CsAl(H₂AsO₄)₂(HAsO₄): a new monoclinic protonated arsenate with decorated kröhnkite-like chains

The crystal structure of hydro­thermally synthesized caesium aluminium bis­[dihydrogen arsenate(V)] hydrogen arsen­ate(V), CsAl(H₂AsO₄)₂(HAsO₄), was determined from single-crystal X-ray diffraction data collected at room temperature. The compound represents a new structure type that is characterized by decorated kröhnkite-like [100] chains of corner-sharing AlO₆ octa­hedra and AsO₄ tetra­hedra. Ten-coordinated Cs atoms are situated between the chains, which are inter­connected by five different hydrogen bonds [O...O = 2.569 (4)–2.978 (4) Å]. All atoms are in general positions. CsAl(H₂AsO₄)₂(HAsO₄) is very closely related to CsGa(H_1.5AsO₄)₂(H₂AsO₄) and isotypic CsCr(H_1.5AsO₄)₂(H₂AsO₄).

S1. Comment

Arsenates, like phosphates and silicates, tend to form tetrahedral–octahedral framework structures with potentially interesting properties, such as ion conductivity, ion exchange and catalytic properties. The system M^I—M^III—As—O—H (M^I = Li, Na, K, Rb, Cs, Ag, Tl, NH₄ and H₃O; M^III = Al, Ga, In, Sc, Cr and Fe) is currently being closely studied by the authors to extend the knowledge on possible new arsenates and structure types. A larger number of novel structure types and new compounds has been prepared and structurally characterized so far (Kolitsch, 2004; Kolitsch & Schwendtner, 2004, 2005; Schwendtner & Kolitsch, 2004a,b, 2005a,b, c, 2006; Baran et al., 2006; Schwendtner, 2006; Schwendtner et al., 2006a,b).

The new compound CsAl(H₂AsO₄)₂(HAsO₄) is monoclinic (space group Cc) and represents a new chain-based structure type, very closely related to CsGa(H_1.5AsO₄)₂(H₂AsO₄) and isotypic CsCr(H_1.5AsO₄)₂(H₂AsO₄) (Schwendtner & Kolitsch, 2005a). The asymmetric unit contains 22 atoms, all of which lie in general positions. Al1 is octahedrally coordinated (mean Al1—O = 1.904 Å), Table 1) and corner-linked to six AsO₄ tetrahedra to form decorated kröhnkite-like chains (Kolitsch & Fleck, 2006 and references therein) running parallel to [100] (Fig. 1). Ten-coordinated Cs1 atoms (mean Cs1—O = 3.254 Å) lie between these chains. All three crystallographically different AsO₄ tetrahedra are protonated and highly distorted. The As—OH bonds are considerably elongated compared with the As—O bonds, which is typical for protonated AsO₄ tetrahedra (Ferraris, 1970; Ferraris & Ivaldi, 1984). The mean As—O bond length in the three AsO₄ tetrahedra ranges from 1.689 - 1.690 Å, which is considerably longer than the average length for AsO₄ tetrahedra of 1.682 Å (Baur, 1981) and a result of the protonation. As3O₄ is doubly protonated and is involved in one strong (O12···O3 = 2.641 (4) Å) and one very strong H-bond (O11···O3 = 2.569 (4) Å). As2O₄ is also doubly protonated with two medium-strength H-bonds (O8···O4 = 2.791 (4) and O7···O10 = 2.801 (4) Å), whereas As1O₄ is only involved in one weak H-bond (O4···O11 = 2.978 (4) Å). The hydrogen bond between O11 and O3 is strongly disordered between the two O atoms. Bond-valence calculations suggest that both atoms act as donor and as acceptor, since both atoms are highly underbonded (1.35 and 1.25 valence units, for atoms O3 and O11, respectively). The longer As—O bond length to O11 and the smaller bond valence may indicate that the H atom is located on a positon closer to the O11 atom, at least statistically and has therefore been attached to O11 in the refinement process.

Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991) for Al and Brown & Altermatt (1985) for As and Cs. They are 1.08 (Cs1), 3.03 (Al1), 4.93 (As1, As2), 4.94 (As3), 1.95 (O1), 1.80 (O2), 1.35 (O3), 1.17 (O4), 1.91 (O5), 1.89 (O6), 1.30 (O7), 1.27 (O8), 1.96 (O9), 1.76 (O10), 1.25 (O11) and 1.30 (O12) valence units. For the cations they are close to the ideal valences. The low sums for O3, O4, O7, O8, O11 and O12 demonstrate that these belong to OH groups. The hydrogen bonds donated by the OH groups are accepted by O3, O4, O10 and O11 (Table 2).

The similarities between CsAl(H₂AsO₄)₂(HAsO₄) and CsM^III(H_1.5AsO₄)₂(H₂AsO₄) (M^III = Ga, Cr) (C2/c) (Schwendtner & Kolitsch, 2005) are striking. Firstly, they have very similar unit-cell parameters [a = 4.634 (1), 4.714 (1) and 4.744 (1); b = 14.672 (3), 14.674 (3) and 14.625 (3); c = 15.143 (3), 15.162 (3) and 15.127 (3) Å; β = 93.11 (3), 93.31 (3) and 93.48 (3) °, V = 1028.7 (4), 1047.1 (4) and 1047.6 (4) ų for the CsAl, CsGa and CsCr compounds, respectively]. Both Cc and C2/c structure types, contain decorated kröhnkite-like chains running parallel [100]. Owing to the smaller size of the Al atom compared wizh Cr or Ga, the structure is distorted, which results in a loss of the twofold axis. Polyhedra As1O₄ (1 times protonated) and As3O₄ (2 times protonated) in the title compound are crystallographically equivalent to the As1O₄ polyhedra (1.5 times protonated) in the CsGa- and CsCr-compounds. The formerly symmetry-restricted split double-well H position in the C2/c-type is not clearly detectable, but has probably now shifted towards As3 in the Cc-type (compare Figs. 1 and 2). Another difference concerns the double protonated As2O₄ groups in both structure types. In the C2/c-type As2 is located on a twofold axis and the two H2 atoms bonded to O6 are crystallographically equivalent (Fig. 1 b, 2 b). This is not the case in the Cc-type structure, where atom H7 behaves very similarly to what we see in the C2/c-type, but atom H8 shifts considerably and its hydrogen bond is accepted by a different O atom, O4 (compare Figs. 1a and 1 b). Th Cs atom in the C2/c-type structure is located on the origin (0,0,0), whereas it is slightly offset in direction b in the Cc-type structure [y = -0.009538 (17)] (compare Figs. 1 and 2), which is in concordance with the lower space group symmetry. The ellipsoid drawing of CsAl(H₂AsO₄)₂(HAsO₄) is given in Fig. 3.

Related literature

For related literature, see: Baran et al. (2006); Baur (1981); Brese & O'Keeffe (1991); Brown & Altermatt (1985); Ferraris (1970); Ferraris & Ivaldi (1984); Kolitsch (2004); Kolitsch & Fleck (2006); Kolitsch & Schwendtner (2004, 2005); Lii & Wu (1994); Schwendtner (2006); Schwendtner & Kolitsch (2004a, 2004b, 2005a, 2005b, 2005c, 2006); Schwendtner et al. (2006a, 2006b); Spek (2003).

S2. Experimental

The title compound was prepared hydrothermally (Teflon-lined stainless steel bomb, 493 K, nine days, slow furnace cooling) from a mixture of Cs₂CO₃, Al₂O₃ and H₃AsO₄·0.5H₂O, with a volume ratio of approximately 1:1:3. The Teflon containers were then filled with distilled water to about 80% of their inner volume. The pH value of the starting and final solutions was approximately 1 and 1/2, respectively. CsAl(H₂AsO₄)₂(HAsO₄) formed colourless prismatic crystals up to 0.8 mm in length (yield ca 60%) and was acompanied by hexagonal tabular crystals of RbFe(HPO₄)₂)-type (Lii & Wu, 1994) CsAl(HAsO₄)₂ (ca 5%) and an uninvestigated amorphous mass (ca 35%) (Schwendtner & Kolitsch, 2005c).

S3. Refinement

The choice of space group Cc was based on the following evidence: (i) A refinement in space group C2/c and using the model of CsGa(H_1.5AsO₄)₂(H₂AsO₄) (Schwendtner & Kolitsch, 2005a) ?? resulted in R(F) = 0.15 and high residual electron densities (6–3 e Å^-3); application of various probable twin laws did not lead to any improvement. (ii) All intensity statistics showed a distinct preference for non-centrosymmetry, whereas those of CsM^III(H_1.5AsO₄)₂(H₂AsO₄) (M^III = Ga and Cr; Schwendtner & Kolitsch, 2005a) ?? clearly indicated centrosymmetry. (iii) A search for higher symmetry using PLATON (Spek, 2003) was unsuccessful. (iv) The correlation parameters calculated from the last refinement step were negligible (the largest correlation was between the z and y coordinates of atom H2. (v) The structure model and the inferred hydrogen-bonding scheme (different from that of the C2/c model) are highly plausible. O—H bond lengths for atoms H1, H2, H4 and H5 were restrained to a length of 0.90 (1) Å. The position of atom H3 was taken from a difference Fourier map and fixed. H1 was constrained to have a fixed U_iso(H) value of 0.05 Ų. The crystal was refined as a racemic twin.

Computing details

Data collection: COLLECT (Nonius, 2003); cell refinement: SCALEPACK (Otwinowski et al., 2003); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. (a) A view of CsAl(H2AsO4)2(HAsO4) along [100]. Decorated kröhnkite-like chains run parallel [100] and are connected through five different hydrogen bonds. The unit cell is outlined. (b) A view of CsGa(H1.5AsO4)2(H2AsO4) (Schwendtner & Kolitsch, 2005a) along [100]. The main difference in comparison with (a) can be seen in the hydrogen bonds, the slight tilting of the AsO4 tetrahedra and the location of the Cs atoms.
	Fig. 2. (a) A view of CsAl(H2AsO4)2(HAsO4) along [110]. Decorated kröhnkite-like chains run parallel [100] and are connected through five different hydrogen bonds. The unit cell is outlined. (b). A view of CsGa(H1.5AsO4)2(H2AsO4) (Schwendtner & Kolitsch, 2005a) along [110]. Decorated kröhnkite-like chains run parallel to [100].
	Fig. 3. Displacment ellipsoid drawing of CsAl(H2AsO4)2(HAsO4) at the 70% probability level. [Symmetry code: (i) x - 1, y, z.] CHNAGE TO iv

caesium aluminium bis[dihydrogen arsenate(V)] hydrogen arsenate(V)

Crystal data

CsAl(H2AsO4)2(HAsO4)	F(000) = 1072
Mr = 581.69	Dx = 3.756 Mg m^−3
Monoclinic, Cc	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C -2yc	Cell parameters from 2339 reflections
a = 4.634 (1) Å	θ = 2.0–35.0°
b = 14.672 (3) Å	µ = 13.32 mm^−1
c = 15.153 (3) Å	T = 293 K
β = 93.11 (3)°	Prism, colorless
V = 1028.7 (4) Å^3	0.10 × 0.09 × 0.02 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	4456 independent reflections
Radiation source: fine-focus sealed tube	4221 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
ϕ and ω scans	θmax = 35.0°, θmin = 2.7°
Absorption correction: multi-scan (SCALEPACK; Otwinowski et al., 2003)	h = −7→7
Tmin = 0.349, Tmax = 0.777	k = −23→23
4467 measured reflections	l = −24→24

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.024	w = 1/[σ^2(Fo^2) + (0.0327P)^2 + 4.0552P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.061	(Δ/σ)max = 0.024
S = 1.02	Δρmax = 0.81 e Å^−3
4456 reflections	Δρmin = −1.08 e Å^−3
170 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
4 restraints	Extinction coefficient: 0.00190 (12)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 2188 Friedel pairs?
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.507 (9)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cs1	−0.00002 (7)	−0.009538 (17)	0.00001 (2)	0.02820 (7)
Al1	−0.0307 (3)	0.14086 (6)	0.24624 (8)	0.00818 (13)
As1	0.49219 (5)	0.21759 (2)	0.36942 (2)	0.00885 (6)
As2	0.47210 (7)	−0.00622 (2)	0.24955 (3)	0.00921 (6)
As3	0.44465 (5)	0.20973 (2)	0.11976 (2)	0.00878 (6)
O1	0.7498 (5)	0.14258 (16)	0.34791 (16)	0.0107 (4)
O2	0.2138 (5)	0.23278 (16)	0.29550 (17)	0.0113 (4)
O3	0.3831 (6)	0.1974 (2)	0.47177 (17)	0.0185 (5)
O4	0.6303 (6)	0.32747 (18)	0.3707 (2)	0.0224 (6)
H4	0.7859	0.3283	0.3992	0.034*
O5	0.7187 (5)	0.04969 (17)	0.19569 (17)	0.0111 (4)
O6	0.2129 (5)	0.04928 (17)	0.30031 (17)	0.0116 (4)
O7	0.3175 (7)	−0.07964 (19)	0.1726 (2)	0.0198 (5)
H7	0.2986	−0.1300	0.1951	0.030*
O8	0.6294 (6)	−0.0709 (2)	0.3336 (2)	0.0194 (5)
H8	0.7681	−0.0985	0.3154	0.029*
O9	0.1886 (5)	0.13530 (17)	0.14408 (17)	0.0120 (4)
O10	0.7244 (5)	0.23282 (16)	0.19080 (16)	0.0108 (4)
O11	0.5659 (6)	0.1710 (2)	0.02259 (19)	0.0202 (5)
H11	0.6817	0.2076	0.0039	0.030*
O12	0.2957 (6)	0.31461 (18)	0.0985 (2)	0.0179 (5)
H12	0.1507	0.3090	0.0654	0.027*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs1	0.04127 (16)	0.02128 (11)	0.02123 (10)	−0.00028 (11)	−0.00575 (9)	−0.00348 (9)
Al1	0.0062 (3)	0.0095 (3)	0.0089 (3)	0.0002 (3)	0.0004 (2)	0.0002 (3)
As1	0.00701 (13)	0.01019 (12)	0.00932 (14)	0.00092 (9)	0.00010 (10)	−0.00164 (9)
As2	0.00697 (11)	0.00857 (11)	0.01210 (12)	0.00053 (11)	0.00069 (9)	0.00054 (11)
As3	0.00777 (13)	0.01035 (12)	0.00817 (14)	−0.00140 (9)	−0.00011 (10)	0.00064 (10)
O1	0.0079 (9)	0.0130 (10)	0.0115 (10)	0.0035 (7)	0.0030 (8)	0.0027 (8)
O2	0.0081 (10)	0.0109 (9)	0.0145 (11)	0.0001 (7)	−0.0027 (8)	0.0007 (8)
O3	0.0179 (12)	0.0294 (13)	0.0082 (10)	0.0044 (10)	0.0019 (9)	−0.0011 (9)
O4	0.0170 (12)	0.0117 (11)	0.0379 (17)	−0.0040 (9)	−0.0035 (11)	−0.0045 (10)
O5	0.0079 (9)	0.0126 (9)	0.0132 (10)	−0.0027 (7)	0.0023 (8)	−0.0005 (8)
O6	0.0095 (10)	0.0127 (10)	0.0127 (10)	0.0031 (7)	0.0027 (8)	0.0020 (8)
O7	0.0215 (13)	0.0128 (10)	0.0245 (13)	−0.0054 (9)	−0.0044 (10)	−0.0053 (9)
O8	0.0162 (12)	0.0189 (12)	0.0228 (13)	0.0060 (9)	0.0000 (10)	0.0089 (10)
O9	0.0077 (10)	0.0155 (10)	0.0127 (11)	−0.0032 (7)	0.0018 (8)	−0.0002 (8)
O10	0.0080 (10)	0.0116 (9)	0.0123 (10)	−0.0013 (7)	−0.0037 (8)	0.0019 (8)
O11	0.0191 (12)	0.0285 (13)	0.0138 (11)	−0.0085 (10)	0.0078 (9)	−0.0047 (10)
O12	0.0167 (12)	0.0126 (10)	0.0236 (13)	0.0010 (8)	−0.0072 (9)	0.0056 (9)

Geometric parameters (Å, º)

Cs1—O7	3.107 (3)	Al1—O6	1.911 (3)
Cs1—O9	3.137 (3)	Al1—O10^iv	1.926 (3)
Cs1—O12^i	3.152 (3)	As1—O1	1.669 (2)
Cs1—O1^ii	3.190 (2)	As1—O2	1.677 (2)
Cs1—O8^ii	3.199 (3)	As1—O3	1.683 (3)
Cs1—O6^iii	3.285 (3)	As1—O4	1.734 (3)
Cs1—O3^iii	3.319 (3)	As2—O5	1.657 (2)
Cs1—O11^iv	3.355 (3)	As2—O6	1.673 (2)
Cs1—O4^v	3.386 (3)	As2—O7	1.716 (3)
Cs1—O5^iv	3.415 (3)	As2—O8	1.718 (3)
Cs1—O11	3.730 (3)	As3—O9	1.668 (2)
Al1—O2	1.889 (3)	As3—O10	1.674 (2)
Al1—O1^iv	1.892 (3)	As3—O11	1.702 (3)
Al1—O9	1.899 (3)	As3—O12	1.710 (3)
Al1—O5^iv	1.905 (3)
O2—Al1—O1^iv	90.35 (12)	O2—As1—O4	98.87 (13)
O2—Al1—O9	90.96 (12)	O3—As1—O4	106.38 (16)
O1^iv—Al1—O9	178.30 (11)	O5—As2—O6	121.14 (11)
O2—Al1—O5^iv	179.04 (12)	O5—As2—O7	104.46 (14)
O1^iv—Al1—O5^iv	89.65 (12)	O6—As2—O7	109.54 (14)
O9—Al1—O5^iv	89.06 (12)	O5—As2—O8	111.23 (13)
O2—Al1—O6	90.28 (12)	O6—As2—O8	102.39 (14)
O1^iv—Al1—O6	89.51 (11)	O7—As2—O8	107.56 (14)
O9—Al1—O6	89.40 (12)	O9—As3—O10	121.71 (12)
O5^iv—Al1—O6	90.67 (10)	O9—As3—O11	104.18 (13)
O2—Al1—O10^iv	89.93 (10)	O10—As3—O11	110.07 (14)
O1^iv—Al1—O10^iv	90.90 (11)	O9—As3—O12	110.25 (13)
O9—Al1—O10^iv	90.18 (12)	O10—As3—O12	103.28 (13)
O5^iv—Al1—O10^iv	89.12 (11)	O11—As3—O12	106.61 (15)
O6—Al1—O10^iv	179.54 (13)	As1—O4—H4	109.5
O1—As1—O2	119.33 (12)	As2—O7—H7	109.5
O1—As1—O3	108.52 (13)	As2—O8—H8	109.5
O2—As1—O3	112.35 (13)	As3—O11—H11	109.5
O1—As1—O4	110.36 (14)	As3—O12—H12	109.5

Symmetry codes: (i) x−1/2, y−1/2, z; (ii) x−1, −y, z−1/2; (iii) x, −y, z−1/2; (iv) x−1, y, z; (v) x−1/2, −y+1/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O11^vi	0.82	2.22	2.978 (4)	154
O7—H7···O10^i	0.82	2.04	2.801 (4)	154
O8—H8···O4^vii	0.82	2.13	2.791 (4)	137
O11—H11···O3^viii	0.82	1.76	2.569 (4)	169
O12—H12···O3^v	0.82	1.84	2.641 (4)	167

Symmetry codes: (i) x−1/2, y−1/2, z; (v) x−1/2, −y+1/2, z−1/2; (vi) x+1/2, −y+1/2, z+1/2; (vii) x+1/2, y−1/2, z; (viii) x+1/2, −y+1/2, z−1/2.