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Rubidium aluminium borate, Rb2Al2B2O7, is characterized by an association of AlO4 tetrahedra and BO3 triangles which form a complete three-dimensional aluminium borate framework. Rb+ cations occupy eight- and nine-coordinate positions within the three-dimensional channel system created by the framework.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536802015659/tk6074sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536802015659/tk6074Isup2.hkl
Contains datablock I

Key indicators

  • Single-crystal X-ray study
  • T = 293 K
  • Mean [sigma](O-B) = 0.005 Å
  • R factor = 0.030
  • wR factor = 0.075
  • Data-to-parameter ratio = 19.2

checkCIF results

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ADDSYM reports no extra symmetry








Comment top

The phase Rb2Al2B2O7 is a new phase first described here following a study of the system M2O—Al2O3—B2O3, where M = Na, K, Rb. Rb2Al2B2O7 crystallizes in the monoclinic space group P21/c and is characterized by a three-dimensional framework built from corner-sharing AlO4 tetrahedra and BO3 triangles surrounding a three-dimensional channel system in which the Rb atoms are located. Two crystallographically distinct Al atoms and two distinct B atoms are present in distorted tetrahedral AlO4 and trigonal-planar BO3 groups (Fig. 1). Each AlO4 group is connected to three BO3 groups and one AlO4 group to form an Al2O7 unit in which the Al–O–Al bond angle is 146.9 (2)°.

The structure can be considered to be built up from ten-membered Al6B4O10 rings, generated from corner-sharing AlO4 and BO3 polyhedra. The rings are linked in herring-bone fashion to form sheets in the bc plane (Fig. 2). Adjacent sheets are connected in a staggered formation through four-membered Al2B2O4 rings and eight-membered Al4B4O8 rings perpendicular to the b and c axes, respectively. Both crystallographically distinct Rb atoms have site symmetry 1. Rb1 is eight-coordinate within a coordination sphere of 3.5 Å and has a calculated bond valence of +1.01 (1). Rb2 is nine-coordinate within a 3.5 Å coordination sphere and has a calculated bond valence of +0.88 (1). Bond valences consistent with expected integral values are computed for each of the remaining atoms in the structure (Brese & O'Keeffe, 1991).

The structure of the material M2Al2B2O7 depends on the nature of the M cation. Na, K and Rb analogues assume three different structures, even when synthesized under identical conditions. Both the Na and K analogues of M2Al2B2O7 crystallize in trigonal space groups [P31c, a = 4.8087 (6), c = 15.2734 (6) Å and Z = 2 (Chang, 1998; He et al., 2001); P321, a = 8.5657 (9), c = 8.463 (2) Å and Z = 3 (Hu et al., 1998)]. Their structures are characterized by six-membered Al3B3O6 rings, built from AlO4 tetrahedra and BO3 triangles, that are linked together to form nearly planar sheets in the ab plane. In the Na analogue, these sheets are connected in pairs through linear Al–O–Al bonds to form layers which are linked through Na atoms to form a three-dimensional structure. In the K analogue, a three-dimensional Al–B–O framework is generated by Al–O–Al bonds between adjacent sheets and the K atoms are located in the space between these sheets.

We have found that up to 2.5% of the Rb atoms in Rb2Al2B2O7 can be replaced by either Na or K and the three-dimensional monoclinic structure is retained with essentially unchanged cell dimensions. Substitution of greater amounts of either Na or K cause the material to assume a structure more closely related to that of K2Al2B2O7.

Experimental top

Single crystals of Rb2Al2B2O7 were grown in a covered Pt crucible by melting a mixture of 42.0 wt% Rb2CO3 (99.8%, Alfa), 18.6 wt% Al2O3 (99.997%, Alfa), 13.3 wt% B2O3 (99.98%, Alfa) and 26.1 wt% LiBO2 (99.995%, Alfa), which acts as a flux to ensure congruent melting. The melt was heated at 1373 K for 16 h to ensure homogeneity, it was then cooled to room temperature at a rate or 0.07 K min−1. Numerous crystals formed in the crucible and a clear colourless block was physically separated from the matrix for analysis.

Refinement top

The Rb and Al atoms were located using the direct methods program SHELXS97 (Sheldrick, 1997) and the remaining atoms were placed following analysis of difference electron-density maps.

Computing details top

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1999); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN for Windows (Molecular Structure Corporation,1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Shape Software, 1998); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. The local coordination of atoms in Rb2Al2B2O7 (50% probability ellipsoids).
[Figure 2] Fig. 2. View of Rb2Al2B2O7 along [100] showing the three-dimensional Al–B–O framework and the Rb atoms in the channels. Yellow spheres = Rb atoms, grey tetrahedra = AlO4 and brown triangles = BO3 (ATOMS; Shape Software, 2002).
(I) top
Crystal data top
Rb2Al2B2O7F(000) = 664
Mr = 358.52Dx = 3.072 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71069 Å
a = 8.901 (2) ÅCell parameters from 21 reflections
b = 7.539 (1) Åθ = 15–20°
c = 11.905 (2) ŵ = 12.85 mm1
β = 103.97 (1)°T = 293 K
V = 775.3 (2) Å3Block, colourless
Z = 40.20 × 0.15 × 0.10 mm
Data collection top
Rigaku AFC-6R
diffractometer
1541 reflections with I > 2σ(I)
Radiation source: Rigaku rotating anodeRint = 0.058
Graphite monochromatorθmax = 30.1°, θmin = 3.2°
ω/2θ scansh = 1212
Absorption correction: ψ scan
(North et al., 1968)
k = 1010
Tmin = 0.113, Tmax = 0.277l = 1616
4763 measured reflections3 standard reflections every 400 reflections
2281 independent reflections intensity decay: 0.6%
Refinement top
Refinement on F20 restraints
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0237P)2 + 0.6214P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.030(Δ/σ)max < 0.001
wR(F2) = 0.075Δρmax = 0.65 e Å3
S = 1.01Δρmin = 0.57 e Å3
2281 reflectionsExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
119 parametersExtinction coefficient: 0.0097 (5)
Crystal data top
Rb2Al2B2O7V = 775.3 (2) Å3
Mr = 358.52Z = 4
Monoclinic, P21/cMo Kα radiation
a = 8.901 (2) ŵ = 12.85 mm1
b = 7.539 (1) ÅT = 293 K
c = 11.905 (2) Å0.20 × 0.15 × 0.10 mm
β = 103.97 (1)°
Data collection top
Rigaku AFC-6R
diffractometer
1541 reflections with I > 2σ(I)
Absorption correction: ψ scan
(North et al., 1968)
Rint = 0.058
Tmin = 0.113, Tmax = 0.2773 standard reflections every 400 reflections
4763 measured reflections intensity decay: 0.6%
2281 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.030119 parameters
wR(F2) = 0.0750 restraints
S = 1.01Δρmax = 0.65 e Å3
2281 reflectionsΔρmin = 0.57 e Å3
Special details top

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 (Å2) top
xyzUiso*/Ueq
Rb10.03371 (4)0.14003 (5)0.62650 (3)0.02092 (11)
Rb20.45632 (4)0.13338 (6)0.85435 (3)0.02143 (11)
Al10.34247 (12)0.12434 (14)0.62207 (8)0.0101 (2)
Al20.18525 (12)0.08342 (14)0.86588 (8)0.0113 (2)
O10.2027 (3)0.1892 (3)0.3953 (2)0.0176 (5)
O20.3084 (3)0.0199 (4)0.7370 (2)0.0191 (6)
O30.0064 (3)0.0407 (4)0.8620 (2)0.0180 (6)
O40.4631 (3)0.1894 (4)0.5892 (2)0.0195 (6)
O50.2828 (3)0.0324 (4)0.4962 (2)0.0226 (6)
O60.2373 (3)0.1790 (3)0.8455 (2)0.0167 (5)
O70.2294 (3)0.5381 (4)0.4795 (2)0.0249 (7)
B10.1552 (5)0.0873 (5)0.9114 (3)0.0134 (8)
B20.3191 (5)0.1147 (5)0.5634 (3)0.0129 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Rb10.0239 (2)0.02130 (18)0.01522 (17)0.00202 (16)0.00023 (14)0.00159 (15)
Rb20.02018 (19)0.0257 (2)0.01822 (17)0.00420 (16)0.00437 (13)0.00026 (15)
Al10.0111 (5)0.0101 (5)0.0085 (4)0.0004 (4)0.0016 (4)0.0002 (4)
Al20.0115 (5)0.0122 (5)0.0101 (5)0.0013 (4)0.0025 (4)0.0005 (4)
O10.0185 (13)0.0151 (11)0.0212 (13)0.0038 (11)0.0086 (11)0.0047 (10)
O20.0177 (13)0.0190 (13)0.0185 (12)0.0021 (11)0.0001 (10)0.0093 (11)
O30.0118 (12)0.0217 (13)0.0209 (13)0.0033 (11)0.0048 (10)0.0019 (11)
O40.0141 (12)0.0216 (13)0.0214 (13)0.0024 (11)0.0018 (10)0.0014 (11)
O50.0311 (16)0.0193 (13)0.0214 (13)0.0061 (12)0.0140 (12)0.0098 (11)
O60.0228 (13)0.0163 (13)0.0109 (11)0.0081 (11)0.0037 (10)0.0016 (9)
O70.0190 (14)0.0343 (16)0.0197 (14)0.0047 (13)0.0012 (11)0.0138 (13)
B10.0122 (18)0.0109 (17)0.0171 (18)0.0008 (14)0.0035 (15)0.0013 (14)
B20.0177 (19)0.0136 (18)0.0080 (14)0.0002 (16)0.0042 (14)0.0028 (14)
Geometric parameters (Å, º) top
Rb1—O62.808 (2)Rb2—B13.361 (4)
Rb1—O1i2.946 (3)Rb2—B23.387 (4)
Rb1—O32.968 (3)Rb2—O7v3.405 (3)
Rb1—O3ii3.048 (3)Rb2—O4viii3.455 (3)
Rb1—O13.056 (3)Rb2—Al2vi3.5577 (13)
Rb1—O53.105 (3)Al1—O21.716 (3)
Rb1—O5i3.126 (3)Al1—O6v1.746 (3)
Rb1—B13.326 (4)Al1—O4ix1.749 (3)
Rb1—O7iii3.403 (3)Al1—O5i1.762 (3)
Rb1—B2i3.403 (4)Al2—O21.725 (3)
Rb1—B23.410 (4)Al2—O31.747 (3)
Rb1—Al2ii3.5976 (12)Al2—O7x1.755 (3)
Rb2—O7iv2.924 (3)Al2—O1x1.764 (3)
Rb2—O2v3.009 (3)O3—B11.360 (5)
Rb2—O2vi3.014 (3)O6—B11.380 (5)
Rb2—O63.043 (3)O7—B1ii1.359 (5)
Rb2—O4vii3.086 (3)O1—B2i1.370 (5)
Rb2—O43.200 (3)O4—B21.366 (5)
Rb2—O1i3.297 (3)O5—B21.360 (5)
O2—Al1—O6v112.30 (13)O3—Al2—O1x108.52 (13)
O2—Al1—O4ix109.32 (14)O7x—Al2—O1x107.89 (14)
O6v—Al1—O4ix105.08 (14)O7v—B1—O3122.8 (4)
O2—Al1—O5i111.00 (15)O7v—B1—O6118.6 (3)
O6v—Al1—O5i105.17 (14)O3—B1—O6118.5 (3)
O4ix—Al1—O5i113.82 (14)O5—B2—O4123.0 (4)
O2—Al2—O3109.87 (14)O5—B2—O1i116.9 (3)
O2—Al2—O7x109.16 (15)O4—B2—O1i120.0 (3)
O3—Al2—O7x109.03 (14)Al1—O2—Al2146.85 (18)
O2—Al2—O1x112.30 (13)
Symmetry codes: (i) x, y, z+1; (ii) x, y+1/2, z+3/2; (iii) x, y+1, z+1; (iv) x+1, y+1/2, z+1/2; (v) x, y1/2, z+3/2; (vi) x+1, y, z; (vii) x, y1/2, z+1/2; (viii) x+1, y+1/2, z+3/2; (ix) x1, y, z; (x) x, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaRb2Al2B2O7
Mr358.52
Crystal system, space groupMonoclinic, P21/c
Temperature (K)293
a, b, c (Å)8.901 (2), 7.539 (1), 11.905 (2)
β (°) 103.97 (1)
V3)775.3 (2)
Z4
Radiation typeMo Kα
µ (mm1)12.85
Crystal size (mm)0.20 × 0.15 × 0.10
Data collection
DiffractometerRigaku AFC-6R
diffractometer
Absorption correctionψ scan
(North et al., 1968)
Tmin, Tmax0.113, 0.277
No. of measured, independent and
observed [I > 2σ(I)] reflections
4763, 2281, 1541
Rint0.058
(sin θ/λ)max1)0.705
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.030, 0.075, 1.01
No. of reflections2281
No. of parameters119
Δρmax, Δρmin (e Å3)0.65, 0.57

Computer programs: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1999), MSC/AFC Diffractometer Control Software, TEXSAN for Windows (Molecular Structure Corporation,1997), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ATOMS (Shape Software, 1998), WinGX (Farrugia, 1999).

Selected geometric parameters (Å, º) top
Rb1—O62.808 (2)Rb2—O4viii3.455 (3)
Rb1—O1i2.946 (3)Al1—O21.716 (3)
Rb1—O32.968 (3)Al1—O6v1.746 (3)
Rb1—O3ii3.048 (3)Al1—O4ix1.749 (3)
Rb1—O13.056 (3)Al1—O5i1.762 (3)
Rb1—O53.105 (3)Al2—O21.725 (3)
Rb1—O5i3.126 (3)Al2—O31.747 (3)
Rb1—O7iii3.403 (3)Al2—O7x1.755 (3)
Rb2—O7iv2.924 (3)Al2—O1x1.764 (3)
Rb2—O2v3.009 (3)O3—B11.360 (5)
Rb2—O2vi3.014 (3)O6—B11.380 (5)
Rb2—O63.043 (3)O7—B1ii1.359 (5)
Rb2—O4vii3.086 (3)O1—B2i1.370 (5)
Rb2—O43.200 (3)O4—B21.366 (5)
Rb2—O1i3.297 (3)O5—B21.360 (5)
Rb2—O7v3.405 (3)
O2—Al1—O6v112.30 (13)O3—Al2—O1x108.52 (13)
O2—Al1—O4ix109.32 (14)O7x—Al2—O1x107.89 (14)
O6v—Al1—O4ix105.08 (14)O7v—B1—O3122.8 (4)
O2—Al1—O5i111.00 (15)O7v—B1—O6118.6 (3)
O6v—Al1—O5i105.17 (14)O3—B1—O6118.5 (3)
O4ix—Al1—O5i113.82 (14)O5—B2—O4123.0 (4)
O2—Al2—O3109.87 (14)O5—B2—O1i116.9 (3)
O2—Al2—O7x109.16 (15)O4—B2—O1i120.0 (3)
O3—Al2—O7x109.03 (14)Al1—O2—Al2146.85 (18)
O2—Al2—O1x112.30 (13)
Symmetry codes: (i) x, y, z+1; (ii) x, y+1/2, z+3/2; (iii) x, y+1, z+1; (iv) x+1, y+1/2, z+1/2; (v) x, y1/2, z+3/2; (vi) x+1, y, z; (vii) x, y1/2, z+1/2; (viii) x+1, y+1/2, z+3/2; (ix) x1, y, z; (x) x, y+1/2, z+1/2.
 

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