996) 479. [17] W.T.A. Harrison, T.E. Gier, G.D. Stucky, T. Vogt, unpublished work.
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[18] W.T.A. Harrison, M.L.F. Phillips, A.V. Chavez, T.M. Neno, J. Mater. Chem. 9 (1999) 3087. [19] M.J. Buerger, W.A. Dollase, I. Garaycochea-Wittke, Zeit. Kristallogr. 125 (1967) 92. [20] W.T.A. Harrison, T.E. Gier, G.D. Stucky, Zeolites 15 (1995) 408. [21] E.A. Behrens, D.A. Poojary, A. Clear®eld, Chem. Mater. 8 (1996) 1236. [22] M. Schindler, F.C. Hawthorne, W.H. Baur, Acta Crystallogr. B55 (1999) 811. [23] E.A. Behrens, D.M. Poojary, A. Clear®eld, Chem. Mater. 10 (1998) 959.