Acta Cryst. (2007). E63, i203-i204 [ doi:10.1107/S1600536807060436 ]
Potassium calcium diniobium hexaoxide fluoride, KCaNb2O6F, crystallizes in the cubic pyrochlore-type structure. In the crystal structure, NbO6 octahedra are linked together by common corners to form a three-dimensional [Nb2O6]2- network. (K,Ca) and F atoms, which are located in the voids of the [Nb2O6]2- network, form zigzag [(K,Ca)F]2+ chains. The K and Ca atoms are distributed statistically over the same crystallographic position (site-occupancy factor = 0.5), having an eightfold coordination by six O atoms from the NbO6 octahedra and by two F atoms. All atoms lie on special positions, viz. Nb has 
m, the (K,Ca) site has m, O has 2mm and F has 
3m site symmetry.
KCaNb2O6F was prepared using a solid state reaction between KF and CaNb2O6. CaNb2O6 was prepared by firing a stoichiometric mixture of CaCO3 and Nb2O5 at 1373 K for 2 d with intermediate grinding. The resulting CaNb2O6 was then thoroughly mixed with KF and pressed into pellets in a glovebox under an anhydrous Ar atmosphere. The pellets were placed inside a sealed gold tube and heated at 1023 K for 12 h, before cooled to room temperature at a rate of 2 K min−1. The composition of the product was confirmed with energy-dispersive X-ray analysis (Jeol JSM-5600 scanning electron microscope fitted with a Be window detector, Oxford Instruments). The synchrotron X-ray powder diffraction (SXPD) measurement was performed on beamline 8 C2-HRPD at Pohang Accelerator Laboratory, Pohang, Korea. The incident X-rays were vertically collimated by a mirror, and monochromated to the wavelength of 1.5422 Å by a double-crystal Si(111) monochromator. A dataset was collected in the range of 10°≤2θ≤130° with a step size of 0.01° (2θ angle). The powder neutron diffraction (PND) data were collected on the high-resolution powder diffractometer with a 32 He-3 multi-detector system and a Ge(331) monochromator, installed at the Korea Atomic Energy Research Institute, Daejeon.
Measured reflections were indexed with DICVOL (Boultif & Louër, 2004) and the cubic symmetry was confirmed from both SXRD and NPD data. Additional peaks due to symmetry lowering or impurity phase were not detected. The figures of merit were M(20) = 49.7, F(20) = 24.0 (0.0083, 100) for NPD and M(20) = 55.1, F(20) = 25.5 (0.0050, 158) for SXRD measurements. Systematic absences suggested two possible cubic space groups, viz. centrosymmetric Fd3m (No. 227) and non-centrosymmetric Fd3 (No. 203). Both space groups turned out later to give basically the same structure solution. Thus, the higher symmetric Fd3m was chosen. The positions of the (K/Ca) and Nb atoms were determined employing direct methods using the SXRD data, for which a total of 500 'Fobs' amplitude factors were converted into structure factors and used as an input for SHELXS97 (Sheldrick, 1997). The positions of anions were then determined by difference Fourier analyses of both SXRD and PND data. After this step, the anions were removed from the refinement and the residual density was calculated. From the difference Fourier maps it was suggested that all atomic positions are identical with those of the classical pyrochlore structure: 16d for (K, Ca), 16c for Nb, 48f for O and 8b for the F atom. The anion positions were confirmed from crystal chemical considerations and from BVS calculations (Brese & O'Keeffe, 1991). Structure refinements were carried out by the Rietveld method using Fullprof (Rodríguez-Carvajal, 2001) with pseudo-Voigt peak shapes and refined backgrounds. Refinement of atomic positions and isotropic displacement parameters gave the goodness of fit, S = 2.38. The refinements of the site occupation factors (SOFs) led to 0.502 (1), 0.502 (1) and 1.03 (2) for K, Ca and F atoms, respectively, which were in good agreement with the nominal composition. In the final step, the SOFs were fixed to the ideal values and the anisotropic thermal displacement factors were refined for all atoms, with constraints for the K and Ca atoms. The structure was refined with origin choice 2 of space group Fd3m. The refinement plots for SXRD and NPD data are shown Fig. 2.
Data collection: HANARO HRPD beamline software; cell refinement: FULLPROF (Rodríguez-Carvajal, 2001); data reduction: FULLPROF (Rodríguez-Carvajal, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: FULLPROF (Rodríguez-Carvajal, 2001); molecular graphics: DIAMOND (Bergerhoff, 1996); software used to prepare material for publication: FULLPROF (Rodríguez-Carvajal, 2001).
![[Figure 1]](./wm2150fig1thm.gif)
| Fig. 1. The local structure around the (K, Ca) site (a), and a schematic view projected along [101] (b). Octahedra represent NbO6, large and small ellipsoids represent F and (K, Ca) atoms, respectively. |
![[Figure 2]](./wm2150fig2thm.gif)
| Fig. 2. Rietveld refinement plots of KCaNb2O6F based on synchrotron X-ray (a) and neutron (b) powder diffraction data. |
| KCaNb2O6F | Z = 8 |
| Mr = 380 | Dx = 4.294 Mg m−3 |
| Cubic, Fd3m | Neutron radiation λ = 1.83480 Å |
| Hall symbol: -F 4vw 2vw 3 | µ = 0.09 mm−1 |
| a = 10.55376 (12) Å | T = 298 K |
| b = 10.55376 (12) Å | Specimen shape: cylinder |
| c = 10.55376 (12) Å | 10 × 10 × 30 mm |
| α = 90º | Specimen prepared at 101 kPa |
| β = 90º | Specimen prepared at 1023 K |
| γ = 90º | Particle morphology: particle, white |
| V = 1175.50 (2) Å3 |
| HANARO high-resolution powder diffractometer | T = 298 K |
| Radiation source: neutron | P = 101 kPa |
| Monochromator: Ge(331) | Absorption correction: for a cylinder mounted on the φ axis (Rodríguez-Carvajal, 1990) |
| Specimen mounting: vanadium can | Tmin = ?, Tmax = ? |
| Specimen mounted in transmission mode | 2θmin = 0.000, 2θmax = 160.00º |
| Scan method: step | Increment in 2θ = 0.05º |
| Refinement on Inet | Excluded region(s): 2θ < 15°, 2θ > 140° |
| Rp = 4.08 | Profile function: pseudo-Voigt |
| Rwp = 5.64 | 25 parameters |
| Rexp = 3.58 | Weighting scheme based on measured s.u.'s ? |
| RB = 2.53 | (Δ/σ)max < 0.001 |
| S = 1.57 | Extinction correction: none |
| Wavelength of incident radiation: 1.83480 Å | Preferred orientation correction: none |
| KCaNb2O6F | Z = 8 |
| Mr = 380 | Neutron radiation λ = 1.83480 Å |
| Cubic, Fd3m | µ = 0.09 mm−1 |
| a = 10.55376 (12) Å | T = 298 K |
| b = 10.55376 (12) Å | Specimen shape: cylinder |
| c = 10.55376 (12) Å | 10 × 10 × 30 mm |
| α = 90º | Specimen prepared at 101 kPa |
| β = 90º | Specimen prepared at 1023 K |
| γ = 90º | Particle morphology: particle, white |
| V = 1175.50 (2) Å3 |
| HANARO high-resolution powder diffractometer | Absorption correction: for a cylinder mounted on the φ axis (Rodríguez-Carvajal, 1990) |
| Specimen mounting: vanadium can | Tmin = ?, Tmax = ? |
| Specimen mounted in transmission mode | 2θmin = 0.000, 2θmax = 160.00º |
| Scan method: step | Increment in 2θ = 0.05º |
| Rp = 4.08 | Wavelength of incident radiation: 1.83480 Å |
| Rwp = 5.64 | Excluded region(s): 2θ < 15°, 2θ > 140° |
| Rexp = 3.58 | Profile function: pseudo-Voigt |
| RB = 2.53 | 25 parameters |
| S = 1.57 | Preferred orientation correction: none |
| x | y | z | Uiso*/Ueq | Occ. (<1) | |
| K | 0.25000 | 0.25000 | 0.50000 | 0.0180 (7) | 0.50000 |
| Ca | 0.25000 | 0.25000 | 0.50000 | 0.0180 (7) | 0.50000 |
| Nb | 0.50000 | 0.00000 | 0.50000 | 0.0124 (3) | |
| O | 0.31488 (7) | 0.12500 | 0.12500 | 0.0118 (4) | |
| F | 0.37500 | 0.37500 | 0.37500 | 0.0399 (8) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| K | 0.0180 (7) | 0.0180 (7) | 0.0180 (7) | −0.0013 (5) | −0.0013 (5) | −0.0013 (5) |
| Ca | 0.0180 (7) | 0.0180 (7) | 0.0180 (7) | −0.0013 (5) | −0.0013 (5) | −0.0013 (5) |
| Nb | 0.0124 (3) | 0.0124 (3) | 0.0124 (3) | −0.0002 (4) | −0.0002 (4) | −0.0002 (4) |
| O | 0.0128 (4) | 0.0114 (3) | 0.0114 (3) | 0.00000 | 0.00000 | 0.0046 (4) |
| F | 0.0399 (8) | 0.0399 (8) | 0.0399 (8) | 0.00000 | 0.00000 | 0.00000 |
| (K,Ca)—F1i | 2.28500 (10) | (K,Ca)—O1x | 2.7014 (5) |
| (K,Ca)—F1ii | 2.28500 (10) | (K,Ca)—O1xi | 2.7014 (5) |
| (K,Ca)—F1iii | 2.28500 (10) | Nb1—O1xii | 1.9873 (3) |
| (K,Ca)—F1iv | 2.28500 (10) | Nb1—O1xiii | 1.9873 (3) |
| (K,Ca)—F1v | 2.28500 (10) | Nb1—O1xiv | 1.9873 (3) |
| (K,Ca)—F1 | 2.28500 (10) | Nb1—O1xi | 1.9873 (3) |
| (K,Ca)—F1vi | 2.28500 (10) | Nb1—O1xv | 1.9873 (3) |
| (K,Ca)—F1vii | 2.28500 (10) | Nb1—O1xvi | 1.9873 (3) |
| (K,Ca)—O1viii | 2.7014 (5) | Nb1—(K,Ca)xvii | 3.7313 (4) |
| (K,Ca)—O1ix | 2.7014 (5) | ||
| F1i—(K,Ca)—F1ii | 180.000 (12) | F1iii—(K,Ca)—O1x | 81.59 (1) |
| F1i—(K,Ca)—F1iii | 9.6 (3) | F1iv—(K,Ca)—O1x | 98.42 (1) |
| F1ii—(K,Ca)—F1iii | 180.00 | F1v—(K,Ca)—O1x | 81.59 (1) |
| F1i—(K,Ca)—F1iv | 180.00 | F1—(K,Ca)—O1x | 98.42 (1) |
| F1ii—(K,Ca)—F1iv | 9.6 (3) | F1vi—(K,Ca)—O1x | 81.6 (2) |
| F1iii—(K,Ca)—F1iv | 180 (4) | F1vii—(K,Ca)—O1x | 98.42 (1) |
| F1i—(K,Ca)—F1v | 9.6 (3) | O1viii—(K,Ca)—O1x | 62.11 (1) |
| F1ii—(K,Ca)—F1v | 180.00 | O1ix—(K,Ca)—O1x | 117.89 (1) |
| F1iii—(K,Ca)—F1v | 9.6 (3) | F1i—(K,Ca)—O1xi | 98.42 (1) |
| F1iv—(K,Ca)—F1v | 180.00 | F1ii—(K,Ca)—O1xi | 81.59 (1) |
| F1i—(K,Ca)—F1 | 180.00 | F1iii—(K,Ca)—O1xi | 98.42 (1) |
| F1ii—(K,Ca)—F1 | 9.6 (3) | F1iv—(K,Ca)—O1xi | 81.59 (1) |
| F1iii—(K,Ca)—F1 | 180.00 | F1v—(K,Ca)—O1xi | 98.42 (1) |
| F1iv—(K,Ca)—F1 | 9.6 (3) | F1—(K,Ca)—O1xi | 81.59 (1) |
| F1v—(K,Ca)—F1 | 180.000 (8) | F1vi—(K,Ca)—O1xi | 98.42 (1) |
| F1i—(K,Ca)—F1vi | 5.5 (3) | F1vii—(K,Ca)—O1xi | 81.6 (2) |
| F1ii—(K,Ca)—F1vi | 180.00 | O1viii—(K,Ca)—O1xi | 117.89 (1) |
| F1iii—(K,Ca)—F1vi | 5.5 (3) | O1ix—(K,Ca)—O1xi | 62.11 (1) |
| F1iv—(K,Ca)—F1vi | 180.00 | O1x—(K,Ca)—O1xi | 180.000 (13) |
| F1v—(K,Ca)—F1vi | 5.5 (3) | O1xii—Nb1—O1xiii | 180.000 (12) |
| F1—(K,Ca)—F1vi | 180.00 | O1xii—Nb1—O1xiv | 89.04 (2) |
| F1i—(K,Ca)—F1vii | 180.00 | O1xiii—Nb1—O1xiv | 90.96 (2) |
| F1ii—(K,Ca)—F1vii | 5.5 (3) | O1xii—Nb1—O1xi | 90.96 (2) |
| F1iii—(K,Ca)—F1vii | 180.00 | O1xiii—Nb1—O1xi | 89.04 (2) |
| F1iv—(K,Ca)—F1vii | 5.5 (3) | O1xiv—Nb1—O1xi | 180.000 (5) |
| F1v—(K,Ca)—F1vii | 180.00 | O1xii—Nb1—O1xv | 90.96 (2) |
| F1—(K,Ca)—F1vii | 5.5 (3) | O1xiii—Nb1—O1xv | 89.04 (2) |
| F1vi—(K,Ca)—F1vii | 180.000 (8) | O1xiv—Nb1—O1xv | 89.04 (2) |
| F1i—(K,Ca)—O1viii | 98.42 (1) | O1xi—Nb1—O1xv | 90.96 (2) |
| F1ii—(K,Ca)—O1viii | 81.59 (1) | O1xii—Nb1—O1xvi | 89.04 (2) |
| F1iii—(K,Ca)—O1viii | 98.42 (1) | O1xiii—Nb1—O1xvi | 90.96 (2) |
| F1iv—(K,Ca)—O1viii | 81.59 (1) | O1xiv—Nb1—O1xvi | 90.96 (2) |
| F1v—(K,Ca)—O1viii | 98.42 (1) | O1xi—Nb1—O1xvi | 89.04 (2) |
| F1—(K,Ca)—O1viii | 81.59 (1) | O1xv—Nb1—O1xvi | 180.00 (2) |
| F1vi—(K,Ca)—O1viii | 98.42 (1) | Nb1ii—O1—Nb1xviii | 139.69 (4) |
| F1vii—(K,Ca)—O1viii | 81.6 (2) | Nb1ii—O1—(K,Ca)ii | 104.44 (1) |
| F1i—(K,Ca)—O1ix | 81.59 (1) | Nb1xviii—O1—(K,Ca)ii | 104.44 (1) |
| F1ii—(K,Ca)—O1ix | 98.42 (1) | Nb1ii—O1—(K,Ca)xviii | 104.44 (1) |
| F1iii—(K,Ca)—O1ix | 81.59 (1) | Nb1xviii—O1—(K,Ca)xviii | 104.44 (1) |
| F1iv—(K,Ca)—O1ix | 98.42 (1) | (K,Ca)ii—O1—(K,Ca)xviii | 87.36 (2) |
| F1v—(K,Ca)—O1ix | 81.59 (1) | (K,Ca)—F1—(K,Ca)iv | 109.47 |
| F1—(K,Ca)—O1ix | 98.42 (1) | (K,Ca)—F1—(K,Ca)ii | 109.47 |
| F1vi—(K,Ca)—O1ix | 81.6 (2) | (K,Ca)iv—F1—(K,Ca)ii | 109.47 |
| F1vii—(K,Ca)—O1ix | 98.42 (1) | (K,Ca)—F1—(K,Ca)vii | 109.47 |
| O1viii—(K,Ca)—O1ix | 180.000 (1) | (K,Ca)iv—F1—(K,Ca)vii | 109.47 |
| F1i—(K,Ca)—O1x | 81.59 (1) | (K,Ca)ii—F1—(K,Ca)vii | 109.47 |
| F1ii—(K,Ca)—O1x | 98.42 (1) |
| Symmetry codes: (i) x−1/4, −y+1/2, z+1/4; (ii) −x+3/4, y, −z+3/4; (iii) −x+1/2, y−1/4, z+1/4; (iv) x, −y+3/4, −z+3/4; (v) −x+1/2, −y+1/2, −z+1; (vi) x−1/4, y−1/4, −z+1; (vii) −x+3/4, −y+3/4, z; (viii) −y+1/2, −z+1/2, −x+1; (ix) y, z, x; (x) y, −x+3/4, −z+3/4; (xi) −y+1/2, x−1/4, z+1/4; (xii) −y+1/2, z−1/4, x+1/4; (xiii) y+1/2, −z+1/4, −x+3/4; (xiv) y+1/2, −x+1/4, −z+3/4; (xv) x+1/4, y−1/4, −z+1/2; (xvi) −x+3/4, −y+1/4, z+1/2; (xvii) −x+3/4, y−1/2, −z+5/4; (xviii) −x+3/4, −y+1/4, z−1/2. |
| (K,Ca)—F1 | 2.28500 (10) | Nb1—O1ii | 1.9873 (3) |
| (K,Ca)—O1i | 2.7014 (5) |
| Symmetry codes: (i) y, z, x; (ii) −x+3/4, −y+1/4, z+1/2. |
This work was supported by Ajou University. The authors are also grateful to Pohang Accelerator Laboratory and HANARO Center in the Korea Atomic Energy Research Institute for synchrotron X-ray and neutron diffraction measurements.
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KCaNb2O6F is isotypic with NaCaNb2O6F (von Gaertner, 1930), typical examples of cubic pyrochlore-type structures. A general description of structures and physical properties of pyrochlore-type compounds was given by Subramanian et al. (1983). Oxyfluoride compounds with general composition A2Nb2O6F (A = alkali metal and alkaline-earth metal) crystallize in different structural types, depending on the size of the A-cation. The layered perovskite-type structure is the most stable structure for large cations as observed in KSrNb2O6F (Yoo et al., 2007) and RbSrNb2O6F (Choy et al., 2001), while the pyrochlore structure is favorable for small A-cations as in NaCaNb2O6F and LiCaNb2O6F (Le Berre et al., 2007). The structural variation on composition in these oxyfluoride homologues has been discussed in the literature (Kim et al. 2002).
In the title compound, the (K, Ca) atoms are located at 16d sites, Nb atoms at 16c, O atoms at 48f and F atoms at 8b sites. (K, Ca) atoms are coordinated to two F atoms and six O atoms, forming a puckered hexagonal bipyramid which is axially compressed with the two F atoms being at considerably shorter distances than the six O atoms. The bond compression inhibits the displacement of the (K, Ca) atoms toward the two closest F atoms, which results in the large anisotropic displacement ellipsoid of the F atom as shown in Fig. 1(a). The Nb atoms are bonded to six O atoms at equal distances forming nearly regular octahedra. By neglecting the K/Ca—O interaction, the structure can be regarded as two interpenetrating networks of Nb2O6 and (K, Ca)2F units, the latter with an anti-cristobalite type arrangement (Fig. 1 b). The bond valence sums (BVS) calculated from the bond distances using the parameters of Brese and O'Keeffe (1991) are (K+, Ca2+) = 1.81, Nb5+ = 4.86, O2- = 1.97 and F− = 1.51. These values imply that the (K, Ca)–F bond is compressed while the Nb—O bond is slightly stretched. Such strong bond compression has been commonly observed in A2B2O7 pyrochlores containing large A-cations. For example, the BVS for O at the 8b site shows a variation from 2.00 for Lu2Sn2O7 to 2.60 for La2Sn2O7 (Kennedy et al., 1997). Taking into consideration that KCaNb2O6F is located near the upper boundary of the stability range for the pyrochlore structures due to the large size of the A-cations, the strong compression of the K/Ca–F bond could be anticipated. The origin of the large variation in BVS for the F ion is, however, still unclear. In this regard, solid state 19F-NMR spectroscopy may be helpful for further studies.