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The crystal structure of tripotassium octa­fluorido­tantalate, K3TaF8, determined from laboratory powder diffraction data by the simulated annealing method and refined by total energy minimization in the solid state, is built from discrete potassium cations, fluoride anions and monocapped trigonal-prismatic [TaF7]2- ions. All six atoms in the asymmetric unit are in special positions of the P63mc space group: the Ta and one F atom in the 2b (3m) sites, the K and two F atoms in the 6c (m) sites, and one F atom in the 2a (3m) site. The structure consists of face-sharing K6 octa­hedra with a fluoride anion at the center of each octa­hedron, forming chains of composition [FK3]2+ running along [001] with isolated [TaF7]2- trigonal prisms in between. The structure of the title compound is different from the reported structure of Na3TaF8 and represents a new structure type.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109055140/sq3230sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108270109055140/sq3230Isup2.rtv
Contains datablock I

Comment top

In the course of our investigation of the compounds formed in the K–Ta–F system we have synthesized the title compound, (I), crystallizing in the binary KF–K2TaF7 system as a congruently melting product of the two components (Boča et al., 2007; Netriová et al., 2009). The existence of this compound has been suggested previously (Efros & Lantratov, 1963; Kovalev et al., 1973), but up till now no structural data have been given.

Since (I) has been obtained only in the form of a fine powder, its structure was solved from laboratory X-ray powder data (Fig. 1). All six atoms in the asymmetric unit were found to be in the special positions of the P63mc space group: atoms Ta1 and F3 in the 2b position, atoms K1, F2 and F4 in the 6c position, and, finally, atom F1 in the 2a position.

The structure is composed of monocapped [TaF7]2- trigonal prisms (Fig. 2) and octahedra of K+ cations, each with an F- anion at the center. The F-centered K6 octahedra share faces to create infinite chains of composition [FK3]2+ along the c axis (Fig. 3). The discrete [TaF7]2- anions are located between the cationic chains. The individual Ta—F bond distances in the monocapped trigonal prisms (Table 1) are in good agreement with the range of 1.919 (3)–1.976 (2) Å found for the similar polyhedron in the low-temperature phase of K2TaF7 (Torardi & Brixner, 1987). The arrangement of the K atoms in (I) is dictated by the presence of the eighth fluoride anion. While in K2TaF7 two independent K+ cations are nine-coordinated (K—F < 3.0 Å) by the F- anions, all of which are shared with the TaV atoms, in (I) the F1 anions bond exclusively to the unique K1 cation and the remaining seven fluoride anions are shared with atom Ta1. The [FK6] octahedra show only a slight deviation from the ideal geometry as the individual K1—F1 bond distances (Table 1) are close to one another and the deviations from the ideal octahedral K—F—K bond angles are less than 2°. These K—F bond distances agree well with the octahedral K—F distances (2.674 Å) in the structure of carobbiite, KF (Wyckoff, 1963). Different bonding conditions in (I) and K2TaF7 have also led to the different K···K separations, which are reduced from 4.08 Å in K2TaF7 to 3.83 Å in (I). To our knowledge, K3TaF8 belongs to a new structure type.

Our structure determination has ruled out the hypothesis of eightfold fluoride coordination of the Ta atoms as identified in the structure of trisodium octafluorotantalate, Na3TaF8 (Hoard et al., 1956). In the Na3TaF8 structure, solved only from the h0l and hk0 projections calculated using film data and not followed by any refinement, the [TaF8]3- groups are suggested to have a configuration of a square antiprism, with the sodium cations surrounded by six fluoride anions forming an octahedron. However, the uncertainities in the choice of the cell, indicating possible problems with pseudosymmetry (i.e. a monoclinic β angle of ca 121°), a rather low number of structure factors and the absence of any refinement throw some doubts on the accuracy of the structure of that compound.

Related literature top

For related literature, see: Boča et al. (2007); Bylander et al. (1990); Efros & Lantratov (1963); Hoard et al. (1956); Kovalev et al. (1973); Kresse & Furthmüller (1996); Kresse & Hafner (1993); Kresse & Joubert (1999); Netriová et al. (2009); Perdew et al. (1992); Rodriguez-Carvajal (1993); Smrčok et al. (2007, 2008); Spek (2003); Teter et al. (1989); Torardi & Brixner (1987); Visser (1969).

Experimental top

K2TaF7 (2.613 g) and KF (0.387 g) were placed in a Pt crucible, gently mixed and finally hermetically sealed in. The system was heated to 1123 K in a furnace for 30 min, followed by slow cooling at 1 K min-1 to 973 K and at 5 K min-1 to 473 K. After the system had cooled to room temperature, a white polycrystalline sample was removed and powdered under dry nitrogen atmosphere.

Refinement top

The X-ray powder diffraction pattern was indexed using the program ITO (Visser, 1969) via the positions of 20 diffraction peaks. The accuracy of the lattice parameters was improved by several cycles of LeBail decomposition of the pattern as implemented in the FULLPROF code (Rodriguez-Carvajal, 1993). The systematic absences suggested extinction symbol Pc. A satisfactory structure solution was obtained in the space group P63mc by using the FOX program (Favre-Nicolin & Černý, 2002).

The solved structure was then expanded to P1 symmetry and the atoms moved from the respective special positions and refined using total energy minimization with the VASP code (Kresse & Furthmüller, 1996; Kresse & Hafner, 1993). The main advantage of such an approach is that the contributions of the atoms to the electron density distribution are not weighted by their scattering power, in contrast with X-ray diffraction. Second, this approach provides a remedy to the chronic difficulty of standard powder refinements, which is the lack of information extractable from a powder pattern (see e.g. Smrčok et al., 2007, 2008).

Geometry analysis using the PLATON program (Spek, 2009) revealed that the symmetry of the optimized structure could be increased to P63mc as the r.m.s. deviations between the atomic coordinates of the P1 structural units, expected to be symmetrically equivalent in P63mc, were negligible. Furthermore, the typical deviation of the optimized fractional coordinates from the expected special positions were only ~0.004 in the P1 model. In the final step, the optimized atomic coordinates were transformed back to the P63mc space group and introduced into a Rietveld refinement; three groups (Ta, K and one for all F atoms) of isotropic displacement parameters plus the scale parameter were refined, keeping the profile and atomic parameters fixed. The refined values of the isotropic displacement parameters, i.e. 0.080 (2)Å2 for Ta, 0.090 (3)Å2 for K and 0.091 (5)Å2 for F, do not show any anomalies that would indicate possible problems with the structural model. The overall fit (Rwp = 0.16) is shown in Fig. 1.

Theoretical calculations were carried out using the VASP package (Kresse & Furthmüller, 1996; Kresse & Hafner, 1993). The calculations were based on the density functional theory with periodic boundary conditions (Jones & Gunnarsson, 1989) using the generalized gradient approximation in the exchange-correlation functional (Perdew et al., 1992). The interactions between ions and electrons were described using the projector augumented wave method (Kresse & Joubert, 1999) with a plane-wave cutoff of 400 eV. The optimization of the structure was performed by the method of conjugated gradient in 4k points (Teter et al., 1989; Bylander et al., 1990).

Computing details top

Data collection: X-POW (Stoe & Cie, DATE?); cell refinement: ITO (Visser, 1969); data reduction: X-POW; program(s) used to solve structure: Fox (Favre-Nicolin & Černý, 2002); program(s) used to refine structure: VASP (Kresse & Furthmüller, 1996; Kresse & Hafner, 1993) and Fullprof (Rodriguez-Carvajal, 1993); molecular graphics: DIAMOND (Brandenburg, 2000); software used to prepare material for publication: PLATON (Spek, 2009).

Figures top
[Figure 1] Fig. 1. The results of Rietveld refinement. Data are given as filled circles, the calculated profile as a solid line and the difference profile as a solid line below. Vertical markers above the difference profile indicate the calculated Bragg reflection positions.
[Figure 2] Fig. 2. The [TaF7]2- polyhedron. The triangles F2/F2i/F4ii and F3/F4/F4i form the bases of a distorted trigonal prism capped by F2ii. [Symmetry codes: (i) -y + 1, x - y, z; (ii) -x + y + 1, -x + 1, z.] DON'T MATCH TABLE
[Figure 3] Fig. 3. (a) A polyhedral presentation of the structure of (I) as viewed along the c axis. [The smaller [TaF7]2- polyhedra are shown in green in the electronic version of the journal, while the larger [FK6] octahedra appear in yellow.] (b) A polyhedral presentation of the structure of (I) as viewed along the b axis. The F-centered K6 octahedra share faces to form infinite chains of composition [FK3]2+ running along [001], with isolated [TaF7]2- anions positioned in between the chains.
tripotassium octafluorotantalate top
Crystal data top
K3TaF8Z = 2
Mr = 450.25Dx = 4.011 Mg m3
Hexagonal, P63mc? radiation, λ = 1.78892 Å
Hall symbol: P 6c -2cT = 298 K
a = 8.2533 (4) Åwhite
c = 6.3196 (4) Åflat sheet, ? × 0.1 mm
V = 372.79 (3) Å3
Data collection top
Stoe Stadi P
diffractometer
Data collection mode: transmission
Radiation source: sealed X-ray tubeScan method: step
Ge(111) monochromator2θmin = 7.12°, 2θmax = 69.98°, 2θstep = 0.02°
Specimen mounting: The sample was mounted between two Mylar foils.
Refinement top
Least-squares matrix: FullProfile function: Modified Lorentzian
Rp = 12.7254 parameters
Rwp = 15.8700 restraints
Rexp = 12.4900 constraints
RBragg = 0.1Weighting scheme based on measured s.u.'s
χ2 = 2.560(Δ/σ)max = 0.01
3144 data pointsBackground function: Linear interpolation
Excluded region(s): nonePreferred orientation correction: None
Crystal data top
K3TaF8V = 372.79 (3) Å3
Mr = 450.25Z = 2
Hexagonal, P63mc? radiation, λ = 1.78892 Å
a = 8.2533 (4) ÅT = 298 K
c = 6.3196 (4) Åflat sheet, ? × 0.1 mm
Data collection top
Stoe Stadi P
diffractometer
Scan method: step
Specimen mounting: The sample was mounted between two Mylar foils.2θmin = 7.12°, 2θmax = 69.98°, 2θstep = 0.02°
Data collection mode: transmission
Refinement top
Rp = 12.725χ2 = 2.560
Rwp = 15.8703144 data points
Rexp = 12.4904 parameters
RBragg = 0.10 restraints
Special details top

Geometry. Note that the refinement method does not provide variances of the optimized parameters. The esd's given below are calculated only from the esd's of the lattice parameters.

Refinement. The number of refined parameters is that in the last cycle.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ta10.333330.666670.740700.080 (2)*
K10.154700.309400.243100.090 (3)*
F10.000000.000000.002200.091 (5)*
F20.439000.561000.534400.091 (5)*
F30.333330.666671.054700.091 (5)*
F40.200300.799700.818500.091 (5)*
Geometric parameters (Å, º) top
Ta1—F21.9953 (1)K1—F4iv2.6427 (2)
Ta1—F31.9844 (1)K1—F12.6848 (2)
Ta1—F41.9643 (1)K1—F1v2.7517 (2)
Ta1—F2i1.9953 (1)K1—F4vi2.7614 (2)
Ta1—F4i1.9643 (1)K1—F3vii2.8175 (2)
Ta1—F2ii1.9953 (1)K1—F22.8869 (2)
Ta1—F4ii1.9643 (1)K1—F2ii2.8869 (2)
K1—F4iii2.6427 (2)
F2—Ta1—F3130.80F2i—Ta1—F478.29
F2—Ta1—F4153.70F4—Ta1—F4i113.96
F2—Ta1—F2i81.93F2ii—Ta1—F478.29
F2—Ta1—F4i78.29F4—Ta1—F4ii113.96
F2—Ta1—F2ii81.93F2i—Ta1—F4i153.70
F2—Ta1—F4ii78.29F2i—Ta1—F2ii81.93
F3—Ta1—F475.50F2i—Ta1—F4ii78.29
F2i—Ta1—F3130.80F2ii—Ta1—F4i78.29
F3—Ta1—F4i75.50F4i—Ta1—F4ii113.96
F2ii—Ta1—F3130.80F2ii—Ta1—F4ii153.70
F3—Ta1—F4ii75.50
Symmetry codes: (i) y+1, xy+1, z; (ii) x+y, x+1, z; (iii) x, y+1, z1/2; (iv) xy+1, x, z1/2; (v) xy, x, z+1/2; (vi) y+1, xy+1, z1; (vii) x, y, z1.

Experimental details

Crystal data
Chemical formulaK3TaF8
Mr450.25
Crystal system, space groupHexagonal, P63mc
Temperature (K)298
a, c (Å)8.2533 (4), 6.3196 (4)
V3)372.79 (3)
Z2
Radiation type?, λ = 1.78892 Å
µ (mm1)?
Specimen shape, size (mm)Flat sheet, ? × 0.1
Data collection
DiffractometerStoe Stadi P
diffractometer
Specimen mountingThe sample was mounted between two Mylar foils.
Data collection modeTransmission
Scan methodStep
2θ values (°)2θmin = 7.12 2θmax = 69.98 2θstep = 0.02
Refinement
R factors and goodness of fitRp = 12.725, Rwp = 15.870, Rexp = 12.490, RBragg = 0.1, χ2 = 2.560
No. of data points3144
No. of parameters4

Computer programs: X-POW (Stoe & Cie, DATE?), ITO (Visser, 1969), X-POW, Fox (Favre-Nicolin & Černý, 2002), VASP (Kresse & Furthmüller, 1996; Kresse & Hafner, 1993) and Fullprof (Rodriguez-Carvajal, 1993), DIAMOND (Brandenburg, 2000), PLATON (Spek, 2009).

Selected bond lengths (Å) top
Ta1—F21.9953 (1)K1—F1ii2.7517 (2)
Ta1—F31.9844 (1)K1—F4iii2.7614 (2)
Ta1—F41.9643 (1)K1—F3iv2.8175 (2)
K1—F4i2.6427 (2)K1—F22.8869 (2)
K1—F12.6848 (2)
Symmetry codes: (i) xy+1, x, z1/2; (ii) xy, x, z+1/2; (iii) y+1, xy+1, z1; (iv) x, y, z1.
 

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