Crystal structure of Na4(As2O5)(H2O)0.5 and a survey of the pyroarsenite anion, (As2O5)4−

Wolflehner, T.; Weil, M.

doi:10.1107/S2056989026002859

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Volume 82| Part 4| April 2026| Pages 379-383

https://doi.org/10.1107/S2056989026002859

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Crystal structure of Na₄(As₂O₅)(H₂O)_0.5 and a survey of the pyroarsenite anion, (As₂O₅)⁴⁻

Tobias Wolflehner ^a and Matthias Weil ^a ^*

^aTU Wien, Institute for Chemical Technologies and Analytics, Division of Applied Solid State Chemistry, Getreidemarkt 9/E164-05-1, 1060 Vienna, Austria
^*Correspondence e-mail: [email protected]

Edited by L. Suescun, Universidad de la República, Uruguay (Received 16 February 2026; accepted 18 March 2026; online 24 March 2026)

The title compound, tetrasodium pyroarsenite hemihydrate, [Na₄(As₂O₅)(H₂O)_0.5], represents the first pyroarsenite compound of an alkali metal. The asymmetric unit comprises four Na, two As, five O sites and one H site in a general position, and one water O atom located on a twofold rotation axis in space group C2/c. The (As₂O₅)^4– anion is made up from two trigonal-pyramidal {AsO₃} units sharing a corner. In the crystal structure, all O atoms of the anion are also part of a framework defined by one [NaO₄] and three [NaO₆] coordination polyhedra. Almost linear hydrogen-bonding interactions of medium strength [O⋯O 2.651 (4) Å; O—H⋯O 171 (6)°] between the water molecule and one of the terminal O atoms of the pyroarsenite anion consolidate the crystal structure. Data mining for structural data on isolated pyroarsenite anions resulted in 30 (As₂O₅)^4– groups present in various crystal structures. In these pyroarsenite anions, the mean As—O distance to terminal oxygen atoms is 1.764 (33) Å and to bridging oxygen atoms 1.856 (64) Å. The values of the As—O—As bridging angle are highly variable [range 107.78 (13) to 144.12 (5)°], with that of Na₄(As₂O₅)(H₂O)_0.5 having by far the smallest value of all structures chacterized so far.

Keywords: crystal structure; diarsenite; hydrogen bonding; hemihydrate; hydroflux synthesis; data mining.

CCDC reference: 2538842

1. Chemical context

Formation studies of alkali transition-metal oxidoantimonates(V) and related oxidoarsenates(V) led to the unexpected discovery of the oxidoarsenate(III) compound K₃FeAs₂O₆ under hydroflux conditions (Wolflehner, 2026 ). A comprehensive review of this synthesis method has been given recently by He et al. (2023 ). To obtain the hypothetical sodium variant of K₃FeAs₂O₆, the starting materials used under similar hydroflux conditions have been adjusted. However, the corresponding experiment did not yield the planned target phase, but rather the transition metal-free compound Na₄(As₂O₅)(H₂O)_0.5, the crystal structure of which is reported here, together with a survey on structural details regarding the pyroarsenite anion, (As₂O₅)^4–.

2. Structural commentary

The asymmetric unit of Na₄(As₂O₅)(H₂O)_0.5 comprises four Na, two As, six O and one H-atom positions. With the exception of the water O atom O1W (site symmetry 2; multiplicity 4, Wyckoff letter e), all atoms are located at sites corresponding to a general position (8 f) of space group C2/c.

The two As^III atoms each are coordinated by three oxygen atoms in a trigonal–pyramidal shape, typical for arsenite groups. Two such trigonal pyramids share an O atom (O3), resulting in the formation of the condensed (As₂O₅)^4– anion. The As—O bond lengths distribution in Na₄(As₂O₅)(H₂O)_0.5 (Table 1) is typical for a condensed system consisting of two oxido anions, for example, for pyro-anions consisting of two tetrahedral groups, e.g. phosphates (Durif, 1995 ) or silicates (Liebau, 1985 ), here with four shorter As—O bonds [average value 1.730 (5) Å] to terminal O atoms and two longer As—O bonds to the bridging O atom [average value 1.903 (12) Å]. The mean As—O bond length of 1.788 (82) Å in the anion of the title compound agrees very well with the literature value of 1.782 Å for the averaged As^III—O distance in the crystal structures of arsenite compounds (Majzlan et al., 2014 ). Structural data specific to pyroarsenite anions are discussed in more detail in section 3.

Table 1
Selected geometric parameters (Å, °)

As1—O1	1.732 (3)	As2—O5	1.723 (3)
As1—O2	1.737 (3)	As2—O4	1.728 (3)
As1—O3	1.915 (3)	As2—O3	1.891 (3)

O1—As1—O2	102.68 (15)	O5—As2—O3	97.79 (16)
O1—As1—O3	98.42 (16)	O4—As2—O3	98.38 (12)
O2—As1—O3	97.34 (12)	As2—O3—As1	107.78 (13)
O5—As2—O4	102.92 (16)

If the non-bonding 4s² electron lone pair ψ of the As^III atom is also taken into account, [ψAsO₃] polyhedra with the shapes of flattened tetrahedra are formed. The positions of ψ were calculated with the LPLoc program (Hamani et al., 2020 ) resulting in the following fractional coordinates: x = 0.10919, y = −0.13221, z = 0.64501 for ψ1 located at the As1 atom [distance As1—ψ1 = 1.240 Å; radius(ψ1) = 1.19 Å], and x = 0.04042, y = 1.15526, z = 0.46720 for ψ2 located at the As2 atom [distance As2—ψ2 = 1.276 Å; radius(ψ2) = 1.19 Å].

The pyroarsenite anion exhibits a half-eclipsed conformation as evidenced by the torsion angles O1—As1⋯As2—O4 of 82.71 (19)° (synclinal), O2—As1⋯As2—O5 of 81.15 (18)° (synclinal), O2—As1⋯As2—O4 of −30.5 (2)° (boundary between synperiplanar and synclinal) and O1—As1⋯As2—O5 of −165.61 (13)° (antiperiplanar). The free electron pairs (taking into account the coordinates given above) have a torsion angle ψ1—As2⋯As2—ψ2 of −80.34° and are therefore also synclinal to each other (Fig. 1).

Figure 1
The (As₂O₅)^4– anion in the title compound in a side view (top) and approximately along the As1⋯As2 axis (bottom). Displacement ellipsoids are drawn at the 90% probability level; the calculated electron lone pairs ψ are indicated as spheres of arbitrary radius.

The Na⁺ sites show coordination numbers of 4 (Na1) and 6 (Na2, Na3 and Na4), and their coordination polyhedra are shown in Fig. 2. The description of the closest matching ideal polyhedron for each site and quantification of the distortion (δ) from it is based on the Polynator program (Link & Niewa, 2023 ), and numerical data considering Na—O distances up to 3.0 Å as relevant are compiled in Table 2, including averaged Na—O bond lengths. The latter are in reasonable agreement with literature values (Gagné & Hawthorne, 2016 ) of 2.359 (76) Å for coordination number 4, and 2.441 (112) Å for coordination number 6.

Table 2
Coordination environments (Å) around the Na⁺ cations in Na₄(As₂O₅)(H₂O)_0.5

Atom	Coordination number	Polyhedron with idealized point group symmetry [in brackets] and deviation δ (in parentheses) from it	Range of Na—O bond lengths	Average Na—O bond length	Number of water molecules in the first coordination sphere	Bond valence/valence units
Na1	4	heterodisphenoid [mm2] (4.159)	2.306 (4) – 2.559 (3)	2.408	1; O1W	0.76
Na2	6	didigonal scalenohedron [ $[\overline{4}]$ 2m] (10.947)	2.259 (3) – 2.775 (3)	2.488	0; –	1.00
Na3	6	twisted trigonal prism [32] (3.999)	2.342 (3) – 2.613 (5)	2.464	0; –	0.98
Na4	6	monocapped trigonal antifrustum [3m] (28.070)	2.409 (9) – 2.623 (4)	2.496	1; O1W	0.86

Figure 2
Coordination environments for the four Na⁺ positions. Displacement ellipsoids are as in Fig. 1

. [Symmetry codes: (i) x, −y + 1, z − $[{1\over 2}]$ ; (ii) −x, y + 1, −z + $[{1\over 2}]$ ; (iii) x, −y, z − $[{1\over 2}]$ ; (iv) −x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z + 1; (v) −x + $[{1\over 2}]$ , −y − $[{1\over 2}]$ , −z + 1; (vi) −x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (vii) −x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (viii) x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z.]

In the extended structure, the diarsenite groups are isolated from each other and arranged via inversion centres to form opposite pairs that are stacked along [010] (Fig. 3), whereby the anions are embedded in a framework of corner- and edge-sharing [NaO₄] and [NaO₆] polyhedra. The electron lone pairs of the As^III atoms are stereochemically active and point into the free space of the framework. The crystal structure of Na₄(As₂O₅)(H₂O)_0.5 is consolidated by O—H⋯O hydrogen bonds between the water molecule and a terminal O atom (O4) of the diarsenite anion as the acceptor atom (Table 3). Based on the D⋯A distance, the hydrogen-bonding interaction is classified as of medium strength (Jeffrey, 1997 ).

Table 3
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1W—H1⋯O4	0.86 (1)	1.80 (1)	2.651 (4)	171 (6)

Figure 3
The crystal structure of Na₄(As₂O₅)(H₂O)_0.5 in a projection along [0 $[\overline{1}]$ 0]. The (As₂O₅)^4– anion is given in polyhedral representation; displacement ellipsoids are as in Fig. 1

. Na—O bonds < 3.0 Å are displayed, and O—H⋯O hydrogen-bonding interactions are shown as yellow lines.

To verify the validity of the structure model, bond-valence sums (BVS; Brown, 2002 ) were calculated using the ECoN21 program (Ilinca, 2022 ). The BVS values (in valence units, v. u.) of the Na sites are listed in Table 1; those of the As and O atoms are As1 2.92, As2 3.01, O1 1.92, O2 1.84, O3 1.97, O4 1.61, O5 1.92, and O1W 0.50. The calculated values correspond to expectations [1.00 v. u. for Na, 3.00 v. u. for As, 2.00 v. u. for O] and also reflect the role of individual oxygen atoms in hydrogen-bonding interactions. Since the contributions of the H atoms to the bonding were not taken into account in the BVS calculations, the O1W atom of the water molecule has a very low BVS value, and the O4 atom, which acts as the acceptor atom of the medium-strong hydrogen bond (Table 3), has a value significantly below 2.

3. Database survey

A search of the Inorganic Crystal Structure Database (ICSD; data release 2025-1; Zagorac et al., 2019 ) for compounds in the system Na₂O–As^III₂O₃–H₂O revealed five phases, viz. NaAsO₂ (Menary, 1958 ; Lee & Harrison, 2004 ), Na₂(H₂As₄O₈), NaAsO₂·4H₂O, Na₂(HAsO₃)·5H₂O and Na₅(HAsO₃)(AsO₃)·12H₂O (Sheldrick & Häusler, 1987 ). The first three phases consist of chains of polymetaarsenite anions, whereas the latter two phases contain discrete arsenite anions. Sodium compounds with pyroarsenite anions have not been reported to date, nor have those of other alkali metals. The title compound thus represents the first structurally characterized pyroarsenite of the alkali metals.

A further database search for inorganic compounds containing discrete pyroarsenite anions in their crystal structure yielded over 20 hits compiled in Table 4, including several minerals. Together with the title compound, this results in 30 individual (As₂O₅)^4– anions, whose averaged As—O bond lengths to terminal and bridging oxygen atoms and As—O—As bridging angles are listed. The structural features described in section 2 can also be observed for the vast majority of the other crystal structures comprising pyroarsenite anions, i.e., the presence of significantly longer As—O bonds to the bridging oxygen atoms compared to those involving terminal oxygen atoms. The mean As—O_terminal and As—O_bridging distances in all 30 pyroarsenite anions are 1.764 (33) Å and 1.856 (64) Å [overall mean As—O bond length 1.795 (63) Å]. However, the values of the As—O—As bridging angle in the listed pyroarsenite anions are highly variable [range 107.78 (13) to 144.12 (5)°], whereby Na₄(As₂O₅)(H₂O)_0.5 has by far the smallest value of all structures. Histograms illustrating these features graphically can be found in Fig. 4.

Table 4
As—O bond lengths (Å) and bridging angles (°) in the crystal structures of compounds with isolated pyroarsenite (As₂O₅)^4– groups

Compound (mineral name)	As—O_terminal	As—O_bridging	As—O—As	Reference
Na₄(As₂O₅)·0.5H₂O	1.732 (3), 1.737 (3), 1.723 (3), 1.728 (3)	1.915 (3), 1.891 (3)	107.78 (13)	This work
BaCo(As₂O₅)	2×1.716 (3), 2×1.736 (3)	1.837 (5), 1.809 (5)	130.9 (3)	David et al. (2014 )
BaFe₂(As₂O₅)(AsO₃)(OH)	2×1.745 (13), 2×1.757 (12)	2×1.816 (7)	134.7 (10)	Leclercq et al. (2020 )
Ba₂Fe₂O(As₂O₅)₂	4×1.7503 (12)	2×1.8391 (11)	130.24 (14)	Leclercq et al. (2020)
Ba₂(Ti⁴⁺V³⁺)(As₂O₅)₂OF (bianchiniite)	4×1.7397 (15)	2×1.8377 (13)	127.12 (16)	Biagioni et al. (2021 )
CaSb⁵⁺₂(As₂O₅)₂O₂·10H₂O (prachařite)	1.7633 (17), 1.7661 (16), 1.7611 (17), 1.7641 (17)	1.8079 (19), 1.8185 (18)	128.55 (9)	Kolitsch et al. (2023 )
Fe²⁺Fe³⁺₃(As₂O₅)₂(AsO₃) (schneiderhöhnite)	1.7680 (15), 1.7997 (16), 1.7936 (15), 1.7968 (15); 1.7689 (16), 1.7608 (15), 1.7485 (15), 1.7844 (15)	1.7926 (16), 1.7648 (15); 1.8610 (15), 1.8075 (16)	132.9 (2), 136.8 (2)	Cooper & Hawthorne (2016 )
Fe³⁺₃(AsO₂)₄(As₂O₅)(OH) (karibibite)	2×1.77 (2), 2×1.79 (2)	2×1.77 (2)	141 (3)	Colombo et al. (2017 )
Fe₃(As₂O₅)(AsO₃)Cl	1.779 (7), 1.772 (8), 1.825 (7), 1.766 (7)	1.834 (7), 1.811 (9)	125.2 (4)	Leclercq et al. (2020)
In₂(As₂O₅)Cl₂	1.756 (5), 1.804 (8), 1.742 (6), 1.786 (7)	1.912 (7), 1.827 (6)	124.9 (5)	Jiang et al. (2011 )
In₄(As₂O₅)(As₃O₇)Br₃	1.734 (9), 1.782 (8), 1.777 (10), 1.780 (8)	1.881 (9), 1.814 (8)	124.3 (5)	Jiang et al. (2011)
Mn₂(As₂O₅)	1.727 (4), 1.736 (4), 1.740 (3), 1.752 (4), 1.709 (4), 1.763 (3), 1.722 (4), 1.779 (3)	1.872 (4), 1.838 (4), 1.860 (4), 1.834 (4)	116.04 (16), 137.33 (19)	Priestner et al. (2019 )
Nd₄(A₂O₅)₂(As₄O₈)	1.716 (3), 1.778 (4), 1.719 (4), 1.783 (4)	1.861 (4), 1.880 (4)	118.2 (2)	Ben Hamida et al. (2005 )
[(Mo⁶⁺O₂)₂(H₂O)₂(As₂O₅]·3H₂O (vajdakite)	1.750 (6), 1.822 (6), 1.778 (6), 1.793 (6)	1.786 (5), 1.817 (5)	127.6 (3)	Ondruš et al. (2002 )
Pb₂As₂O₅ (paulmooreite)	1.747 (9), 1.750 (9), 1.733 (9), 1.772 (8)	1.826 (9), 1.842 (9)	123.0 (5)	Araki et al. (1980 )
Pb₈OCl₆(As₂O₅)₂ (gebhardite)	1.762 (2), 1.823 (2), 1.674 (2), 1.792 (2); 1.757 (2), 1.757 (2), 1.756 (2), 1.866 (2)	1.888 (2), 1.6323 (19); 1.693 (2), 1.890 (2)	132.85 (6); 144.12 (5)	Klaska & Gebert (1982 )
RE₃Cl₂(AsO₃)(As₂O₅) RE = Eu; Gd	1.776 (5), 1.827 (5), 1.751 (5), 1.753 (5); 1.772 (2), 1.831 (2), 1.741 (2); 1.749 (2)	1.864 (5), 1.968 (5); 1.855 (2), 1.972 (2)	116.0 (3), 115.97 (13)	Schander et al. (2024 )
RE₃Br₂(AsO₃)(As₂O₅) RE = Y, Dy–Yb	1.736 (10) – 1.858 (9)	1.858 (9) – 1.971 (7)	115.2 (5) – 116.0 (5)	Locke et al. (2025 ).
Sm₃Cl₂(As₂O₅)(AsO₃)	1.769 (7), 1.833 (7), 1.753 (6), 1.765 (7)	1.866 (6), 1.969 (6)	116.2 (3)	Goerigk et al. (2020 )
Sm₄(A₂O₅)₂(As₄O₈)	1.719 (2), 1.787 (2), 1.714 (2), 1.774 (3)	1.883 (2), 1.862 (3)	117.7 (2)	Kang & Schleid (2006 )
Sm₄(A₂O₅)₂(As₄O₈)	1.709 (7), 1.778 (8), 1.721 (8), 1.783 (8)	1.850 (8), 1.886 (8)	117.7 (4)	Ben Hamida et al. (2005)

Figure 4
Histograms for the As—O_terminal and As—O_bridging bond lengths and As—O—As bridging angles in the crystal structures listed in Table 4

; red colors refer to the title compound.

4. Synthesis and crystallization

Na₈(As₂O₅)₂·H₂O was first obtained serendipitously under hydroflux conditions. Powders of Fe₂O₃ and As₂O₃ were mixed in a 1:2 ratio and combined with an excess of NaOH (98.5%_wt) as a flux and a suitable amount of water to achieve a molar NaOH:H₂O ratio of approximately 1:1. The reaction was carried out in a Teflon container placed in a steel autoclave that was heated to 483 K for 2 d. An off-white, highly water-soluble solid product was obtained, which also dissolved when placed in a methanol solution for a prolonged period of time. To remove the NaOH flux, the product was finally washed quickly in two stages, twice with dry methanol and twice with dry acetone. The shape of the obtained colourless crystals was rather unspecific, mostly plate- to block-like with rounded edges.

Na₄(As₂O₅)·(H₂O)_0.5 was obtained specifically, i.e., without the addition of iron oxide, when As₂O₃ was heated directly with an excess of NaOH (approximate molar ratios As₂O₃: NaOH 1:12 and H₂O: NaOH 1.5:1) in an autoclave under the same conditions.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 5. The position of the water H atom was clearly discernible from a difference-Fourier map. The O—H distance was restrained to 0.85 (1) Å, while the U_iso(H) parameter was refined freely.

Table 5
Experimental details

Crystal data
Chemical formula	[Na₄(As₂O₅)(H₂O)_0.5]
M_r	330.81
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	301
a, b, c (Å)	18.283 (3), 5.0747 (9), 14.740 (3)
β (°)	91.256 (6)
V (Å³)	1367.3 (4)
Z	8
Radiation type	Mo Kα
μ (mm⁻¹)	10.00
Crystal size (mm)	0.04 × 0.03 × 0.02

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.660, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	10323, 1910, 1193
R_int	0.069
(sin θ/λ)_max (Å⁻¹)	0.695

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.033, 0.058, 1.02
No. of reflections	1910
No. of parameters	109
No. of restraints	1
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.72, −0.79
Computer programs: APEX4 and SAINT (Bruker, 2022 ), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), ATOMS for Windows (Dowty, 2006 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2538842

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989026002859/oo2017sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989026002859/oo2017Isup2.hkl

checkCIF report

Computing details top

Tetrasodium pyroarsenite hemihydrate top

Crystal data top

[Na₄(As₂O₅)(H₂O)_0.5]	F(000) = 1240
M_r = 330.81	D_x = 3.214 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 18.283 (3) Å	Cell parameters from 1379 reflections
b = 5.0747 (9) Å	θ = 2.2–28.9°
c = 14.740 (3) Å	µ = 10.00 mm⁻¹
β = 91.256 (6)°	T = 301 K
V = 1367.3 (4) Å³	Block, colourless
Z = 8	0.04 × 0.03 × 0.02 mm

Data collection top

Bruker APEXII CCD diffractometer	1193 reflections with I > 2σ(I)
ω– and φ–scans	R_int = 0.069
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 29.6°, θ_min = 2.2°
T_min = 0.660, T_max = 0.746	h = −25→25
10323 measured reflections	k = −7→7
1910 independent reflections	l = −20→20

Refinement top

Refinement on F²	1 restraint
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.033	All H-atom parameters refined
wR(F²) = 0.058	w = 1/[σ²(F_o²) + 4.6824P] where P = (F_o² + 2F_c²)/3
S = 1.02	(Δ/σ)_max < 0.001
1910 reflections	Δρ_max = 0.72 e Å⁻³
109 parameters	Δρ_min = −0.79 e Å⁻³

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
Na1	0.03814 (10)	0.4524 (4)	0.09242 (14)	0.0176 (4)
Na2	0.23554 (8)	0.0157 (5)	0.32203 (11)	0.0163 (4)
Na3	0.30984 (9)	0.0008 (5)	0.01553 (11)	0.0149 (4)
Na4	0.37938 (11)	0.0460 (4)	0.21031 (13)	0.0207 (5)
As1	0.15826 (2)	0.01960 (12)	0.62085 (3)	0.00966 (12)
As2	0.08337 (2)	−0.01329 (12)	0.43240 (3)	0.01006 (11)
O1	0.14842 (16)	0.3569 (6)	0.6337 (2)	0.0111 (8)
O2	0.24805 (15)	−0.0336 (7)	0.65721 (19)	0.0132 (7)
O3	0.17524 (14)	0.0122 (8)	0.49315 (17)	0.0133 (6)
O4	0.11217 (15)	0.0392 (7)	0.3230 (2)	0.0157 (7)
O5	0.07415 (16)	−0.3510 (7)	0.4372 (2)	0.0132 (8)
O1W	0.000000	0.3006 (10)	0.250000	0.0189 (12)
H1	0.034 (2)	0.202 (9)	0.273 (4)	0.044 (19)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Na1	0.0124 (8)	0.0218 (10)	0.0184 (9)	0.0026 (10)	−0.0012 (7)	−0.0004 (10)
Na2	0.0115 (8)	0.0186 (10)	0.0186 (9)	−0.0027 (11)	−0.0024 (7)	0.0006 (11)
Na3	0.0146 (8)	0.0152 (9)	0.0149 (9)	−0.0013 (11)	0.0007 (7)	−0.0017 (11)
Na4	0.0240 (9)	0.0239 (12)	0.0143 (10)	0.0033 (11)	0.0035 (8)	−0.0003 (10)
As1	0.0102 (2)	0.0097 (3)	0.0091 (2)	−0.0005 (2)	0.00026 (17)	−0.0004 (3)
As2	0.0089 (2)	0.0107 (2)	0.0106 (2)	0.0009 (2)	0.00064 (17)	0.0001 (2)
O1	0.0125 (18)	0.0092 (18)	0.0118 (18)	0.0013 (12)	0.0030 (14)	−0.0007 (13)
O2	0.0124 (14)	0.0161 (17)	0.0109 (14)	0.0038 (15)	−0.0026 (11)	−0.0028 (15)
O3	0.0119 (14)	0.0204 (17)	0.0076 (13)	−0.0027 (18)	−0.0007 (11)	0.0005 (17)
O4	0.0134 (15)	0.0215 (19)	0.0122 (15)	0.0007 (15)	0.0018 (12)	0.0038 (15)
O5	0.0101 (19)	0.013 (2)	0.016 (2)	−0.0009 (13)	0.0010 (16)	−0.0006 (13)
O1W	0.019 (3)	0.017 (3)	0.020 (3)	0.000	−0.007 (2)	0.000

Geometric parameters (Å, º) top

Na1—O1ⁱ	2.306 (4)	Na3—O3^vi	2.613 (5)
Na1—O5ⁱⁱ	2.316 (4)	Na4—O5^vi	2.409 (4)
Na1—O5ⁱⁱⁱ	2.450 (4)	Na4—O1^iv	2.416 (4)
Na1—O1W	2.559 (3)	Na4—O2ⁱⁱⁱ	2.510 (3)
Na2—O4	2.259 (3)	Na4—O4^vi	2.556 (4)
Na2—O1^iv	2.299 (3)	Na4—O1W^viii	2.588 (3)
Na2—O2ⁱⁱⁱ	2.447 (3)	Na4—O4^vii	2.623 (4)
Na2—O2^v	2.483 (4)	As1—O1	1.732 (3)
Na2—O2^iv	2.662 (4)	As1—O2	1.737 (3)
Na2—O3	2.775 (3)	As1—O3	1.915 (3)
Na3—O5^vi	2.342 (3)	As2—O5	1.723 (3)
Na3—O2ⁱⁱⁱ	2.402 (3)	As2—O4	1.728 (3)
Na3—O1^vii	2.455 (4)	As2—O3	1.891 (3)
Na3—O3ⁱⁱⁱ	2.477 (3)	O1W—H1	0.860 (10)
Na3—O3^vii	2.498 (5)	O1W—H1^ix	0.860 (10)

O1ⁱ—Na1—O5ⁱⁱ	129.43 (13)	Na2^iv—O1—Na3^vi	82.39 (11)
O1ⁱ—Na1—O5ⁱⁱⁱ	94.78 (12)	Na1^x—O1—Na3^vi	85.74 (12)
O5ⁱⁱ—Na1—O5ⁱⁱⁱ	99.78 (12)	Na4^iv—O1—Na3^vi	150.20 (15)
O1ⁱ—Na1—O1W	98.11 (11)	As1—O2—Na3^xi	100.39 (13)
O5ⁱⁱ—Na1—O1W	92.44 (11)	As1—O2—Na2^xi	101.09 (13)
O5ⁱⁱⁱ—Na1—O1W	150.34 (15)	Na3^xi—O2—Na2^xi	156.45 (14)
O4—Na2—O1^iv	153.99 (15)	As1—O2—Na2^v	107.58 (15)
O4—Na2—O2ⁱⁱⁱ	96.86 (11)	Na3^xi—O2—Na2^v	96.75 (13)
O1^iv—Na2—O2ⁱⁱⁱ	99.53 (12)	Na2^xi—O2—Na2^v	85.87 (12)
O4—Na2—O2^v	99.75 (13)	As1—O2—Na4^xi	172.49 (19)
O1^iv—Na2—O2^v	97.70 (12)	Na3^xi—O2—Na4^xi	78.92 (10)
O2ⁱⁱⁱ—Na2—O2^v	98.35 (13)	Na2^xi—O2—Na4^xi	78.52 (10)
O4—Na2—O2^iv	93.27 (13)	Na2^v—O2—Na4^xi	79.91 (10)
O1^iv—Na2—O2^iv	65.71 (11)	As1—O2—Na2^iv	89.16 (13)
O2ⁱⁱⁱ—Na2—O2^iv	93.71 (13)	Na3^xi—O2—Na2^iv	88.71 (12)
O2^v—Na2—O2^iv	161.01 (14)	Na2^xi—O2—Na2^iv	82.07 (11)
O4—Na2—O3	65.04 (10)	Na2^v—O2—Na2^iv	161.02 (14)
O1^iv—Na2—O3	97.26 (11)	Na4^xi—O2—Na2^iv	83.35 (11)
O2ⁱⁱⁱ—Na2—O3	161.88 (10)	As2—O3—As1	107.78 (13)
O2^v—Na2—O3	86.05 (12)	As2—O3—Na3^xi	158.62 (15)
O2^iv—Na2—O3	87.01 (12)	As1—O3—Na3^xi	92.96 (10)
O5^vi—Na3—O2ⁱⁱⁱ	99.15 (12)	As2—O3—Na3^vi	98.08 (15)
O5^vi—Na3—O1^vii	93.73 (11)	As1—O3—Na3^vi	92.93 (14)
O2ⁱⁱⁱ—Na3—O1^vii	163.93 (14)	Na3^xi—O3—Na3^vi	85.65 (12)
O5^vi—Na3—O3ⁱⁱⁱ	159.87 (15)	As2—O3—Na3^vii	90.08 (14)
O2ⁱⁱⁱ—Na3—O3ⁱⁱⁱ	68.45 (10)	As1—O3—Na3^vii	94.99 (14)
O1^vii—Na3—O3ⁱⁱⁱ	101.55 (11)	Na3^xi—O3—Na3^vii	82.85 (12)
O5^vi—Na3—O3^vii	103.59 (12)	Na3^vi—O3—Na3^vii	166.34 (14)
O2ⁱⁱⁱ—Na3—O3^vii	99.63 (12)	As2—O3—Na2	86.35 (10)
O1^vii—Na3—O3^vii	67.81 (10)	As1—O3—Na2	165.85 (13)
O3ⁱⁱⁱ—Na3—O3^vii	94.35 (12)	Na3^xi—O3—Na2	73.01 (8)
O5^vi—Na3—O3^vi	66.47 (11)	Na3^vi—O3—Na2	84.33 (11)
O2ⁱⁱⁱ—Na3—O3^vi	91.46 (12)	Na3^vii—O3—Na2	85.28 (11)
O1^vii—Na3—O3^vi	102.52 (11)	As2—O4—Na2	108.81 (14)
O3ⁱⁱⁱ—Na3—O3^vi	97.15 (12)	As2—O4—Na4^vii	92.81 (15)
O3^vii—Na3—O3^vi	166.34 (14)	Na2—O4—Na4^vii	83.25 (12)
O5^vi—Na4—O1^iv	154.17 (13)	As2—O4—Na4^vi	110.26 (16)
O5^vi—Na4—O2ⁱⁱⁱ	94.47 (11)	Na2—O4—Na4^vi	89.30 (13)
O1^iv—Na4—O2ⁱⁱⁱ	94.71 (12)	Na4^vii—O4—Na4^vi	156.92 (15)
O5^vi—Na4—O4^vi	65.80 (12)	As2—O5—Na1^xii	120.53 (16)
O1^iv—Na4—O4^vi	89.87 (12)	As2—O5—Na3^vii	104.04 (15)
O2ⁱⁱⁱ—Na4—O4^vi	91.49 (12)	Na1^xii—O5—Na3^vii	135.32 (16)
O5^vi—Na4—O1W^viii	89.59 (10)	As2—O5—Na4^vii	98.18 (15)
O1^iv—Na4—O1W^viii	94.54 (10)	Na1^xii—O5—Na4^vii	93.74 (13)
O2ⁱⁱⁱ—Na4—O1W^viii	149.65 (16)	Na3^vii—O5—Na4^vii	82.18 (11)
O4^vi—Na4—O1W^viii	117.37 (14)	As2—O5—Na1^xi	105.99 (15)
O5^vi—Na4—O4^vii	91.18 (12)	Na1^xii—O5—Na1^xi	80.22 (12)
O1^iv—Na4—O4^vii	113.12 (13)	Na3^vii—O5—Na1^xi	85.10 (11)
O2ⁱⁱⁱ—Na4—O4^vii	88.68 (11)	Na4^vii—O5—Na1^xi	154.79 (16)
O4^vi—Na4—O4^vii	156.92 (15)	Na1^ix—O1W—Na1	144.9 (2)
O1W^viii—Na4—O4^vii	61.14 (12)	Na1^ix—O1W—Na4^vi	84.13 (10)
O1—As1—O2	102.68 (15)	Na1—O1W—Na4^vi	79.19 (10)
O1—As1—O3	98.42 (16)	Na1^ix—O1W—Na4^xiii	79.19 (10)
O2—As1—O3	97.34 (12)	Na1—O1W—Na4^xiii	84.13 (10)
O5—As2—O4	102.92 (16)	Na4^vi—O1W—Na4^xiii	122.5 (2)
O5—As2—O3	97.79 (16)	Na1^ix—O1W—H1	91 (4)
O4—As2—O3	98.38 (12)	Na1—O1W—H1	109 (4)
As1—O1—Na2^iv	102.22 (15)	Na4^vi—O1W—H1	66 (4)
As1—O1—Na1^x	118.57 (16)	Na4^xiii—O1W—H1	166 (4)
Na2^iv—O1—Na1^x	138.84 (16)	Na1^ix—O1W—H1^ix	109 (4)
As1—O1—Na4^iv	109.29 (15)	Na1—O1W—H1^ix	91 (4)
Na2^iv—O1—Na4^iv	83.37 (12)	Na4^vi—O1W—H1^ix	166 (4)
Na1^x—O1—Na4^iv	87.96 (12)	Na4^xiii—O1W—H1^ix	66 (4)
As1—O1—Na3^vi	99.26 (14)	H1—O1W—H1^ix	109 (8)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1···O4	0.86 (1)	1.80 (1)	2.651 (4)	171 (6)

Coordination environments (Å) around the Na⁺ cations in Na₄(As₂O₅)(H₂O)_0.5 top

Atom	Coordination number	Polyhedron with idealized point group symmetry [in brackets] and deviation δ (in parentheses) from it	Range of Na—O bond lenghts	Average Na—O bond length	Number of water molecules in the first coordination sphere	Bond valence/valence units (without contribution of H atoms)
Na1	4	heterodisphenoid [mm2] (4.159)	2.306 (4)–2.559 (3)	2.408	1; O1W	0.76
Na2	6	didigonal scalenohedron [42m] (10.947)	2.259 (3) – 2.775 (3)	2.488	0; –	1.00
Na3	6	twisted trigonal prism [32] (3.999)	2.342 (3) – 2.613 (5)	2.464	0; –	0.98
Na4	6	monocapped trigonal antifrustum [3m] (28.070)	2.409 (9) – 2.623 (4)	2.496	1; O1W	0.86

As—O bond lengths (Å) and bridging angles (°) in the crystal structures of compounds with isolated pyroarsenite (As₂O₅)^4– groups top

Compound (mineral name)	As—O_terminal	As—O_bridging	As—O—As	Reference
Na₄(As₂O₅)·0.5H₂O	1.732 (3), 1.737 (3), 1.723 (3), 1.728 (3)	1.915 (3), 1.891 (3)	107.78 (13)	This work
BaCo(As₂O₅)	2×1.716 (3), 2×1.736 (3)	1.837 (5), 1.809 (5)	130.9 (3)	David et al. (2014)
BaFe₂(As₂O₅)(AsO₃)(OH)	2×1.745 (13), 2×1.757 (12)	2×1.816 (7)	134.7 (10)	Leclercq et al. (2020)
Ba₂Fe₂O(As₂O₅)₂	4×1.7503 (12)	2×1.8391 (11)	130.24 (14)	Leclercq et al. (2020)
Ba₂(Ti⁴⁺V³⁺)(As₂O₅)₂OF (bianchiniite)	4×1.7397 (15)	2×1.8377 (13)	127.12 (16)	Biagioni et al. (2021)
CaSb⁵⁺₂(As₂O₅)₂O₂·10H₂O (prachařite)	1.7633 (17), 1.7661 (16), 1.7611 (17), 1.7641 (17)	1.8079 (19), 1.8185 (18)	128.55 (9)	Kolitsch et al. (2023)
Fe²⁺Fe³⁺₃(As₂O₅)₂(AsO₃) (schneiderhöhnite)	1.7680 (15), 1.7997 (16), 1.7936 (15), 1.7968 (15); 1.7689 (16), 1.7608 (15), 1.7485 (15), 1.7844 (15)	1.7926 (16), 1.7648 (15); 1.8610 (15), 1.8075 (16)	132.9 (2), 136.8 (2)	Cooper & Hawthorne (2016)
Fe³⁺₃(AsO₂)₄(As₂O₅)(OH) (karibibite)	2×1.77 (2), 2×1.79 (2)	2×1.77 (2)	141 (3)	Colombo et al. (2017)
Fe₃(As₂O₅)(AsO₃)Cl	1.779 (7), 1.772 (8), 1.825 (7), 1.766 (7)	1.834 (7), 1.811 (9)	125.2 (4)	Leclercq et al. (2020)
In₂(As₂O₅)Cl₂	1.756 (5), 1.804 (8), 1.742 (6), 1.786 (7)	1.912 (7), 1.827 (6)	124.9 (5)	Jiang et al. (2011)
In₄(As₂O₅)(As₃O₇)Br₃	1.734 (9), 1.782 (8), 1.777 (10), 1.780 (8)	1.881 (9), 1.814 (8)	124.3 (5)	Jiang et al. (2011)
Mn₂(As₂O₅)	1.727 (4), 1.736 (4), 1.740 (3), 1.752 (4), 1.709 (4), 1.763 (3), 1.722 (4), 1.779 (3)	1.872 (4), 1.838 (4), 1.860 (4), 1.834 (4)	116.04 (16), 137.33 (19)	Priestner et al. (2019)
Nd₄(A₂O₅)₂(As₄O₈)	1.716 (3), 1.778 (4), 1.719 (4), 1.783 (4)	1.861 (4), 1.880 (4)	118.2 (2)	Ben Hamida et al. (2005)
[(Mo⁶⁺O₂)₂(H₂O)₂(As₂O₅]·3H₂O (vajdakite)	1.750 (6), 1.822 (6), 1.778 (6), 1.793 (6)	1.786 (5), 1.817 (5)	127.6 (3)	Ondruš et al. (2002)
Pb₂As₂O₅ (paulmooreite)	1.747 (9), 1.750 (9), 1.733 (9), 1.772 (8)	1.826 (9), 1.842 (9)	123.0 (5)	Araki et al. (1980)
Pb₈OCl₆(As₂O₅)₂ (gebhardite)	1.762 (2), 1.823 (2), 1.674 (2), 1.792 (2); 1.757 (2), 1.757 (2), 1.756 (2), 1.866 (2)	1.888 (2), 1.6323 (19); 1.693 (2), 1.890 (2)	132.85 (6); 144.12 (5)	Klaska & Gebert (1982)
RE₃Cl₂(AsO₃)(As₂O₅) RE = Eu; Gd	1.776 (5), 1.827 (5), 1.751 (5), 1.753 (5); 1.772 (2), 1.831 (2), 1.741 (2); 1.749 (2)	1.864 (5), 1.968 (5); 1.855 (2), 1.972 (2)	116.0 (3), 115.97 (13)	Schander et al. (2024)
RE₃Br₂(AsO₃9(As₂O₅) RE = Y, Dy–Yb	1.736 (10) – 1.858 (9)	1.858 (9) – 1.971 (7)	115.2 (5) – 116.0 (5)	Locke et al. (2025).
Sm₃Cl₂(As₂O₅)(AsO₃)	1.769 (7), 1.833 (7), 1.753 (6), 1.765 (7)	1.866 (6), 1.969 (6)	116.2 (3)	Goerigk et al. (2020)
Sm₄(A₂O₅)₂(As₄O₈)	1.719 (2), 1.787 (2), 1.714 (2), 1.774 (3)	1.883 (2), 1.862 (3)	117.7 (2)	Kang & Schleid (2006)
Sm₄(A₂O₅)₂(As₄O₈)	1.709 (7), 1.778 (8), 1.721 (8), 1.783 (8)	1.850 (8), 1.886 (8)	117.7 (4)	Ben Hamida et al. (2005)

Acknowledgements

The authors acknowledge the X-ray Centre of TU Wien for providing access to instrumentation and analysis software and TU Wien Bibliothek for financial support through its Open Access Funding Program.

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