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ISSN: 2056-9890

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

aBayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany, bESRF, 38043 Grenoble, France, cDepartment of Chemistry, Ludwig Maximilian University, 81377 Munich, Germany, and dMaterial Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany
*Correspondence e-mail: maxim.bykov@uni-bayreuth.de

Edited by M. Weil, Vienna University of Technology, Austria (Received 10 August 2015; accepted 8 October 2015; online 14 October 2015)

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 PO2N2 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 I-42d 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.

1. Chemical context

The pseudo-binary system P3N5/P2O5 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 thin-film Li/Li-ion batteries. Phospho­rus oxonitride is an isoelectronic analogue of silica (SiO2) with the charge-balanced substitution P5+ + N3− = Si4+ + O2−. The crystal structures of the polymorphic forms of SiO2 and PON are built of tetra­hedral SiO4 and PO2N2 units, respectively. At present, five modifications of PON have been identified. Four of them are isostructural to known silica polymorphs, viz. α-quartz- (Léger et al., 1999[Léger, J.-M., Haines, J., de Oliveira, L. S., Chateau, C., Le Sauze, A., Marchand, R. & Hull, S. (1999). J. Phys. Chem. Solids, 60, 145-152.]), β-cristobalite- (Léger et al., 2001[Léger, J. M., Haines, J., Chateau, C., Bocquillon, G., Schmidt, M. W., Hull, S., Gorelli, F., Lesauze, A. & Marchand, R. (2001). Phys. Chem. Miner. 28, 388-398.]), moganite- (Chateau et al., 1999[Chateau, C., Haines, J., Léger, J. M., Lesauze, A. & Marchand, R. (1999). Am. Mineral. 84, 207-210.]) and coesite-type (Baumann et al., 2015[Baumann, D., Niklaus, R. & Schnick, W. (2015). Angew. Chem. Int. Ed. 54, 4388-4391.]). The fifth one, δ-PON, has a structure type different from any of the silica modifications (Baumann et al., 2012[Baumann, D., Sedlmaier, S. J. & Schnick, W. (2012). Angew. Chem. Int. Ed. 51, 4707-4709.]). 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 P3N5/P2O5 system are usually obtained either in an amorphous state or in the form of powders consisting of very small crystallites. We succeeded in 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.

2. Structural commentary

The structure of cri-PON (Fig. 1[link]a) can be derived from that of β-cristobalite by tilting each PO2N2 tetra­hedron about the [\overline{4}] axes alternately clockwise and anti­clockwise. This leads to the lowering of symmetry from Fd[\overline{3}]m to I[\overline{4}]2d, 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[Huminicki, D. M. C. & Hawthorne, F. C. (2002). Rev. Mineral. Geochem. 48, 123-253.]). All P—(O,N) distances within the PO2N2 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.

[Figure 1]
Figure 1
Crystal structures of cri-PON (a) and coe-PON (b) shown in polyhedral representation. Displacement parameters are drawn at the 50% probability level. Mixed (N,O) sites are shown in red; P atoms are shown in brown.

The structure of coe-PON (Fig. 1[link]b) is isotypic with coesite (SiO2) (Angel et al., 2003[Angel, R. J., Shaw, C. S. J. & Gibbs, G. V. (2003). Phys. Chem. Miner. 30, 167-176.]). The framework of coe-PON is constructed of four-member rings comprised of corner-sharing PO2N2 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[Baumann, D., Niklaus, R. & Schnick, W. (2015). Angew. Chem. Int. Ed. 54, 4388-4391.]) 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 (Léger et al., 2001[Léger, J. M., Haines, J., Chateau, C., Bocquillon, G., Schmidt, M. W., Hull, S., Gorelli, F., Lesauze, A. & Marchand, R. (2001). Phys. Chem. Miner. 28, 388-398.]; Baumann et al., 2015[Baumann, D., Niklaus, R. & Schnick, W. (2015). Angew. Chem. Int. Ed. 54, 4388-4391.]), single-crystal 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.

3. Synthesis and crystallisation

Cristobalite-type PON was synthesized from phospho­ric tri­amide by a two-step condensation process. POCl3 (99%, Sigma Aldrich) was reacted with liquid NH3 (5.0, Air Liquide) to yield a mixture of PO(NH2)3 and NH4Cl, 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.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1[link]. Structure refinements of both coe-PON and cri-PON were performed using occupancies of oxygen and nitro­gen atoms fixed to 0.5 for each site. As a result of the very similar scattering powers of N and O atoms, an attempt to refine the occupancies resulted in unreliable values with large standard uncertainties. The cri-PON crystal 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·Å−3 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.

Table 1
Experimental details

  cri-PON coe-PON
Crystal data
Chemical formula PON PON
Mr 60.98 60.98
Crystal system, space group Tetragonal, I[\overline{4}]2d Monoclinic, C2/c
Temperature (K) 293 100
a, b, c (Å) 4.6135 (2), 4.6135 (2), 6.9991 (5) 6.9464 (6), 12.0340 (4), 6.9463 (5)
α, β, γ (°) 90, 90, 90 90, 119.914 (10), 90
V3) 148.97 (2) 503.30 (7)
Z 4 16
Radiation type Mo Kα Synchrotron, λ = 0.69428 Å
μ (mm−1) 1.24 1.35
Crystal size (mm) 0.02 × 0.02 × 0.02 0.02 × 0.02 × 0.02
 
Data collection
Diffractometer Bruker SMART APEX CCD PILATUS@SNBL
Absorption correction Multi-scan (CrysAlis PRO; Agilent, 2014[Agilent (2014). CrysAlis PRO. Agilent Technologies, Yarnton, England.]) Multi-scan (CrysAlis PRO; Agilent, 2014[Agilent (2014). CrysAlis PRO. Agilent Technologies, Yarnton, England.])
Tmin, Tmax 0.791, 1.000 0.949, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections 445, 92, 92 2415, 535, 469
Rint 0.016 0.038
(sin θ/λ)max−1) 0.666 0.640
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.016, 0.043, 1.45 0.037, 0.102, 1.05
No. of reflections 92 535
No. of parameters 8 57
Δρmax, Δρmin (e Å−3) 0.21, −0.28 1.41, −0.54
Absolute structure Refined as a perfect inversion twin.
Absolute structure parameter 0.5
Computer programs: CrysAlis PRO (Agilent, 2014[Agilent (2014). CrysAlis PRO. Agilent Technologies, Yarnton, England.]), SHELXS (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), SHELXL2014/7 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]), DIAMOND (Brandenburg, 2006[Brandenburg, K. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Computing details top

For both compounds, data collection: CrysAlis PRO (Agilent, 2014); cell refinement: CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

(cri-PON) Phosphorus oxonitride top
Crystal data top
NOPDx = 2.719 Mg m3
Mr = 60.98Mo Kα radiation, λ = 0.71069 Å
Tetragonal, I42dCell parameters from 431 reflections
a = 4.6135 (2) Åθ = 5.3–28.2°
c = 6.9991 (5) ŵ = 1.24 mm1
V = 148.97 (2) Å3T = 293 K
Z = 4Prism, colourless
F(000) = 1200.02 × 0.02 × 0.02 mm
Data collection top
Three-circle
diffractometer
92 independent reflections
Radiation source: rotating-anode X-ray tube, Rigaku Rotor Flex FR-D92 reflections with I > 2σ(I)
Detector resolution: 16.6 pixels mm-1Rint = 0.016
ω scansθmax = 28.3°, θmin = 5.3°
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2014)
h = 55
Tmin = 0.791, Tmax = 1.000k = 56
445 measured reflectionsl = 59
Refinement top
Refinement on F20 restraints
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0229P)2 + 0.0508P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.016(Δ/σ)max < 0.001
wR(F2) = 0.043Δρmax = 0.21 e Å3
S = 1.45Δρmin = 0.28 e Å3
92 reflectionsAbsolute structure: Refined as a perfect inversion twin.
8 parametersAbsolute structure parameter: 0.5
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refined as a 2-component perfect inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
P0.50000.50000.00000.0106 (3)
N0.3630 (5)0.25000.12500.0155 (5)0.5
O0.3630 (5)0.25000.12500.0155 (5)0.5
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
P0.0112 (4)0.0112 (4)0.0094 (4)0.0000.0000.000
N0.0151 (11)0.0149 (12)0.0165 (9)0.0000.0000.0065 (10)
O0.0151 (11)0.0149 (12)0.0165 (9)0.0000.0000.0065 (10)
Geometric parameters (Å, º) top
P—Oi1.5796 (10)P—Oiii1.5796 (10)
P—Ni1.5796 (10)P—Niii1.5796 (10)
P—Oii1.5796 (10)P—N1.5796 (10)
P—Nii1.5796 (10)N—Piv1.5796 (10)
Oi—P—Ni0.0Ni—P—Niii107.86 (2)
Oi—P—Oii107.86 (2)Oii—P—Niii112.7
Ni—P—Oii107.86 (2)Nii—P—Niii112.73 (5)
Oi—P—Nii107.9Oiii—P—Niii0.0
Ni—P—Nii107.86 (2)Oi—P—N112.7
Oii—P—Nii0.0Ni—P—N112.73 (5)
Oi—P—Oiii107.86 (2)Oii—P—N107.9
Ni—P—Oiii107.86 (2)Nii—P—N107.86 (2)
Oii—P—Oiii112.73 (5)Oiii—P—N107.9
Nii—P—Oiii112.73 (5)Niii—P—N107.86 (2)
Oi—P—Niii107.9P—N—Piv132.83 (16)
Symmetry codes: (i) x+1, y+1, z; (ii) y, x+1, z; (iii) y+1, x, z; (iv) x, y+1/2, z+1/4.
(coe-PON) Phosphorus oxonitride top
Crystal data top
NOPF(000) = 480
Mr = 60.98Dx = 3.219 Mg m3
Monoclinic, C2/cSynchrotron radiation, λ = 0.69428 Å
a = 6.9464 (6) ÅCell parameters from 1202 reflections
b = 12.0340 (4) Åθ = 3.3–26.3°
c = 6.9463 (5) ŵ = 1.35 mm1
β = 119.914 (10)°T = 100 K
V = 503.30 (7) Å3Prism, colourless
Z = 160.02 × 0.02 × 0.02 mm
Data collection top
PILATUS@SNBL
diffractometer
535 independent reflections
Radiation source: Beamline BM1A, SNBL ESRF, Grenoble, France469 reflections with I > 2σ(I)
Detector resolution: 5.8 pixels mm-1Rint = 0.038
φ scansθmax = 26.4°, θmin = 3.3°
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2014)
h = 88
Tmin = 0.949, Tmax = 1.000k = 1515
2415 measured reflectionsl = 88
Refinement top
Refinement on F257 parameters
Least-squares matrix: full0 restraints
R[F2 > 2σ(F2)] = 0.037 w = 1/[σ2(Fo2) + (0.054P)2 + 4.3556P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.102(Δ/σ)max < 0.001
S = 1.05Δρmax = 1.41 e Å3
535 reflectionsΔρmin = 0.54 e Å3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
P10.28266 (16)0.09026 (8)0.04006 (16)0.0067 (4)
P20.31812 (17)0.35749 (7)0.42525 (17)0.0084 (4)
N10.2117 (5)0.4603 (2)0.4818 (6)0.0148 (8)0.5
O10.2117 (5)0.4603 (2)0.4818 (6)0.0148 (8)0.5
N20.25000.25000.50000.0116 (10)0.5
O20.25000.25000.50000.0116 (10)0.5
N30.2322 (6)0.3532 (3)0.1704 (5)0.0188 (8)0.5
O30.2322 (6)0.3532 (3)0.1704 (5)0.0188 (8)0.5
N40.50000.1336 (3)0.25000.0110 (10)0.5
O40.50000.1336 (3)0.25000.0110 (10)0.5
N50.0792 (5)0.1273 (3)0.0656 (6)0.0186 (8)0.5
O50.0792 (5)0.1273 (3)0.0656 (6)0.0186 (8)0.5
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
P10.0054 (7)0.0057 (6)0.0070 (7)0.0003 (3)0.0016 (5)0.0013 (3)
P20.0075 (7)0.0071 (6)0.0095 (7)0.0000 (4)0.0035 (5)0.0011 (4)
N10.0191 (18)0.0047 (15)0.0250 (19)0.0010 (12)0.0142 (16)0.0018 (12)
O10.0191 (18)0.0047 (15)0.0250 (19)0.0010 (12)0.0142 (16)0.0018 (12)
N20.013 (2)0.006 (2)0.016 (2)0.0007 (16)0.008 (2)0.0032 (17)
O20.013 (2)0.006 (2)0.016 (2)0.0007 (16)0.008 (2)0.0032 (17)
N30.028 (2)0.0177 (16)0.0062 (18)0.0054 (14)0.0048 (16)0.0015 (13)
O30.028 (2)0.0177 (16)0.0062 (18)0.0054 (14)0.0048 (16)0.0015 (13)
N40.008 (2)0.009 (2)0.013 (2)0.0000.003 (2)0.000
O40.008 (2)0.009 (2)0.013 (2)0.0000.003 (2)0.000
N50.0034 (17)0.0255 (18)0.0213 (19)0.0014 (13)0.0020 (15)0.0079 (15)
O50.0034 (17)0.0255 (18)0.0213 (19)0.0014 (13)0.0020 (15)0.0079 (15)
Geometric parameters (Å, º) top
P1—O3i1.568 (3)P2—O5iii1.584 (3)
P1—N3i1.568 (3)P2—N5iii1.584 (3)
P1—O1ii1.573 (3)P2—N11.588 (3)
P1—N1ii1.573 (3)N1—P1iv1.574 (3)
P1—N51.574 (3)N2—P2v1.5530 (9)
P1—N41.5755 (17)N3—P1i1.568 (3)
P2—N21.5530 (9)N4—P1vi1.5755 (17)
P2—N31.562 (3)N5—P2vii1.584 (3)
O3i—P1—N3i0.0N2—P2—N3110.10 (13)
O3i—P1—O1ii109.55 (17)N2—P2—O5iii109.69 (13)
N3i—P1—O1ii109.55 (17)N3—P2—O5iii112.0 (2)
O3i—P1—N1ii109.55 (17)N2—P2—N5iii109.69 (13)
N3i—P1—N1ii109.55 (17)N3—P2—N5iii112.0 (2)
O1ii—P1—N1ii0.0O5iii—P2—N5iii0.0
O3i—P1—N5109.7 (2)N2—P2—N1108.03 (12)
N3i—P1—N5109.7 (2)N3—P2—N1110.13 (18)
O1ii—P1—N5111.04 (17)O5iii—P2—N1106.79 (19)
N1ii—P1—N5111.04 (17)N5iii—P2—N1106.79 (19)
O3i—P1—N4107.83 (16)P1iv—N1—P2135.5 (2)
N3i—P1—N4107.83 (16)P2—N2—P2v180.0
O1ii—P1—N4111.09 (19)P2—N3—P1i148.5 (2)
N1ii—P1—N4111.09 (19)P1vi—N4—P1141.3 (3)
N5—P1—N4107.57 (15)P1—N5—P2vii141.3 (2)
Symmetry codes: (i) x+1/2, y+1/2, z; (ii) x+1/2, y1/2, z+1/2; (iii) x+1/2, y+1/2, z+1/2; (iv) x+1/2, y+1/2, z+1/2; (v) x+1/2, y+1/2, z+1; (vi) x+1, y, z+1/2; (vii) x1/2, y+1/2, z1/2.
 

Acknowledgements

We gratefully acknowledge financial support by the Fonds der Chemischen Industrie (FCI) and the Deutsche Forschungsgemeinschaft (DFG) (priority program SPP1236, project SCHN 377–13). ND thanks the German Research Foundation for financial support through the DFG Heisenberg Program. ND and LD gratefully acknowledge the Federal Ministry of Education and Research (BMBF, Germany) for funding.

References

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