research papers
We report the crystal structure and electron density of samarium titanium oxysulfide, Sm2Ti2S2O4.9, photocatalyst obtained through the Rietveld analysis, maximum-entropy method (MEM) and MEM-based pattern fitting of the high-resolution synchrotron powder diffraction data taken at 298.7 K. The Sm2Ti2S2O4.9 has a tetragonal structure with the space group I4/mmm. Refined occupancy factors at the `equatorial' O1 and `apical' O2 sites were 0.994 (3) and 0.944 (12), respectively, which strongly suggest oxygen deficiency at the O2 site. Electron-density analyses based on the synchrotron diffraction data of Sm2Ti2S2O4.9 in combination with density-functional theory (DFT) calculations of stoichiometric Sm2Ti2S2O5 reveal covalent bonds between Ti and O atoms, while the Sm and S atoms are more ionic. The presence of S 3p and O 2p orbitals results in increased dispersion of the valence band, raising the top of the valence band and making the material active at visible wavelengths. The present DFT calculations of stoichiometric Sm2Ti2S2O5 indicate the possibility of overall splitting of water, although Sm2Ti2S2O4.9 works as a visible-light-responsive photocatalyst in aqueous solutions only in the presence of sacrificial electron donors or acceptors. The oxygen deficiency and cocatalyst seem to be factors affecting the catalytic activity.
Keywords: electron density; light-responsive photocatalyst; synchrotron diffraction; density-functional theory.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768108007532/og5028sup1.cif | |
Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768108007532/og5028sup2.rtv |
Computing details top
Data collection: RIGAKU software (Toraya et al., 1996); cell refinement: RIETAN-2000 (Izumi and Ikeda, 2000); program(s) used to solve structure: RIETAN-2000; program(s) used to refine structure: RIETAN-2000; molecular graphics: VESTA [Momma and Izumi (2006)].
samarium titanium oxysulfide top
Crystal data top
O9.84S4Sm4Ti4 | Z = 1 |
Mr = 1078.61 | Dx = 5.342 Mg m−3 |
Tetragonal, I4/mmm | Synchrotron radiation, λ = 1.20643 Å |
Hall symbol: -I 4 2 | T = 299 K |
a = 3.82123 (2) Å | yellow to yellowish-green |
c = 22.96371 (12) Å | circular flat plate, 20 × 0.5 mm |
V = 335.31 (1) Å3 | Specimen preparation: Prepared at 1173 K and 100 kPa, cooled at 10 K min−1 |
Data collection top
A multiple-detector system with Ge(111) analyzer crystals, Soller slits and
scintillation counters (Toraya et al., 1996) diffractometer | Data collection mode: reflection |
Ge(111) analyzer crystals monochromator | Scan method: step |
Specimen mounting: flat specimen | 2θmin = 7.051°, 2θmax = 154.887°, 2θstep = 0.01° |
Refinement top
Rp = 0.054 | Excluded region(s): none |
Rwp = 0.069 | Profile function: Split Pearson (Toraya, 1990) |
Rexp = ? | 10 parameters |
χ2 = 2.434 | |
? data points |
Crystal data top
O9.84S4Sm4Ti4 | V = 335.31 (1) Å3 |
Mr = 1078.61 | Z = 1 |
Tetragonal, I4/mmm | Synchrotron radiation, λ = 1.20643 Å |
a = 3.82123 (2) Å | T = 299 K |
c = 22.96371 (12) Å | circular flat plate, 20 × 0.5 mm |
Data collection top
A multiple-detector system with Ge(111) analyzer crystals, Soller slits and
scintillation counters (Toraya et al., 1996) diffractometer | Scan method: step |
Specimen mounting: flat specimen | 2θmin = 7.051°, 2θmax = 154.887°, 2θstep = 0.01° |
Data collection mode: reflection |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
x | y | z | Biso*/Beq | Occ. (<1) | |
Sm | 0 | 0 | 0.33382 (1) | 0.253 (9)* | |
Ti | 0 | 0 | 0.07819 (4) | 0.309 (15)* | |
S | 0 | 0 | 0.20364 (7) | 0.360 (18)* | |
O1 | 0 | 0.50000 | 0.09734 (9) | 0.53 (5)* | 0.994 (3) |
O2 | 0 | 0 | 0 | 0.73 (11)* | 0.944 (12) |
Atomic displacement parameters (Å2) top
U11 | U22 | U33 | U12 | U13 | U23 | |
Sm | 0.003202 | 0.003202 | 0.003202 | 0 | 0 | 0 |
Ti | 0.003912 | 0.003912 | 0.003912 | 0 | 0 | 0 |
S | 0.004564 | 0.004564 | 0.004564 | 0 | 0 | 0 |
O1 | 0.006700 | 0.006700 | 0.006700 | 0 | 0 | 0 |
O2 | 0.009267 | 0.009267 | 0.009267 | 0 | 0 | 0 |
Experimental details
Crystal data | |
Chemical formula | O9.84S4Sm4Ti4 |
Mr | 1078.61 |
Crystal system, space group | Tetragonal, I4/mmm |
Temperature (K) | 299 |
a, c (Å) | 3.82123 (2), 22.96371 (12) |
V (Å3) | 335.31 (1) |
Z | 1 |
Radiation type | Synchrotron, λ = 1.20643 Å |
Specimen shape, size (mm) | Circular flat plate, 20 × 0.5 |
Data collection | |
Diffractometer | A multiple-detector system with Ge(111) analyzer crystals, Soller slits and
scintillation counters (Toraya et al., 1996) diffractometer |
Specimen mounting | Flat specimen |
Data collection mode | Reflection |
Scan method | Step |
2θ values (°) | 2θmin = 7.051 2θmax = 154.887 2θstep = 0.01 |
Refinement | |
R factors and goodness of fit | Rp = 0.054, Rwp = 0.069, Rexp = ?, χ2 = 2.434 |
No. of data points | ? |
No. of parameters | 10 |
No. of restraints | ? |
Computer programs: RIGAKU software (Toraya et al., 1996), RIETAN-2000 (Izumi and Ikeda, 2000), RIETAN-2000, VESTA [Momma and Izumi (2006)].