Zn(AsO₃)₂, a unique structure among arsenates with composition M^IIAs^V₂O₆

Zinc bis­[catena-arsenate(V)] or catena-poly[zinc-di-μ-arsenato(V)-κ⁴O:O], [Zn(AsO₃)₂]_n, is isotypic with the low-temperature modification of Zn(PO₃)₂. The asymmetric unit comprises one Zn atom situated on a twofold rotation axis and one As and three O atoms in general positions. In the crystal structure, the principal building units, ZnO₆ octa­hedra and AsO₄ tetra­hedra, are condensed into ¹_∞[ZnO_4/2O_2/1] zigzag chains and (AsO₃)₂ polyarsenate chains with a periodicity of two AsO₄ tetra­hedra, respectively. The two types of chains extend parallel to [001] and are fused into a compact framework structure. Compared with divalent transition metal arsenates and other arsenates of the formula M^IIAs₂O₆, Zn(AsO₃)₂ is unique in that arsenic is present exclusively in a tetra­hedral coordination and not in a mixed tetra­hedral/octa­hedral or purely octa­hedral coordination.

1. Chemical context

The zinc arsenate Zn(AsO₃)₂ has been reported as one of the phases existing in the pseudo-binary system ZnO–As₂O₅ (Kasenov et al., 1996). Whereas for the other phases reported in this system complete crystal structure determinations have been carried out in the me­antime (details are given in the database survey), for Zn(AsO₃)₂ only unit-cell parameters were given on basis of a powder X-ray diffraction study: crystal system ortho­rhom­bic with a = 8.625, b = 9.035, c = 12.494 Å, V = 933 ų (Mustafin & Kasenov, 1994, 1996). Recent results of crystal structure prediction methods claimed the possible existence of Zn(AsO₃)₂ (or as stated there of ZnAs₂O₆). Whereas one study states that this phase has an existence probability of 0.9105 with a polar structure (Gake et al., 2025; data given in the supporting information) the other states that it crystallizes in the centrosymmetric PbSb₂O₆ structure type with trigonal symmetry (Griesemer et al., 2025; supplementary material of this article: OQMD ID 1348395).

We sought to investigate these conflicting claims and carried out experiments on the crystal growth of Zn(AsO₃)₂. Using a reaction similar to that described by Kasenov et al. (1996), we were able to isolate single crystals of Zn(AsO₃)₂ (I) and report here on the results of the crystal structure analysis.

2. Structural commentary

Our study does not confirm any of the previous statements made regarding the ortho­rhom­bic unit cell or possible crystal structures. Compound (I) crystallizes in the monoclinic system in the centrosymmetric space group C2/c and is isotypic with the corresponding phosphate, i.e. low-temperature Zn(PO₃)₂ (Averbuch-Pouchot et al., 1983). The unit cell of Zn(AsO₃)₂ reported by Mustafin & Kasenov (1994, 1996) has a doubled volume compared to the unit cell determined here, however, with no apparent metrical relation beyond this.

In the crystal structure of Zn(AsO₃)₂, the Zn atom is located on a twofold rotation axis (multiplicity 4, Wyckoff letter e), and the As and the three O atom sites on general positions (8f) of space group C2/c. The zinc atom in (I) exhibits a distorted octa­hedral coordination environment with an average Zn—O bond length of 2.090 Å (Table 1), in good agreement with the literature value of 2.110 (86) Å derived from 193 [ZnO₆] coordination polyhedra (Gagné & Hawthorne, 2020). The [ZnO₆] octa­hedron shares two of its edges with neighbouring octa­hedra, forming ¹_∞[ZnO_4/2O_2/1] zigzag-chains extending parallel to [001] (Fig. 1a). The arsenate tetra­hedron condenses into a polyarsenate chain with a periodicity of two tetra­hedra (Fig. 1b). The bond length distribution (Table 1) in this chain follows the usual pattern, with significantly longer As—O distances to the bridging oxygen atoms (O1, average 1.735 Å) than to the terminal O atoms (O2, O3; average 1.646 Å). The mean average As—O bond length within the AsO₄ tetra­hedron (1.690 Å) complies with the literature value [1.687 (27) Å from 508 arsenate tetra­hedra; Gagné & Hawthorne, 2018]. The terminal oxygen atoms of the polyarsenate chain are also the ones that bind to the zinc atoms in adjacent zinc oxide chains, so that the polyarsenate chains are aligned in the same direction as the zinc oxide chains, which leads to a very compact packing in this type of structure (Durif, 1995). In the crystal structure of (I) (Fig. 2), each zinc oxide chain is surrounded by six polyarsenate chains, and each polyarsenate chain is surrounded by three zinc oxide chains, corresponding to the 1:2 ratio of Zn:As in the formula.

Table 1
Selected bond lengths (Å)

Zn1—O3	2.0276 (10)	As1—O2	1.6518 (10)
Zn1—O2^i	2.0786 (10)	As1—O1^iii	1.7319 (10)
Zn1—O2^ii	2.1640 (10)	As1—O1^iv	1.7379 (10)
As1—O3	1.6336 (10)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 1 (a) One ^1∞[ZnO4/2O2/1] zigzag chain, and (b) one polyarsenate chain crossing the unit cell of (I), as shown in projections down [00]. Displacement ellipsoids are drawn at the 97% probability level. [Symmetry codes: (i) x + , y + , z; (ii) −x + 1, −y + 1, −z + 1; (iii) −x + 1, y, −z + ; (iv) x, −y + 1, z − ; (v) −x + , y + , −z + ; (vi) −x + , −y + , −z.]

Figure 2 The crystal structure of (I) in polyhedral representation (ZnO6 octa­hedra blue, AsO4 tetra­hedra red), viewed down [00]. Displacement ellipsoids are drawn at the 97% probability level.

The structure model was confirmed using bond-valence-sum calculations (Brown, 2002) performed with the ECoN21 program (Ilinca, 2022). The calculated bond-valence sums (Table 2) are close to the expected values of 2, 5 and 2 valence units for Zn, As and O atoms, respectively, and the global instability index (GII) of 0.11 valence units indicates a stable and not particularly strained crystal structure (Salinas-Sanchez et al., 1992; Brown, 2009).

Table 2
Bond-valence-sum calculations (in valence units) for Zn(AsO₃)₂

	Zn1	As1	Σ
O1		1.10, 1.08	2.18
O2	0.36 (1×→, 2×↓), 0.29 (1×→, 2×↓)	1.38	2.03
O3	0.41 (1×→, 2×↓)	1.45	1.86
Σ	2.12	5.01

Using the compstru program (de la Flor et al., 2016) that is available at the Bilbao Crystallographic Server (Aroyo et al., 2006), a qu­anti­tative comparison of the isotypic structures of low-temperature Zn(PO₃)₂ and Zn(AsO₃)₂ was made. The arithmetic mean of the distance between paired atoms is 0.1148 Å, and the absolute distances between paired atoms are 0.0925 Å for Zn1, 0.0598 Å for As1/P1, 0.0901 Å for O1, 0.1146 Å for O2, and the largest distance of 0.2057 Å for O3. The latter O atoms are those which, in both structures, have both the shortest Zn—O and the shortest X—O (X = As, P) bond lengths. The degree of lattice distortion (0.0264), and the low value for the measure of similarity (0.023) indicate a high degree of similarity for the two crystal structures.

3. Database survey

The Inorganic Crystal Structure Database (ICSD, data release 2025.1; Zagorac et al., 2019) was used for the database search. Known structurally characterized phases in the ZnO–As₂O₅ system include the high- and low-temperature modifications of tetra­zinc bis­(arsenate(V)) oxide, Zn₄(AsO₄)₂O (Frerichs et al., 1996a), dimorphic trizinc orthoarsenate, Zn₃(AsO₄)₂ (Frerichs et al., 1996b), and the pyroarsenate Zn₂As₂O₇ with three modifications existing at different temperatures, one of which is commensurately and one is incommensurately modulated (Weil & Stöger, 2010).

As already mentioned, Zn(AsO₃)₂ is isotypic with Zn(PO₃)₂, in its low-temperature modification (Averbuch-Pouchot et al., 1983). The high-temperature modification of Zn(PO₃)₂ can be obtained by quenching from the recrystallized melt and contains a similar polyphosphate chain but in contrast to the low-temperature modification with two unique zinc cations in tetra­hedral coordination (Weil, 2004). According to the information on the system ZnO–As₂O₅ provided by Kasenov et al. (1996), there are no indications of a possible high-temperature form of Zn(AsO₃)₂. Other M(PO₃)₂ polyphosphates isotypic with low-temperature Zn(PO₃)₂ and Zn(AsO₃)₂ can only be prepared by using high pressure (∼80 kbar) and temperatures (∼1270 K) and are reported for M = Ni, Mg, Cu, Co, Fe, Mn, and Cd (Bagieu-Beucher et al., 1976).

Zn(AsO₃)₂ is the only arsenate phase with an M:As^V ratio of 1:2 where solely tetra­hedral AsO₄ units are present in the polymeric arsenate anion. Except for M = Ba with a catena-arsenate anion consisting of AsO₄ tetra­hedra and AsO₆ octa­hedra (Weil, 2016), all other MAs₂O₆ phases crystallize in the PbSb₂O₆ type of structure (Hill, 1987) in the trigonal space group type P1m. This structure type is also referred to as the rosiaite structure type after the mineral of this name (Basso et al., 1996) and is based on a hexa­gonal array of O atoms with the As atom octa­hedrally coordinated in form of honeycomb layers. This structure type is realized for M = Ni, Co, Mn (Nakua & Greedan, 1995), Cd (Weil, 2001), Hg (Weil, 2000), Pd (Orosel & Jansen, 2006), Ca, Pb (Losilla et al., 1995), and Sr. The strontium phase is dimorphic and as well as in a rosiaite-type structure also crystallizes in a hexa­gonal superstructure with a doubled unit cell (Weil et al., 2009).

4. Synthesis and crystallization

Crystal growth procedures were carried out in sealed and evacuated silica ampoules using commercially available chemicals as received. A molar 1:1 mixture of ZnO (≥ 99%, Carl Roth GmbH) and As₂O₅ [99.9% (metals basis), abcr] was heated at 750 K for about 336 h. X-ray powder diffraction of the bulk product revealed Zn(AsO₃OH)·H₂O (mineral name koritnigite; Keller et al., 1980) and Zn(AsO₃)₂ as the main phases in an approximate ratio of 1:2. From this mixture, yellowish crystals of Zn(AsO₃)₂ were isolated for the single-crystal X-ray diffraction study. The presence of the hydrous phase Zn(AsO₃OH)·H₂O indicates that the hygroscopic starting material As₂O₅ was indeed not free of water.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Atomic coordinates and labelling were adapted from the isotypic low-temperature phase of Zn(PO₃)₂ (Averbuch-Pouchot et al., 1983).

Table 3
Experimental details

Crystal data
Chemical formula	[Zn(AsO3)2]
Mr	311.21
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	300
a, b, c (Å)	10.1061 (10), 9.0308 (8), 5.2801 (5)
β (°)	108.374 (7)
V (Å^3)	457.33 (8)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	19.68
Crystal size (mm)	0.04 × 0.02 × 0.01
Data collection
Diffractometer	Stoe Stadivari
Absorption correction	Multi-scan (LANA; Koziskova et al., 2016)
Tmin, Tmax	0.492, 0.654
No. of measured, independent and observed [I > 2σ(I)] reflections	3846, 797, 770
Rint	0.017
(sin θ/λ)max (Å^−1)	0.762
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.010, 0.028, 1.13
No. of reflections	797
No. of parameters	43
Δρmax, Δρmin (e Å^−3)	0.40, −0.37
Computer programs: X-AREA (Folkers-Karlsson et al., 2026), SHELXL (Sheldrick, 2015), OLEX2 (Dolomanov et al., 2009), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

Supporting information

Computing details

catena-Poly[zinc-di-µ-arsenato(V)-κ⁴O:O]

Crystal data

[Zn(AsO3)2]	F(000) = 576
Mr = 311.21	Dx = 4.520 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 10.1061 (10) Å	Cell parameters from 5088 reflections
b = 9.0308 (8) Å	θ = 3.1–32.8°
c = 5.2801 (5) Å	µ = 19.68 mm^−1
β = 108.374 (7)°	T = 300 K
V = 457.33 (8) Å^3	Shard, yellow
Z = 4	0.04 × 0.02 × 0.01 mm

Data collection

Stoe Stadivari diffractometer	797 independent reflections
Radiation source: Axo_Mo	770 reflections with I > 2σ(I)
Graded multilayer mirror monochromator	Rint = 0.017
Detector resolution: 13.33 pixels mm^-1	θmax = 32.8°, θmin = 3.1°
rotation method, ω scans	h = −14→15
Absorption correction: multi-scan (LANA; Koziskova et al., 2016)	k = −12→13
Tmin = 0.492, Tmax = 0.654	l = −7→4
3846 measured reflections

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0127P)^2 + 0.509P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.010	(Δ/σ)max = 0.001
wR(F^2) = 0.028	Δρmax = 0.40 e Å^−3
S = 1.13	Δρmin = −0.36 e Å^−3
797 reflections	Extinction correction: SHELXL (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
43 parameters	Extinction coefficient: 0.0035 (2)
0 restraints

Special details

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

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

	x	y	z	Uiso*/Ueq
Zn1	0.000000	0.10801 (2)	0.250000	0.00841 (6)
As1	0.21468 (2)	0.40539 (2)	0.24523 (3)	0.00640 (5)
O1	0.36157 (10)	−0.00460 (12)	0.04492 (19)	0.0121 (2)
O2	0.38566 (10)	0.42724 (11)	0.35206 (19)	0.00870 (17)
O3	0.15428 (11)	0.23634 (11)	0.1990 (2)	0.0132 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.00662 (11)	0.00985 (11)	0.00828 (10)	0.000	0.00168 (8)	0.000
As1	0.00579 (7)	0.00774 (8)	0.00560 (7)	−0.00148 (4)	0.00169 (5)	−0.00034 (4)
O1	0.0078 (4)	0.0209 (5)	0.0074 (4)	0.0001 (4)	0.0021 (3)	0.0056 (4)
O2	0.0057 (4)	0.0118 (4)	0.0077 (4)	−0.0008 (3)	0.0008 (3)	0.0008 (3)
O3	0.0139 (5)	0.0094 (4)	0.0172 (5)	−0.0052 (4)	0.0061 (4)	−0.0020 (4)

Geometric parameters (Å, º)

Zn1—O3	2.0276 (10)	Zn1—O2^v	2.1640 (10)
Zn1—O3^i	2.0276 (10)	As1—O3	1.6336 (10)
Zn1—O2^ii	2.0786 (10)	As1—O2	1.6518 (10)
Zn1—O2^iii	2.0786 (10)	As1—O1^vi	1.7319 (10)
Zn1—O2^iv	2.1640 (10)	As1—O1^vii	1.7379 (10)
O3—Zn1—O3^i	110.28 (6)	O2^iii—Zn1—O2^v	88.05 (3)
O3—Zn1—O2^ii	99.15 (4)	O2^iv—Zn1—O2^v	82.06 (5)
O3^i—Zn1—O2^ii	90.92 (4)	O3—As1—O2	117.65 (5)
O3—Zn1—O2^iii	90.92 (4)	O3—As1—O1^vi	108.08 (5)
O3^i—Zn1—O2^iii	99.15 (4)	O2—As1—O1^vi	111.48 (5)
O2^ii—Zn1—O2^iii	162.38 (6)	O3—As1—O1^vii	108.20 (5)
O3—Zn1—O2^iv	163.24 (4)	O2—As1—O1^vii	110.65 (5)
O3^i—Zn1—O2^iv	84.54 (4)	O1^vi—As1—O1^vii	99.15 (3)
O2^ii—Zn1—O2^iv	88.05 (3)	As1^vi—O1—As1^v	130.01 (6)
O2^iii—Zn1—O2^iv	78.64 (4)	As1—O2—Zn1^iii	121.02 (5)
O3—Zn1—O2^v	84.54 (4)	As1—O2—Zn1^viii	126.04 (5)
O3^i—Zn1—O2^v	163.24 (4)	Zn1^iii—O2—Zn1^viii	101.35 (4)
O2^ii—Zn1—O2^v	78.64 (4)	As1—O3—Zn1	141.36 (6)

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