Date Published: August 01, 2019
Publisher: International Union of Crystallography
Author(s): Karolina Schwendtner, Uwe Kolitsch.
The crystal structures of Cs(H2AsO4)(H3AsO4)2, NH4(H2AsO4)(H3AsO4) and Li2(H2PO4)2 were determined from single-crystal X-ray diffraction data. The two alkali compounds represent novel structure types, while the ammonium compound is homeotypic with its Rb analogue.
M+ phosphates and arsenates, and their crystal structures and physicochemical properties, have been extensively studied. Several compounds exhibit interesting properties, such as protonic conductivity (Chouchene et al., 2017a ▸,b ▸; Volkov et al., 1995 ▸, 1997 ▸; Voronov et al., 2013 ▸; Dekhili et al., 2018 ▸) or nonlinear optical properties (Dhouib et al., 2014a ▸, 2017 ▸; Kumaresan et al., 2008 ▸).
The asymmetric unit of Cs(H2AsO4)(H3AsO4)2 contains one Cs, three As, 12 O and eight H atoms (Fig. 1 ▸). The Cs atom is 12-coordinated, with the Cs—O bond lengths varying between 3.1202 (17) and 3.934 (3) Å (Table 2 ▸). The average Cs—O bond length (3.458 Å) is considerably longer than the statistical average of 3.377 Å for 12-coordinated Cs atoms (Gagné & Hawthorne, 2016 ▸), explaining the low bond-valence sum (BVS; Gagné & Hawthorne, 2015 ▸) of 0.85 v.u. The As—O bond lengths are very similar for the doubly (As3) and triply protonated (As1 and As2) As atoms (1.683–1.681 Å) and slightly shorter than the statistical average of 1.687 Å (Gagné & Hawthorne, 2018a ▸). Since two/three O atoms of the coordination polyhedra are protonated, the As—O bond lengths are only slightly elongated compared to unprotonated O atoms. The BVSs of the three As atoms are between 5.06 and 5.09 v.u. and thus close to the expected value, whereas all O atoms are considerably underbonded, with BVSs ranging from 1.22 to 1.53 v.u., and are all either donors or acceptors of hydrogen bonds. The latter are strong (compared to the other H3AsO4-containing compounds cited above), with O—H⋯O distances in the range 2.524 (2)–2.664 (2) Å (Table 3 ▸) and connect the individual protonated AsO4 tetrahedra into a three-dimensional (3D) network (Figs. 2 ▸a–c). In the  direction, the structure forms tunnels walled by AsO4 tetrahedra in which the Cs atom is located (Fig. 2 ▸d).
Several statistical analyses of bond lengths in As5+O4 polyhedra have been published recently. Gagné & Hawthorne (2018a ▸) reported average As—O bond lengths of 1.687 (27) Å in AsO4 and 1.830 (28) Å in AsO6 groups, derived from 508 and 13 polyhedra, respectively. Schwendtner (2008 ▸) found similar values of 1.686 (29) and 1.827 (29) Å for a larger sample size of 704 AsO4 and 40 AsO6 polyhedra, respectively. An analysis of As—O bond lengths in minerals by Majzlan et al. (2014 ▸) gave a very similar value of 1.685 Å (no s.u. given) for the average As—O bond length and a value of 1.727 Å (no s.u. given) for As—OH bonds. Data for As—O bond lengths in multiply protonated As5+Ox (x = 4 and 6) polyhedra are scarce (especially those for H3AsO4 groups) due to the rare occurrence of compounds containing such polyhedra. An earlier attempt by Ichikawa (1988 ▸) to carry out a statistical analysis of the hydrogen-bond-length dependence of the distortion in HnAsO4 (n = 1–3) tetrahedra was severely hampered for the doubly and triply protonated representatives, since data for only six H2AsO4 and two H3AsO4 groups were available, and no pertinent conclusions were possible. As the number of synthetic compounds and minerals containing HnAsO4 (n = 1–3) groups has considerably increased in the last three decades, we were able to perform a detailed analysis of As—O/OH bonds in HnAsO4 (n = 1–3) groups using data from the ICSD database (FIZ, 2018 ▸) (conventional R value < 5, full occupancy of As and O sites), expanded by the published data for known H3AsO4-containing inorganic compounds mentioned in the Introduction (§1), and the two novel title arsenate compounds (Table 8 ▸ and Fig. 8 ▸). Source: http://doi.org/10.1107/S2053229619008489