KMo₅O₁₃ and KNb_1.76Sb_3.24O₁₃

The title compounds, potassium pentamolybdenum oxide, KMo₅O₁₃, and potassium niobate antimonate or potassium niobium antimony oxide (1/1.76/3.24), KNb_1.76Sb_3.24O₁₃, were synthesized by solid-state reactions and are isomorphous in space group Cmcm. The structure of the Mo complex has three unique Mo atoms and consists of MoO₆ octahedra sharing edges to form Mo₂O₆ pairs and Mo₃O₉ triplets, which, in turn, form layers by sharing corners. These layers are linked together in the [100] direction, yielding a three-dimensional network similar to that of KSb₅O₁₃. This framework delimits interconnected tunnels, running approximately along the [110] and [10] directions, in which K⁺ ions are located. In the isomorphous KNb_1.76Sb_3.24O₁₃ structure, one of the Mo sites in KMo₅O₁₃ is replaced by Sb and the other two Mo sites have been replaced by Nb/Sb.

S1. Comment

KMo₅O₁₃ crystallizes in the orthorhombic space group Cmcm and is isotypic with KSb₅O₁₃ (Bodenstein et al., 1983). The structure possesses a three-dimensional network, which can be described by the succession of MoO₆ octahedra sharing edges to form [Mo2Mo1Mo2]O₉ triplets. These triplets are associated by sharing O1 corners, yielding zigzag chains running along the [001] direction. These chains are, in turn, associated by sharing axial oxygen corners, with pairs of Mo3₂O₆ octahedra sharing edges to form layers parallel to the (100) plane (Fig. 1).

In the [100] direction, these layers are associated with an ABA ordering. Each Mo3₂O₆ pair of one layer links two [Mo2Mo1Mo2]O₉ triplets of the two neighbouring layers by sharing each of its three equatorial oxygen corners with the axial corners of each triplet.

This arrangement of octahedra generates interconnected tunnels, parallel to the [110] and [110] directions, in which the K⁺ ions are located (Fig. 2).

In the structure of KMo₅O₁₃, all Mo—O bond distances are similar to those customarily encountered with Mo^V and O. However, we note the existence of longer bond distances for Mo1—O4, Mo2—O4 and Mo3—O4 (Table 1) that can be explained by the sharing of atom O4 with the three Mo atoms (Mo1, Mo2 and Mo3). This makes the interaction between the metal atoms and the O atom weaker. The K⁺ ions are coordinated by seven O atoms.

The crystal structure of KNb_1.76Sb_3.24O₁₃ is isomorphous with that of KMo₅O₁₃. The characteristic feature of this structure is the double occupancy of sites 2 and 3 by Sb and Nb atoms; the occupancies of the Sb and Nb atoms in both sites are 0.56 and 0.44, respectively.

For both compounds, atom O2 lies on a general position, atom O3 lies on a twofold axis, K and O1 have mm symmetry, Mo1 and Sb1 have 2/m symmetry and all other atoms have m symmetry.

S2. Experimental

Single crystals of KMo₅O₁₃ were prepared from a mixture of K₂CO₃, (NH₄)₆Mo₇O₂₄·4H₂O and H₃BO₃ in a molar ratio of 5:1:5. The mixture was ground and then heated in a platinium crucible at 1073 K for 40 h. The mixture was then cooled to room temperature at a rate of 0.1 K min^-1. Colourless plates were extracted from the boron glass using hot water. Qualitative analysis of the sample by electron microscope probe revealed that it contained K and Mo.

Transparent and colourless single crystals of KNb_1.76Sb_3.24O₁₃ were prepared from a mixture of K₂CO₃, Sb₂O₃ and Nb₂O₅ in a molar ratio of 5:2:2. The powder was ground and homogenized with boric acid (H₃BO₃) as a flux, then heated in a porcelain crucible in air to 1273 K. This temperature was held for 20 h, then the mixture was cooled to 773 K at a rate of 6 K h^-1 and maintained at that temperature for 2 h, before being cooled to room temperature at a rate of 30 K h^-1. Single crystals were extracted from the boron glass using hot water. Qualitative analysis of the single crystals by electron microscope probe revealed that they contained K, Nb and Sb.

Computing details

For both compounds, data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992). Cell refinement: CAD-4 EXPRESS for (I); CAD-4 EXPRESS and WinGX (Farrugia, 1999) for (II). For both compounds, data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A layer of the KNb1.76Sb3.24O13 structure (similar to the KMo5O13 structure), viewed along the [100] direction. The Sb1O6 octahedra (Mo1O6 octahedra in the KMo5O13 structure) are hatched, and the (Nb2,Sb2)O6 (Mo2 in KMo5O13) and (Nb3,Sb3)O6 (Mo3 in KMo5O13) octahedra are grey. Open circles denote K atoms.
	Fig. 2. A projection of the structures of KNb1.76Sb3.24O13 (similar to the KMo5O13 structure) along the [110] direction, showing the tunnels. The shading key is the same as for Fig. 1.

(I) Potassium closo-13-oxopentamolybdate

Crystal data

KMo5O13	F(000) = 1331
Mr = 726.79	Dx = 4.847 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2	Cell parameters from 24 reflections
a = 6.6027 (10) Å	θ = 9.1–14.0°
b = 8.9552 (10) Å	µ = 6.62 mm^−1
c = 16.844 (2) Å	T = 293 K
V = 996.0 (2) Å^3	Plate, colourless
Z = 4	0.09 × 0.07 × 0.02 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	740 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.008
Graphite monochromator	θmax = 30.0°, θmin = 2.4°
ω/2θ scans	h = −1→9
Absorption correction: ψ scan (North et al., 1968)	k = 0→12
Tmin = 0.601, Tmax = 0.910	l = 0→23
2790 measured reflections	2 standard reflections every 120 min
812 independent reflections	intensity decay: 0.6%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.037	w = 1/[σ^2(Fo^2) + (0.0499P)^2 + 28.8946P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.100	(Δ/σ)max < 0.001
S = 1.22	Δρmax = 1.94 e Å^−3
812 reflections	Δρmin = −2.48 e Å^−3
58 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0062 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mo1	0.0000	0.0000	0.5000	0.0048 (2)
Mo2	0.0000	0.23165 (7)	0.36166 (4)	0.0048 (2)
Mo3	0.0000	0.38575 (7)	0.57726 (4)	0.0049 (2)
K	0.0000	0.1278 (4)	0.7500	0.0309 (8)
O1	0.0000	0.1878 (13)	0.2500	0.023 (2)
O2	0.2035 (9)	0.2732 (6)	0.6322 (3)	0.0223 (11)
O3	−0.3110 (12)	0.0000	0.5000	0.0195 (14)
O4	0.0000	0.2256 (8)	0.4907 (5)	0.0202 (15)
O5	0.0000	0.5535 (8)	0.6499 (5)	0.0213 (15)
O6	0.0000	0.0147 (8)	0.3869 (4)	0.0205 (15)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mo1	0.0062 (4)	0.0046 (4)	0.0035 (4)	0.000	0.000	0.0007 (3)
Mo2	0.0064 (3)	0.0041 (3)	0.0038 (3)	0.000	0.000	0.0002 (2)
Mo3	0.0064 (4)	0.0042 (3)	0.0042 (3)	0.000	0.000	0.0003 (2)
K	0.047 (2)	0.0259 (17)	0.0192 (16)	0.000	0.000	0.000
O1	0.030 (6)	0.028 (6)	0.012 (4)	0.000	0.000	0.000
O2	0.025 (3)	0.022 (2)	0.020 (2)	0.001 (2)	−0.004 (2)	0.0027 (19)
O3	0.023 (3)	0.020 (3)	0.015 (3)	0.000	0.000	0.003 (3)
O4	0.026 (4)	0.017 (3)	0.018 (4)	0.000	0.000	0.001 (3)
O5	0.030 (4)	0.014 (3)	0.020 (3)	0.000	0.000	0.001 (3)
O6	0.032 (4)	0.016 (3)	0.013 (3)	0.000	0.000	0.002 (3)

Geometric parameters (Å, º)

Mo1—O6^i	1.909 (7)	Mo2—O4	2.174 (8)
Mo1—O4^i	2.026 (7)	Mo3—O2	1.917 (5)
Mo1—O3^i	2.054 (8)	Mo3—O5	1.937 (8)
Mo1—Mo2^i	3.1198 (7)	Mo3—O4	2.046 (8)
Mo2—O1	1.921 (3)	Mo3—O3^iv	2.073 (5)
Mo2—O5^ii	1.934 (7)	K—O6^v	2.636 (7)
Mo2—O2^iii	1.961 (6)	K—O2^vi	2.727 (6)
Mo2—O6	1.989 (7)	K—O1^i	2.826 (12)
O2—Mo3—O2^vii	89.0 (3)	O4^i—Mo1—O4	180.0 (4)
O2—Mo3—O5	95.9 (2)	O6^i—Mo1—O3^i	90.000 (1)
O2^vii—Mo3—O5	95.9 (2)	O6—Mo1—O3^i	90.000 (1)
O2—Mo3—O4	88.6 (2)	O4^i—Mo1—O3^i	90.000 (1)
O2^vii—Mo3—O4	88.6 (2)	O4—Mo1—O3^i	90.000 (2)
O5—Mo3—O4	173.7 (3)	O6^i—Mo1—O3	90.000 (1)
O2—Mo3—O3^iv	98.1 (3)	O6—Mo1—O3	90.000 (1)
O2^vii—Mo3—O3^iv	169.8 (2)	O4^i—Mo1—O3	90.000 (2)
O5—Mo3—O3^iv	90.79 (18)	O4—Mo1—O3	90.000 (1)
O4—Mo3—O3^iv	84.17 (17)	O3^i—Mo1—O3	180.000 (1)
O2—Mo3—O3^viii	169.8 (2)	O1—Mo2—O5^ii	96.0 (4)
O2^vii—Mo3—O3^viii	98.1 (3)	O1—Mo2—O2^iii	92.70 (15)
O5—Mo3—O3^viii	90.79 (18)	O5^ii—Mo2—O2^iii	91.57 (16)
O4—Mo3—O3^viii	84.17 (17)	O1—Mo2—O2^ix	92.70 (15)
O3^iv—Mo3—O3^viii	74.0 (4)	O5^ii—Mo2—O2^ix	91.57 (16)
O2—Mo3—K	45.19 (17)	O2^iii—Mo2—O2^ix	173.4 (3)
O2^vii—Mo3—K	45.19 (17)	O1—Mo2—O6	90.6 (4)
O5—Mo3—K	89.3 (2)	O5^ii—Mo2—O6	173.4 (3)
O4—Mo3—K	97.0 (2)	O2^iii—Mo2—O6	88.11 (16)
O3^iv—Mo3—K	143.00 (18)	O2^ix—Mo2—O6	88.11 (16)
O3^viii—Mo3—K	143.00 (18)	O1—Mo2—O4	166.8 (4)
O6^i—Mo1—O6	180.000 (1)	O5^ii—Mo2—O4	97.2 (3)
O6^i—Mo1—O4^i	81.6 (3)	O2^iii—Mo2—O4	86.94 (15)
O6—Mo1—O4^i	98.4 (3)	O2^ix—Mo2—O4	86.94 (15)
O6^i—Mo1—O4	98.4 (3)	O6—Mo2—O4	76.2 (3)
O6—Mo1—O4	81.6 (3)

Symmetry codes: (i) −x, −y, −z+1; (ii) −x, −y+1, −z+1; (iii) −x+1/2, −y+1/2, −z+1; (iv) x+1/2, y+1/2, z; (v) −x, −y, z+1/2; (vi) x, y, −z+3/2; (vii) −x, y, z; (viii) −x−1/2, −y+1/2, −z+1; (ix) x−1/2, −y+1/2, −z+1.

(II) Potassium niobium antimony oxide (1/1.76/3.24)

Crystal data

KNb1.76Sb3.24O13	F(000) = 1441.5
Mr = 804.67	Dx = 5.186 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2c 2	Cell parameters from 25 reflections
a = 6.697 (1) Å	θ = 10.0–14.5°
b = 9.027 (1) Å	µ = 10.74 mm^−1
c = 17.047 (2) Å	T = 293 K
V = 1030.6 (2) Å^3	Parallelepiped, colourless
Z = 4	0.20 × 0.05 × 0.04 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	743 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	θmax = 29.9°, θmin = 2.4°
Graphite monochromator	h = −9→0
ω/2θ scans	k = 0→12
Absorption correction: ψ scan (North et al., 1968)	l = −23→0
Tmin = 0.567, Tmax = 0.643	2 standard reflections every 120 min
832 measured reflections	intensity decay: 0.5%
832 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.023	w = 1/[σ^2(Fo^2) + (0.0389P)^2 + 3.9495P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.072	(Δ/σ)max < 0.001
S = 1.18	Δρmax = 1.34 e Å^−3
832 reflections	Δρmin = −1.53 e Å^−3
66 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
2 restraints	Extinction coefficient: 0.00129 (12)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sb1	0.0000	0.0000	0.5000	0.01077 (19)
Nb2	0.0000	0.230 (4)	0.359 (4)	0.0131 (12)	0.44 (4)
Sb2	0.0000	0.227 (2)	0.364 (2)	0.0131 (12)	0.56 (3)
Nb3	0.0000	0.3735 (14)	0.5748 (12)	0.0102 (6)	0.44 (3)
Sb3	0.0000	0.3911 (11)	0.5762 (7)	0.0102 (6)	0.56 (3)
K	0.0000	0.1318 (2)	0.7500	0.0264 (5)
O1	0.0000	0.1933 (6)	0.2500	0.0120 (11)
O2	0.2051 (4)	0.2846 (3)	0.63116 (15)	0.0112 (5)
O3	−0.3078 (6)	0.0000	0.5000	0.0096 (7)
O4	0.0000	0.2240 (4)	0.4895 (2)	0.0099 (8)
O5	0.0000	0.5611 (4)	0.6419 (2)	0.0119 (8)
O6	0.0000	0.0126 (4)	0.3873 (2)	0.0082 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sb1	0.0109 (3)	0.0106 (3)	0.0108 (3)	0.000	0.000	0.00117 (16)
Nb2	0.0155 (2)	0.0081 (7)	0.016 (4)	0.000	0.000	0.0012 (12)
Sb2	0.0155 (2)	0.0081 (7)	0.016 (4)	0.000	0.000	0.0012 (12)
Nb3	0.0107 (3)	0.0040 (18)	0.0158 (4)	0.000	0.000	−0.0022 (12)
Sb3	0.0107 (3)	0.0040 (18)	0.0158 (4)	0.000	0.000	−0.0022 (12)
K	0.0516 (14)	0.0160 (8)	0.0115 (8)	0.000	0.000	0.000
O1	0.014 (3)	0.010 (2)	0.013 (3)	0.000	0.000	0.000
O2	0.0089 (12)	0.0106 (11)	0.0140 (12)	0.0002 (10)	−0.0004 (10)	0.0010 (9)
O3	0.0071 (17)	0.0101 (15)	0.0115 (17)	0.000	0.000	0.0027 (12)
O4	0.0137 (19)	0.0089 (18)	0.0071 (16)	0.000	0.000	0.0002 (13)
O5	0.020 (2)	0.0033 (16)	0.0126 (17)	0.000	0.000	−0.0003 (13)
O6	0.0135 (18)	0.0051 (16)	0.0062 (16)	0.000	0.000	0.0016 (12)

Geometric parameters (Å, º)

Sb1—O6	1.924 (4)	Sb2—O4	2.14 (3)
Sb1—O4^i	2.030 (4)	Nb3—O2^iv	1.858 (12)
Sb1—O3	2.061 (4)	Nb3—O4	1.984 (17)
Nb2—O1	1.89 (6)	Nb3—O5	2.044 (15)
Nb2—O5^ii	1.89 (4)	Nb3—O3^v	2.142 (14)
Nb2—O2^iii	1.987 (6)	Sb3—O5	1.900 (11)
Nb2—O6	2.02 (4)	Sb3—O2^iv	1.921 (8)
Nb2—O4	2.22 (6)	Sb3—O3^v	2.076 (9)
Sb2—O5^ii	1.92 (2)	Sb3—O4	2.112 (11)
Sb2—O1	1.96 (3)	K—O6^i	2.679 (4)
Sb2—O6	1.98 (2)	K—O2^vi	2.810 (3)
Sb2—O2^iii	1.980 (3)	K—O1^i	2.935 (6)
O6—Sb1—O6^i	180.0	O5^ii—Nb2—O2^viii	93.8 (11)
O6—Sb1—O4^i	98.46 (15)	O2^iii—Nb2—O2^viii	168 (3)
O6^i—Sb1—O4^i	81.54 (15)	O1—Nb2—O6	94 (2)
O6—Sb1—O4	81.54 (15)	O5^ii—Nb2—O6	167 (4)
O6^i—Sb1—O4	98.46 (15)	O2^iii—Nb2—O6	85.2 (11)
O4^i—Sb1—O4	180.0	O2^viii—Nb2—O6	85.2 (11)
O6—Sb1—O3	90.000 (1)	O1—Nb2—O4	169 (2)
O6^i—Sb1—O3	90.000 (1)	O5^ii—Nb2—O4	92 (2)
O4^i—Sb1—O3	90.000 (1)	O2^iii—Nb2—O4	85.1 (17)
O4—Sb1—O3	90.0	O2^viii—Nb2—O4	85.1 (17)
O6—Sb1—O3^i	90.000 (1)	O6—Nb2—O4	74.8 (17)
O6^i—Sb1—O3^i	90.000 (1)	O5^ii—Sb2—O1	96.0 (13)
O4^i—Sb1—O3^i	90.0	O5^ii—Sb2—O6	171 (2)
O4—Sb1—O3^i	90.000 (1)	O1—Sb2—O6	92.8 (12)
O3—Sb1—O3^i	180.0	O5^ii—Sb2—O2^iii	93.2 (6)
O2^iv—Nb3—O4	94.9 (5)	O1—Sb2—O2^iii	92.0 (10)
O2—Nb3—O4	94.9 (5)	O6—Sb2—O2^iii	86.5 (6)
O2^iv—Nb3—O5	93.9 (7)	O5^ii—Sb2—O2^viii	93.2 (6)
O2—Nb3—O5	93.9 (7)	O1—Sb2—O2^viii	92.0 (10)
O4—Nb3—O5	166.9 (10)	O6—Sb2—O2^viii	86.5 (6)
O2^iv—Nb3—O3^v	95.38 (17)	O2^iii—Sb2—O2^viii	172.1 (15)
O2—Nb3—O3^v	169.2 (7)	O5^ii—Sb2—O4	93.6 (13)
O4—Nb3—O3^v	85.8 (7)	O1—Sb2—O4	170.4 (12)
O5—Nb3—O3^v	83.8 (4)	O6—Sb2—O4	77.6 (10)
O2^iv—Nb3—O3^vii	169.2 (7)	O2^iii—Sb2—O4	87.5 (10)
O2—Nb3—O3^vii	95.38 (17)	O2^viii—Sb2—O4	87.5 (10)
O4—Nb3—O3^vii	85.8 (7)	O6^i—K—O6^ix	121.80 (17)
O5—Nb3—O3^vii	83.8 (4)	O6^i—K—O2^vi	150.33 (6)
O3^v—Nb3—O3^vii	73.9 (6)	O6^ix—K—O2^vi	66.97 (8)
O5—Sb3—O2^iv	96.7 (4)	O6^i—K—O2^x	150.33 (6)
O5—Sb3—O2	96.7 (4)	O6^ix—K—O2^x	66.97 (8)
O2^iv—Sb3—O2	91.3 (5)	O2^vi—K—O2^x	58.52 (11)
O5—Sb3—O3^v	89.2 (4)	O6^i—K—O2^iv	66.97 (8)
O2^iv—Sb3—O3^v	95.67 (14)	O6^ix—K—O2^iv	150.33 (6)
O2—Sb3—O3^v	170.3 (5)	O2^vi—K—O2^iv	121.18 (13)
O5—Sb3—O3^vii	89.2 (4)	O2^x—K—O2^iv	92.28 (11)
O2^iv—Sb3—O3^vii	170.3 (5)	O6^i—K—O2	66.97 (8)
O2—Sb3—O3^vii	95.67 (14)	O6^ix—K—O2	150.33 (6)
O3^v—Sb3—O3^vii	76.6 (4)	O2^vi—K—O2	92.28 (12)
O5—Sb3—O4	171.7 (6)	O2^x—K—O2	121.18 (13)
O2^iv—Sb3—O4	89.1 (4)	O2^iv—K—O2	58.52 (11)
O2—Sb3—O4	89.1 (4)	O6^i—K—O1^i	60.90 (9)
O3^v—Sb3—O4	84.3 (4)	O6^ix—K—O1^i	60.90 (9)
O3^vii—Sb3—O4	84.3 (4)	O2^vi—K—O1^i	119.41 (6)
O1—Nb2—O5^ii	100 (2)	O2^x—K—O1^i	119.41 (7)
O1—Nb2—O2^iii	94.1 (17)	O2^iv—K—O1^i	119.41 (7)
O5^ii—Nb2—O2^iii	93.8 (11)	O2—K—O1^i	119.41 (6)
O1—Nb2—O2^viii	94.1 (17)

Symmetry codes: (i) −x, −y, −z+1; (ii) −x, −y+1, −z+1; (iii) x−1/2, −y+1/2, −z+1; (iv) −x, y, z; (v) −x−1/2, −y+1/2, −z+1; (vi) x, y, −z+3/2; (vii) x+1/2, y+1/2, z; (viii) −x+1/2, −y+1/2, −z+1; (ix) −x, −y, z+1/2; (x) −x, y, −z+3/2.