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The structure of a new layered oxyfluoride, viz. potassium strontium diniobium hexa­oxide fluoride, KSrNb2O6F, was refined from powder neutron diffraction data in the ortho­rhom­bic space group Immm. The oxyfluoride compound is an n = 2 member of the Dion-Jacobson-type family of general formula A[A'n-1BnX3n+1], which consists of double layered perovskite slabs, [SrNb2O6F]-, between which K+ ions are located. Within the perovskite slabs, the NbO5F octa­hedra are significantly distorted and tilted about the a axis. A bond-valence-sum calculation gives evidence for O/F ordering in KSrNb2O6F, with the F- ions located in the central sites of the corner-sharing NbO5F octa­hedra along the b axis. All atoms lie on special positions, namely Nb on m, Sr on mmm, K on m2m, F on mm2, and O on sites of symmetry m and m2m.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270107029563/fa3094sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108270107029563/fa3094Isup2.rtv
Contains datablock I

Comment top

Dion–Jacobson (DJ)-type compounds with the general formula A[A'n-1BnX3n + 1] have attracted great interest because they have a wide range of properties including ion exchange, intercalation behavior and ion conductivity (Dion et al., 1981). Among many synthetic approaches developed for the preparation of new DJ-phases, the replacement of O2- by F- could be a promising method (Choy et al., 2001). We describe here the crystal structure of a new oxyfluoride compound, KSrNb2O6F, based on neutron diffraction analysis. This oxyfluoride compound was obtained by replacement of O2- and La3+ ions in the well known oxide, KLaNb2O7 (Gopalakrishnan et al., 1987) by F-and Sr2+ ions.

The crystal structure of KSrNb2O6F is closely related to that of KLaNb2O7, which is an n = 2 member of the DJ series. In a previous study (Sato et al., 1992), the structure of KLaNb2O7 was refined in space group C222 with a 3.91 Å, b 21.60 Å and c 3.89 Å. An attempt to fit the neutron diffraction data for KSrNb2O6F using this model was successful only for the major reflections and could not explain some weak reflections, while we found that an orthorhombic unit cell involving doubling of the c axis was adequate to fit the phase. Space group Immm was chosen from the reflection conditions h + k + l = 2n. The starting model was deduced from the structure of KLaNb2O7 with consideration of the cell doubling. The observed, calculated and difference patterns from the Rietveld refinement of the neutron diffraction pattern are shown in Fig. 1. Selected interatomic distances and angles are summarized in Table 1. This refinement was performed on the assumption that the F- ions lie in the central positions of the corner-sharing NbO5F octahedra along the b axis (4j sites) and the O2- ions occupy the other anionic sites. The ordered O/F distribution was confirmed by bond-valence sum (BVS) calculations (Brese & O'Keeffe, 1991). The BVS values for the F- ion (1.09) and the O2- ions (1.93–2.14) are in agreement with the formal charge of both ions within error ranges below 10%. Several models with different O/F distributions were also tested but they gave relatively large deviations of the BVS values from the formal charges for the anions. For example, in the `random O/F distribution' model, the formal charge of each anion site is 1.86 because the O2- and F- ions are assumed to occupy all the sites with the statistical proportion (2×6/7 + 1×1/7). The observed BVS values, however, cover the range 1.12–2.06. Noticeably, the anion at the 4j site exhibits a significantly small BVS value (1.12). The BVS values for KSrNb2O6F deduced from the two models with different O/F distributions are summarized in Table 2.

As shown in Fig. 2, the structure of KSrNb2O6F is composed of two-dimensional double perovksite layers and interlayer K+ ions. The adjacent perovskite layers are stacked along the b axis with a displacement vector of (a+c)/2. The K+ ions are coordinated by six O2- anions to form two short K—O bonds and four long K—O bonds. A similar environment around the K+ ions has been observed in other K-containing compounds, such as KCa2Nb3O10 (Fukuoka et al., 2000). The distortion of the trigonal prismatic coordination of the K+ ions in KSrNb2O6F is attributed to the displacement of the apical O2- ions from their positions in the ideal perovskite structure of the [SrNb2O6F]- moiety. In the perovskite slabs, the NbO5F octahedra are tilted about the a axis, to the left and right alternately (as shown in Fig. 2), giving rise to a diminution of the Nb—F—Nb bond angle. The cooperative tilting of the NbO5F octahedra results in the corrugation of the perovskie slab along the c direction, doubling the c axis, which thus becomes twice as long as a. Recently, the same tilt was reported for BaSrNb2O7 (Le Berre et al., 2004).

The Nb5+ ion is significantly displaced from the center of the NbO5F octahedron, leading to four equatorial Nb—O distances of nearly equal length (1.98–1.99 Å), a short Nb—O bond (1.77 Å) and a long opposite Nb—F bond (2.40 Å). Such distortion, leading to long and short bonds along the b axis, is well known in layered perovskites. However, it is noteworthy that the Nb—F bond in KSrNb2O6F is much longer than the Nb—Ocentral bonds (2.25–2.28 Å) found in other DJ-type oxides, such as KLaNb2O7 and RbLaNb2O7 (Armstrong & Anderson, 1994). The apical Nb—O bond and the equatorial Nb—O bonds in KSrNb2O6F show almost the same distances as those of the oxide analogues. The characteristic distribution of interatomic distances also supports the conclusion that the bridging sites of the corner-sharing NbO5F bioctahedra aligned with the b axis are occupied by F- ions in the structure of KSrNb2O6F.

Related literature top

For related literature, see: Armstrong & Anderson (1994); Brese & O'Keeffe (1991); Choy et al. (2001); Dion et al. (1981); Fukuoka et al. (2000); Gopalakrishnan et al. (1987); Le Berre, Crosnier-Lopez & Fourquet (2004); Rodríguez-Carvajal (1990); Sato et al. (1992); Subramanian et al. (1983).

Experimental top

A polycrystalline sample of KSrNb2O6F was prepared from SrNb2O6 and KF. The precursor SrNb2O6 was prepared by firing the stoichiometric mixture of SrCO3 and Nb2O5 at 1373 K for 2 d. The SrNb2O6 was thoroughly mixed with KF and pressed into pellets in a glove-box. The pellets were placed inside a sealed gold tube. The tube was heated at 2 k min-1 to the reaction temperature of 1143 K, held for 24 h and then cooled at a rate of 2 K min-1. Energy-dispersive X-ray analysis indicated that the ratio of atoms [K:Sr:Nb:O:F = 1.03 (3):1.01 (3):2:5.6 (5):0.97 (3)] matched the nominal composition (1:1:2:6:1) within experimental error.

Refinement top

Structure refinement was carried out by the Rietveld method using the FULLPROF program (Rodríguez-Carvajal, 1990) with pseudo-Voigt peak shapes and refined backgrounds. In the Rietveld refinement, isotropic displacement parameters were used for all atoms. The equatorial O atoms were constrained to have a common displacement parameter. Attempts to refine independent values for these parameters led to model instability, which may result from micro-twinning or stacking faults, often encountered in powder samples with layered structures. The diffraction pattern also includes peaks from a small amount of KNb2O5F (Fd3m, a = 10.58 Å; Subramanian et al., 1983) as an impurity, but the impurity peaks are well separated from those of the KSrNb2O6F phase, at least in the low- and medium-scattering angle region. The total concentration of the impurities was estimated to be below 3%.

Computing details top

Data collection: HANARO HRPD beamline software; cell refinement: FULLPROF (Rodríguez-Carvajal, 1990); data reduction: FULLPROF; program(s) used to solve structure: FULLPROF; program(s) used to refine structure: FULLPROF; molecular graphics: ATOMS; software used to prepare material for publication: FULLPROF.

Figures top
[Figure 1]
[Figure 2]
Figure 1.

A Rietveld refinement plot of KSrNb2O6F. The inset shows the indexation of a selected portion of the diffraction pattern.

Figure 2.

Projections along the a axis (left) and the c axis (right) of the structure of KSrNb2O6F.
potassium strontium diniobium hexaoxide fluoride top
Crystal data top
KSrNb2O6FDx = 4.303 Mg m3
Mr = 427.54Neutron radiation, λ = 1.8371 Å
Orthorhombic, Immmµ = 0.87 mm1
a = 3.8604 (2) ÅT = 298 K
b = 22.220 (1) ÅParticle morphology: plate-like
c = 7.6932 (3) Åwhite
V = 659.91 (5) Å3cylinder, 10 × 10 mm
Z = 4Specimen preparation: Prepared at 1143 K and 101 kPa, cooled at 100 K min1
Data collection top
HANARO high-resolution powder
diffractometer
Data collection mode: transmission
Ge(331) monochromatorScan method: step
Specimen mounting: vanadium can2θmin = 0°°, 2θmax = 160°, 2θstep = 0.05°
Refinement top
Refinement on InetProfile function: pseudo-Voigt
Rp = 0.04939 parameters
Rwp = 0.0630 restraints
Rexp = 0.0320 constraints
RBragg = 0.039Weighting scheme based on measured s.u.'s
χ2 = 3.881(Δ/σ)max < 0.001
3200 data pointsBackground function: polynomial function
Excluded region(s): 2θ < 15°, 2θ > 140°Preferred orientation correction: none
Crystal data top
KSrNb2O6FV = 659.91 (5) Å3
Mr = 427.54Z = 4
Orthorhombic, ImmmNeutron radiation, λ = 1.8371 Å
a = 3.8604 (2) ŵ = 0.87 mm1
b = 22.220 (1) ÅT = 298 K
c = 7.6932 (3) Åcylinder, 10 × 10 mm
Data collection top
HANARO high-resolution powder
diffractometer
Scan method: step
Specimen mounting: vanadium can2θmin = 0°°, 2θmax = 160°, 2θstep = 0.05°
Data collection mode: transmission
Refinement top
Rp = 0.049χ2 = 3.881
Rwp = 0.0633200 data points
Rexp = 0.03239 parameters
RBragg = 0.0390 restraints
Special details top

Experimental. Energy-dispersive X-ray analysis (EDX) was performed using a Jeol JSM-5600 scanning electron microscope fitted with a Be window detector (Oxford Instruments).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
K10.50000.2630 (3)0.50000.0094 (12)*
Sr10.50000.50000.50000.0102 (6)*
Sr20.50000.50000.00000.0102 (6)*
Nb10.00000.39227 (9)0.2542 (6)0.0054 (4)*
F10.00000.50000.2355 (8)0.0078 (7)*
O10.50000.41316 (10)0.2496 (9)0.0072 (3)*
O20.00000.4100 (3)0.00000.0072 (3)*
O30.00000.4191 (3)0.50000.0072 (3)*
O40.00000.31290 (13)0.2705 (6)0.0138 (6)*
Geometric parameters (Å, º) top
K1—O4i2.679 (6)Sr2—F12.647 (4)
K1—O42.841 (4)Nb1—O11.9854 (7)
Sr1—O12.722 (5)Nb1—O21.995 (5)
Sr1—O32.638 (5)Nb1—O31.983 (5)
Sr1—F12.805 (4)Nb1—O41.768 (4)
Sr2—O12.727 (5)Nb1—F12.398 (2)
Sr2—O22.779 (5)
O4i—K1—O4ii102.0 (2)O4—Nb1—O1103.56 (9)
O4i—K1—O476.29 (10)O3—Nb1—O187.0 (2)
O4ii—K1—O4136.76 (10)O1—Nb1—O1xii152.88 (15)
O4iii—K1—O4134.1 (3)O4—Nb1—O2105.5 (3)
O4—K1—O4iv76.84 (13)O3—Nb1—O2151.1 (3)
O4—K1—O4v85.59 (13)O1—Nb1—O286.4 (2)
O3—Sr1—O3vi85.93 (15)O4—Nb1—F1179.4 (3)
O3—Sr1—O3v94.07 (15)O3—Nb1—F175.9 (3)
O3—Sr1—O161.16 (9)O1—Nb1—F176.44 (8)
O3—Sr1—O1vii118.84 (9)O2—Nb1—F175.2 (3)
O1—Sr1—O1viii90.10 (15)Nb1viii—F1—Nb1173.1 (3)
O1—Sr1—O1iii89.91 (15)Nb1—F1—Sr292.35 (12)
O3—Sr1—F159.76 (8)Sr2xii—F1—Sr293.63 (14)
O1—Sr1—F159.16 (8)Nb1—F1—Sr187.51 (14)
O1—Sr1—F1vi120.84 (8)Sr2—F1—Sr189.70 (1)
F1—Sr1—F1vi93.02 (12)Sr2—F1—Sr1xii176.68 (18)
O3—Sr1—F1v120.24 (8)Sr1—F1—Sr1xii86.98 (12)
F1—Sr1—F1v86.98 (12)Nb1—O1—Nb1v152.88 (15)
F1v—Sr2—F193.63 (14)Nb1—O1—Sr2100.27 (19)
F1ix—Sr2—F186.37 (14)Nb1—O1—Sr198.8 (2)
F1—Sr2—O1x118.87 (8)Sr2—O1—Sr189.81 (7)
F1—Sr2—O161.14 (8)Nb1xiii—O2—Nb1157.2 (4)
O1viii—Sr2—O190.28 (15)Nb1—O2—Sr298.17 (13)
O1x—Sr2—O189.72 (15)Sr2—O2—Sr2xii87.97 (13)
F1—Sr2—O2xi120.42 (8)Nb1—O3—Nb1iv145.0 (4)
O1—Sr2—O2xi120.67 (8)Nb1—O3—Sr1101.83 (11)
F1—Sr2—O259.58 (8)Sr1—O3—Sr1xii94.1 (2)
O1—Sr2—O259.34 (8)Nb1—O4—K1i125 (4)
O2—Sr2—O2ix92.03 (13)Nb1—O4—K1115.7 (2)
O2—Sr2—O2v87.97 (13)K1i—O4—K1103.71 (15)
O4—Nb1—O3103.4 (3)K1—O4—K1xii85.59 (13)
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x+1, y, z+1; (iv) x, y, z+1; (v) x+1, y, z; (vi) x, y+1, z+1; (vii) x+1, y+1, z+1; (viii) x, y+1, z; (ix) x, y+1, z; (x) x+1, y, z; (xi) x+1, y+1, z; (xii) x1, y, z; (xiii) x, y, z.

Experimental details

Crystal data
Chemical formulaKSrNb2O6F
Mr427.54
Crystal system, space groupOrthorhombic, Immm
Temperature (K)298
a, b, c (Å)3.8604 (2), 22.220 (1), 7.6932 (3)
V3)659.91 (5)
Z4
Radiation typeNeutron, λ = 1.8371 Å
µ (mm1)0.87
Specimen shape, size (mm)Cylinder, 10 × 10
Data collection
DiffractometerHANARO high-resolution powder
diffractometer
Specimen mountingVanadium can
Data collection modeTransmission
Scan methodStep
2θ values (°)2θmin = 0° 2θmax = 160 2θstep = 0.05
Refinement
R factors and goodness of fitRp = 0.049, Rwp = 0.063, Rexp = 0.032, RBragg = 0.039, χ2 = 3.881
No. of data points3200
No. of parameters39

Computer programs: HANARO HRPD beamline software, FULLPROF (Rodríguez-Carvajal, 1990), FULLPROF, ATOMS.

Selected geometric parameters (Å, º) top
K1—O4i2.679 (6)Sr2—F12.647 (4)
K1—O42.841 (4)Nb1—O11.9854 (7)
Sr1—O12.722 (5)Nb1—O21.995 (5)
Sr1—O32.638 (5)Nb1—O31.983 (5)
Sr1—F12.805 (4)Nb1—O41.768 (4)
Sr2—O12.727 (5)Nb1—F12.398 (2)
Sr2—O22.779 (5)
Nb1ii—F1—Nb1173.1 (3)Nb1iv—O2—Nb1157.2 (4)
Nb1—O1—Nb1iii152.88 (15)Nb1—O3—Nb1v145.0 (4)
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x, y+1, z; (iii) x+1, y, z; (iv) x, y, z; (v) x, y, z+1.
Bond-valence sums (V*) and the formal charges (V) for KSrNb2O6F depending on O/F distribution top
ordereddisordered
siteatomV*VatomV*V
4gK11.041K10.991
2aSr12.242Sr12.182
2cSr22.172Sr22.202
8lNb14.975Nb14.905
8lO12.0226/7O +1/7F1.991.86
4gO21.9326/7O +1/7F1.921.86
4hO32.1426/7O +1/7F2.061.86
8lO41.9926/7O +1/7F1.951.86
4jF11.0916/7O +1/7F1.121.86
 

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