Tetragonal form of barium cobalt disilicate, Ba₂CoSi₂O₇

The structure of Ba₂CoSi₂O₇ consists of layers made from corner-sharing of [SiO₄] and [CoO₄] polyhedra. These layers, lying parallel to (001), are linked by Ba...O interactions. Co²⁺ forms flattened CoO₄ tetrahedra (point symmetry ), which are isolated in the structure. The blue color of the solid might be explained by the coordination mode of the Co atoms.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Si-O) = 0.006 Å
• R factor = 0.025
• wR factor = 0.064
• Data-to-parameter ratio = 16.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level B:



PLAT_213  Alert B Atom O1      has ADP max/min Ratio ...........       4.40 prolate

General Notes



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           30.48
         From the CIF: _reflns_number_total                575
         Count of symmetry unique reflns          359
         Completeness (_total/calc)            160.17%
         TEST3: Check Friedels for noncentro structure
         Estimate of Friedel pairs measured       216
         Fraction of Friedel pairs measured     0.602
         Are heavy atom types Z>Si present          yes
          Please check that the estimate of the number of Friedel pairs is
          correct. If it is not, please give the correct count in the
          _publ_section_exptl_refinement section of the submitted CIF.


 0 Alert Level A = Potentially serious problem

 1 Alert Level B = Potential problem

 0 Alert Level C = Please check

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S1. Comment

In the mixed-silicate system MO–TO–SiO₂, where M is an alkaline earth metal and T is a divalent 3 d metal atom, many compounds of type (M,T)₃Si₂O₇ have been reported in the literature: Ca₂CoSi₂O₇ (Kimata, 1982, 1983; Hagiya et al.. 1993), Ca_2.33Mn_0.67Si₂O₇ (Kimata, 1989), Ca₂ZnSi₂O₇ (Warren et al., 1930; Louisnathan 1969), Sr₂CuSi₂O₇ (Tovar et al., 1998), BaCo₂Si₂O₇ (Adams et al., 1993), Ba₂CoSi₂O₇ (Adams et al., 1996), Ba₂CuSi₂O₇ (Malinovskii, 1984), BaCu₂Si₂O₇ (Janczak et al., 1990), BaZn₂Si₂O₇ (Lin et al., 1999). Adams et al. (1996) did not mention any structure transformation when they reported the monoclinic form Ba₂CoSi₂O₇. The present paper deals with a new tetragonal form of this mixed silicate, which is isostructutral with Ca₂MSi₂O₇ (M = Co and Zn). Crystals of the new phase were obtained quite by chance during the melting of a phosphate powder in a crucible made of quartz. The structure of Ba₂CoSi₂O₇ can de described as a two-dimensional framework of [SiO₄] and [CoO₄] tetrahedra sharing corners. These layers, lying parallel to the (001) plane at around x = 1/2, are interconnected by barium ions, as shown in Fig. 1. Alternatively, the structure can be described as being composed of BaO₈ polyhedra sharing edges and faces to form a sheet. These Ba sheets share corners with Si₂O₇ moieties to delimit tunnels, parallel to [001], which contain Co²⁺ cations. Fig. 2 depicts the projection of the structure on to the ab plane.

Co²⁺ forms flattened CoO₄ tetrahedra (point symmetry 4), the four apices of the tetrahedron belonging to four Si₂O₇^-4 groups, as shown on Fig. 3. The average Co—O distance of 1.964 Å which can be compared with the value of 1.926 Å in Ca₂CoSi₂O₇. Co²⁺ polyhedra are isolated in the structure, the shortest Co···Co distance is 5.778 Å, larger than that reported in Ca₂CoSi₂O₇ (5.015 Å; Kimata, 1982). The blue colour is a consequence of the geometry of Co²⁺. In general, for many compounds containing this metal in the 2+ oxidation state, the rose color points to an octahedral conformation, while a blue color indicates tetrahedral geometry.

Ba⁺² occupies an eight-coordinated site. The average Ba···O distance is 2.763 Å, a value close to the value of 2.814 Å reported for Ba₂CuSi₂O₇, but it is shorter than the value of 2.927 Å in monoclinic Ba₂CoSi₂O₇. Such a value is also reported for phosphates like Ba₂Ni(PO₄)₂ (2.765 Å; El Bali et al., 1994).

Tetrahedral Si⁴⁺ in two neighbouring SiO₄ units share O1 to form the pyrosilicate [Si₂O₇]^4- group. Si—O distances range from 1.592 to 1.664 Å, with an average of 1.633 Å, similar to other mixed pyrosilicates; e.g. 1.636 Å in Ca₂ZnSi₂O₇ and 1.628 Å in Ba₂CuSi₂O₇. Si₂O₇ can be also characterized by its almost eclipsed conformation and the bridging angle ϕ(Si,O,Si) of 142.8 (5)°, a value close to those reported in the homologous phases; e.g. 142.2° in monoclinic Ba₂CoSi₂O₇, 142° in Ba₂CuSi₂O₇ and 141.5° in Ba₂ZnSi₂O₇. Fig. 4 shows a fragment of Ba₂CoSi₂O₇ with the atomic connectivity.

S2. Experimental

A mixture of BaCO₃, CoCO₃ and (NH₄)₂HPO₄, in a 2:1:2 molar ratio, was ground and heated progressively to 1173 K. The resulting powder was then melted at 1423 K in a crucible made of quartz. Two materials were obtained, viz. blue crystals of the title compound and an unknown amorphous substance.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty,1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Perspective view of the crystal structure of Ba2CoSi2O7. Dashed yellow polyhedra [Si2O7], dashed blue polyhedra [CoO4] and grey balls Ba^+2.
	Fig. 2. Projection of the structure on to the ab plane. Blue polyhedra [BaO8], yellow polyhedra [Si2O7] and pink ball Co^2+.
	Fig. 3. Coordination of Co^2+ in Ba2CoSi2O7. Colors as in Fig. 2.
	Fig. 4. Perspective view of a fragment of Ba2CoSi2O7, showing the atomic connectivity, with the atom labeling. Displacement ellipsoids are drawn at 50% probability levels. Symmetry codes for equivalent atoms: (i) x - 1, y, z; (ii) x, y, z - 1; (iii) 1 - x, -y, z; (iv) 1 - x, -y, z - 1; (v) y, 1 - x, -z; (vi) y, 1 - x, 1 - z; (vii) -y, x - 1, 1 - z; (viii) x - 0.5, -y + 0.5, 1 - z; (ix) y + 0.5, x - 0.5, z; (x) y + 0.5, x - 0.5, z - 1.

Barium cobalt disilicate

Crystal data

Ba2CoSi2O7	Dx = 4.677 Mg m^−3
Mr = 501.79	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P421m	Cell parameters from 922 reflections
Hall symbol: P -4 2ab	θ = 3.6–30.3°
a = 8.1709 (7) Å	µ = 13.56 mm^−1
c = 5.3374 (7) Å	T = 293 K
V = 356.34 (6) Å^3	Prism, intense blue
Z = 2	0.13 × 0.05 × 0.05 mm
F(000) = 446

Data collection

Bruker SMART CCD area-detector diffractometer	575 independent reflections
Radiation source: fine-focus sealed tube	536 reflections with I > \2s(I)
Graphite monochromator	Rint = 0.026
ω scans	θmax = 30.5°, θmin = 3.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −11→6
Tmin = 0.392, Tmax = 0.522	k = −8→11
1383 measured reflections	l = −7→7

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)] = 0.025	w = 1/[σ^2(Fo^2) + (0.0223P)^2 + 0.683P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.064	(Δ/σ)max < 0.001
S = 1.18	Δρmax = 1.04 e Å^−3
575 reflections	Δρmin = −0.87 e Å^−3
34 parameters	Absolute structure: Flack (1983), 0000 Friedel pairs
0 restraints	Absolute structure parameter: −0.12 (7)

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
Ba1	0.33427 (4)	0.16573 (4)	0.00875 (10)	0.01252 (14)
Co1	0.0000	0.0000	0.5000	0.0087 (3)
Si1	0.63650 (18)	0.13650 (18)	0.5426 (4)	0.0060 (4)
O1	0.5000	0.0000	0.6421 (15)	0.0090 (16)
O2	0.8056 (5)	0.0780 (5)	0.6806 (8)	0.0094 (9)
O3	0.6393 (6)	0.1393 (6)	0.2445 (11)	0.0130 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.01256 (16)	0.01256 (16)	0.0124 (2)	0.00251 (16)	0.00097 (16)	−0.00097 (16)
Co1	0.0068 (4)	0.0068 (4)	0.0126 (7)	0.000	0.000	0.000
Si1	0.0060 (5)	0.0060 (5)	0.0060 (11)	−0.0012 (7)	0.0006 (5)	0.0006 (5)
O1	0.013 (3)	0.013 (3)	0.001 (4)	−0.006 (4)	0.000	0.000
O2	0.006 (2)	0.011 (2)	0.011 (2)	0.0016 (17)	0.0021 (17)	−0.0017 (16)
O3	0.017 (2)	0.017 (2)	0.006 (3)	−0.006 (3)	0.0017 (16)	0.0017 (16)

Geometric parameters (Å, º)

Ba1—O3^i	2.628 (6)	Si1—O3	1.592 (6)
Ba1—O2^ii	2.681 (4)	Si1—O2^x	1.637 (5)
Ba1—O2^iii	2.681 (4)	Si1—O2	1.637 (5)
Ba1—O1^iv	2.738 (6)	Si1—O1	1.664 (3)
Ba1—O3	2.800 (5)	O1—Si1^v	1.664 (3)
Ba1—O3^v	2.800 (5)	O1—Ba1^xi	2.738 (6)
Ba1—O2^vi	2.888 (4)	O1—Ba1^xii	2.738 (6)
Ba1—O2^vii	2.888 (4)	O2—Co1^xiii	1.964 (4)
Co1—O2^viii	1.964 (4)	O2—Ba1^xiv	2.681 (4)
Co1—O2^v	1.964 (4)	O2—Ba1^xi	2.888 (4)
Co1—O2^iii	1.964 (4)	O3—Ba1^xv	2.628 (6)
Co1—O2^ix	1.964 (4)	O3—Ba1^v	2.800 (5)
O3^i—Ba1—O2^ii	77.97 (15)	O2^viii—Co1—O2^iii	121.2 (3)
O3^i—Ba1—O2^iii	77.97 (15)	O2^v—Co1—O2^iii	103.93 (12)
O2^ii—Ba1—O2^iii	58.74 (19)	O2^viii—Co1—O2^ix	103.93 (12)
O3^i—Ba1—O1^iv	103.42 (17)	O2^v—Co1—O2^ix	121.2 (3)
O2^ii—Ba1—O1^iv	150.63 (10)	O2^iii—Co1—O2^ix	103.93 (12)
O2^iii—Ba1—O1^iv	150.63 (10)	O3—Si1—O2^x	116.2 (2)
O3^i—Ba1—O3	144.85 (11)	O3—Si1—O2	116.2 (2)
O2^ii—Ba1—O3	81.96 (13)	O2^x—Si1—O2	106.9 (3)
O2^iii—Ba1—O3	115.08 (14)	O3—Si1—O1	109.7 (4)
O1^iv—Ba1—O3	80.96 (13)	O2^x—Si1—O1	103.1 (2)
O3^i—Ba1—O3^v	144.85 (11)	O2—Si1—O1	103.1 (2)
O2^ii—Ba1—O3^v	115.08 (14)	Si1^v—O1—Si1	142.8 (5)
O2^iii—Ba1—O3^v	81.96 (13)	Si1^v—O1—Ba1^xi	103.17 (16)
O1^iv—Ba1—O3^v	80.96 (13)	Si1—O1—Ba1^xi	103.17 (16)
O3—Ba1—O3^v	70.2 (2)	Si1^v—O1—Ba1^xii	103.17 (16)
O3^i—Ba1—O2^vi	82.31 (12)	Si1—O1—Ba1^xii	103.17 (16)
O2^ii—Ba1—O2^vi	151.53 (5)	Ba1^xi—O1—Ba1^xii	88.8 (2)
O2^iii—Ba1—O2^vi	97.26 (18)	Si1—O2—Co1^xiii	123.8 (3)
O1^iv—Ba1—O2^vi	54.63 (10)	Si1—O2—Ba1^xiv	97.1 (2)
O3—Ba1—O2^vi	124.86 (13)	Co1^xiii—O2—Ba1^xiv	119.09 (19)
O3^v—Ba1—O2^vi	71.90 (14)	Si1—O2—Ba1^xi	98.0 (2)
O3^i—Ba1—O2^vii	82.31 (12)	Co1^xiii—O2—Ba1^xi	113.19 (18)
O2^ii—Ba1—O2^vii	97.26 (18)	Ba1^xiv—O2—Ba1^xi	101.42 (14)
O2^iii—Ba1—O2^vii	151.53 (5)	Si1—O3—Ba1^xv	122.1 (3)
O1^iv—Ba1—O2^vii	54.63 (10)	Si1—O3—Ba1	116.0 (2)
O3—Ba1—O2^vii	71.90 (14)	Ba1^xv—O3—Ba1	105.18 (15)
O3^v—Ba1—O2^vii	124.86 (13)	Si1—O3—Ba1^v	116.0 (2)
O2^vi—Ba1—O2^vii	100.25 (17)	Ba1^xv—O3—Ba1^v	105.18 (15)
O2^viii—Co1—O2^v	103.93 (12)	Ba1—O3—Ba1^v	86.30 (18)
O3—Si1—O1—Si1^v	0.0	O2^x—Si1—O1—Si1^v	124.43 (18)
O2—Si1—O1—Si1^v	−124.43 (18)

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