research papers
The incommensurate structure of (Sr0.13Ca0.87)2CoSi2O7 at room temperature has been determined from single-crystal X-ray diffraction data. The compound has a non-centrosymmetric tetragonal basic cell of a = 7.8743 (4) and c = 5.0417 (2) Å with the space group P421m. The refinements of the basic structure converged to R = 0.038 for 757 main reflections. The two-dimensional incommensurate structure is characterized by the wavevectors q1 = 0.286 (3)(a* + b*) and q2 = 0.286 (3)(-a* + b*), where a*, b* are the reciprocal lattice vectors of the basic structure. With the (3 + 2)-dimensional superspace group Pp4mgP421m, the refinements converged to R = 0.071 for 1697 observed reflections (757 main and 940 satellite reflections). The structure is described in terms of displacement of the atoms, rotation, distortion of CoO4 and SiO4 tetrahedra, and the partial ordering of the Sr and Ca atoms accompanied with the modulation. Correlated evolution of these features throughout the crystal gives rise to various oxygen coordination around Ca/Sr. Comparison of the derived modulated structure to that of Ca2CoSi2O7 clarified that the partial substitution of Ca by large alkaline-earth atoms such as Sr should decrease the distortion of the polyhedra around the cations.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768100006716/sn0002sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108768100006716/sn0002sup2.hkl |
Computing details top
(I) top
Crystal data top
(Sr0.13Ca0.87)2CoSi2O7 | V = 312.61 (3) Å3 |
Mr = 319.6 | Z = 2 |
Tetragonal, P421m | F(000) = 311 |
q1 = 0.28600a* + 0.28600b*; q2 = -0.28600a* + 0.28600b* | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P -4 2ab | µ = 6.74 mm−1 |
a = 7.8743 (4) Å | , blue |
c = 5.0417 (2) Å | 0.15 mm (radius) |
Data collection top
CAD4 diffractometer | θmax = 50.0°, θmin = 1.5° |
ω/2θ scans | h = ?→? |
Absorption correction: analytical ? | k = ?→? |
Tmin = 0.225, Tmax = 0.273 | l = ?→? |
8207 independent reflections | 2 standard reflections |
1697 reflections with 2.5 | intensity decay: 1.3, 1.5% |
Refinement top
Refinement on F | |
R[F2 > 2σ(F2)] = 0.070 | Δρmax = 2.59 e Å−3 |
wR(F2) = 0.052 | Δρmin = −2.94 e Å−3 |
1697 reflections | Extinction correction: B-C type 1 Lorentzian isotropic |
107 parameters | Extinction coefficient: 0.102492 |
Unit |
Crystal data top
(Sr0.13Ca0.87)2CoSi2O7 | V = 312.61 (3) Å3 |
Mr = 319.6 | Z = 2 |
Tetragonal, P421m | Mo Kα radiation |
q1 = 0.28600a* + 0.28600b*; q2 = -0.28600a* + 0.28600b* | µ = 6.74 mm−1 |
a = 7.8743 (4) Å | 0.15 mm (radius) |
c = 5.0417 (2) Å |
Data collection top
CAD4 diffractometer | 1697 reflections with 2.5 |
Absorption correction: analytical ? | 2 standard reflections |
Tmin = 0.225, Tmax = 0.273 | intensity decay: 1.3, 1.5% |
8207 independent reflections |
Refinement top
R[F2 > 2σ(F2)] = 0.070 | 107 parameters |
wR(F2) = 0.052 | Δρmax = 2.59 e Å−3 |
1697 reflections | Δρmin = −2.94 e Å−3 |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Sr | 0.33234 | 0.16766 | 0.5058 | 0.01645 | 0.128 |
Ca | 0.33234 (4) | 0.16766 (4) | 0.5058 (1) | 0.01845 (8) | 0.872 (3) |
Co | 0 | 0 | 0 | 0.01184 (7) | |
Si | 0.13915 (7) | 0.36085 (7) | 0.9382 (1) | 0.0113 (1) | |
O1 | 0 | 0.5 | 0.8254 (6) | 0.0354 (8) | |
O2 | 0.1401 (2) | 0.3599 (2) | 0.2547 (4) | 0.0244 (4) | |
O3 | 0.0798 (3) | 0.1894 (2) | 0.7863 (3) | 0.0289 (5) |
Atomic displacement parameters (Å2) top
U11 | U22 | U33 | U12 | U13 | U23 | |
Sr | 0.0199 (2) | 0.0199 (2) | 0.0100 (2) | 0.0089 (2) | 0.0015 (1) | −0.0015 (1) |
Ca | 0.0199 (2) | 0.0199 (2) | 0.0100 (2) | 0.0089 (2) | 0.0015 (1) | −0.0015 (1) |
Co | 0.0109 (1) | 0.0109 (1) | 0.0109 (1) | 0 | 0 | 0 |
Si | 0.0112 (2) | 0.0112 (2) | 0.0076 (2) | 0.0014 (2) | 0.0000 (1) | 0.0000 (1) |
O1 | 0.0450 (16) | 0.0450 (16) | 0.0070 (9) | 0.0330 (16) | 0 | 0 |
O2 | 0.0303 (8) | 0.0303 (8) | 0.0073 (7) | 0.0140 (10) | −0.0017 (5) | 0.0017 (5) |
O3 | 0.0420 (10) | 0.0155 (6) | 0.0141 (8) | −0.0120 (7) | 0.0064 (8) | −0.0014 (6) |
Experimental details
Crystal data | |
Chemical formula | (Sr0.13Ca0.87)2CoSi2O7 |
Mr | 319.6 |
Crystal system, space group | Tetragonal, P421m |
Temperature (K) | ? |
Wave vectors | q1 = 0.28600a* + 0.28600b*; q2 = -0.28600a* + 0.28600b* |
a, c (Å) | 7.8743 (4), 5.0417 (2) |
V (Å3) | 312.61 (3) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 6.74 |
Crystal size (mm) | 0.15 (radius) |
Data collection | |
Diffractometer | CAD4 diffractometer |
Absorption correction | Analytical |
Tmin, Tmax | 0.225, 0.273 |
No. of measured, independent and observed (2.5) reflections | ?, 8207, 1697 |
Rint | ? |
(sin θ/λ)max (Å−1) | 1.078 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.070, 0.052, ? |
No. of reflections | 1697 |
No. of parameters | 107 |
No. of restraints | ? |
Δρmax, Δρmin (e Å−3) | 2.59, −2.94 |