inorganic papers X-ray powder diffraction study of LiCrP2O7
Acta Crystallographica Section E
Structure Reports Online ISSN 1600-5368
Ludmila S. Ivashkevich,* Kirill A. Selevich, Anatoly I. Lesnikovich and Anatoly F. Selevich Research Institute for Physico-Chemical Problems, Belarusian State University, Leningradskaya Street 14, Minsk 220030, Belarus Correspondence e-mail:
[email protected]
Key indicators
The monoclinic crystal structure of lithium chromium(III) diphosphate, LiCrP2O7, isotypic with other members of the series LiMIIIP2O7 (MIII = Mn, Fe, V, Mo, Sc and In), was refined from laboratory X-ray powder diffraction data using the Rietveld method. The Cr3+ cation is bonded to six O atoms from five diphosphate anions to form a distorted octahedron. Links between the bent diphosphate anions and the Cr3+ cations result in a three-dimensional network, with tunnels filled by the Li+ cations in a considerably distorted tetrahedral environment of O atoms.
Received 31 January 2007 Accepted 2 February 2007
Comment
Powder X-ray study T = 295 K ˚ Mean (P–O) = 0.015 A R factor = 0.029 wR factor = 0.038 Data-to-parameter ratio = 6.02 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e.
Double diphosphates LiMIIIP2O7 (MIII is a trivalent metal) are of interest because of their ionic conductivity at high temperatures (Vıtin sˇ et al., 2000). The crystal structures of LiMIIIP2O7 compounds have already been described for MIII = In (Tran Qui et al., 1987), Fe (Riou et al., 1990), Mo (Ledain et al., 1996), Sc (Vıtin sˇ et al., 2000), V (Rousse et al., 2001) and Mn (Ivashkevich et al., 2006). All these compounds crystallize in the monoclinic space group P21 and are isotypic. Here, we report the crystal structure of the CrIII member of this series, LiCrP2O7, (I), refined from laboratory X-ray powder diffraction data. The asymmetric unit of (I) comprises one Li, one Cr, two P and seven O atoms, all in general positions. The conformation of the diphosphate anion is close to eclipsed and exhibits a bent P—O—P angle of 127.2 (5) . The average of the dihedral angles O1—P1 P2—O4, O2—P1 P2—O6 and O3— P1 P2—O5 is 9.1 (Figs. 1 and 2). All other distances and angles are in the usual ranges observed for many other diphosphates (Durif, 1995).
Figure 1 # 2007 International Union of Crystallography All rights reserved
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The conformation of the diphosphate anion in (I), viewed approximately perpendicular to the P1/O7/P2 plane. Displacement ellipsoids are drawn at the 50% probability level. doi:10.1107/S1600536807005752
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inorganic papers
Figure 2 The conformation of the diphosphate anion in (I), viewed approximately along the P1 P2 backbone. Displacement ellipsoids are drawn at the 50% probability level.
Figure 4 Final plots of the Rietveld refinement, showing the experimental (circles) and calculated (line) intensities of (I) in the range 2 = 12–72 . The difference plot appears below. Vertical markers refer to the calculated positions of the Bragg reflections.
shows that the geometric features of the diphosphate anions (terminal and bridging P—O bond lengths, P—O—P and O— P—O angles, and torsion angles) and of the LiO4 polyhedron are quite similar in all structures. In the isotypic series, there is ˚ 3) in the a monotonic increase of the unit-cell volume (A III following sequence of M salts: 249.32 (Cr), 254.77 (Mn), 255.22 (Fe), 256.02 (V), 272.85 (Mo), 274.05 (Sc) and 274.28 (In), which is in accordance with the increase of the ionic radii (Shannon, 1976) from Cr3+ to In3+, provided that Mn3+ and Fe3+ are in their high-spin state (otherwise LiMnP2O7 and LiFeP2O7 would have the smallest cell volumes). The assumption of the presence of Fe3+ high-spin cations in LiFeP2O7 has been confirmed by magnetic measurements (Whangbo et al., 2004).
Experimental
Figure 3 The crystal structure of (I), viewed along the c axis. The atom-numbering scheme is shown for the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level.
The Cr atoms have a distorted octahedral environment of O atoms from five diphosphate groups, with an average Cr—O ˚ . The Cr3+ cations connect the diphosphate distance of 1.973 A anions to form a three-dimensional network, with channels filled by Li+ cations (Fig. 3). The Li+ cations have a strongly distorted tetrahedral environment of four O atoms from four diphosphate anions (Table 1), with one O—Li—O angle of 175.8 (12) significantly different from the ideal tetrahedral angle. A detailed comparison of (I) with the isotypic structures of the LiMIIIP2O7 diphosphates (MIII = Mn, Fe, V, Mo, Sc, In) Acta Cryst. (2007). E63, i70–i72
The title compound was prepared by the technique described previously by Selevich et al. (2006). A mixture of CrCl36H2O (2.40 g), Li2CO3 (0.33 g) and (NH4)2HPO4 (2.38 g) was placed in a platinum crucible and heated to 1173 K at a rate of 5 K min1. After cooling to room temperature, washing with water and drying at 373 K, a colourless microcrystalline solid was obtained. The compound was further identified by chemical analysis and quantitative thin-layer chromatography. Crystal data LiCrP2O7 Mr = 232.88 Monoclinic, P21 ˚ a = 6.9019 (10) A ˚ b = 7.9919 (11) A ˚ c = 4.7807 (7) A = 109.003 (4) ˚3 V = 249.32 (6) A Z=2
Dx = 3.102 Mg m3 Cu K radiation ˚ = 1.5418 A T = 295 (2) K Specimen shape: flat sheet 30 30 1 mm Particle morphology: no specific habit,, colourless
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inorganic papers Data collection Carl Zeiss HZG-4A diffractometer Specimen mounting: packed powder pellet Specimen mounted in reflection mode
Scan method: step 2min = 12.0 , 2max = 100 Increment in 2 = 0.02 Excluded region(s): none
Refinement 307 reflections 51 parameters w = 1/(Yobs) (/)max = 0.02 Preferred orientation correction: none
Refinement on Inet Rp = 0.029 Rwp = 0.038 Rexp = 0.101 RB = 0.064 S = 0.35 Profile function: pseudo-Voigt profile function, = 0.61(6)
Table 1 ˚ , ). Selected geometric parameters (A Cr—O1 Cr—O4 Cr—O3i Cr—O6ii Cr—O5iii Cr—O2iv P1—O1 P1—O2 P1—O3
1.99 (2) 1.996 (19) 1.91 (3) 2.040 (19) 1.980 (16) 1.924 (11) 1.529 (17) 1.542 (11) 1.53 (2)
P1—O7 P2—O4 P2—O5 P2—O6 P2—O7 Li—O2 Li—O4v Li—O6iv Li—O5vi
1.629 (12) 1.487 (8) 1.55 (2) 1.499 (17) 1.611 (12) 2.102 (12) 2.049 (11) 1.912 (16) 1.966 (17)
O1—P1—O2 O1—P1—O3 O1—P1—O7 O2—P1—O3 O2—P1—O7 O3—P1—O7 O4—P2—O5 O4—P2—O6 O4—P2—O7
110.8 112.8 107.6 115.2 101.4 108.1 110.1 114.1 108.1
O5—P2—O6 O5—P2—O7 O6—P2—O7 O2—Li—O4v O2—Li—O5vi O2—Li—O6iv O4v—Li—O5vi O4v—Li—O6iv O5vi—Li—O6iv
113.5 111.0 99.5 92.6 76.9 175.8 107.7 83.3 104.0
(12) (12) (9) (12) (6) (11) (8) (8) (8)
(7) (9) (7) (8) (12) (12) (11) (10) (8)
Symmetry codes: (i) x; y; z 1; (ii) x; y 12; z; (iii) x; y 12; z þ 1; (iv) x þ 1; y 12; z þ 1; (v) x þ 1; y; z þ 1; (vi) x þ 1; y; z.
The diffraction pattern of (I) could be indexed with a monoclinic cell and reasonable reliability factors (M20 = 43, F20 = 54, M40 = 26, F40 = 35) using the program TREOR90 (Werner et al., 1985). The unit-cell dimensions obtained indicated isotypism with compounds of the type LiMIIIP2O7 that crystallize in the monoclinic space group P21. Therefore, this space group and the atomic coordinates of LiMnP2O7
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(Ivashkevich et al., 2006) were used as starting parameters for the refinement of the structure using the Rietveld method as implemented in the FULLPROF program (Rodrı´guez-Carvajal, 2001). Because space group P21 is polar, the y coordinate of the Cr atom was fixed. A correction for profile asymmetry was made for reflections up to 2 = 40 . In view of an unstable refinement of the displacement parameters of some atoms, they were fixed with values similar to those for other isotypic compounds [Uiso(Cr, P) = 0.005, Uiso(O) = ˚ 2]. No preferred orientation of grains in 0.0095 and Uiso(Li) = 0.025 A the sample was found. During structure refinement, soft restraints for the interatomic distances of the diphosphate group based on a geometric analysis of a large number of diphosphates (Durif, 1995), and also for Li—O distances, were used. Fig. 4 shows the observed and calculated diffraction patterns of (I) for the final refinement. Data collection: local program; cell refinement: FULLPROF (Rodrı´guez-Carvajal, 2001); data reduction: local program; method used to solve structure: coordinates taken from an isotypic compound (Ivashkevich et al., 2006); program(s) used to refine structure: FULLPROF; molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: FULLPROF and PLATON (Spek, 2003).
References Durif, A. (1995). Crystal Chemistry of Condensed Phosphates, pp. 73–77. New York: Plenum Press. Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. Ivashkevich, L. S., Selevich, K. A., Lesnikovich, A. I., Selevich, A. F. & Lyakhov, A. S. (2006). Z. Kristallogr. 221, 115–121. Ledain, S., Leclaire, A., Borel, M. M. & Raveau, B. (1996). Acta Cryst. C52, 1593–1594. Riou, D., Nguyen, N., Benloucif, R. & Raveau, B. (1990). Mater. Res. Bull. 25, 1363–1369. Rodrı´guez-Carvajal, J. (2001). FULLPROF. Version 1.9c. CEA/Saclay, France. Rousse, G., Wurm, C., Morcrette, M., Rodriguez-Carvajal, J., Gaubicher, J. & Masquelier, C. (2001). Int. J. Inorg. Mater. 3, 881–887. Selevich, K. A., Lesnikovich, A. I., Grushevich, E. V. & Selevich, A. F. (2006). Belarussian Patent Appl. A20061215. Shannon, R. D. (1976). Acta Cryst. A32, 751–767. Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13. Tran Qui, D., Hamdoune, S. & Le Page, Y. (1987). Acta Cryst. C43, 201–202. Vıtin sˇ, G., Kanepe, Z., Vıtin sˇ, A., Ronis, J., Dindu ne, A. & Lu sis, A. (2000). J. Solid State Electrochem. 4, 146–152. Werner, P.-E., Eriksson, L. & Westdahl, M. (1985). J. Appl. Cryst. 18, 367–370. Whangbo, M.-H., Dai, D. & Koo, H.-J. (2004). J. Chem. Soc. Dalton Trans. pp. 3019–3025.
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