Crystal structure of Zn2(HTeO3)(AsO4)

The crystal structure of Zn2(HTeO3)(AsO4) consists of ∞ 2[ZnO3/2(OH)1/2O1/1] layers extending parallel to (001) that are linked by [TeIVO3OH] and oxidoarsenate(V) groups.

Single crystals of Zn 2 (HTeO 3 )(AsO 4 ), dizinc(II) hydroxidodioxidotellurate(IV) oxidoarsenate(V), were obtained as one of the by-products in a hydrothermal reaction between Zn(NO 3 ) 2 Á6H 2 O, TeO 2 , H 3 AsO 4 and NH 3 in molar ratios of 2:1:2:10 at 483 K for seven days. The asymmetric unit of Zn 2 (HTeO 3 )(AsO 4 ) contains one Te (site symmetry m), one As (m), one Zn (1), five O (three m, two 1) and one H (m) site. The Zn II atom exhibits a coordination number of 5 and is coordinated by four oxygen atoms and a hydroxide group, forming a distorted trigonal bipyramid. The hydroxide ion is positioned at a significantly larger distance on one of the axial positions of the bipyramid. The [ZnO 4 OH] polyhedra are connected to each other by corner-sharing to form 1 2 [ZnO 3/2 (OH) 1/2 O 1/1 ] layers extending parallel to (001). The Te IV atom is coordinated by three oxygen atoms and a hydroxide group in a one-sided manner in the shape of a bisphenoid, revealing stereochemical activity of its 5s 2 electron lone pair. The As V atom is coordinated by four oxygen atoms to form the tetrahedral oxidoarsenate(V) anion. By corner-sharing, [TeO 3 OH] and [AsO 4 ] groups link adjacent 1 2 [ZnO 3/2 (OH) 1/2 O 1/1 ] layers along [001] into a three-dimensional framework structure.

Chemical context
Only a few elements have such a diverse crystal chemistry as tellurium, especially in its +IV oxidation state. This can be attributed to the stereochemically active non-bonding 5s 2 electron pair of Te IV (Galy et al., 1975) that has a similar space requirement as coordinating ligands and therefore often results in one-sided and low-symmetry coordination spheres around Te IV atoms. An extensive review of the rich crystal chemistry of oxidotellurates(IV) was published recently by Christy et al. (2016).
The peculiar crystal chemistry of Te IV makes it an interesting building block in the search for new compounds with crystal structures lacking inversion symmetry. As a prerequisite, a compound must be non-centrosymmetric in order to have ferro-, piezo-or pyroelectric properties or to possess non-linear optical properties (Ok et al., 2006). Another effect of the electron lone pair and its large space consumption is the frequent formation of open-framework structures in (transition) metal oxidotellurates(IV). Different structure units such as clusters, chains, layers or channels resulting from the presence of oxidotellurate(IV) anions are observed in various crystal structures (Stö ger & Weil, 2013 Zimmermann et al., 2011] or, very recently, arsenates [Cu 5 (TeO 3 ) 2 (AsO 4 ) 2 ; Missen et al., 2020]. Crystals of Cu 5 (TeO 3 ) 2 (AsO 4 ) 2 have been grown by a chemical transport reaction (Binnewies et al., 2012), starting from CuO, TeO 2 and As 2 O 5 at temperatures of 1023 K (source) and 953 K (sink). The title compound, Zn 2 (HTeO 3 )(AsO 4 ), however, was obtained at much milder temperatures (483 K) under hydrothermal conditions.
The zinc cation (Zn1) is coordinated by five oxygen atoms with one (O3, as part of the hydroxy group) being at a significantly longer distance [2.3259 (18) Å ] than the other four [1.979 (3)-2.0486 (16) Å ]. The resulting polyhedron has the shape of a distorted trigonal bipyramid, with the remote O3 site occupying one of the axial positions and the equatorial positions being slightly tilted towards it (Fig. 1). The geometry index 5 (Addison et al., 1984), which is 0 for an ideal square pyramid and 1 for an ideal trigonal bipyramid, amounts to 0.665 for the [ZnO 4 OH] polyhedron. The [ZnO 4 OH] polyhedra are connected to each other by sharing four corners with neighbouring polyhedra to form 1 2 [ZnO 3/2 (OH) 1/2 O 1/1 ] layers extending parallel to (001). The bond-valence sum (BVS; Brown, 2002) of Zn1 was calculated to be 1.98 valence units (v.u.) using the values of Brese & O'Keeffe (1991).
The tellurium(IV) atom (Te1) is coordinated by four oxygen atoms with bond lengths in the range 1.880 (2) Table 1 Selected bond lengths (Å ).

Figure 1
The This way, a three-dimensional framework structure is established (Fig. 4).
In the crystal structure, the spatial requirements of the 5s 2 electron lone pairs at the Te IV atoms lead to the formation of   Channels in the crystal structure of Zn 2 (HTeO 3 )(AsO 4 ) running parallel to [110]. Colour codes and displacement ellipsoids are as in Fig. 3. O-HÁ Á ÁO hydrogen bonds are shown as orange lines.

Figure 5
Channels in the structure of Zn 2 (HTeO 3 )(AsO 4 ) running parallel to [100]. Colour codes and displacement ellipsoids are as in Fig. 3.

Synthesis and crystallization
Crystals of Zn 2 (HTeO 3 )(AsO 4 ) were obtained by hydrothermal synthesis. The reactants, 0.1949 g of Zn(NO 3 ) 2 Á6H 2 O (0.670 mmol), 0.0512 g of TeO 2 (0.321 mmol), 0.1365 g 80% wt of H 3 AsO 4 (aq) (0.713 mmol) and 0.22 g of 25% wt NH 3 (aq) (3.23 mmol) were weighed into a small Teflon vessel with an inner volume of ca 3 ml. The vessel was filled with deionized water to three-quarters of its volume and the reactants were mixed by manual stirring. The Teflon vessel was then put into a steel autoclave and heated to 483 K for 7 d at autogenous pressure. Afterwards, the autoclave was cooled to room temperature within about 4 h. The resulting product was a colourless multi-phase solid. In the X-ray powder pattern of the bulk, Zn 2 (HTeO 3 )(AsO 4 ) was found as a by-product, in addition to (NH 4 )Zn(AsO 4 ) (Feng et al., 2001) and the educt TeO 2 (-TeO 2 ; Stehlik & Balak, 1948). Under a polarizing microscope, small colourless block-shaped crystals of Zn 2 (HTeO 3 )(AsO 4 ) were visible that were manually separated for the single-crystal X-ray diffraction study.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Atom labels and coordinates were standardized with Structure Tidy (Gelato & Parthé, 1987) implemented in PLATON (Spek, 2020). The H atom of the hydroxy group was located in a difference-Fourier map and was refined freely. The crystal structure was refined under consideration of twinning by inversion, revealing a minor contribution of 3.2 (12)% for the inversion-related twin component.