Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010500867X/sk1825sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S010827010500867X/sk1825Isup2.hkl |
Single crystals for TiOBr were grown by gas transport following published procedures (Schäfer et al., 1958). The starting materials for synthesis were Ti (Alpha, 99.99% purity), TiO2 (Alpha, 99.99%) and TiBr4 (Aldrich, 99.99%). They were mixed with a 40% surplus of Ti and TiBr4. The sealed evacuated (p = 1.5 10−2 hPa) quartz glass tube was heated in a temperature gradient of 923:823 K for 72 h. The tube was cooled to room-temperature at a rate of 15.6 K h−1. The yellow–brown crystals have a needle-to-plate-like habit and develop a grey–white patina in air. Therefore they were further handled under inert gasses.
Diffraction at room-temperature confirmed the FeOCl structure type with an orthorhombic lattice and space group Pmmn. Diffraction at 17.5 K indicated the presence of superlattice reflections at positions q = (0, 1/2, 0) from the main reflections. Accordingly, all Bragg reflections at low temperature were indexed with respect to the orthorhombic unit cell of the room-temperature structure and the modulation wavevector q, employing four integers (hklm). The main reflections are described by m = 0, while m = 1 indicates the superlattice reflections.
Owing to the presence of the cryostat, only a limited range of setting angles of the crystal could be reached. This allowed the measurement of most Bragg reflections in one octant that represents the unique reflections with respect to mmm symmetry. Furthermore, 16 observed main reflections and 24 observed satellite reflections could be measured within a second octant that would be required for a complete unique data set within monoclinic symmetry.
The structure was refined within the superspace formalism as a commensurately modulated structure with superspace group Pmmn(0β0), with β = 1/2 and t0 = 1/8. Section t0 = 1/8 of superspace indicates monoclinic symmetry for the supercell. Therefore, one parameter was introduced for pseudo-merohedral twinning of the monoclinic structure on the pseudo-orthorhombic lattice. The refined volume fraction of the second twin domain was obtained as 0.492 (7). This implies a 1:1 ratio of volumes of the two twin domains and an apparent mmm symmetry of the diffraction pattern. Therefore, the measured single octant represents a complete data set for the twinned crystal, as was confirmed by the good fit to the reflections measured for the second octant.
The superspace refinement allowed the structural parameters to be separated into parameters of the basic structure, as defined by the strong main reflections, and modulation parameters that give rise to the weak superstructure reflections (satellites). One modulation wave was refined, leading to two modulation parameters for each atom. Partial R values at convergence R(main reflections) = 1.59% and R(satellites) = 4.86% show that a good fit to the superstructure reflections was obtained.
Subsequently the structure was transformed to a twofold superstructure, leading to two independent atoms in the supercell instead of one in the basic structure unit cell. To allow for direct comparison of the superstructure and the room-temperature structure, a non-standard setting `monoclinic a unique' with inversion center at (1/4 3/8 0) was used. An ordinary structure refinement was performed in the supercell but with additional restrictions as they were derived from the superspace approach; displacement parameters of the two independent atoms of the same species were made equal, the displacement parameter U23 of all atoms was set equal to 0, and y(A2) = [1/2 + yave − y(A1)], were yave is the y coordinate of atom A in the room-temperature structure with values yave = 0.5 when A is Ti, and yave = 0 when A is O or Br. Omitting these restrictions leads to severe correlations between the parameters as a result of the pseudo-orthorhombic nature of the structure.
Data collection: DIF4 (Eichhorn, 1996); cell refinement: DIF4; data reduction: REDUCE (Eichhorn, 1991) and JANA2000 (Petricek et al., 2000); program(s) used to solve structure: program (reference)?; program(s) used to refine structure: JANA2000; molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: JANA2000.
TiOBr | F(000) = 260 |
Mr = 143.8 | pseudo-orthorhombic, non-standard setting "monoclinic a unique",
origin shift by (1/4 1/8 0), therefore inversion center at (1/4 3/8 0). The standard uncertainty of the monoclinic angle α was estimated on the basis of the width of the reflections. Splitting of reflections was not observed. Mass absorbtion coefficients and anomalous dispersion parameters were calculated by the refinement program JANA2000 (Petricek et al., 2000), using a table of the coefficients for a range of wavelengths and interpolating between these values in order to obtain the values for any desired wavelength. The source of the tables is: CrossSec_McMaster.dat: Photon-Atom Cross Section and attenuation coefficients (McMaster); and f1f2_asf_Kissel.dat: Elastic Photon-Atom Scattering, anomalous scattering factors. The files belong to the DABAX library created at ESRF. More information on DABAX can be found at: https://www.esrf.fr/computing/scientific/dabax/ |
Monoclinic, P21/m11 | Dx = 4.277 Mg m−3 |
Hall symbol: P -2x 2xabv | Synchrotron radiation, λ = 0.5 Å |
a = 3.7852 (12) Å | Cell parameters from 24 reflections |
b = 6.9366 (9) Å | θ = 10.0–23.0° |
c = 8.501 (3) Å | µ = 8.35 mm−1 |
β = 90° | T = 18 K |
V = 223.21 (10) Å3 | Platelet, translucent light brown |
Z = 4 | 0.27 × 0.13 × 0.002 mm |
Huber four-circle diffractometer | 336 reflections with I > 3σ(I) |
Radiation source: synchrotron | Rint = 0.054 |
Double-crystal flat Si(111) monochromator | θmax = 20.5°, θmin = 1.7° |
Profile data with ω scans | h = −5→5 |
Absorption correction: gaussian (Jana2000; Petricek et al., 2000) | k = −9→3 |
Tmin = 0.475, Tmax = 0.978 | l = −10→0 |
989 measured reflections | 3 standard reflections every 90 min |
416 independent reflections | intensity decay: none |
Refinement on F | 0 constraints |
R[F2 > 2σ(F2)] = 0.018 | Weighting scheme based on measured s.u.'s w = 1/[σ2(F) + 0.0004F2] |
wR(F2) = 0.029 | (Δ/σ)max = 0.0001 |
S = 1.03 | Δρmax = 0.53 e Å−3 |
416 reflections | Δρmin = −0.57 e Å−3 |
21 parameters | Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974) |
0 restraints | Extinction coefficient: 0.09 (2) |
TiOBr | V = 223.21 (10) Å3 |
Mr = 143.8 | Z = 4 |
Monoclinic, P21/m11 | Synchrotron radiation, λ = 0.5 Å |
a = 3.7852 (12) Å | µ = 8.35 mm−1 |
b = 6.9366 (9) Å | T = 18 K |
c = 8.501 (3) Å | 0.27 × 0.13 × 0.002 mm |
β = 90° |
Huber four-circle diffractometer | 336 reflections with I > 3σ(I) |
Absorption correction: gaussian (Jana2000; Petricek et al., 2000) | Rint = 0.054 |
Tmin = 0.475, Tmax = 0.978 | 3 standard reflections every 90 min |
989 measured reflections | intensity decay: none |
416 independent reflections |
R[F2 > 2σ(F2)] = 0.018 | 21 parameters |
wR(F2) = 0.029 | 0 restraints |
S = 1.03 | Δρmax = 0.53 e Å−3 |
416 reflections | Δρmin = −0.57 e Å−3 |
Experimental. Asingle crystal of TiOBr was glued onto a carbon fiber that was attached to a closed-cycle helium cryostat mounted on a four-circle Huber diffractometer. X-ray diffraction with synchrotron radiation was measured at beamline D3 of Hasylab in Hamburg. |
Refinement. The crystal is twinned by pseudo-merohedry and it contains two twin domains. The twinning matrix for the second domain is: | 1.000 0.000 0.000 | | 0.000 1.000 0.000 | | 0.000 0.000 − 1.000 | The refined volume fractions are v1=0.508 (7) and v2=0.492 (7) for the first and second domains, respectively. The restriction v1+v2=1 was applied. |
x | y | z | Uiso*/Ueq | ||
Ti1 | 0 | 0.25414 (8) | 0.11268 (8) | 0.00280 (18) | |
Ti2 | 0 | 0.74586 (8) | 0.10897 (7) | 0.00280 (18) | |
Br1 | 0 | −0.00149 (4) | 0.32743 (4) | 0.00364 (13) | |
Br2 | 0 | 0.50149 (4) | 0.33158 (4) | 0.00364 (13) | |
O1 | 0 | 0.0025 (2) | −0.0486 (3) | 0.0040 (6) | |
O2 | 0 | 0.4975 (2) | −0.0534 (3) | 0.0040 (6) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Ti1 | 0.0020 (3) | 0.0020 (3) | 0.0045 (3) | 0 | 0 | 0 |
Ti2 | 0.0020 (3) | 0.0020 (3) | 0.0045 (3) | 0 | 0 | 0 |
Br1 | 0.0050 (2) | 0.0017 (2) | 0.0042 (2) | 0 | 0 | 0 |
Br2 | 0.0050 (2) | 0.0017 (2) | 0.0042 (2) | 0 | 0 | 0 |
O1 | 0.0033 (11) | 0.0021 (10) | 0.0066 (9) | 0 | 0 | 0 |
O2 | 0.0033 (11) | 0.0021 (10) | 0.0066 (9) | 0 | 0 | 0 |
Ti1—Ti2 | 3.4110 (8) | Ti2—O2 | 2.208 (2) |
Ti1—Ti2i | 3.5259 (8) | Ti1—Br1 | 2.5451 (7) |
Ti1—Ti1ii | 3.7852 (13) | Ti1—Br2 | 2.5312 (7) |
Ti2—Ti2ii | 3.7852 (13) | Ti2—Br1v | 2.5535 (7) |
Ti1—Ti1iii | 3.1723 (7) | Ti2—Br2 | 2.5406 (7) |
Ti1—Ti2iii | 3.1843 (7) | Ti1—Ti1iv | 3.1723 (7) |
Ti2—Ti2iii | 3.1976 (7) | Ti1—Ti2iv | 3.1843 (7) |
Ti1—O1 | 2.219 (2) | Ti2—Ti2vi | 3.1976 (7) |
Ti1—O2 | 2.201 (2) | O1—O1vii | 2.7192 (19) |
Ti1—O2iii | 1.9586 (6) | O1—O1viii | 2.7192 (19) |
Ti1—O2iv | 1.9586 (6) | O1—O2iii | 2.7093 (19) |
Ti2—O1v | 2.228 (2) | O1—O2iv | 2.7093 (19) |
Ti2—O1iii | 1.9611 (6) | O2—O2iii | 2.7006 (19) |
Ti2—O1iv | 1.9611 (6) | O2—O2iv | 2.7006 (19) |
Ti1—Br2—Ti2 | 84.53 (2) | O1iii—Ti2—O2 | 80.83 (6) |
Ti1—Br1—Ti2i | 87.51 (2) | O1iv—Ti2—O1v | 80.68 (6) |
Ti1—O2—Ti2 | 101.39 (9) | O1iv—Ti2—O1iii | 149.63 (10) |
Ti1—O1—Ti2i | 104.91 (10) | O1iv—Ti2—O2 | 80.83 (6) |
Ti1—O1—Ti2iv | 99.05 (6) | Ti1—O1—O1vii | 108.92 (8) |
Ti2i—O1—Ti2iv | 99.32 (6) | Ti1—O1—O1viii | 108.92 (8) |
Ti2iii—O1—Ti2iv | 149.63 (13) | Ti1—O1—O2iii | 45.49 (4) |
Ti1—O2—Ti1iv | 99.25 (6) | Ti1—O1—O2iv | 45.49 (4) |
Ti1iii—O2—Ti1iv | 150.17 (13) | Ti2i—O1—O1vii | 45.37 (4) |
Ti1iv—O2—Ti2 | 99.52 (6) | Ti2i—O1—O1viii | 45.37 (4) |
Ti1—O1—Ti2iii | 99.05 (6) | Ti2i—O1—O2iii | 108.62 (8) |
Ti2i—O1—Ti2iii | 99.32 (6) | Ti2i—O1—O2iv | 108.62 (8) |
Ti2iii—O1—Ti2i | 99.32 (6) | Ti2iii—O1—O1vii | 53.95 (4) |
Ti2iv—O1—Ti2i | 99.32 (6) | Ti2iii—O1—O1viii | 139.02 (8) |
Ti2iv—O1—Ti2iii | 149.63 (13) | Ti2iii—O1—O2iii | 53.56 (4) |
Ti1iv—O2—Ti1 | 99.25 (6) | Ti2iii—O1—O2iv | 138.96 (8) |
Ti1—O2—Ti1iii | 99.25 (6) | Ti2iv—O1—O1vii | 139.02 (8) |
Ti1iii—O2—Ti1 | 99.25 (6) | Ti2iv—O1—O1viii | 53.95 (4) |
Ti1iv—O2—Ti1iii | 150.17 (13) | Ti2iv—O1—O2iii | 138.96 (8) |
Ti1iii—O2—Ti2 | 99.52 (6) | Ti2iv—O1—O2iv | 53.56 (4) |
Br1—Ti1—Br2 | 86.84 (2) | O1vii—O1—O1viii | 88.22 (7) |
Br1—Ti1—O1 | 83.98 (6) | O1vii—O1—O2iii | 80.39 (5) |
Br1—Ti1—O2 | 174.08 (6) | O1vii—O1—O2iv | 143.66 (11) |
Br1—Ti1—O2iii | 100.40 (6) | O1viii—O1—O1vii | 88.22 (7) |
Br1—Ti1—O2iv | 100.40 (6) | O1viii—O1—O2iii | 143.66 (11) |
Br2—Ti1—O1 | 170.82 (6) | O1viii—O1—O2iv | 80.39 (5) |
Br2—Ti1—O2 | 87.24 (6) | O2iii—O1—O2iv | 88.62 (7) |
Br2—Ti1—O2iii | 101.13 (6) | O2iv—O1—O2iii | 88.62 (7) |
Br2—Ti1—O2iv | 101.13 (6) | Ti1—O2—O1iii | 106.60 (8) |
O1—Ti1—O2 | 101.94 (8) | Ti1—O2—O1iv | 106.60 (8) |
O1—Ti1—O2iii | 80.60 (6) | Ti1—O2—O2iii | 45.71 (4) |
O1—Ti1—O2iv | 80.60 (6) | Ti1—O2—O2iv | 45.71 (4) |
O2—Ti1—O2iii | 80.75 (6) | Ti1iii—O2—O1iii | 53.91 (4) |
O2—Ti1—O2iv | 80.75 (6) | Ti1iii—O2—O1iv | 139.55 (8) |
O2iii—Ti1—O2 | 80.75 (6) | Ti1iii—O2—O2iii | 53.54 (4) |
O2iii—Ti1—O2iv | 150.17 (10) | Ti1iii—O2—O2iv | 139.48 (8) |
O2iv—Ti1—O2 | 80.75 (6) | Ti1iv—O2—O1iii | 139.55 (8) |
O2iv—Ti1—O2iii | 150.17 (10) | Ti1iv—O2—O1iv | 53.91 (4) |
Br1v—Ti2—Br2 | 85.19 (2) | Ti1iv—O2—O2iii | 139.48 (8) |
Br1v—Ti2—O1v | 83.61 (6) | Ti1iv—O2—O2iv | 53.54 (4) |
Br1v—Ti2—O1iii | 100.75 (6) | Ti2—O2—O1iii | 45.61 (4) |
Br1v—Ti2—O1iv | 100.75 (6) | Ti2—O2—O1iv | 45.61 (4) |
Br1v—Ti2—O2 | 172.04 (6) | Ti2—O2—O2iii | 106.31 (8) |
Br2—Ti2—O1v | 168.80 (6) | Ti2—O2—O2iv | 106.31 (8) |
Br2—Ti2—O1iii | 101.47 (6) | O1iii—O2—O1iv | 88.62 (7) |
Br2—Ti2—O1iv | 101.47 (6) | O1iii—O2—O2iii | 78.79 (5) |
Br2—Ti2—O2 | 86.85 (6) | O1iii—O2—O2iv | 141.69 (11) |
O1v—Ti2—O1iii | 80.68 (6) | O1iv—O2—O1iii | 88.62 (7) |
O1v—Ti2—O1iv | 80.68 (6) | O1iv—O2—O2iii | 141.69 (11) |
O1v—Ti2—O2 | 104.35 (8) | O1iv—O2—O2iv | 78.79 (5) |
O1iii—Ti2—O1v | 80.68 (6) | O2iii—O2—O2iv | 88.98 (7) |
O1iii—Ti2—O1iv | 149.63 (10) | O2iv—O2—O2iii | 88.98 (7) |
Symmetry codes: (i) x, y−1, z; (ii) x−1, y, z; (iii) x−1/2, −y+3/4, −z; (iv) x+1/2, −y+3/4, −z; (v) x, y+1, z; (vi) x+1/2, −y+7/4, −z; (vii) x−1/2, −y−1/4, −z; (viii) x+1/2, −y−1/4, −z. |
Experimental details
Crystal data | |
Chemical formula | TiOBr |
Mr | 143.8 |
Crystal system, space group | Monoclinic, P21/m11 |
Temperature (K) | 18 |
a, b, c (Å) | 3.7852 (12), 6.9366 (9), 8.501 (3) |
β (°) | 90.00 (2), 90, 90 |
V (Å3) | 223.21 (10) |
Z | 4 |
Radiation type | Synchrotron, λ = 0.5 Å |
µ (mm−1) | 8.35 |
Crystal size (mm) | 0.27 × 0.13 × 0.002 |
Data collection | |
Diffractometer | Huber four-circle diffractometer |
Absorption correction | Gaussian (Jana2000; Petricek et al., 2000) |
Tmin, Tmax | 0.475, 0.978 |
No. of measured, independent and observed [I > 3σ(I)] reflections | 989, 416, 336 |
Rint | 0.054 |
(sin θ/λ)max (Å−1) | 0.700 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.018, 0.029, 1.03 |
No. of reflections | 416 |
No. of parameters | 21 |
Δρmax, Δρmin (e Å−3) | 0.53, −0.57 |
Computer programs: DIF4 (Eichhorn, 1996), DIF4, REDUCE (Eichhorn, 1991) and JANA2000 (Petricek et al., 2000), program (reference)?, JANA2000, DIAMOND (Brandenburg, 1999).
Ti1—Ti2 | 3.4110 (8) | Ti1—O2iv | 1.9586 (6) |
Ti1—Ti2i | 3.5259 (8) | Ti2—O1v | 2.228 (2) |
Ti1—Ti1ii | 3.7852 (13) | Ti2—O1iii | 1.9611 (6) |
Ti2—Ti2ii | 3.7852 (13) | Ti2—O1iv | 1.9611 (6) |
Ti1—Ti1iii | 3.1723 (7) | Ti2—O2 | 2.208 (2) |
Ti1—Ti2iii | 3.1843 (7) | Ti1—Br1 | 2.5451 (7) |
Ti2—Ti2iii | 3.1976 (7) | Ti1—Br2 | 2.5312 (7) |
Ti1—O1 | 2.219 (2) | Ti2—Br1v | 2.5535 (7) |
Ti1—O2 | 2.201 (2) | Ti2—Br2 | 2.5406 (7) |
Ti1—O2iii | 1.9586 (6) | ||
Ti1—Br2—Ti2 | 84.53 (2) | Ti2i—O1—Ti2iv | 99.32 (6) |
Ti1—Br1—Ti2i | 87.51 (2) | Ti2iii—O1—Ti2iv | 149.63 (13) |
Ti1—O2—Ti2 | 101.39 (9) | Ti1—O2—Ti1iv | 99.25 (6) |
Ti1—O1—Ti2i | 104.91 (10) | Ti1iii—O2—Ti1iv | 150.17 (13) |
Ti1—O1—Ti2iv | 99.05 (6) | Ti1iv—O2—Ti2 | 99.52 (6) |
Symmetry codes: (i) x, y−1, z; (ii) x−1, y, z; (iii) x−1/2, −y+3/4, −z; (iv) x+1/2, −y+3/4, −z; (v) x, y+1, z. |
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TiOBr (von Schnering et al., 1972) and TiOCl (Schäfer et al., 1958) are isostructural compounds that crystallize in the FeOCl structure type (Fig. 1). On cooling, they undergo two phase transitions, as evidenced by anomalies in the temperature dependencies of the magnetic susceptibilites (Seidel et al., 2003; Kato et al., 2005; Lemmens et al., 2005). Previously, we have shown that in the lowest-temperature phase (T < 67 K) the crystal structure of TiOCl is a twofold superstructure of the structure at room temperature (Shaz et al., 2005). The superstructure involves the formation of Ti—Ti dimers on the chains of Ti atoms parallel to [010], thus suggesting a spin-Peierls state by direct exchange on the quasi-one-dimensional chains of Ti atoms (Shaz et al., 2005). Recently Sasaki et al. (2005) reported superlattice reflections for TiOBr at low temperatures, but they did not report a satisfactory structure model because of the limited number of measured reflections (two reflections).
We report here the low-temperature twofold superstructure of TiOBr at T = 17.5 K. It is found that the pattern of displacements in TiOBr is similar to that in TiOCl, but with amplitudes that are only about half of the displacement amplitudes in TiOCl (Fig. 2 and Table 1). These results indicate that TiOBr is in a spin-Peierls state at low temperatures, similar to that of TiOCl but with the relevant interactions smaller than in TiOCl, in accordance with the transition temperatures being lower in TiOBr (Tc1 = 27 K and Tc2 = 47 K) than in TiOCl (Tc1 = 67 K and Tc2 = 90 K; Seidel et al., 2003; Shaz et al., 2005).