Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109054341/fn3043sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270109054341/fn3043Isup2.hkl |
The title crystal was obtained from the following pure starting chemicals: MgF2 (55.8 g), BaCO3 (177.6 g), H3BO3 (111.24 g), BaF2 (31.6 g), KF.2H2O (42.3 g). The mixture of the starting materials was put in a platinum crucible, heated to 1163 K and stirred for 24 h untill the mixture became a clear and homogeneous melt. A platinum wire was dipped into the melt and rotated at a speed of 15 r min-1. The column-shaped KBa7Mg2B14O28F5 crystal crystallized onto the platinum wire when the melt was cooled to between 1143 and 1133 K at a rate of 0.3–0.5 K d-1. The crystal growth was ended by raising the Pt wire, followed by cooling the furnace to room temperature within 10 h.
Based on the E2-1 (1.013) and powder SHG tests, the centrosymmetric space group C2/c was adopted for the structure solution. Five Ba atoms were originally found by direct methods (SHELX-97) with one of them, Ba5, having unusually large displacement factors. K and Ba atoms were allowed to co-occupy the Ba5 site and all the rest of the atoms were found by subsequent difference Fourier synthesis. Refinement with anisotropic displacement parameters resulted in some of the atoms having non-positive definite ADPs [anisotropic displacement parameters?]. Close inspection showed that the displacement parameter of the statistically occupied site Ba/K5 was still large. Therefore, K and Ba were allowed to occupy split sites with constrained ADPs. The refinement proceeded smoothly and led to a 0.3 Å separation between K and Ba and a K-to-Ba occupation ratio of 0.39/0.61. In order to maintain charge balance, we let the rest of the K atom distribute statistically over the other Ba sites. The final refinements gave the following agreement indices Rw = 0.0411 and RI=0.0213 [I > 2σ(I)]. The final difference electron-density map shows a highest peak of 1.43 eÅ-3 located 0.74 Å from Ba5 and a deepest hole of -1.06 eÅ-3 located 0.55 Å from Ba5.
Data collection: CrystalClear (Rigaku, 2002); cell refinement: CrystalClear (Rigaku, 2002); data reduction: CrystalClear (Rigaku, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: publCIF (Westrip, 2009).
Fig. 1. The unit-cell content of KBa7Mg2B14O28F5. | |
Fig. 2. The heptaborate group [B7O14]7-. |
KBa7Mg2B14O28F5 | F(000) = 3096 |
Mr = 1743.44 | Dx = 3.934 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 4934 reflections |
a = 16.638 (3) Å | θ = 3.0–27.5° |
b = 13.609 (2) Å | µ = 9.54 mm−1 |
c = 15.214 (3) Å | T = 93 K |
β = 121.309 (2)° | Block, colourless |
V = 2943.3 (9) Å3 | 0.24 × 0.23 × 0.20 mm |
Z = 4 |
Rigaku AFC10 diffractometer | 3301 independent reflections |
Radiation source: rotating anode | 2980 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.029 |
Detector resolution: 28.5714 pixels mm-1 | θmax = 27.5°, θmin = 3.0° |
dtprofit.ref scans | h = −17→21 |
Absorption correction: numerical (CrystalClear; Rigaku, 2002) | k = −17→17 |
Tmin = 0.121, Tmax = 0.148 | l = −18→19 |
11738 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.021 | w = 1/[σ2(Fo2) + (0.017P)2] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.041 | (Δ/σ)max = 0.002 |
S = 1.08 | Δρmax = 1.43 e Å−3 |
3301 reflections | Δρmin = −1.06 e Å−3 |
267 parameters | Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.000756 (18) |
KBa7Mg2B14O28F5 | V = 2943.3 (9) Å3 |
Mr = 1743.44 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 16.638 (3) Å | µ = 9.54 mm−1 |
b = 13.609 (2) Å | T = 93 K |
c = 15.214 (3) Å | 0.24 × 0.23 × 0.20 mm |
β = 121.309 (2)° |
Rigaku AFC10 diffractometer | 3301 independent reflections |
Absorption correction: numerical (CrystalClear; Rigaku, 2002) | 2980 reflections with I > 2σ(I) |
Tmin = 0.121, Tmax = 0.148 | Rint = 0.029 |
11738 measured reflections |
R[F2 > 2σ(F2)] = 0.021 | 267 parameters |
wR(F2) = 0.041 | 0 restraints |
S = 1.08 | Δρmax = 1.43 e Å−3 |
3301 reflections | Δρmin = −1.06 e Å−3 |
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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Ba1 | 0.5000 | 0.72228 (2) | 0.2500 | 0.00495 (11) | 0.911 (3) |
K1 | 0.5000 | 0.72228 (2) | 0.2500 | 0.00495 (11) | 0.089 (3) |
Ba2 | 0.5000 | 0.30580 (2) | 0.2500 | 0.00661 (11) | 0.966 (3) |
K2 | 0.5000 | 0.30580 (2) | 0.2500 | 0.00661 (11) | 0.034 (3) |
Ba3 | 0.594198 (15) | 0.988148 (15) | 0.670651 (16) | 0.00548 (8) | 0.983 (2) |
K3 | 0.594198 (15) | 0.988148 (15) | 0.670651 (16) | 0.00548 (8) | 0.017 (2) |
Ba4 | 0.121159 (16) | 0.995774 (16) | 0.500581 (18) | 0.00862 (9) | 0.968 (2) |
K4 | 0.121159 (16) | 0.995774 (16) | 0.500581 (18) | 0.00862 (9) | 0.032 (2) |
Ba5 | 0.22825 (7) | 0.78502 (5) | 0.23190 (7) | 0.00804 (14) | 0.61 |
K5 | 0.2413 (4) | 0.7665 (4) | 0.2429 (5) | 0.00804 (14) | 0.39 |
Mg7 | 0.38306 (8) | 0.51312 (8) | 0.26628 (9) | 0.0048 (3) | |
F1 | 0.5000 | 0.5189 (2) | 0.2500 | 0.0094 (6) | |
F2 | 0.44791 (15) | 0.39789 (15) | 0.36531 (16) | 0.0113 (5) | |
F3 | 0.28076 (15) | 0.49212 (16) | 0.29321 (17) | 0.0152 (5) | |
B1 | 0.5692 (3) | 0.8371 (3) | 0.4857 (3) | 0.0056 (8) | |
B2 | 0.6492 (3) | 0.8770 (3) | 0.3916 (3) | 0.0077 (8) | |
B3 | 0.2532 (3) | 0.8933 (3) | 0.4293 (3) | 0.0048 (8) | |
B4 | 0.7449 (3) | 0.8693 (3) | 0.5816 (3) | 0.0076 (8) | |
B5 | 0.4729 (3) | 0.6842 (3) | 0.4405 (3) | 0.0072 (8) | |
B6 | 0.3537 (3) | 0.8489 (3) | 0.6115 (3) | 0.0057 (8) | |
B7 | 0.4156 (3) | 0.8302 (3) | 0.4907 (3) | 0.0046 (8) | |
O1 | 0.49468 (15) | 0.88121 (18) | 0.49524 (17) | 0.0050 (5) | |
O2 | 0.36528 (16) | 0.83399 (18) | 0.70324 (18) | 0.0076 (5) | |
O3 | 0.26750 (17) | 0.88056 (18) | 0.52755 (19) | 0.0087 (5) | |
O4 | 0.66122 (16) | 0.85521 (17) | 0.58093 (18) | 0.0065 (5) | |
O5 | 0.73738 (17) | 0.88040 (18) | 0.48597 (19) | 0.0093 (6) | |
O6 | 0.45713 (17) | 0.59401 (18) | 0.39737 (19) | 0.0087 (5) | |
O7 | 0.64413 (17) | 0.86425 (18) | 0.30245 (19) | 0.0098 (6) | |
O8 | 0.32835 (17) | 0.88381 (18) | 0.41565 (18) | 0.0070 (5) | |
O9 | 0.55586 (17) | 0.73070 (18) | 0.46604 (19) | 0.0096 (5) | |
O10 | 0.16597 (16) | 0.91342 (18) | 0.35236 (18) | 0.0073 (5) | |
O11 | 0.56883 (16) | 0.88330 (18) | 0.39698 (18) | 0.0067 (5) | |
O12 | 0.42665 (16) | 0.83225 (18) | 0.59339 (18) | 0.0073 (5) | |
O13 | 0.82708 (17) | 0.87013 (18) | 0.6684 (2) | 0.0111 (6) | |
O14 | 0.40550 (17) | 0.72923 (18) | 0.4527 (2) | 0.0104 (6) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Ba1 | 0.00696 (19) | 0.00417 (16) | 0.00424 (17) | 0.000 | 0.00329 (14) | 0.000 |
K1 | 0.00696 (19) | 0.00417 (16) | 0.00424 (17) | 0.000 | 0.00329 (14) | 0.000 |
Ba2 | 0.00658 (19) | 0.00386 (16) | 0.00557 (17) | 0.000 | 0.00047 (13) | 0.000 |
K2 | 0.00658 (19) | 0.00386 (16) | 0.00557 (17) | 0.000 | 0.00047 (13) | 0.000 |
Ba3 | 0.00709 (13) | 0.00591 (12) | 0.00288 (12) | 0.00134 (8) | 0.00219 (9) | 0.00037 (8) |
K3 | 0.00709 (13) | 0.00591 (12) | 0.00288 (12) | 0.00134 (8) | 0.00219 (9) | 0.00037 (8) |
Ba4 | 0.00425 (14) | 0.01220 (13) | 0.00972 (13) | 0.00128 (8) | 0.00384 (10) | 0.00234 (9) |
K4 | 0.00425 (14) | 0.01220 (13) | 0.00972 (13) | 0.00128 (8) | 0.00384 (10) | 0.00234 (9) |
Ba5 | 0.0133 (4) | 0.0050 (3) | 0.0098 (3) | −0.0034 (2) | 0.0088 (3) | −0.0031 (2) |
K5 | 0.0133 (4) | 0.0050 (3) | 0.0098 (3) | −0.0034 (2) | 0.0088 (3) | −0.0031 (2) |
Mg7 | 0.0040 (6) | 0.0058 (6) | 0.0046 (6) | 0.0003 (4) | 0.0023 (5) | −0.0006 (4) |
F1 | 0.0073 (16) | 0.0070 (14) | 0.0159 (17) | 0.000 | 0.0074 (14) | 0.000 |
F2 | 0.0144 (12) | 0.0105 (11) | 0.0096 (11) | 0.0029 (9) | 0.0065 (10) | 0.0033 (8) |
F3 | 0.0061 (12) | 0.0311 (14) | 0.0086 (11) | −0.0016 (9) | 0.0039 (10) | −0.0032 (9) |
B1 | 0.004 (2) | 0.0086 (19) | 0.005 (2) | 0.0013 (15) | 0.0030 (17) | 0.0002 (15) |
B2 | 0.009 (2) | 0.0063 (18) | 0.009 (2) | −0.0007 (16) | 0.0061 (19) | 0.0016 (15) |
B3 | 0.006 (2) | 0.0055 (18) | 0.0030 (19) | −0.0013 (14) | 0.0022 (17) | 0.0002 (14) |
B4 | 0.010 (2) | 0.0053 (18) | 0.010 (2) | 0.0007 (15) | 0.0075 (19) | 0.0023 (15) |
B5 | 0.008 (2) | 0.0073 (19) | 0.0028 (19) | 0.0012 (16) | 0.0007 (17) | 0.0008 (15) |
B6 | 0.008 (2) | 0.0018 (17) | 0.008 (2) | 0.0012 (15) | 0.0049 (18) | 0.0003 (14) |
B7 | 0.004 (2) | 0.0064 (19) | 0.0039 (19) | 0.0005 (15) | 0.0027 (17) | −0.0006 (15) |
O1 | 0.0040 (13) | 0.0058 (12) | 0.0070 (13) | −0.0001 (9) | 0.0041 (11) | 0.0006 (9) |
O2 | 0.0073 (13) | 0.0122 (13) | 0.0021 (12) | 0.0026 (10) | 0.0015 (11) | 0.0018 (10) |
O3 | 0.0063 (14) | 0.0155 (13) | 0.0051 (13) | 0.0037 (10) | 0.0034 (12) | 0.0031 (10) |
O4 | 0.0040 (13) | 0.0099 (12) | 0.0041 (13) | −0.0008 (10) | 0.0011 (11) | 0.0010 (10) |
O5 | 0.0063 (14) | 0.0132 (13) | 0.0095 (14) | 0.0005 (10) | 0.0049 (12) | 0.0046 (10) |
O6 | 0.0077 (14) | 0.0101 (13) | 0.0048 (13) | 0.0012 (10) | 0.0009 (11) | −0.0038 (10) |
O7 | 0.0134 (14) | 0.0091 (13) | 0.0115 (14) | 0.0055 (10) | 0.0097 (12) | 0.0047 (10) |
O8 | 0.0032 (13) | 0.0132 (13) | 0.0042 (13) | 0.0034 (10) | 0.0017 (11) | 0.0017 (10) |
O9 | 0.0092 (14) | 0.0073 (12) | 0.0137 (14) | −0.0009 (10) | 0.0069 (12) | −0.0025 (10) |
O10 | 0.0051 (13) | 0.0096 (13) | 0.0057 (13) | 0.0015 (10) | 0.0017 (11) | 0.0025 (10) |
O11 | 0.0048 (13) | 0.0102 (12) | 0.0051 (13) | 0.0005 (10) | 0.0027 (11) | 0.0017 (10) |
O12 | 0.0065 (13) | 0.0115 (13) | 0.0062 (13) | 0.0026 (10) | 0.0048 (11) | 0.0010 (10) |
O13 | 0.0040 (13) | 0.0115 (13) | 0.0098 (14) | −0.0001 (10) | −0.0019 (11) | −0.0048 (10) |
O14 | 0.0064 (13) | 0.0090 (13) | 0.0184 (15) | −0.0027 (10) | 0.0082 (12) | −0.0049 (11) |
Ba1—F1 | 2.768 (3) | Ba5—F3xiv | 2.838 (2) |
Ba1—O13i | 2.773 (2) | Ba5—F2xiv | 2.938 (2) |
Ba1—O13ii | 2.773 (2) | Ba5—O10 | 3.076 (2) |
Ba1—O7 | 2.852 (3) | Ba5—O13i | 3.122 (3) |
Ba1—O7iii | 2.852 (3) | K5—O4i | 2.679 (7) |
Ba1—O11 | 2.907 (2) | K5—O2xii | 2.683 (7) |
Ba1—O11iii | 2.907 (2) | K5—O7iii | 2.692 (7) |
Ba1—O9 | 2.916 (3) | K5—O8 | 2.757 (7) |
Ba1—O9iii | 2.916 (3) | K5—O13i | 2.909 (6) |
Ba2—F2 | 2.642 (2) | K5—O14 | 2.975 (6) |
Ba2—F2iii | 2.642 (2) | Mg7—F3 | 1.968 (2) |
Ba2—O2iv | 2.737 (2) | Mg7—O6 | 2.038 (3) |
Ba2—O2v | 2.737 (2) | Mg7—O13i | 2.042 (3) |
Ba2—O12iv | 2.770 (2) | Mg7—F2 | 2.050 (2) |
Ba2—O12v | 2.770 (2) | Mg7—O10vi | 2.058 (3) |
Ba2—O10vi | 2.780 (2) | Mg7—F1 | 2.0867 (12) |
Ba2—O10vii | 2.780 (2) | B1—O1 | 1.450 (4) |
Ba2—F1 | 2.900 (3) | B1—O9 | 1.472 (4) |
Ba3—O7viii | 2.649 (2) | B1—O4 | 1.483 (4) |
Ba3—F3ix | 2.677 (2) | B1—O11 | 1.487 (4) |
Ba3—O2x | 2.682 (2) | B2—O7 | 1.326 (5) |
Ba3—O1 | 2.718 (2) | B2—O11 | 1.384 (5) |
Ba3—O1xi | 2.800 (2) | B2—O5 | 1.424 (5) |
Ba3—O4 | 2.822 (2) | B3—O10 | 1.336 (5) |
Ba3—O8xi | 2.860 (2) | B3—O8 | 1.376 (4) |
Ba3—O11xi | 2.929 (2) | B3—O3 | 1.397 (4) |
Ba3—O12 | 3.201 (2) | B4—O13 | 1.318 (5) |
Ba4—F3xii | 2.685 (2) | B4—O4 | 1.401 (5) |
Ba4—O6xiii | 2.692 (2) | B4—O5 | 1.401 (5) |
Ba4—O3 | 2.742 (2) | B5—O6 | 1.352 (5) |
Ba4—O6xii | 2.775 (2) | B5—O14 | 1.373 (4) |
Ba4—O5xi | 2.815 (2) | B5—O9 | 1.378 (5) |
Ba4—F2xiii | 2.855 (2) | B6—O2 | 1.322 (5) |
Ba4—O10 | 2.940 (2) | B6—O12 | 1.396 (4) |
Ba4—F2xii | 3.164 (2) | B6—O3 | 1.404 (5) |
Ba4—O14xii | 3.226 (3) | B7—O1 | 1.456 (4) |
Ba5—O7iii | 2.664 (3) | B7—O14 | 1.466 (4) |
Ba5—O4i | 2.738 (3) | B7—O12 | 1.476 (4) |
Ba5—O8 | 2.749 (3) | B7—O8 | 1.490 (4) |
Ba5—O2xii | 2.760 (3) | ||
O1—B1—O9 | 112.4 (3) | O10—B3—O8 | 123.0 (3) |
O1—B1—O4 | 109.7 (3) | O10—B3—O3 | 117.8 (3) |
O9—B1—O4 | 109.4 (3) | O8—B3—O3 | 119.1 (3) |
O1—B1—O11 | 108.4 (3) | O13—B4—O4 | 121.2 (3) |
O9—B1—O11 | 107.6 (3) | O13—B4—O5 | 121.7 (3) |
O4—B1—O11 | 109.3 (3) | O4—B4—O5 | 117.1 (3) |
O7—B2—O11 | 121.3 (4) | O6—B5—O14 | 120.3 (3) |
O7—B2—O5 | 121.3 (3) | O6—B5—O9 | 117.9 (3) |
O11—B2—O5 | 117.3 (3) | O14—B5—O9 | 121.7 (3) |
Symmetry codes: (i) x−1/2, −y+3/2, z−1/2; (ii) −x+3/2, −y+3/2, −z+1; (iii) −x+1, y, −z+1/2; (iv) x, −y+1, z−1/2; (v) −x+1, −y+1, −z+1; (vi) −x+1/2, y−1/2, −z+1/2; (vii) x+1/2, y−1/2, z; (viii) x, −y+2, z+1/2; (ix) x+1/2, −y+3/2, z+1/2; (x) −x+1, y, −z+3/2; (xi) −x+1, −y+2, −z+1; (xii) −x+1/2, −y+3/2, −z+1; (xiii) x−1/2, y+1/2, z; (xiv) −x+1/2, y+1/2, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | KBa7Mg2B14O28F5 |
Mr | 1743.44 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 93 |
a, b, c (Å) | 16.638 (3), 13.609 (2), 15.214 (3) |
β (°) | 121.309 (2) |
V (Å3) | 2943.3 (9) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 9.54 |
Crystal size (mm) | 0.24 × 0.23 × 0.20 |
Data collection | |
Diffractometer | Rigaku AFC10 diffractometer |
Absorption correction | Numerical (CrystalClear; Rigaku, 2002) |
Tmin, Tmax | 0.121, 0.148 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 11738, 3301, 2980 |
Rint | 0.029 |
(sin θ/λ)max (Å−1) | 0.649 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.021, 0.041, 1.08 |
No. of reflections | 3301 |
No. of parameters | 267 |
Δρmax, Δρmin (e Å−3) | 1.43, −1.06 |
Computer programs: CrystalClear (Rigaku, 2002), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997), publCIF (Westrip, 2009).
Borate crystals have found a wide range of applications as laser hosts, phosphors and nonlinear optcial (NLO) materials. Owing to the relatively large electron affinity difference between boron and oxygen, borate crystals often show a wide transparency range and a high resistance to laser or other radiation damage. Studies of borate systems have led to [the discovery of?] many new crystals possessing nonlinear optical properties, among which the low-temperature form LT-BaB2O4 (BBO; Chen et al., 1985) and LiB3O5 (LBO; Chen et al., 1989) are important industrial materials. Recently, fluoroborates have become a prime focus of our laboratory (Chen et al., 2006) because of their transmission cut-off at the deep ultraviolet (DUV) range (λ < 200 nm). For example, KBe2BO3F (KBBF; Chen et al., 1996) and RbBe2BO3F (RBBF; Chen et al., 2009) were found to be the only NLO crystals capable of generating a laser output below 200 nm by direct second-harmonic generation (SHG). In an attempt to grow crystals of a recently found noncentrosymmetric fluoroborate, BaMgBO3F, with KF flux, a single crystal of the title compound, KBa7Mg2B14O28F5, (I), was obtained.
Fig. 1 illustrates the unit-cell contents of (I), . A notable feature in the structure of (I) is the [B7O14]7- group (Fig. 2) which is composed of the combination of two B3O7 rings and one B3O8 ring. According to the scheme proposed by Christ & Clark (1977), the [B7O14]7- group can be classified as 7:[5Δ + 2T]. It is worth noting that all the terminal B—O bonds [1.318 (5)–1.352 (5) Å, average 1.331 (11) Å] in [B7O14]7- are significantly shorter than those within the rings [average 1.393 (11) Å for BO3 and 1.473 (19) Å for BO4]. The systematic variations of the B—O bond lengths within the rings (BT—TOT 1.453 Å, BT—TOΔ 1.479 Å; BΔ—ΔOΔ 1.407 Å, BΔ—ΔOT 1.385 Å) are also in good agreement with the scheme proposed by Filatov & Bubnova (2000). Though the B—O bond lengths display large variations [1.318 (5)–1.490 (4) Å], a bond valence sum (BVS) calculation (Brown & Altermatt, 1985) of B gives values from 2.962 to 3.043 (average 3.012), in good agreement with the ideal B valence. According to a recent survey of borate structure types (Touboul et al., 2003), such heptaborate groups have not previously been reported in any anhydrous borates. Only very recently, similar heptaborate groups with terminal hydroxyl groups (OH) were found in two hydrated alkaline borates (Liu & Li, 2006; Liu et al., 2006) and some organic tempelated hydrated borates (Schubert & Knobler, 2009).
Another distinctive feature of the structure is the double perovskite unit [Mg2O6F5]13-. The six O atoms in the unit come from six [B7O14]7- groups. By sharing these O atoms, [B7O14]7- groups connect [Mg2O6F5]13- to form a three-dimensional net with Ba atoms (partially K) occupying the voids. In the double perovskite units, two MgO3F3 octahedra share a common atom F1 and are related by a twofold rotation axis through F1. Along the axial direction of the double perovskite unit, Mg—F bonds show alternating short [1.968 (2) Å]–long [2.0867 (12) Å]–short [1.968 (2) Å] bond lengths. In contrast, the equatorial Mg—F and Mg—O bond lengths show little variation [(2.038 (3)–2.058 (3) Å].
Among the five cations, attention should be paid to the Ba5/K5 site. BVS calculations of Ba2+ showed that this site has significant valence deficiency (1.742) compared with the other Ba sites (1.913–2.386), indicating a strong tendency to host a lower valent cation such as K+, which is consistent with our refinement. Close inspection shows that the Ba5 site is located at a crossing of two large voids running along the a and c directions, leaving room for the split Ba/K position to optimize both Ba and K coordination.
A preliminary study shows that the sodium analog of the title compound can also be grown similarly from NaF flux. The structure reveals that it has a more disordered Ba/Na site with a larger Ba—Na separation than that of Ba—K in the title compound. It will be interesting to see whether other common perovskite units (e.g. BaTi2O5 or KNi2F5 etc) can also be included in the structure of the present compound.