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
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.
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%.
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.
potassium strontium diniobium hexaoxide fluoride
top
Crystal data top
KSrNb2O6F | Dx = 4.303 Mg m−3 |
Mr = 427.54 | Neutron radiation, λ = 1.8371 Å |
Orthorhombic, Immm | µ = 0.87 mm−1 |
a = 3.8604 (2) Å | T = 298 K |
b = 22.220 (1) Å | Particle morphology: plate-like |
c = 7.6932 (3) Å | white |
V = 659.91 (5) Å3 | cylinder, 10 × 10 mm |
Z = 4 | Specimen preparation: Prepared at 1143 K and 101 kPa, cooled at 100 K min−1 |
Data collection top
HANARO high-resolution powder diffractometer | Data collection mode: transmission |
Ge(331) monochromator | Scan method: step |
Specimen mounting: vanadium can | 2θmin = 0°°, 2θmax = 160°, 2θstep = 0.05° |
Refinement top
Refinement on Inet | Profile function: pseudo-Voigt |
Rp = 0.049 | 39 parameters |
Rwp = 0.063 | 0 restraints |
Rexp = 0.032 | 0 constraints |
RBragg = 0.039 | Weighting scheme based on measured s.u.'s |
χ2 = 3.881 | (Δ/σ)max < 0.001 |
3200 data points | Background function: polynomial function |
Excluded region(s): 2θ < 15°, 2θ > 140° | Preferred orientation correction: none |
Crystal data top
KSrNb2O6F | V = 659.91 (5) Å3 |
Mr = 427.54 | Z = 4 |
Orthorhombic, Immm | Neutron radiation, λ = 1.8371 Å |
a = 3.8604 (2) Å | µ = 0.87 mm−1 |
b = 22.220 (1) Å | T = 298 K |
c = 7.6932 (3) Å | cylinder, 10 × 10 mm |
Data collection top
HANARO high-resolution powder diffractometer | Scan method: step |
Specimen mounting: vanadium can | 2θmin = 0°°, 2θmax = 160°, 2θstep = 0.05° |
Data collection mode: transmission | |
Refinement top
Rp = 0.049 | χ2 = 3.881 |
Rwp = 0.063 | 3200 data points |
Rexp = 0.032 | 39 parameters |
RBragg = 0.039 | 0 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 | x | y | z | Uiso*/Ueq | |
K1 | 0.5000 | 0.2630 (3) | 0.5000 | 0.0094 (12)* | |
Sr1 | 0.5000 | 0.5000 | 0.5000 | 0.0102 (6)* | |
Sr2 | 0.5000 | 0.5000 | 0.0000 | 0.0102 (6)* | |
Nb1 | 0.0000 | 0.39227 (9) | 0.2542 (6) | 0.0054 (4)* | |
F1 | 0.0000 | 0.5000 | 0.2355 (8) | 0.0078 (7)* | |
O1 | 0.5000 | 0.41316 (10) | 0.2496 (9) | 0.0072 (3)* | |
O2 | 0.0000 | 0.4100 (3) | 0.0000 | 0.0072 (3)* | |
O3 | 0.0000 | 0.4191 (3) | 0.5000 | 0.0072 (3)* | |
O4 | 0.0000 | 0.31290 (13) | 0.2705 (6) | 0.0138 (6)* | |
Geometric parameters (Å, º) top
K1—O4i | 2.679 (6) | Sr2—F1 | 2.647 (4) |
K1—O4 | 2.841 (4) | Nb1—O1 | 1.9854 (7) |
Sr1—O1 | 2.722 (5) | Nb1—O2 | 1.995 (5) |
Sr1—O3 | 2.638 (5) | Nb1—O3 | 1.983 (5) |
Sr1—F1 | 2.805 (4) | Nb1—O4 | 1.768 (4) |
Sr2—O1 | 2.727 (5) | Nb1—F1 | 2.398 (2) |
Sr2—O2 | 2.779 (5) | | |
| | | |
O4i—K1—O4ii | 102.0 (2) | O4—Nb1—O1 | 103.56 (9) |
O4i—K1—O4 | 76.29 (10) | O3—Nb1—O1 | 87.0 (2) |
O4ii—K1—O4 | 136.76 (10) | O1—Nb1—O1xii | 152.88 (15) |
O4iii—K1—O4 | 134.1 (3) | O4—Nb1—O2 | 105.5 (3) |
O4—K1—O4iv | 76.84 (13) | O3—Nb1—O2 | 151.1 (3) |
O4—K1—O4v | 85.59 (13) | O1—Nb1—O2 | 86.4 (2) |
O3—Sr1—O3vi | 85.93 (15) | O4—Nb1—F1 | 179.4 (3) |
O3—Sr1—O3v | 94.07 (15) | O3—Nb1—F1 | 75.9 (3) |
O3—Sr1—O1 | 61.16 (9) | O1—Nb1—F1 | 76.44 (8) |
O3—Sr1—O1vii | 118.84 (9) | O2—Nb1—F1 | 75.2 (3) |
O1—Sr1—O1viii | 90.10 (15) | Nb1viii—F1—Nb1 | 173.1 (3) |
O1—Sr1—O1iii | 89.91 (15) | Nb1—F1—Sr2 | 92.35 (12) |
O3—Sr1—F1 | 59.76 (8) | Sr2xii—F1—Sr2 | 93.63 (14) |
O1—Sr1—F1 | 59.16 (8) | Nb1—F1—Sr1 | 87.51 (14) |
O1—Sr1—F1vi | 120.84 (8) | Sr2—F1—Sr1 | 89.70 (1) |
F1—Sr1—F1vi | 93.02 (12) | Sr2—F1—Sr1xii | 176.68 (18) |
O3—Sr1—F1v | 120.24 (8) | Sr1—F1—Sr1xii | 86.98 (12) |
F1—Sr1—F1v | 86.98 (12) | Nb1—O1—Nb1v | 152.88 (15) |
F1v—Sr2—F1 | 93.63 (14) | Nb1—O1—Sr2 | 100.27 (19) |
F1ix—Sr2—F1 | 86.37 (14) | Nb1—O1—Sr1 | 98.8 (2) |
F1—Sr2—O1x | 118.87 (8) | Sr2—O1—Sr1 | 89.81 (7) |
F1—Sr2—O1 | 61.14 (8) | Nb1xiii—O2—Nb1 | 157.2 (4) |
O1viii—Sr2—O1 | 90.28 (15) | Nb1—O2—Sr2 | 98.17 (13) |
O1x—Sr2—O1 | 89.72 (15) | Sr2—O2—Sr2xii | 87.97 (13) |
F1—Sr2—O2xi | 120.42 (8) | Nb1—O3—Nb1iv | 145.0 (4) |
O1—Sr2—O2xi | 120.67 (8) | Nb1—O3—Sr1 | 101.83 (11) |
F1—Sr2—O2 | 59.58 (8) | Sr1—O3—Sr1xii | 94.1 (2) |
O1—Sr2—O2 | 59.34 (8) | Nb1—O4—K1i | 125 (4) |
O2—Sr2—O2ix | 92.03 (13) | Nb1—O4—K1 | 115.7 (2) |
O2—Sr2—O2v | 87.97 (13) | K1i—O4—K1 | 103.71 (15) |
O4—Nb1—O3 | 103.4 (3) | K1—O4—K1xii | 85.59 (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) x−1, y, z; (xiii) −x, y, −z. |
Experimental details
Crystal data |
Chemical formula | KSrNb2O6F |
Mr | 427.54 |
Crystal system, space group | Orthorhombic, Immm |
Temperature (K) | 298 |
a, b, c (Å) | 3.8604 (2), 22.220 (1), 7.6932 (3) |
V (Å3) | 659.91 (5) |
Z | 4 |
Radiation type | Neutron, λ = 1.8371 Å |
µ (mm−1) | 0.87 |
Specimen shape, size (mm) | Cylinder, 10 × 10 |
|
Data collection |
Diffractometer | HANARO high-resolution powder diffractometer |
Specimen mounting | Vanadium can |
Data collection mode | Transmission |
Scan method | Step |
2θ values (°) | 2θmin = 0° 2θmax = 160 2θstep = 0.05 |
|
Refinement |
R factors and goodness of fit | Rp = 0.049, Rwp = 0.063, Rexp = 0.032, RBragg = 0.039, χ2 = 3.881 |
No. of data points | 3200 |
No. of parameters | 39 |
Selected geometric parameters (Å, º) topK1—O4i | 2.679 (6) | Sr2—F1 | 2.647 (4) |
K1—O4 | 2.841 (4) | Nb1—O1 | 1.9854 (7) |
Sr1—O1 | 2.722 (5) | Nb1—O2 | 1.995 (5) |
Sr1—O3 | 2.638 (5) | Nb1—O3 | 1.983 (5) |
Sr1—F1 | 2.805 (4) | Nb1—O4 | 1.768 (4) |
Sr2—O1 | 2.727 (5) | Nb1—F1 | 2.398 (2) |
Sr2—O2 | 2.779 (5) | | |
| | | |
Nb1ii—F1—Nb1 | 173.1 (3) | Nb1iv—O2—Nb1 | 157.2 (4) |
Nb1—O1—Nb1iii | 152.88 (15) | Nb1—O3—Nb1v | 145.0 (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 | | ordered | | | disordered | |
site | atom | V* | V | atom | V* | V |
4g | K1 | 1.04 | 1 | K1 | 0.99 | 1 |
2a | Sr1 | 2.24 | 2 | Sr1 | 2.18 | 2 |
2c | Sr2 | 2.17 | 2 | Sr2 | 2.20 | 2 |
8l | Nb1 | 4.97 | 5 | Nb1 | 4.90 | 5 |
8l | O1 | 2.02 | 2 | 6/7O +1/7F | 1.99 | 1.86 |
4g | O2 | 1.93 | 2 | 6/7O +1/7F | 1.92 | 1.86 |
4h | O3 | 2.14 | 2 | 6/7O +1/7F | 2.06 | 1.86 |
8l | O4 | 1.99 | 2 | 6/7O +1/7F | 1.95 | 1.86 |
4j | F1 | 1.09 | 1 | 6/7O +1/7F | 1.12 | 1.86 |
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.