Re-investigation of Cd₂B₂O₅: evidence of a centre of symmetry

The previously determined structure of cadmium diborate, Cd₂B₂O₅, has been re-investigated and a change in space-group symmetry from P1 to P evidenced. Cd₂B₂O₅ crystallizes isotypically with other triclinic members of the M₂B₂O₅ (M = Mg, Mn, Co, Fe) family and is composed of slightly distorted [CdO₆] octahedra, forming ribbons built of four condensed single-chains and diborate anions, B₂O₅^2-, which are composed of corner-sharing BO₃ triangles and which interconnect adjacent ribbons.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](O-B) = 0.002 Å
• R factor = 0.022
• wR factor = 0.056
• Data-to-parameter ratio = 31.1

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

General Notes



ABSTM_02  The ratio of expected to reported Tmax/Tmin(RR) is > 1.10
          Tmin and Tmax reported:      0.267     0.473
          Tmin and Tmax expected:      0.199     0.496
          RR =      1.408
          Please check that your absorption correction is appropriate.

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S1. Comment

Cd₂B₂O₅, (I), is an interesting host material for luminescent applications when doped with tranistion metal or RE ions. Its structure has been determined by Sokolova et al. (1979) and was described in space group P1. Comparison of the symmetry and lattice parameters of other M₂B₂O₅ members [M = Co (Berger, 1950), Mg, Fe, Mn (Block et al., 1959), Mg (Guo et al., 1995a)] and solid solutions MM'B₂O₅ [M = Mn, M' = Mg, Co (Utzolino & Bluhm, 1996); M = Zn, M' = Co, Ni (Busche & Bluhm, 1995)], which all crystallize with similar lattice parameters in the centrosymmetric space group P1, suggested a possible change in the space-group symmetry. Therefore the published atomic coordinates (Sokolova et al., 1979) were checked with the PLATON program (Spek, 2003) which, in fact, indicated a centre of symmetry within the default tolerances of the program. For experimental proof, single crystals of Cd₂B₂O₅ were grown and the structure was re-determined in space group P1.

Cd₂B₂O₅ crystallizes isotypically with the triclinic representatives of the M₂B₂O₅ family mentioned above. For all these structures, two angles close to 90° are observed, indicating a possible phase transition to the monoclinic crystal system. At least for Mg₂B₂O₅, a synthetic monoclinic polymorph is described (Guo et al., 1995b) which is identical to the mineral Suanite (Takeuchi, 1952).

The crystal structure is composed of two crystallographically independent and distorted [CdO₆] octahedra, and B₂O₅ groups as the main building units. The [CdO₆] octahedra have mean Cd—O distances of 2.303 Å (Cd1) and 2.334 Å (Cd2) and build chains running parallel to the [100] direction by edge-sharing (Fig. 1). Four of these chains are connected to form ribbons along the [011] direction (Fig. 2). Adjacent ribbons are held together by the interstitial B₂O₅ anions.

As in other diborate structures with two condensed BO₃ triangles, the corresponding polyhedra are substantially distorted. The distances from B to the bridging atom O4 are considerably longer than to the terminal atoms O1, O2, O3 and O5 (see Table 1). The B—O distances for the two independent BO₃ triangles are very similar, and the average B—O distances ¯d(B1—O) = 1.377 Å and ¯d(B2—O) = 1.381 Å are in good agreement with the data for many other borate structures with BO₃ groups (Zobetz, 1982). The B₂O₅ anion (Fig. 3) deviates significantly from coplanarity; the dihedral angle between the two slanting BO₃ triangles is 13.1 (1)°.

The O atoms O1, O3 and O5 exhibit coordination number 4 and are each surrounded by three Cd and one B atom. O2 has three coordination partners (2 x Cd and B) with two short Cd—O distances and a short B—O distance. O4 is the bridging atom of the diborate group and has an additional Cd atom with a long Cd—O distance in its coordination environment.

S2. Experimental

Stoichiometric amounts of CdCO₃ (Merck, p·A.) and H₃BO₃ (10% excess, Merck, p·A.) were ground together finely in an agate mortar and charged in a platinum crucible which was heated to 1353 K over the course of 5 h, kept at that temperature for 1 h and cooled to 973 K over 4 d. The furnace was then shut off. After leaching with boiling demineralized water, colourless single crystals of Cd₂B₂O₅, with mainly plate-like habit and an edge-length of up to 2 mm, were isolated.

S3. Refinement

The structure was refined with the atomic coordinates of the isomorphous solid solution MnMgB₂O₅ (Utzolino & Bluhm, 1996) as starting parameters. The refined positional parameters were afterwards standardized using the program STRUCTURE-TIDY (Gelato & Parthé, 1987). The highest difference peak is located at a distance of 0.59 Å from Cd1, and the deepest hole 0.63 Å from this atom.

Computing details

Data collection: CAD-4 Software (Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA implemented in PLATON (Spek, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Slice through the structure, approximately parallel to (011). The structure is plotted in polyhedral representation; [Cd1O6] octahedra are orange, [Cd2O6] octahedra are yellow and B2O5 groups are blue.
	Fig. 2. Projection of the crystal structure in polyhedral representation, viewed along [100]. [Cd1O6] octahedra are orange, [Cd2O6] octahedra are yellow and B2O5 groups are blue.
	Fig. 3. ORTEP plot of the diborate anion, with anisotropic displacement ellipsoids drawn at the 90% probability level.

Cadmium diborate

Crystal data

Cd2(B2O5)	Z = 2
Mr = 326.42	F(000) = 292
Triclinic, P1	Dx = 5.157 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 3.4490 (2) Å	Cell parameters from 25 reflections
b = 6.3603 (5) Å	θ = 12.4–14.9°
c = 9.9502 (8) Å	µ = 10.02 mm^−1
α = 105.441 (8)°	T = 293 K
β = 90.807 (6)°	Plate, colourless
γ = 91.933 (6)°	0.22 × 0.14 × 0.07 mm
V = 210.22 (3) Å^3

Data collection

Enraf-Nonius CAD-4 diffractometer	2464 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.018
Graphite monochromator	θmax = 39.9°, θmin = 2.1°
ω/2θ scans	h = −6→6
Absorption correction: numerical The crystal shape was optimized by minimizing the R-value of selected ψ-scanned reflections using the program HABITUS (Herrendorf, 1993-97). The habit so derived was used for the numerical absorption correction.	k = −11→11
Tmin = 0.267, Tmax = 0.473	l = −17→17
5166 measured reflections	3 standard reflections every 500 min
2584 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0307P)^2 + 0.3225P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.022	(Δ/σ)max = 0.001
wR(F^2) = 0.056	Δρmax = 2.33 e Å^−3
S = 1.13	Δρmin = −2.03 e Å^−3
2584 reflections	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
83 parameters	Extinction coefficient: 0.238 (4)
0 restraints

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
Cd1	0.22524 (3)	0.34751 (2)	0.593004 (13)	0.00856 (4)
Cd2	0.24898 (4)	0.77837 (2)	0.125010 (13)	0.00940 (5)
B1	0.3291 (6)	0.3271 (3)	0.1555 (2)	0.0081 (3)
B2	0.6800 (6)	0.1264 (3)	0.3200 (2)	0.0081 (3)
O1	0.1972 (5)	0.0792 (2)	0.69646 (17)	0.0127 (2)
O2	0.2570 (5)	0.2897 (3)	0.01669 (16)	0.0128 (2)
O3	0.2747 (4)	0.5244 (2)	0.24790 (16)	0.0103 (2)
O4	0.4788 (5)	0.1526 (2)	0.20034 (16)	0.0132 (2)
O5	0.7276 (4)	0.2963 (2)	0.43691 (15)	0.0097 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.00838 (6)	0.00948 (6)	0.00881 (6)	0.00093 (4)	−0.00033 (4)	0.00410 (4)
Cd2	0.00867 (6)	0.01101 (6)	0.00860 (6)	0.00064 (4)	−0.00024 (4)	0.00275 (4)
B1	0.0082 (6)	0.0091 (6)	0.0072 (6)	0.0008 (5)	−0.0005 (5)	0.0027 (5)
B2	0.0092 (6)	0.0076 (6)	0.0078 (6)	0.0020 (5)	0.0009 (5)	0.0025 (5)
O1	0.0181 (6)	0.0097 (5)	0.0122 (6)	0.0055 (4)	0.0028 (5)	0.0055 (4)
O2	0.0111 (5)	0.0194 (6)	0.0081 (5)	0.0015 (5)	−0.0006 (4)	0.0038 (5)
O3	0.0118 (5)	0.0092 (5)	0.0095 (5)	0.0027 (4)	0.0002 (4)	0.0015 (4)
O4	0.0212 (6)	0.0078 (5)	0.0101 (5)	0.0024 (4)	−0.0057 (5)	0.0016 (4)
O5	0.0107 (5)	0.0098 (5)	0.0080 (5)	0.0010 (4)	−0.0005 (4)	0.0013 (4)

Geometric parameters (Å, º)

Cd1—O1	2.2152 (15)	Cd2—O1^iii	2.3787 (16)
Cd1—O5^i	2.2544 (15)	Cd2—O4^vii	2.4004 (15)
Cd1—O3^ii	2.2993 (15)	Cd2—O1^ii	2.5603 (18)
Cd1—O5	2.3162 (15)	Cd2—Cd2^iv	3.4490 (2)
Cd1—O5^ii	2.3631 (15)	Cd2—Cd2^i	3.4490 (2)
Cd1—O3^iii	2.3722 (15)	B1—O2	1.356 (3)
Cd1—Cd1^iii	3.4112 (3)	B1—O3	1.364 (2)
Cd1—Cd1^iv	3.4490 (2)	B1—O4	1.411 (3)
Cd1—Cd1^i	3.4490 (2)	B2—O1^viii	1.357 (2)
Cd2—O2^v	2.1853 (16)	B2—O5	1.364 (3)
Cd2—O2^vi	2.2071 (16)	B2—O4	1.421 (3)
Cd2—O3	2.2742 (15)
O1—Cd1—O5^i	108.32 (6)	O2^vi—Cd2—O1^iii	167.32 (6)
O1—Cd1—O3^ii	82.70 (6)	O3—Cd2—O1^iii	79.56 (5)
O5^i—Cd1—O3^ii	167.91 (6)	O2^v—Cd2—O4^vii	116.07 (6)
O1—Cd1—O5	110.00 (5)	O2^vi—Cd2—O4^vii	88.75 (6)
O5^i—Cd1—O5	97.97 (6)	O3—Cd2—O4^vii	127.06 (5)
O3^ii—Cd1—O5	82.35 (5)	O1^iii—Cd2—O4^vii	79.34 (6)
O1—Cd1—O5^ii	160.36 (6)	O2^v—Cd2—O1^ii	170.77 (6)
O5^i—Cd1—O5^ii	84.78 (5)	O2^vi—Cd2—O1^ii	81.11 (5)
O3^ii—Cd1—O5^ii	83.31 (5)	O3—Cd2—O1^ii	75.95 (5)
O5—Cd1—O5^ii	81.63 (5)	O1^iii—Cd2—O1^ii	88.50 (5)
O1—Cd1—O3^iii	80.89 (5)	O4^vii—Cd2—O1^ii	55.55 (5)
O5^i—Cd1—O3^iii	82.07 (5)	O2—B1—O3	122.46 (17)
O3^ii—Cd1—O3^iii	95.16 (5)	O2—B1—O4	116.53 (17)
O5—Cd1—O3^iii	168.29 (5)	O3—B1—O4	120.97 (17)
O5^ii—Cd1—O3^iii	86.72 (5)	O1^viii—B2—O5	126.35 (18)
O2^v—Cd2—O2^vi	103.48 (6)	O1^viii—B2—O4	112.94 (16)
O2^v—Cd2—O3	110.08 (6)	O5—B2—O4	120.70 (17)
O2^vi—Cd2—O3	104.69 (6)	B1—O4—B2	136.82 (16)
O2^v—Cd2—O1^iii	85.84 (6)

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