Alkali scandium arsenates. II. The framework structures of α- and β-CsSc(HAsO₄)₂

The crystal structures of hydro­thermally synthesized α-, (I), and β-caesium scandium bis­[hydrogen arsenate(V)], (II), both CsSc(HAsO₄)₂, have been determined from single-crystal X-ray diffraction data collected at room temperature. The dimorphs are both characterized by a three-dimensional negatively charged framework of corner-sharing alternating ScO₆ octahedra and HAsO₄ tetrahedra. The charge-balancing Cs⁺ cations are located in a system of three intersecting tunnels in (I) and in tunnels parallel to the a axis in (II). Strong to weak hydrogen bonds reinforce both frameworks. The average Sc—O bond lengths are 2.098 and 2.094 Å, respectively. Compound (I) is triclinic and isotypic with (NH₄)Fe^III(HPO₄)₂, α-A^IV^III(HPO₄)₂ (A is NH₄ or Rb) and α-(NH₄)(Al_0.64Ga_0.36)(HPO₄)₂. Compound (II) is monoclinic and isotypic with (H₃O)Fe^III(HPO₄)₂, β-A^IV^III(HPO₄)₂ (A is NH₄ or Rb), CsIn(HPO₄)₂ and RbSc(HPO₄)₂. Both (I) and (II) represent the first arsenate examples of their structure types. The Cs and Sc atoms in (I) lie on inversion centres. In (II), all atoms are in general positions. A brief overview is presented of the six structure types shown by A^IM^III(HXO₄)₂ compounds (X is P or As).

S1. Comment

We have recently started to investigate the crystallography and topology of metal scandium arsenates, in order to study the crystal-chemical behaviour of Sc^III cations in oxysalts, and to compare it with that of trivalent cations with similar ionic radii (e.g. V^III, Fe^III, Cr^III and Ga^III). In the first part of our series on alkali scandium arsenates (Schwendtner & Kolitsch, 2004), we have reported the acid arsenate KSc(HAsO₄)₂, representing a new microporous structure type designated MCV-3, and the diarsenate RbScAs₂O₇, which represents the first diarsenate known to crystallize with a KAlP₂O₇-type structure (Ng & Calvo, 1973). This second part of our series describes triclinic α-CsSc(HAsO₄)₂, (I), and monoclinic β-CsSc(HAsO₄)₂, (II). Furthermore, a brief overview is presented of the six structure types presently known for the compounds A^IM^III(HXO₄)₂ (A^I is a monovalent cation, M^III is a trivalent cation, and X is P or As). \sch

The crystal structure of (I) contains two Cs, two Sc, three As, twelve O and three H atoms in the asymmetric unit, and has space-group symmetry P1. The structure can be described as a complex three-dimensional negatively charged [Sc(HAsO₄)₂]⁻ framework, built of ScO₆ octahedra sharing corners with (HAsO₄)₂ tetrahedra (Figs. 1 and 2). The polyhedral connectivity results in a complex system of intersecting tunnels extending parallel to all three principal axes (Fig. 1, only the largest tunnel is shown). These tunnels host the Cs⁺ cations, which balance the negative charge of the framework. Each of the two ScO₆ octahedra shares all its vertices with adjacent (HAsO₄)₂ tetrahedra. Each of the three (HAsO₄)₂ tetrahedra shares three (unprotonated) vertices with adjacent ScO₆ octahedra.

The average Sc—O bond lengths are 2.104 (Sc1) and 2.092 Å (Sc2). The average As—O bond lengths in the three protonated arsenate groups (1.680 for As1, 1.687 for As2 and 1.682 Å for As3) show good agreement with the mean length in arsenate compounds (1.682 Å; Baur, 1981). The As—OH bonds are distinctly elongated in comparison with the As—O bonds (Table 1), as is typical of protonated AsO₄ tetrahedra (Ferraris, 1970; Ferraris & Ivaldi, 1984).

The two Cs atoms have coordination numbers of 12 (Cs1) and 11 (Cs2) within 3.9 Å, and show average Cs—O bond lengths of 3.40 and 3.42 Å, respectively. Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991). The values obtained [1.07 (Cs1), 0.95 (Cs2), 3.03 (Sc1), 3.13 (Sc2), 5.08 (As1), 4.98 (As2), 5.05 (As3), 2.12 (O1), 1.32 (O2), 1.95 (O3), 1.98 (O4), 1.96 (O5), 1.90 (O6), 1.86 (O7), 1.30 (O8), 1.78 (O9), 1.84 (O10), 1.94 (O11) and 1.26 (O12) valence units] are all reasonably close to the expected ideal valences, and confirm that O3, O8 and O12 represent hydroxyl groups. The two most underbonded O atoms (O9, O10) are acceptors of two of the three strong to medium-strong hydrogen bonds (Table 2), which reinforce the framework along different vectors rougly parallel to the ab plane. The compound is isotypic with (NH₄)Fe^III(HPO₄)₂ and several other acid phosphates (see overview below for details).

Compound (II) is monoclinic (space group P2₁/c) and the asymmetric unit contains one Cs, one Sc, two As, eight O and two H atoms. Similar to what was observed for (I), the crystal structure is characterized by a three-dimensional negatively charged [Sc(HAsO₄)₂]⁻ framework, built of ScO₆ octahedra sharing corners with (HAsO₄)₂ tetrahedra (Figs. 3 and 4). The ScO₆ shares each of its six vertices with adjacent (HAsO₄)₂ tetrahedra, while each (HAsO₄)₂ tetrahedron shares three (unprotonated) vertices in the construction of the framework. The charge-balancing Cs⁺ cations are located in tunnels which run parallel to the a axis (Fig. 3). These tunnels are defined by the stacking of 24-membered rings, which are constructed from corner-linkage of six ScO₆ octahedra and six (HAsO₄)₂ tetrahedra.

The average Sc—O bond length is 2.094 Å, very similar to that in (I). In KSc(HAsO₄)₂ and RbScAs₂O₇ (Schwendtner & Kolitsch, 2004), the corresponding values are negligibly smaller (2.090 Å). It is worth noting that all these values are consistently smaller than the reported mean Sc—O bond length in oxidic compounds (2.105 Å; Baur, 1981). The two protonated arsenate groups have average As—O bond lengths (1.680 for As1 and 1.683 Å for As2), which agree well with the mean length in arsenate compounds (1.682 Å; Baur, 1981). As in (I), the As—OH bonds are distinctly elongated in comparison with the As—O bonds (Table 3). The Cs atom is coordinated by 13 O atoms within 3.8 Å, and shows an average Cs—O bond length of 3.42 Å, similar to the situation in (I). Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991). They amount to 1.05 (Cs), 3.11 (Sc), 5.07 (As1), 5.05 (As2), 1.79 (O1), 1.91 (O2), 2.03 (O3), 1.36 (O4), 2.02 (O5), 1.81 (O6), 2.07 (O7) and 1.28 (O8) valence units, and confirm that O4 and O8 belong to hydroxyl groups. The three somewhat underbonded O ligands (O1, O2 and O6) are all acceptors of three strong to weak hydrogen bonds (Table 4). The strongest of these bonds reinforces the framework roughly along the c axis. The compound is isotypic with (H₃O)Fe^III(HPO₄)₂ and some other acid phosphates (see overview below for details).

At present, six different structure types are known to exist for compounds with the general formula A^IM^III(HXO₄)₂ (A^I is a monovalent cation, M^III is a trivalent cation, and X is P or As). These types comprise one triclinic type, four monoclinic types and one rhombohedral type.

The triclinic type (space group P1) was originally described for (NH₄)Fe^III(HPO₄)₂ by Yakubovich (1993). (Note that, for reasons which were not given, this author chose an I-centered triclinic cell for the description of the structure). As early as 1959, Smith & Brown had prepared the compounds A^IM^III(HPO₄)₂ (A^I is NH₄ or K, and M^III is Al or Fe) and studied KFe^III(HPO₄)₂ by single-crystal X-ray film methods. The non-standard primitive triclinic cell they found for this phase can be transformed into a standard cell with a = 7.20, b = 8.76 and c = 9.49 Å, and α = 64.58, β = 69.82 and γ = 70.13°, which clearly indicates that KFe^III(HPO₄)₂ also adopts this structure type. Crystals of the other three members were too small to be studied by single-crystal methods (Smith & Brown, 1959). The unindexed X-ray powder diffraction pattern of (NH₄)Fe^III(HPO₄)₂ listed in the ICDD powder diffraction file (ICDD-PDF, ICDD, Year?; No. 44–736) is fairly similar to the theoretical pattern calculated from the crystal-structure data given by Yakubovich (1993). The structure types of the Al analogues A^IAl(HPO₄)₂ (A^I is NH₄ or K) appear uncertain, although the unindexed X-ray powder diffraction patterns for (NH₄)Al^III(HPO₄)₂ (ICDD-PDF No. 44–755) and `(NH₄)Al^III(HPO₄)₂.xH₂O' (ICDD-PDF No. 44–726) suggest that at least the ammonium member may also be isotypic with its Fe^III analogue [no published pattern exists for KAl^III(HPO₄)₂]. Subsequently, the triclinic type was also found for the acid phosphates α-RbV^III(HPO₄)₂ (Lii & Wu, 1994; Haushalter et al., 1995), α-RbFe^III(HPO₄)₂ (Lii & Wu, 1994), α-(NH₄)(Al_0.64Ga_0.36)(HPO₄)₂ (Stalder & Wilkinson, 1998) and α-(NH₄)V^III(HPO₄)₂ (Bircsak & Harrison, 1998). α-CsSc(HAsO₄)₂, (I), is the first acid arsenate crystallizing with this structure type. Note that the prefix α is nowadays commonly added to the chemical formulae in order to distinguish individual phases from their monoclinic β dimorphs (see below).

The first monoclinic type (space group P2₁/c; a ~5, b ~9 and c ~14 Å, and β ~95°) was originally determined for (H₃O)Fe^III(HPO₄)₂ (Vencato et al., 1989). Later, isotypic β-RbV^III(HPO₄)₂ and β-(NH₄)V^III(HPO₄)₂ (Haushalter et al., 1995), CsIn(HPO₄)₂ (ICDD-PDF No. 53–1111) and RbSc(HPO₄)₂ (Bartu & Wildner, 2004) were reported. The prefix β is now commonly added to the chemical formulae in order to distinguish these phases from their triclinic α dimorphs (see above). Note that the X-ray powder data listed for RbSc(HPO₄)₂ in ICDD-PDF No. 53–1110 are indexed with the wrong space group, P2₁/m. A detailed comparison of this type and the above triclinic type is provided by Haushalter et al. (1995). β-CsSc(HAsO₄)₂, (II), is the first acid arsenate adopting this structure type. A structurally similar arsenate is PbFe(AsO₄)(AsO₃OH) (Effenberger et al., 1996), which is monoclinic (space group P2₁/n) and has roughly comparable unit-cell parameters (a = 4.85, b = 8.48 and c 15.56 Å, and β = 92.8°).

The second monoclinic type also has space-group symmetry P2₁/c, but completely different cell parameters (a ~10, b ~8 and c ~10 Å, and β ~116°). It was recently described for three indium acid phosphates A^IIn(HPO₄)₂ (A^I is K, Rb and NH₄) by Filaretov et al. (2002). No arsenate member is presently known. The third monoclinic type (space group C2/c) is shown by KSc(HAsO₄)₂ and was reported in Part I of our series on alkali scandium arsenates (Schwendtner & Kolitsch, 2004). No phosphate member is known at present. The fourth monoclinic type (space group Cc) is represented by NaSc(HPO₄)₂ (Bartu & Wildner, 2004), the only member known. This type is unique among the six recognized structure types because it is the only one showing a non-centrosymmetric atomic arrangement. The sixth type and the only one having a symmetry higher than monoclinic is represented by RbFe^III(HPO₄)₂ (Lii & Wu, 1994), which is rhombohedral (space group R3c) and exhibits a rather long c axis (about 53 Å).

All these six different structure types have one feature in common: they are characterized by three-dimensional frameworks built of strictly alternating M^IIIO₆ octahedra and HXO₄ tetrahedra. The A^I cations occupy voids and (often intersecting) tunnels within the frameworks. The OH groups of the HXO₄ tetrahedra always point towards void areas in the polyhedral arrangement. However, the connectivities of the tetrahedral and octahedral units in each structure type are quite distinct.

The hydrogen bonding in these compounds is also of interest. Haushalter et al. (1995) observed that the hydrogen bonds provided by the HPO₄ groups are very strong in the case of (monoclinic) β-RbV^III(HPO₄)₂ (O···O 2.41 Å, no s.u. given) and β-(NH₄)V^III(HPO₄)₂ (O···O 2.50–3.00 Å, no s.u. given), but weak in the case of (triclinic) α-RbV^III(HPO₄)₂. In contrast, the hydrogen bonds in α-(NH₄)V^III(HPO₄)₂ (Bircsak & Harrison, 1998) were fairly strong [O···O 2.574 (2)–2.742 (3) Å]. The present results also do not confirm the scheme observed by Haushalter et al. (1995). In fact, α-CsSc(HAsO₄)₂ shows slightly stronger hydrogen bonds [O···O 2.623 (3)–2.784 (4) Å, Table 2] than β-CsSc(HAsO₄)₂ [O···O 2.691 (7)–3.199 (7) Å, Table 4]. Thus, the details of the hydrogen bonding seem to be determined by the interplay between the polyhedra involved in the respective structures and cannot be predicted, although it appears that increasing the size of the polyhedra and/or of the A^I cations results in an overall weakening of the hydrogen bonds.

It is worth noting that the structure types one and two (α and β compounds) show a strong tendency to co-crystallize in the same experimental run. This behaviour was observed for dimorphous RbV^III(HPO₄)₂ (Haushalter et al., 1995), (NH₄)V^III(HPO₄)₂ (Bircsak & Harrison, 1998) and CsSc(HAsO₄)₂ (present work, see Experimental). Apparently, the lattice energies of both modifications are very similar.

S2. Experimental

A mixture of Sc₂O₃, arsenic acid, Cs₂CO₃ and water was treated hydrothermally in a Teflon-lined steel autoclave (493 K, 7 d, initial and final pH ~2). The reaction products were compounds (I) and (II). Compound (I) formed small colourless rhomb-shaped plate-like crystals (yield ca 2%). Compound (II) crystallized as small colourless prisms which were invariably twinned by non-merohedry (yield ca 98%). The twinning was often recognizable from re-entrant angles, but the twin law was not determined.

S3. Refinement

The crystal fragment of (II) used for the data collection was partially twinned by non-merohedry (estimated twin ratio > 4:1). A total of 41 reflections most strongly affected by the twinning were omitted in the final refinement steps. For comparison purposes, the atomic coordinates of isotypic α-(NH₄)V^III(HPO₄)₂ (Bircsak & Harrison, 1998) and (H₃O)Fe^III(HPO₄)₂ (Vencato et al., 1989) were used as starting parameters in the final refinements of (I) and (II), respectively. However, the Sc atom in (II) had to be moved to a symmetry-equivalent position in order to obtain a properly connected set of atoms. H atoms in both (I) and (II) were freely refined. The highest electron-density peak in (I), 1.62 e Å⁻³, is at a distance of 0.63 Å from the Cs2 site. The deepest hole, −1.55 e Å⁻³, is at a distance of 0.59 Å from the Cs2 site. The highest electron-density peak in (II), 2.76 e Å⁻³, is at a distance of 1.75 Å from the O4 site. The deepest hole, −1.11 e Å⁻³, is at a distance of 0.69 Å from the Cs site.

Computing details

For both compounds, data collection: COLLECT (Nonius, 2003); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Pennington, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the framework structure of (I), along (a) [100] and (b) [101]. Protonated AsO4 tetrahedra are corner-linked to slightly distorted ScO6 octahedra. A system of intersecting tunnels hosts the charge-balancing Cs^+ cations (dark spheres). The unit cell is outlined, and the hydrogen bonding is shown in (b) (dashed lines).
	Fig. 2. The connectivity in (I), shown with displacement ellipsoids at the 50% probability level. The H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (iv) −x, 1 − y, 1 − z; (vi) −x, 1 − y, −z; (vii) 1 − x, 1 − y, −z; (viii) −x, 2 − y, −z.]
	Fig. 3. The framework structure of (II), projected down [100]. Protonated AsO4 tetrahedra are corner-linked to slightly distorted ScO6 octahedra. Tunnels parallel to [100] host the charge-balancing Cs^+ cations (dark spheres). The unit cell is outlined, and the hydrogen bonding is shown by dashed lines.
	Fig. 4. The connectivity in (II), shown with displacement ellipsoids at the 50% probability level. The H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) 1 − x, y − 1/2, 1/2 − z; (vi) 2 − x, y − 1/2, 1/2 − z; (vii) 1 + x, y, z; (viii) 1 − x, −y, −z.]

(I) caesium scandium hydrogen arsenate(V)

Crystal data

CsSc(HAsO4)2	Z = 3
Mr = 457.73	F(000) = 624
Triclinic, P1	Dx = 3.852 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.520 (2) Å	Cell parameters from 4259 reflections
b = 9.390 (2) Å	θ = 2.0–32.6°
c = 10.050 (2) Å	µ = 13.81 mm^−1
α = 65.48 (3)°	T = 293 K
β = 70.66 (3)°	Tabular, colourless
γ = 70.10 (3)°	0.08 × 0.06 × 0.03 mm
V = 592.0 (3) Å^3

Data collection

Nonius KappaCCD area-detector diffractometer	4302 independent reflections
Radiation source: fine-focus sealed tube	3839 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.022
ψ and ω scans	θmax = 32.6°, θmin = 2.3°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −11→11
Tmin = 0.405, Tmax = 0.682	k = −14→14
8447 measured reflections	l = −15→15

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.023	All H-atom parameters refined
wR(F^2) = 0.057	w = 1/[σ^2(Fo^2) + (0.025P)^2 + 0.84P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max = 0.001
4302 reflections	Δρmax = 1.62 e Å^−3
179 parameters	Δρmin = −1.55 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0055 (3)

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	0.5000	1.0000	0.0000	0.02920 (7)
Cs2	0.37099 (3)	0.83606 (2)	0.47378 (2)	0.02848 (6)
Sc1	0.0000	0.5000	0.5000	0.00784 (11)
Sc2	0.20526 (6)	0.72071 (5)	−0.05559 (5)	0.00787 (8)
As1	0.45377 (3)	0.55611 (3)	0.22012 (3)	0.00789 (6)
As2	−0.08279 (3)	0.87763 (3)	0.23294 (3)	0.00835 (6)
As3	0.09332 (3)	0.34998 (3)	0.21516 (3)	0.00814 (6)
O1	0.4036 (3)	0.6929 (2)	0.0595 (2)	0.0132 (3)
O2	0.6005 (3)	0.6313 (3)	0.2625 (3)	0.0198 (4)
O3	0.5672 (3)	0.3704 (2)	0.21477 (19)	0.0115 (3)
O4	0.2599 (3)	0.5437 (2)	0.3603 (2)	0.0190 (4)
O5	−0.0210 (3)	0.8188 (2)	0.0858 (2)	0.0155 (4)
O6	−0.1513 (3)	0.7356 (2)	0.3933 (2)	0.0144 (3)
O7	−0.2574 (3)	1.0495 (2)	0.2078 (2)	0.0116 (3)
O8	0.1116 (3)	0.9240 (3)	0.2507 (3)	0.0186 (4)
O9	−0.0599 (3)	0.4234 (2)	0.3487 (2)	0.0146 (3)
O10	−0.0109 (3)	0.2370 (2)	0.1872 (2)	0.0130 (3)
O11	0.1761 (3)	0.4888 (2)	0.0605 (2)	0.0160 (4)
O12	0.2905 (3)	0.2076 (2)	0.2818 (2)	0.0166 (4)
H2	0.707 (6)	0.571 (5)	0.280 (4)	0.018 (9)*
H8	0.076 (7)	1.001 (6)	0.233 (6)	0.044 (15)*
H12	0.373 (6)	0.263 (5)	0.263 (4)	0.020 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs1	0.04254 (17)	0.02343 (14)	0.02584 (14)	−0.02199 (12)	−0.00483 (12)	−0.00324 (10)
Cs2	0.03520 (11)	0.02676 (11)	0.01759 (9)	−0.00223 (8)	−0.01020 (8)	−0.00268 (7)
Sc1	0.0093 (3)	0.0069 (3)	0.0068 (2)	−0.0013 (2)	−0.0022 (2)	−0.0019 (2)
Sc2	0.00934 (18)	0.00648 (19)	0.00750 (18)	−0.00145 (14)	−0.00225 (14)	−0.00215 (14)
As1	0.00787 (10)	0.00719 (11)	0.00825 (10)	−0.00142 (8)	−0.00176 (8)	−0.00260 (8)
As2	0.00940 (11)	0.00575 (11)	0.00893 (11)	−0.00060 (8)	−0.00247 (8)	−0.00219 (8)
As3	0.00954 (11)	0.00639 (11)	0.00930 (11)	−0.00227 (8)	−0.00274 (8)	−0.00263 (8)
O1	0.0165 (8)	0.0111 (8)	0.0117 (8)	−0.0019 (6)	−0.0086 (7)	−0.0003 (6)
O2	0.0192 (10)	0.0169 (10)	0.0322 (11)	−0.0009 (8)	−0.0156 (9)	−0.0119 (8)
O3	0.0128 (8)	0.0079 (8)	0.0124 (8)	−0.0005 (6)	−0.0018 (6)	−0.0044 (6)
O4	0.0149 (9)	0.0182 (10)	0.0163 (9)	−0.0052 (7)	0.0069 (7)	−0.0056 (7)
O5	0.0177 (9)	0.0172 (9)	0.0124 (8)	−0.0026 (7)	−0.0004 (7)	−0.0093 (7)
O6	0.0165 (9)	0.0097 (8)	0.0107 (8)	−0.0011 (6)	−0.0029 (7)	0.0008 (6)
O7	0.0129 (8)	0.0055 (7)	0.0127 (8)	0.0003 (6)	−0.0026 (6)	−0.0017 (6)
O8	0.0180 (9)	0.0104 (9)	0.0315 (11)	−0.0019 (7)	−0.0131 (8)	−0.0063 (8)
O9	0.0136 (8)	0.0189 (9)	0.0143 (8)	−0.0011 (7)	−0.0023 (7)	−0.0112 (7)
O10	0.0187 (9)	0.0089 (8)	0.0155 (8)	−0.0031 (6)	−0.0092 (7)	−0.0039 (6)
O11	0.0201 (9)	0.0082 (8)	0.0149 (8)	−0.0056 (7)	−0.0019 (7)	0.0007 (7)
O12	0.0134 (8)	0.0116 (9)	0.0224 (9)	−0.0017 (7)	−0.0086 (7)	−0.0008 (7)

Geometric parameters (Å, º)

Cs1—O1	2.984 (2)	Sc1—O6^vii	2.113 (2)
Cs1—O1^i	2.984 (2)	Sc1—O6	2.113 (2)
Cs1—O8^i	3.231 (3)	Sc1—O9^vii	2.1478 (19)
Cs1—O8	3.231 (3)	Sc1—O9	2.1478 (19)
Cs1—O2	3.388 (3)	Sc2—O5	2.045 (2)
Cs1—O2^i	3.388 (3)	Sc2—O11	2.049 (2)
Cs1—O7^ii	3.436 (2)	Sc2—O1	2.0631 (19)
Cs1—O7^iii	3.436 (2)	Sc2—O10^ix	2.119 (2)
Cs1—O5^ii	3.645 (2)	Sc2—O3^v	2.1319 (19)
Cs1—O5^iii	3.645 (2)	Sc2—O7^iii	2.142 (2)
Cs1—O12^iv	3.720 (2)	As1—O4	1.6566 (19)
Cs1—O12^v	3.720 (2)	As1—O1	1.6587 (19)
Cs2—O3^vi	3.014 (2)	As1—O3	1.6753 (18)
Cs2—O8	3.127 (2)	As1—O2	1.729 (2)
Cs2—O2	3.144 (2)	As2—O5	1.6599 (18)
Cs2—O12^iv	3.164 (2)	As2—O6	1.666 (2)
Cs2—O6^ii	3.294 (2)	As2—O7	1.6825 (18)
Cs2—O9^vii	3.478 (2)	As2—O8	1.740 (2)
Cs2—O7^viii	3.542 (2)	As3—O11	1.653 (2)
Cs2—O10^vii	3.553 (2)	As3—O10	1.6672 (18)
Cs2—O7^ii	3.659 (2)	As3—O9	1.6755 (19)
Cs2—O4	3.785 (2)	As3—O12	1.732 (2)
Cs2—O4^vi	3.820 (3)	O2—H2	0.83 (4)
Cs2—O12^vi	3.909 (2)	O8—H8	0.64 (5)
Sc1—O4	2.051 (2)	O12—H12	0.87 (4)
Sc1—O4^vii	2.051 (2)
O4—Sc1—O4^vii	180.00 (12)	O10^ix—Sc2—O3^v	87.94 (8)
O4—Sc1—O6^vii	88.23 (9)	O5—Sc2—O7^iii	93.22 (8)
O4^vii—Sc1—O6^vii	91.77 (9)	O11—Sc2—O7^iii	170.70 (8)
O4—Sc1—O6	91.77 (9)	O1—Sc2—O7^iii	91.17 (8)
O4^vii—Sc1—O6	88.23 (9)	O10^ix—Sc2—O7^iii	84.71 (8)
O6^vii—Sc1—O6	180.00 (11)	O3^v—Sc2—O7^iii	84.34 (8)
O4—Sc1—O9^vii	87.43 (8)	O4—As1—O1	112.79 (11)
O4^vii—Sc1—O9^vii	92.57 (8)	O4—As1—O3	108.45 (10)
O6^vii—Sc1—O9^vii	88.14 (8)	O1—As1—O3	115.30 (9)
O6—Sc1—O9^vii	91.86 (8)	O4—As1—O2	105.87 (11)
O4—Sc1—O9	92.57 (8)	O1—As1—O2	104.03 (10)
O4^vii—Sc1—O9	87.43 (8)	O3—As1—O2	109.91 (10)
O6^vii—Sc1—O9	91.86 (8)	O5—As2—O6	112.33 (10)
O6—Sc1—O9	88.14 (8)	O5—As2—O7	109.96 (10)
O9^vii—Sc1—O9	180.00 (8)	O6—As2—O7	111.08 (9)
O5—Sc2—O11	95.23 (9)	O5—As2—O8	110.82 (11)
O5—Sc2—O1	92.41 (8)	O6—As2—O8	106.93 (11)
O11—Sc2—O1	92.37 (9)	O7—As2—O8	105.45 (10)
O5—Sc2—O10^ix	89.06 (8)	O11—As3—O10	114.17 (10)
O11—Sc2—O10^ix	91.52 (8)	O11—As3—O9	113.84 (10)
O1—Sc2—O10^ix	175.70 (8)	O10—As3—O9	108.51 (10)
O5—Sc2—O3^v	176.30 (8)	O11—As3—O12	108.03 (10)
O11—Sc2—O3^v	87.03 (8)	O10—As3—O12	101.20 (10)
O1—Sc2—O3^v	90.42 (8)	O9—As3—O12	110.34 (10)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O9^ii	0.83 (4)	1.96 (4)	2.784 (3)	173 (3)
O8—H8···O10^iv	0.64 (5)	1.98 (5)	2.623 (3)	174 (6)
O12—H12···O3	0.87 (4)	1.87 (4)	2.739 (3)	175 (4)

Symmetry codes: (ii) x+1, y, z; (iv) x, y+1, z.

(II) caesium scandium hydrogen arsenate(V)

Crystal data

CsSc(HAsO4)2	F(000) = 832
Mr = 457.73	Dx = 3.905 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 2410 reflections
a = 5.546 (1) Å	θ = 2.0–30.0°
b = 9.375 (2) Å	µ = 14.00 mm^−1
c = 15.034 (3) Å	T = 293 K
β = 95.16 (3)°	Prismatic fragment, colourless
V = 778.5 (3) Å^3	0.10 × 0.02 × 0.02 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	2240 independent reflections
Radiation source: fine-focus sealed tube	1995 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.017
ψ and ω scans	θmax = 30.0°, θmin = 2.6°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −7→7
Tmin = 0.310, Tmax = 0.756	k = −13→13
4355 measured reflections	l = −21→21

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.035	All H-atom parameters refined
wR(F^2) = 0.085	w = 1/[σ^2(Fo^2) + (0.01P)^2 + 16P] where P = (Fo^2 + 2Fc^2)/3
S = 1.18	(Δ/σ)max < 0.001
2240 reflections	Δρmax = 2.76 e Å^−3
118 parameters	Δρmin = −1.11 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00322 (19)

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
Cs	0.23317 (8)	0.00281 (5)	0.39233 (3)	0.02346 (14)
Sc	0.7540 (2)	−0.03569 (12)	0.15359 (7)	0.0085 (2)
As1	0.21617 (11)	0.15097 (7)	0.06033 (4)	0.00899 (14)
As2	0.72220 (11)	0.24835 (7)	0.31057 (4)	0.00855 (14)
O1	−0.0727 (8)	0.1150 (5)	0.0788 (3)	0.0160 (9)
O2	0.4134 (9)	0.0341 (5)	0.1067 (3)	0.0187 (10)
O3	0.2416 (8)	0.1793 (5)	−0.0469 (3)	0.0139 (9)
O4	0.2809 (10)	0.3176 (5)	0.1051 (3)	0.0196 (10)
O5	0.9340 (8)	0.3650 (5)	0.2878 (3)	0.0146 (9)
O6	0.4420 (8)	0.3053 (5)	0.2792 (3)	0.0131 (9)
O7	0.7706 (8)	0.0894 (5)	0.2678 (3)	0.0153 (9)
O8	0.7390 (9)	0.2074 (5)	0.4240 (3)	0.0151 (9)
H1	0.31 (2)	0.318 (15)	0.158 (9)	0.07 (4)*
H2	0.721 (16)	0.279 (10)	0.452 (6)	0.02 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.0213 (2)	0.0219 (2)	0.0261 (2)	0.00285 (17)	−0.00409 (16)	−0.00417 (18)
Sc	0.0083 (5)	0.0088 (5)	0.0084 (5)	0.0002 (4)	0.0010 (4)	0.0008 (4)
As1	0.0093 (3)	0.0093 (3)	0.0083 (3)	−0.0007 (2)	0.0007 (2)	0.0010 (2)
As2	0.0079 (3)	0.0087 (3)	0.0090 (3)	0.0000 (2)	0.0003 (2)	−0.0008 (2)
O1	0.011 (2)	0.018 (2)	0.020 (2)	−0.0018 (17)	0.0061 (17)	0.0074 (19)
O2	0.012 (2)	0.021 (2)	0.023 (2)	0.0065 (18)	−0.0014 (18)	0.006 (2)
O3	0.018 (2)	0.014 (2)	0.010 (2)	0.0008 (17)	0.0041 (17)	0.0003 (17)
O4	0.030 (3)	0.014 (2)	0.015 (2)	−0.005 (2)	0.000 (2)	−0.0049 (19)
O5	0.013 (2)	0.016 (2)	0.015 (2)	−0.0062 (17)	−0.0008 (17)	0.0013 (18)
O6	0.0089 (19)	0.015 (2)	0.015 (2)	0.0050 (16)	−0.0018 (16)	−0.0056 (17)
O7	0.014 (2)	0.013 (2)	0.018 (2)	0.0026 (17)	−0.0004 (17)	−0.0056 (18)
O8	0.021 (2)	0.017 (2)	0.0079 (19)	−0.0043 (19)	0.0018 (17)	−0.0006 (18)

Geometric parameters (Å, º)

Cs—O5^i	3.068 (5)	Sc—O7	2.074 (5)
Cs—O3^ii	3.116 (5)	Sc—O5^vi	2.089 (5)
Cs—O7^iii	3.144 (5)	Sc—O1^vii	2.092 (5)
Cs—O4^i	3.203 (5)	Sc—O3^viii	2.095 (5)
Cs—O4^iv	3.341 (6)	Sc—O6^i	2.150 (4)
Cs—O8^v	3.385 (5)	As1—O3	1.653 (4)
Cs—O8	3.396 (5)	As1—O2	1.657 (5)
Cs—O8^iii	3.413 (5)	As1—O1	1.685 (4)
Cs—O6	3.554 (5)	As1—O4	1.726 (5)
Cs—O4^ii	3.603 (5)	As2—O7	1.654 (5)
Cs—O7	3.749 (5)	As2—O5	1.663 (5)
Cs—O6^i	3.763 (5)	As2—O6	1.671 (4)
Cs—As2^iii	3.7701 (10)	As2—O8	1.742 (5)
Cs—O1^iv	3.777 (5)	O4—H1	0.79 (14)
Sc—O2	2.062 (5)	O8—H2	0.80 (10)
O2—Sc—O7	94.3 (2)	O3^viii—Sc—O6^i	87.43 (18)
O2—Sc—O5^vi	169.7 (2)	O3—As1—O2	114.0 (2)
O7—Sc—O5^vi	85.98 (18)	O3—As1—O1	111.0 (2)
O2—Sc—O1^vii	93.05 (19)	O2—As1—O1	113.6 (2)
O7—Sc—O1^vii	94.3 (2)	O3—As1—O4	101.7 (2)
O5^vi—Sc—O1^vii	97.15 (19)	O2—As1—O4	109.2 (3)
O2—Sc—O3^viii	90.9 (2)	O1—As1—O4	106.5 (3)
O7—Sc—O3^viii	173.62 (19)	O7—As2—O5	111.7 (2)
O5^vi—Sc—O3^viii	88.27 (19)	O7—As2—O6	110.9 (2)
O1^vii—Sc—O3^viii	89.16 (19)	O5—As2—O6	112.9 (2)
O2—Sc—O6^i	83.92 (19)	O7—As2—O8	100.7 (2)
O7—Sc—O6^i	89.41 (19)	O5—As2—O8	111.7 (2)
O5^vi—Sc—O6^i	85.83 (18)	O6—As2—O8	108.2 (2)
O1^vii—Sc—O6^i	175.40 (19)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H1···O6	0.79 (14)	1.90 (14)	2.691 (7)	170 (14)
O8—H2···O1^ix	0.80 (10)	2.36 (9)	2.974 (7)	135 (8)
O8—H2···O2^x	0.80 (10)	2.64 (9)	3.199 (7)	129 (8)

Symmetry codes: (ix) x+1, −y+1/2, z+1/2; (x) −x+1, y+1/2, −z+1/2.