Crystal structures of cristobalite-type and coesite-type PON redetermined on the basis of single-crystal X-ray diffraction data

The crystal structures of two phosphorus oxonitride polymorphs (cristobalite- and coesite-type) were redetermined by means of single-crystal X-ray diffraction data.

Hitherto, phosphorus oxonitride (PON) could not be obtained in the form of single crystals and only powder diffraction experiments were feasible for structure studies. In the present work we have synthesized two polymorphs of phosphorus oxonitride, cristobalite-type (cri-PON) and coesite-type (coe-PON), in the form of single crystals and reinvestigated their crystal structures by means of in house and synchrotron single-crystal X-ray diffraction. The crystal structures of cri-PON and coe-PON are built from PO 2 N 2 tetrahedral units, each with a statistical distribution of oxygen and nitrogen atoms. The crystal structure of the coe-PON phase has the space group C2/c with seven atomic sites in the asymmetric unit [two P and three (N,O) sites on general positions, one (N,O) site on an inversion centre and one (N,O) site on a twofold rotation axis], while the cri-PON phase possesses tetragonal I42d symmetry with two independent atoms in the asymmetric unit [the P atom on a fourfold inversion axis and the (N,O) site on a twofold rotation axis]. In comparison with previous structure determinations from powder data, all atoms were refined with anisotropic displacement parameters, leading to higher precision in terms of bond lengths and angles.

Chemical context
The pseudo-binary system P 3 N 5 /P 2 O 5 has been investigated intensively because the properties of related ceramic materials are promising for industrial applications. A mid-member of this system is phosphorus oxonitride (PON), whose chemical stability is essential for its use as an insulator or for fireproofing. This compound has attracted significant attention as a ternary base compound of electrolytes for rechargeable thinfilm Li/Li-ion batteries. Phosphorus oxonitride is an isoelectronic analogue of silica (SiO 2 ) with the charge-balanced substitution P 5+ + N 3À = Si 4+ + O 2À . The crystal structures of the polymorphic forms of SiO 2 and PON are built of tetrahedral SiO 4 and PO 2 N 2 units, respectively. At present, five modifications of PON have been identified. Four of them are isostructural to known silica polymorphs, viz. -quartz- ), -cristobalite-(Lé ger et al., 2001), moganite-(Chateau et al., 1999 and coesite-type (Baumann et al., 2015). The fifth one, -PON, has a structure type different from any of the silica modifications (Baumann et al., 2012). A rich variety of polymorphs is a result of the many ways in which the tetrahedra can be linked to form corner-sharing networks. Most of the phases in the P 3 N 5 /P 2 O 5 system are usually obtained either in an amorphous state or in the form of powders consisting of very small crystallites. We succeeded in ISSN 2056-9890 synthesizing single crystals of pure cristobalite-(cri) and coesite-type (coe) PON of a size suitable for single-crystal X-ray diffraction and report here the results of the structure refinements.

Structural commentary
The structure of cri-PON (Fig. 1a) can be derived from that of -cristobalite by tilting each PO 2 N 2 tetrahedron about the 4 axes alternately clockwise and anticlockwise. This leads to the lowering of symmetry from Fd3m to I42d, however, the topology remains the same. The length of the P-(O,N) bond in cri-PON is 1.5796 (10) Å , which is in a good agreement with the average of expected P-N (1.626 Å ) and P-O (1.537 Å ) distances (Huminicki & Hawthorne, 2002). All P-(O,N) distances within the PO 2 N 2 units are equal, but there is a noticeable (O,N)-P-(O,N) angle variation between 107.86 (2) and 112.73 (5) due to the compression of the tetrahedra along the c-axis direction.
The structure of coe-PON (Fig. 1b) is isotypic with coesite (SiO 2 ) (Angel et al., 2003). The framework of coe-PON is constructed of four-member rings comprised of corner-sharing PO 2 N 2 tetrahedra. These rings are linked in such a manner that crankshaft-like chains are formed. The average P-(O,N) distance in coe-PON (1.572 Å ) is slightly shorter than that of 1.581 Å reported by Baumann et al. (2015) likely due to the difference in temperatures at which the experiments were conducted. The tetrahedra are irregularly distorted, with P-(O,N) distances varying between 1.5530 (9) and 1.588 (3) Å , and (O,N)-P-(O,N) angles between 106.79 (19) and 112.0 (2) .
In comparison with the refinements from powder diffraction data (Lé ger et al., 2001;Baumann et al., 2015), singlecrystal diffraction data revealed a detailed electron density map, which allowed us in addition to a substitutional O-N disorder, to detect a possible positional disorder (for details see Refinement section), which may affect physical properties of coe-PON.

Synthesis and crystallisation
Cristobalite-type PON was synthesized from phosphoric triamide by a two-step condensation process. POCl 3 (99%, Sigma Aldrich) was reacted with liquid NH 3 (5.0, Air Liquide) to yield a mixture of PO(NH 2 ) 3 and NH 4 Cl, which was subsequently heated to 893 K for 5 h in a stream of dry ammonia. The amorphous reaction product was crystallized at 1023 K for 7 d in an evacuated fused silica ampoule, yielding pure cristobalite-type PON. Coesite-type PON was obtained by high-pressure/high-temperature reaction of cri-PON in a modified Walker-type multi-anvil apparatus. The starting material was tightly packed in a h-BN capsule, which was centered in a MgO:Cr octahedron (Ceramic Substrates & Components, Isle of Wight, UK) with an edge length of 10 mm. The latter was subsequently compressed between eight truncated tungsten carbide cubes (5 mm truncation edge length, Hawedia, Marklkofen, Germany) using a 1000 t hydraulic press (Voggenreiter, Mainleus, Germany). The sample was compressed to 15.5 GPa, the temperature raised to 1573 K within 15 min and held constant for 60 min. The sample was cooled by turning off the heating, decompressed and mechanically isolated.

Refinement
was twinned by inversion with an equal amount of the two twin domains. The refinement of the coe-PON structure revealed a residual electron density peak of 1.41 e À ÁÅ À3 at a distance 1.22 Å from atom P2 and 1.50, 1.65 and 1.65 Å from atoms O1, O2 and O5, respectively. This density may be explained by a static disorder of the P2 atom between two positions. The disorder is, however, too weak to give additional reliable residual density peaks for the assignments of oxygen and nitrogen atoms. used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).    (15) Geometric parameters (Å, º)