Rb2Al2B2O7

Acta Cryst.(2002). E58, i85±i87 DOI: 10.1107/S1600536802015659 Kissick and Keszler Rb₂Al₂B₂O₇

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inorganic papers

Acta Crystallographica Section E Structure Reports Online

ISSN 1600-5368

Rb2Al2B2O7

Judith L. Kissicka_{* and} Douglas A. Keszlerb

a_{Department of Chemistry, University of}

Reading, Whiteknights, Reading RG6 6AD, England, andb_{Department of Chemistry,} Oregon State University, 153 Gilbert Hall, Corvallis, OR 97331, USA

Correspondence e-mail: [email protected]

Key indicators

Single-crystal X-ray study T= 293 K

Mean(O±B) = 0.005 AÊ Rfactor = 0.030 wRfactor = 0.075

Data-to-parameter ratio = 19.2

For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e.

#2002 International Union of Crystallography Printed in Great Britain ± all rights reserved

Rubidium aluminium borate, Rb2Al2B2O7, is characterized by

an association of AlO4 tetrahedra and BO3 triangles which

form a complete three-dimensional aluminium borate frame-work. Rb+_{cations occupy eight- and nine-coordinate positions}

within the three-dimensional channel system created by the framework.

Comment

The phase Rb2Al2B2O7 is a new phase ®rst described here,

following a study of the systemM2O±Al2O3±B2O3, whereM=

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 crystal-lographically distinct Al atoms and two distinct B atoms are present in distorted tetrahedral AlO4and trigonal-planar BO3

groups (Fig. 1). Each AlO4group is connected to three BO3

groups and one AlO4group to form an Al2O7unit in which the

AlÐOÐAl bond angle is 146.9 (2)_.

The structure can be considered to be built up from ten-membered Al6B4O10 rings, generated from corner-sharing

AlO4and BO3polyhedra. The rings are linked in herring-bone

fashion to form sheets in thebcplane (Fig. 2). Adjacent sheets are connected in a staggered formation through four-membered Al2B2O4rings and eight-membered Al4B4O8rings

perpendicular to thebandcaxes, respectively. Both crystal-lographically distinct Rb atoms have site symmetry 1. Rb1 is eight-coordinate within a coordination sphere of 3.5 AÊ and has a calculated bond valence of +1.01 (1). Rb2 is nine-coordinate within a 3.5 AÊ coordination sphere and has a calculated bond

Received 15 August 2002 Accepted 30 August 2002 Online 6 September 2002

Figure 1

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i86

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 materialM2Al2B2O7depends on the

nature of theMcation. 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) AÊ andZ= 2 (Chang, 1998; Heet al., 2001);P321,

a= 8.5657 (9), c= 8.463 (2) AÊ andZ = 3 (Hu et al., 1998)]. Their structures are characterized by six-membered Al3B3O6

rings, built from AlO4tetrahedra and BO3triangles, that are

linked together to form nearly planar sheets in theabplane. In the Na analogue, these sheets are connected in pairs through linear AlÐOÐAl bonds to form layers, which are linked through Na atoms to form a three-dimensional structure. In the K analogue, a three-dimensional AlÐBÐO framework is generated by AlÐOÐAl 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 Rb2Al2B2O7can 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 causes the material to assume a structure more closely related to that of K2Al2B2O7.

Experimental

Single crystals of Rb2Al2B2O7were 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 ¯ux to ensure congruent melting. The melt was heated at 1373 K for 16 h to ensure homo-geneity, it was then cooled to room temperature at a rate of 0.07 K minÿ1_{. Numerous crystals formed in the crucible and a clear} colourless block was physically separated from the matrix for analysis.

Crystal data Rb2Al2B2O7

Mr= 358.52

Monoclinic,P21/c

a= 8.901 (2) AÊ

b= 7.539 (1) AÊ

c= 11.905 (2) AÊ = 103.97 (1) V= 775.3 (2) AÊ3

Z= 4

Dx= 3.072 Mg mÿ3

MoKradiation Cell parameters from 21

re¯ections = 15±20

= 12.85 mmÿ1

T= 293 (2) K Block, colourless 0.200.150.10 mm Data collection

Rigaku AFC-6Rdiffractometer !/2scans

Absorption correction: scan (Northet al., 1968)

Tmin= 0.113,Tmax= 0.277

4763 measured re¯ections 2281 independent re¯ections 1541 re¯ections withI> 2(I)

Rint= 0.058

max= 30.1

h=ÿ12!12

k=ÿ10!10

l=ÿ16!16 3 standard re¯ections

every 400 re¯ections intensity decay: 0.6% Re®nement

Re®nement onF2

R[F2_{> 2(F}2_{)] = 0.030}

wR(F2_{) = 0.075}

S= 1.01 2281 re¯ections 119 parameters

w= 1/[2_(F

o2) + (0.0237P)2

+ 0.6214P]

whereP= (Fo2+ 2Fc2)/3

(/)max< 0.001 max= 0.65 e AÊÿ3 min=ÿ0.57 e AÊÿ3

Extinction correction:SHELXL97 Extinction coef®cient: 0.0097 (5)

Table 1

Selected geometric parameters (AÊ,_).

Rb1ÐO6 2.808 (2) Rb1ÐO1i _{2.946 (3)}

Rb1ÐO3 2.968 (3) Rb1ÐO3ii _{3.048 (3)}

Rb1ÐO1 3.056 (3) Rb1ÐO5 3.105 (3) Rb1ÐO5i _{3.126 (3)}

Rb1ÐO7iii _{3.403 (3)}

Rb2ÐO7iv _{2.924 (3)}

Rb2ÐO2v _{3.009 (3)}

Rb2ÐO2vi _{3.014 (3)}

Rb2ÐO6 3.043 (3) Rb2ÐO4vii _{3.086 (3)}

Rb2ÐO4 3.200 (3) Rb2ÐO1i _{3.297 (3)}

Rb2ÐO7v _{3.405 (3)}

Rb2ÐO4viii _{3.455 (3)}

Al1ÐO2 1.716 (3) Al1ÐO6v _{1.746 (3)}

Al1ÐO4ix _{1.749 (3)}

Al1ÐO5i _{1.762 (3)}

Al2ÐO2 1.725 (3) Al2ÐO3 1.747 (3) Al2ÐO7x _{1.755 (3)}

Al2ÐO1x _{1.764 (3)}

O3ÐB1 1.360 (5) O6ÐB1 1.380 (5) O7ÐB1ii _{1.359 (5)}

O1ÐB2i _{1.370 (5)}

O4ÐB2 1.366 (5) O5ÐB2 1.360 (5)

O2ÐAl1ÐO6v _{112.30 (13)}

O2ÐAl1ÐO4ix _{109.32 (14)}

O6v_ÐAl1ÐO4ix _{105.08 (14)}

O2ÐAl1ÐO5i _{111.00 (15)}

O6v_ÐAl1ÐO5i _{105.17 (14)}

O4ix_ÐAl1ÐO5i _{113.82 (14)}

O2ÐAl2ÐO3 109.87 (14) O2ÐAl2ÐO7x _{109.16 (15)}

O3ÐAl2ÐO7x _{109.03 (14)}

O2ÐAl2ÐO1x _{112.30 (13)}

O3ÐAl2ÐO1x _{108.52 (13)}

O7x_ÐAl2ÐO1x _{107.89 (14)}

O7v_{ÐB1ÐO3 122.8 (4)}

O7v_{ÐB1ÐO6 118.6 (3)}

O3ÐB1ÐO6 118.5 (3) O5ÐB2ÐO4 123.0 (4) O5ÐB2ÐO1i _{116.9 (3)}

O4ÐB2ÐO1i _{120.0 (3)}

Al1ÐO2ÐAl2 146.85 (18)

Symmetry codes: (i) ÿx;ÿy;1ÿz; (ii) ÿx;1

2‡y;32ÿz; (iii) ÿx;1ÿy;1ÿz; (iv)

1‡x;1

2ÿy;12‡z; (v) ÿx;yÿ12;32ÿz; (vi) 1‡x;y;z; (vii) x;ÿ12ÿy;12‡z; (viii)

1ÿx;1

2‡y;32ÿz; (ix)xÿ1;y;z; (x)x;12ÿy;12‡z.

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1999); cell re®nement: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 re®ne structure:SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS(Shape Software, 1998); software used to prepare material for publication:WinGX(Farrugia, 1999).

Figure 2

This work was supported by the Solid State Chemistry Program of the National Science Foundation. JLK would like to thank Dr A. M. Chippindale for helpful discussion about this work.

References

Brese, N. E. & O'Keeffe, M. (1991).Acta Cryst.B47, 192±197. Chang, K.-S. (1998). PhD dissertation, Oregon State University, USA. Farrugia, L. J. (1999).J. Appl. Cryst.32, 837±838.

He, M., Chen, X. L., Zhou, T., Hu, B. Q., Xu, Y. P. & Xu, T. (2001).J. Alloys Compds,327, 210±214.

Hu, Z. G., Higashiyama, T., Yoshimura, M., Yap, Y. K., Mori, Y. & Sasaki, T. (1998).Jpn. J. Appl. Phys. Part2Lett.32, L1093±1094.

Molecular Structure Corporation (1997).TEXSAN for Windows. Version 1.0. MSC, 9009 New Trails Drive, The Woodlands, TX 77381, USA.

Molecular Structure Corporation (1999).MSC/AFC Diffractometer Control Software. MSC, 9009 New Trails Drive, The Woodlands, TX 77381 USA. North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968).Acta Cryst.A24, 351±

359.

Shape Software (1998).ATOMS. Shape Software, 521 Hidden Valley Rd, Kingsport, TN 37663, USA.

Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of GoÈttingen, Germany.

Acta Cryst.(2002). E58, i85±i87 Kissick and Keszler Rb₂Al₂B₂O₇

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Acta Cryst. (2002). E58, i85–i87

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Acta Cryst. (2002). E58, i85–i87 [doi:10.1107/S1600536802015659]

Rb

Al

B

O

Judith L. Kissick and Douglas A. Keszler

S1. Comment

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.

S2. Experimental

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}

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Acta Cryst. (2002). E58, i85–i87

S3. Refinement

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.

Figure 1

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Acta Cryst. (2002). E58, i85–i87

Figure 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)

Crystal data

Rb2Al2B2O7

Mr = 358.52

Monoclinic, P21/c

a = 8.901 (2) Å

b = 7.539 (1) Å

c = 11.905 (2) Å

β = 103.97 (1)°

V = 775.3 (2) Å3

Z = 4

F(000) = 664

Dx = 3.072 Mg m−3

Mo Kα radiation, λ = 0.71069 Å Cell parameters from 21 reflections

θ = 15–20°

µ = 12.85 mm−1

T = 293 K Block, colourless 0.20 × 0.15 × 0.10 mm

Data collection

Rigaku AFC-6R diffractometer

Radiation source: Rigaku rotating anode Graphite monochromator

ω/2θ scans

Absorption correction: ψ scan (North et al., 1968)

Tmin = 0.113, Tmax = 0.277 4763 measured reflections

2281 independent reflections 1541 reflections with I > 2σ(I)

Rint = 0.058

θmax = 30.1°, θmin = 3.2°

h = −12→12

k = −10→10

l = −16→16

3 standard reflections every 400 reflections intensity decay: 0.6%

Refinement

Refinement on F2 Least-squares matrix: full

R[F2 _{> 2σ(F}2_{)] = 0.030}

wR(F2_{) = 0.075}

S = 1.01 2281 reflections 119 parameters 0 restraints

w = 1/[σ2_(F

o2) + (0.0237P)2 + 0.6214P] where P = (Fo2 + 2Fc2)/3

(Δ/σ)max < 0.001 Δρmax = 0.65 e Å−3 Δρmin = −0.57 e Å−3

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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 F}2_, conventional R-factors R are based on F, with F set to zero for negative F2_{. The threshold expression of F}2 _{> σ(F}2_{) 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₎

x y z Uiso*/Ueq

Rb1 0.03371 (4) 0.14003 (5) 0.62650 (3) 0.02092 (11)

Rb2 0.45632 (4) −0.13338 (6) 0.85435 (3) 0.02143 (11)

Al1 −0.34247 (12) −0.12434 (14) 0.62207 (8) 0.0101 (2)

Al2 −0.18525 (12) 0.08342 (14) 0.86588 (8) 0.0113 (2)

O1 −0.2027 (3) 0.1892 (3) 0.3953 (2) 0.0176 (5)

O2 −0.3084 (3) 0.0199 (4) 0.7370 (2) 0.0191 (6)

O3 0.0064 (3) 0.0407 (4) 0.8620 (2) 0.0180 (6)

O4 0.4631 (3) −0.1894 (4) 0.5892 (2) 0.0195 (6)

O5 0.2828 (3) 0.0324 (4) 0.4962 (2) 0.0226 (6)

O6 0.2373 (3) 0.1790 (3) 0.8455 (2) 0.0167 (5)

O7 −0.2294 (3) 0.5381 (4) 0.4795 (2) 0.0249 (7)

B1 0.1552 (5) 0.0873 (5) 0.9114 (3) 0.0134 (8)

B2 0.3191 (5) −0.1147 (5) 0.5634 (3) 0.0129 (7)

Atomic displacement parameters (Å2₎

U11 _U22 _U33 _U12 _U13 _U23

Rb1 0.0239 (2) 0.02130 (18) 0.01522 (17) 0.00202 (16) 0.00023 (14) −0.00159 (15)

Rb2 0.02018 (19) 0.0257 (2) 0.01822 (17) −0.00420 (16) 0.00437 (13) −0.00026 (15)

Al1 0.0111 (5) 0.0101 (5) 0.0085 (4) 0.0004 (4) 0.0016 (4) 0.0002 (4)

Al2 0.0115 (5) 0.0122 (5) 0.0101 (5) −0.0013 (4) 0.0025 (4) −0.0005 (4)

O1 0.0185 (13) 0.0151 (11) 0.0212 (13) 0.0038 (11) 0.0086 (11) 0.0047 (10)

O2 0.0177 (13) 0.0190 (13) 0.0185 (12) 0.0021 (11) −0.0001 (10) −0.0093 (11)

O3 0.0118 (12) 0.0217 (13) 0.0209 (13) −0.0033 (11) 0.0048 (10) −0.0019 (11)

O4 0.0141 (12) 0.0216 (13) 0.0214 (13) −0.0024 (11) 0.0018 (10) −0.0014 (11)

O5 0.0311 (16) 0.0193 (13) 0.0214 (13) 0.0061 (12) 0.0140 (12) 0.0098 (11)

O6 0.0228 (13) 0.0163 (13) 0.0109 (11) −0.0081 (11) 0.0037 (10) −0.0016 (9)

O7 0.0190 (14) 0.0343 (16) 0.0197 (14) 0.0047 (13) 0.0012 (11) −0.0138 (13)

B1 0.0122 (18) 0.0109 (17) 0.0171 (18) −0.0008 (14) 0.0035 (15) −0.0013 (14)

B2 0.0177 (19) 0.0136 (18) 0.0080 (14) −0.0002 (16) 0.0042 (14) −0.0028 (14)

Geometric parameters (Å, º)

Rb1—O6 2.808 (2) Rb2—B1 3.361 (4)

Rb1—O1i _{2.946 (3) Rb2—B2 3.387 (4)}

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Acta Cryst. (2002). E58, i85–i87

Rb1—O3ii _{3.048 (3) Rb2—O4}viii _{3.455 (3)}

Rb1—O1 3.056 (3) Rb2—Al2vi _{3.5577 (13)}

Rb1—O5 3.105 (3) Al1—O2 1.716 (3)

Rb1—O5i _{3.126 (3) Al1—O6}v _{1.746 (3)}

Rb1—B1 3.326 (4) Al1—O4ix _{1.749 (3)}

Rb1—O7iii _{3.403 (3) Al1—O5}i _{1.762 (3)}

Rb1—B2i _{3.403 (4) Al2—O2 1.725 (3)}

Rb1—B2 3.410 (4) Al2—O3 1.747 (3)

Rb1—Al2ii _{3.5976 (12) Al2—O7}x _{1.755 (3)}

Rb2—O7iv _{2.924 (3) Al2—O1}x _{1.764 (3)}

Rb2—O2v _{3.009 (3) O3—B1 1.360 (5)}

Rb2—O2vi _{3.014 (3) O6—B1 1.380 (5)}

Rb2—O6 3.043 (3) O7—B1ii _{1.359 (5)}

Rb2—O4vii _{3.086 (3) O1—B2}i _{1.370 (5)}

Rb2—O4 3.200 (3) O4—B2 1.366 (5)

Rb2—O1i _{3.297 (3) O5—B2 1.360 (5)}

O2—Al1—O6v _{112.30 (13) O3—Al2—O1}x _{108.52 (13)}

O2—Al1—O4ix _{109.32 (14) O7}x_—Al2—O1x _{107.89 (14)}

O6v_—Al1—O4ix _{105.08 (14) O7}v_{—B1—O3 122.8 (4)}

O2—Al1—O5i _{111.00 (15) O7}v_{—B1—O6 118.6 (3)}

O6v_—Al1—O5i _{105.17 (14) O3—B1—O6 118.5 (3)}

O4ix_—Al1—O5i _{113.82 (14) O5—B2—O4 123.0 (4)}

O2—Al2—O3 109.87 (14) O5—B2—O1i _{116.9 (3)}

O2—Al2—O7x _{109.16 (15) O4—B2—O1}i _{120.0 (3)}

O3—Al2—O7x _{109.03 (14) Al1—O2—Al2 146.85 (18)}

O2—Al2—O1x _{112.30 (13)}