Introduction

New methods for growing single crystals of arsenic oxyhydroxides are of interest due to the difficulty inherent to growing arsenic oxyhydroxide crystals using many of the traditional methods. The recently developed hydroflux method [1–3] combines the hydrothermal and hydroxide flux methods to create conditions in which a flux [4] can be melted at relatively low temperatures due to the presence of notable quantities of water. The low temperature both makes conducting experiments more convenient and allows certain crystals to form that would not do so under high temperature crystal growth conditions. We were able to obtain Na4(AsO4)OH single crystals out of a sodium hydroxide hydroflux.

Na4(AsO4)OH is isotypic with Na4(VO4)OH, which was recently crystallized from a Na2O melt by Kerp et al. [5], and with Na4[P0.5Mn0.5O4][O], which was prepared from a NaOH flux and might be a hydroxide as well [6]. Related arsenates include Na5[AsO4][O], reported by Haas et al. [7] and arsenates belonging to the Adelite mineral family reported by Qurashi et al. [8] The Adelite mineral has a composition of CaMg(AsO4)(OH), where the AsO4 3− tetrahedra are isolated by CaO7(OH) and MgO4(OH)2 polyhedra. In the title compound, the AsO4 3− polyhedra are preserved, but the Ca2+ and Mg2+ cations are replaced by four Na+ cations to maintain the charge balance, resulting in Na4(AsO4)(OH), an Adelite related structure type. In this study, we report on the synthesis and crystal structure of Na4(AsO4)OH.

Experimental

Materials and Methods

Single crystals of Na4(AsO4)OH were grown out of a hydroflux of sodium hydroxide containing approximately 34 % water by weight. The reaction was carried out in a 25 mL polytetrafluoroethylene (PTFE) lined stainless steel autoclave. A mixture of 4/3 mmol (0.1320 g) of As2O3 (Alfa Aesar, 99.95 %), 0.1 mol (6.194 g) of Na2O (Alfa Aesar, reagent grade), and ~6 g (6.0027 g) of deionized H2O were added to the autoclave, and the autoclave was sealed. Na2O was used to exclude the CO3 2− contained in commercial NaOH. The autoclave was heated to 230° at 5 °C/min, held for 24 h, and then cooled slowly to 33 °C at a rate of 0.3 °C/min, at which point the oven was shut off. After cooling, the liquid in the autoclave was poured out, and the crystals inside were scraped off the sides and rinsed out of the autoclave with methanol. The yield was approximately 45 % based on As2O3.

Energy-Dispersive Spectroscopy (EDS)

Elemental analysis was performed on the flux-grown crystals using an FEI Quanta 200 scanning electron microscope (SEM) with EDS capabilities. The crystals were mounted on carbon tape and analyzed using a 30 kV accelerating voltage and an accumulation time of 20 s. As a qualitative measure, EDS confirmed the presence of each reported element in the title compound.

Crystallographic Study

X-ray intensity data from a colorless block-like crystal were collected at 100(2) K using a Bruker SMART APEX diffractometer (Mo Ka radiation, λ = 0.71073 Å) [9]. The data collection covered 99.6 % of reciprocal space to 2θ max = 65.2°, with an average reflection redundancy of 6.1, and R int = 0.029 after absorption correction. The raw area detector data frames were reduced and corrected for absorption effects using the SAINT + and SADABS programs [9]. Final unit cell parameters were determined by least-squares refinement of 3,858 reflections from the data set. An initial structural model was obtained by direct methods using SHELXS [10]. Subsequent difference Fourier calculations and full-matrix least-squares refinement against F 2 were performed with SHELXL-2013/4 [10] using the ShelXle interface [11].

The compound crystallizes in the orthorhombic system. The pattern of systematic absences in the intensity data was consistent with the space groups Pnma and Pna21, the former of which was confirmed by structure solution. The asymmetric unit consists of one arsenic atom, three sodium atoms, four oxygen atoms, and one hydroxyl hydrogen atom. Arsenic As(1), sodium Na(2), oxygen atoms O(2), O(3), and O(4), and the hydroxyl hydrogen H(4) are located on mirror planes (site 4c). Na(1) and O(1) are located on general positions. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydroxyl hydrogen atom was located in a Fourier difference map and refined freely. The largest residual electron density peak and hole in the final difference map are +0.79 and −0.84 e−/Å3, located 1.64 and 0.70 Å from As(1), respectively. Final atomic coordinates were standardized using Structure Tidy [12–14].

Results and Discussion

Plentiful clear, colorless crystals were obtained from the hydroflux reaction. To avoid sample decomposition, the crystal structure of the moisture sensitive crystals was determined by minimizing exposure to air prior to data collection. The crystals were quickly mounted and transferred into a nitrogen stream. Na4(AsO4)OH crystallizes in the orthorhombic space group Pnma with a = 8.9467(11) Å, b = 7.3854(9) Å, and c = 8.2395(10) Å. Crystallographic information is summarized in Table 1, the atomic coordinates can be found in Table 2, selected interatomic distances are listed in Table 3, anisotropic displacement parameters are tabulated in Table 4, and the hydrogen bonds in Na4(AsO4)OH are listed in Table 5.

Table 1 Crystal data and structure refinement for Na4(AsO4)OH
Full size table
Table 2 Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for Na4(AsO4)OH
Full size table
Table 3 Selected bond lengths [Å] and angles [°] for Na4(AsO4)OH
Full size table
Table 4 Anisotropic displacement parameters (Å2 × 103) for Na4(AsO4)OH
Full size table
Table 5 Hydrogen bonds for Na4(AsO4)OH [Å and °]
Full size table

The material exhibits a three-dimensional crystal structure consisting of isolated AsO4 3− tetrahedra (Figs. 1, 2, 3), chains of alternating face and corner shared Na(1)O6 polyhedra running down the y-direction, chains of corner sharing Na(2)O6 octahedra running down the x axis (Na(2)O6 polyhedra are not connected to each other along the y-direction), and chains of edge shared Na(3)O6 octahedra running down the y-axis. These sodium-containing chains are in turn connected via corner and edge sharing to create a complex 3-D network in which the AsO4 tetrahedra occupy available cavities. The AsO4 tetrahedra are distorted with bond lengths ranging from 1.681 to 1.705 Å (Fig. 4), and bond angles ranging from 107.44° to 111.414° (Fig. 5). The arsenic AsO4 tetrahedra share corners with Na(3)O6 and Na(2)O6 octahedra and edges with the Na(1)O6 polyhedra (Figs. 1, 2, 3).

Fig. 1
figure 1

Extended view of the crystal structure of Na4(AsO4)OH along the a-axis, with oxygen atoms in red, AsO4 tetrahedra in purple, sodium atoms in green, and hydroxide atoms in blue (Color figure online)

Full size image
Fig. 2
figure 2

The arsenic AsO4 tetrahedra share edges with the Na(1)O6 polyhedra and corners with the Na(2)O6 and Na(3)O6 octahedra. The AsO4 tetrahedron is shown in purple, Na(1), Na(2), and Na(3) polyhedra are shown in blue, yellow, and green, respectively (Color figure online)

Full size image
Fig. 3
figure 3

Depiction of the Na1 (top), Na2 (middle), and Na3 (bottom) chains in Na4(AsO4)OH

Full size image
Fig. 4
figure 4

View of a single AsO4 tetrahedra, with bond lengths labeled

Full size image
Fig. 5
figure 5

View of a single AsO4 tetrahedra, with bond angles labeled

Full size image

As mentioned earlier, the title compound is related to the previously reported compounds, Na4(VO4)OH (isostructural with Na4(AsO4)OH in this work) and Na5(VO4)O, with space groups of Pnma and Pbcm, respectively. Although overall structural features in these two structures, including the isolated MO4 tetrahedra (M = V, As) are similar, the major difference is found in the coordination environment of the O(4) atom that is not bonded to any M atoms. In Na5(VO4)O, the O(4) atom is observed in a distorted ONa6 polyhedra, while in Na4(MO4)OH (M = V, As) the O(4) atom is placed in a severely distorted ONa6 polyhedra due to the presence of the H atom attached to the O(4) atom forming –OH group.

Conclusion

Single crystals of Na4(AsO4)OH were grown from a hydroflux. The title compound exhibits a new composition and is isostructural to a recently reported vanadate, Na4(VO4)OH. The arsenic is located in isolated, distorted AsO4 tetrahedra.

Despite the lack of stability in the title compound, analogs of this compound, such as Na4(AsO4)F, may be stable enough to examine the properties of the structure; thus, this compound has future research potential.

Supporting Information

Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +497247808666; e-mail: crystdata@fiz-karlsruhe.de) on quoting the depository number CSD-428481.