A cubic calcium oxynitrido-silicate, Ca_2.89Si₂N_1.76O_4.24

The title compound, tricalcium oxynitride silicate, with composition Ca_3-xSi₂N_2-2xO_4+2x (x ≃ 0.12), is a perovskite-related calcium oxynitrido silicate containing isolated oxynitrido silicate 12-rings. The N atoms are statistically disordered with O atoms (occupancy ratio N:O = 0.88:0.12) and occupy the bridging positions in the 12 ring oxynitrido silicate anion, while the remaining O atoms are located at the terminal positions of the Si(O,N)₄ tetrahedra. The majority of the Ca²⁺ cations fill the channels along [100] in the packing of the 12-ring anions. The rest of these cations are located at several positions, with partial occupancy, in channels along the body diagonals.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Si-O) = 0.002 Å
• Disorder in main residue
• R factor = 0.032
• wR factor = 0.069
• Data-to-parameter ratio = 17.5

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT029_ALERT_3_C _diffrn_measured_fraction_theta_full Low .......      0.972
PLAT041_ALERT_1_C Calc. and Reported SumFormula    Strings  Differ          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       0.33 Ratio
PLAT068_ALERT_1_C Reported F000 Differs from Calcd (or Missing)...          ?
PLAT143_ALERT_4_C su on c - Axis Small or Missing ................    0.00010 Ang.
PLAT202_ALERT_3_C Isotropic non-H Atoms in Anion/Solvent .........          2
PLAT910_ALERT_3_C Missing # of FCF Reflections Below Th(Min) .....          4
PLAT912_ALERT_4_C Missing # of FCF Reflections Above STh/L=  0.600         46

Alert level G
PLAT005_ALERT_5_G No _iucr_refine_instructions_details in CIF ....          ?
PLAT083_ALERT_2_G SHELXL Second Parameter in WGHT Unusually Large.       5.78
PLAT152_ALERT_1_G The Supplied and Calc. Volume s.u. Differ by ...          3 Units
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported   _diffrn_ambient_temperature        293 K
PLAT301_ALERT_3_G Note: Main Residue  Disorder ...................         23 Perc.
PLAT302_ALERT_4_G Note: Anion/Solvent Disorder ...................         41 Perc.
PLAT396_ALERT_2_G Deviating Si-O-Si Angle from 150 Deg for     <O2      171.9 Deg.
PLAT790_ALERT_4_G Centre of Gravity not Within Unit Cell: Resd.  #          3
              Ca
PLAT790_ALERT_4_G Centre of Gravity not Within Unit Cell: Resd.  #          4
              Ca
PLAT811_ALERT_5_G No ADDSYM Analysis: Too Many Excluded Atoms ....          !

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   8 ALERT level C = Check. Ensure it is not caused by an omission or oversight
  11 ALERT level G = General information/check it is not something unexpected

   6 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   4 ALERT type 3 Indicator that the structure quality may be low
   5 ALERT type 4 Improvement, methodology, query or suggestion
   2 ALERT type 5 Informative message, check

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S1. Comment

The title compound is a crystalline component formed in the melts of oxynitrido glasses, studied at our department. The title compound calcium oxynitrido-silicate contains isolated 12-ring anions with the ideal composition Ca_{3 - x}Si₂N_2–2xO_{4 + 2x} where the title compound have x ≈ 0.12. It should be emphasized that x=0 do not indicate an end member of a possible solid solution series of compounds. The nitrogen atoms in the anions occupies mainly the ring positions in the 12 ring while the oxygen atoms mainly occupy the apex positions. A figure of the oxynitrido-silicate anion is shown in Fig. 1. In Fig. 2 a simplified packing is shown where the arrangement of Ca atoims in channels along <100> as well as the disordered arrangement along the <111> directions. 56 Ca atoms in each unit cell fills channels along <100> in the packing of the 12-ring anions while the rest of the Ca positions are located in channels along <111> and show tendencies to be disordered. The split positions of the Ca cations along the <111> can be viewed as a consequence of the implied Pa-3 symmetry. Whether the space group should better be P2₁3 or even P2₁2₁2₁ with cubic twinning is unfortunately not possible to determine, neither from systematic reflection conditions nor from investigations of the s.u. of the cell parameters. No One can refine orthorhombic unit-cell parameters but if one should beleive the e.s.d.'s is more of an open question. We choose to describe the structure with highest possible symmetry but at the price of some disorder. Similar arrangements of cations and ring formed anions are found in the structurally related compounds K₄SrGe₃O₉ and Na₄CaSi₃O₉ (Fischer & Tillmanns, 1984).

Related literature

For a related pseudo isomorphic silicate as well as a germanate, see: Fischer & Tillmanns (1984) and for a more distantly related calcium aluminate, see: Mondal & Jeffery (1975)

S2. Experimental

The title compound was obtained by slow cooling of a melt with the nominal composition Ca_1.71Si₂O_1.71N_2.67 from 1700° C by 1° C / min in a graphite furnace.

S3. Refinement

In order to match the refined Ca composition the N1 and N2 position were mixed occupied by 88° N and 12°O. Attempts to refine the N/O ratio from the X-ray diffraction data failed using the present single-crystal data as a consequence of the close resemblance of the atomic form factors of N and O. The occupancy factors of the Calcium ions did converge to the composition reported in the title and was fully consistent with results from EDS analyses giving the ratio Ca/Si = 0.59 (1). The refined Ca content 2.88 and 2 Si give the Ca/Si = 0.59. It must be emphasized that the precise occupation of Ca atoms are heavily dependent on the precise model used. Including a larger number of Ca atoms, one could refine the model slightly closer to the ideal composition Ca₃Si₂O₄N₂.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: PLATON (Spek, 2009) and SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The oxynitrido-silicate anion of the title compound (I) with the unique atoms of the oxynitrido-silicate anion labelled. Displacement ellipsoids drawn at 50% probability.
	Fig. 2. Stereoview of the unitcell viewed approximately along [100] with the oxynitrido-silicate anions marked with red colour and the Ca atoms ffwith blue colour. The disordered arrays of Ca atoms along the <111> shown by a solid rod.

tricalcium oxynitride silicate

Crystal data

Ca2.89Si2N1.76O4.24	Dx = 3.087 Mg m^−3
Mr = 264.65	Mo Kα radiation, λ = 0.71073 Å
Cubic, Pa3	Cell parameters from 19622 reflections
Hall symbol: -P 2ac 2ab 3	θ = 3.8–32.2°
a = 15.0626 (1) Å	µ = 3.18 mm^−1
V = 3417.45 (4) Å^3	T = 293 K
Z = 24	Block, colourless
F(000) = 3158	0.10 × 0.06 × 0.02 mm

Data collection

Oxford Diffraction XcaliburIII with Sapphire-3 CCD diffractometer	1982 independent reflections
Radiation source: fine-focus sealed tube	1779 reflections with I > 2s(I)
Graphite monochromator	Rint = 0.027
Detector resolution: 16.5467 pixels mm^-1	θmax = 32.3°, θmin = 3.8°
ω scans at different ϕ	h = −22→18
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2008)	k = −18→21
Tmin = 0.67, Tmax = 0.94	l = −22→21
30793 measured reflections

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)] = 0.032	w = 1/[σ^2(Fo^2) + (0.026P)^2 + 5.7834P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.069	(Δ/σ)max = 0.001
S = 1.22	Δρmax = 0.87 e Å^−3
1982 reflections	Δρmin = −0.76 e Å^−3
113 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00048 (5)

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	Occ. (<1)
Si1	0.00530 (3)	0.27221 (3)	0.76315 (3)	0.00633 (9)
Si2	0.01658 (3)	0.24358 (3)	0.98439 (3)	0.00567 (9)
N1	0.01861 (12)	0.37791 (11)	0.72676 (13)	0.0176 (3)	0.88
O1	0.01861 (12)	0.37791 (11)	0.72676 (13)	0.0176 (3)	0.12
N2	0.00891 (13)	0.26556 (13)	0.87435 (11)	0.0193 (4)	0.88
O2	0.00891 (13)	0.26556 (13)	0.87435 (11)	0.0193 (4)	0.12
O3	0.09048 (10)	0.21494 (9)	0.72534 (9)	0.0131 (3)
O4	−0.09065 (10)	0.23612 (10)	0.72964 (10)	0.0160 (3)
O5	−0.01845 (10)	0.33802 (10)	1.02719 (10)	0.0147 (3)
O6	−0.04423 (10)	0.15759 (10)	1.01040 (10)	0.0153 (3)
Ca1	−0.12893 (2)	0.37107 (2)	1.12893 (2)	0.01028 (12)
Ca2	−0.11371 (3)	0.38333 (3)	0.90382 (3)	0.01162 (8)
Ca3	0.13964 (3)	0.13071 (3)	0.85405 (3)	0.01601 (9)
Ca4	0.24330 (14)	0.25670 (14)	0.74330 (14)	0.0094 (6)*	0.1715 (6)
Ca5	0.1757 (2)	0.3243 (2)	0.6757 (2)	0.0094 (2)*	0.140 (2)
Ca6	0.15523 (5)	0.34477 (5)	0.65523 (5)	0.0094 (2)*	0.670 (3)
Ca7	0.1254 (9)	0.3746 (9)	0.6254 (9)	0.0094 (2)*	0.043 (3)
Ca8	0.1031 (7)	0.3969 (7)	0.6031 (7)	0.0094 (2)*	0.076 (2)
Ca9	0.0819 (5)	0.4181 (5)	0.5819 (5)	0.0094 (2)*	0.086 (3)
Ca10	0.0572 (3)	0.4428 (3)	0.5572 (3)	0.0094 (2)*	0.099 (2)
Ca11	0.0166 (4)	0.4834 (4)	0.5166 (4)	0.0094 (2)*	0.095 (3)
Ca12	0.0000	0.5000	0.5000	0.0094 (2)*	0.603 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Si1	0.0077 (2)	0.0061 (2)	0.0052 (2)	−0.00013 (16)	−0.00028 (15)	−0.00047 (15)
Si2	0.00564 (19)	0.0064 (2)	0.0049 (2)	0.00030 (16)	0.00068 (15)	−0.00020 (16)
N1	0.0206 (8)	0.0063 (7)	0.0260 (9)	0.0011 (6)	0.0043 (7)	0.0004 (6)
O1	0.0206 (8)	0.0063 (7)	0.0260 (9)	0.0011 (6)	0.0043 (7)	0.0004 (6)
N2	0.0240 (9)	0.0274 (9)	0.0066 (7)	−0.0007 (7)	0.0010 (6)	−0.0004 (6)
O2	0.0240 (9)	0.0274 (9)	0.0066 (7)	−0.0007 (7)	0.0010 (6)	−0.0004 (6)
O3	0.0149 (6)	0.0105 (6)	0.0140 (6)	0.0035 (5)	0.0042 (5)	−0.0008 (5)
O4	0.0150 (6)	0.0162 (7)	0.0169 (7)	−0.0055 (5)	−0.0065 (5)	0.0014 (5)
O5	0.0174 (7)	0.0124 (6)	0.0144 (6)	0.0057 (5)	−0.0035 (5)	−0.0062 (5)
O6	0.0157 (7)	0.0132 (6)	0.0170 (7)	−0.0063 (5)	0.0040 (5)	0.0008 (5)
Ca1	0.01028 (12)	0.01028 (12)	0.01028 (12)	−0.00048 (12)	0.00048 (12)	0.00048 (12)
Ca2	0.01262 (17)	0.00917 (16)	0.01306 (17)	0.00006 (12)	−0.00140 (13)	0.00004 (12)
Ca3	0.01781 (19)	0.01586 (19)	0.01435 (18)	0.00563 (14)	−0.00307 (14)	−0.00105 (14)

Geometric parameters (Å, º)

Si1—O4	1.6245 (15)	Si2—O6	1.6341 (15)
Si1—O3	1.6476 (14)	Si2—O5	1.6485 (15)
Si1—N2	1.6788 (18)	Si2—N1^i	1.6899 (17)
Si1—N1	1.6958 (17)	Si2—N2	1.6942 (18)
O4—Si1—O3	114.22 (8)	O6—Si2—N1^i	109.58 (9)
O4—Si1—N2	108.60 (9)	O5—Si2—N1^i	108.13 (9)
O3—Si1—N2	106.76 (9)	O6—Si2—N2	110.56 (9)
O4—Si1—N1	108.59 (9)	O5—Si2—N2	101.08 (9)
O3—Si1—N1	106.72 (8)	O1^i—Si2—N2	113.07 (9)
N2—Si1—N1	112.00 (10)	N1^i—Si2—N2	113.07 (9)
O6—Si2—O5	114.26 (8)	Si2^ii—N1—Si1	142.98 (12)
O6—Si2—O1^i	109.58 (9)	Si1—N2—Si2	171.87 (14)
O5—Si2—O1^i	108.13 (9)
N2—Si1—N1—Si2^ii	−46.7 (2)

Symmetry codes: (i) −y+1/2, z−1/2, x+1; (ii) z−1, −x+1/2, y+1/2.