Ammonium oxofluorotitanate, NH4TiOF3, is probably the best known precursor for the synthesis of anatase mesocrystals. Transformation of NH4TiOF3 into TiO2 through thermal decomposition, accompanied by hydrolysis, preserves some structural features of the precursor. Currently, any discussion of the mechanism of this transformation is difficult, as the exact crystal structure of the starting compound is not available and no intermediate structures are known. This article describes the outcome of single-crystal and powder X-ray diffraction studies, revealing the existence of two polymorphs of the parent NH4TiOF3 at different temperatures. A second-order phase transition from the polar Pca21 α phase (1), stable at room temperature, to the Pma2 β phase (2) above ∼433 K has been demonstrated. The direction of the pseudo-fourfold axis in NH4TiOF3 coincides with the orientation of the fourfold axis of anatase mesocrystals, consistent with a topotactical transformation.
Supporting information
CCDC references: 1880514; 1880515
For both structures, program(s) used to refine structure: FULLPROF.
Crystal data top
F3OTi·H4N | c = 7.58450 (12) Å |
Mr = 138.9 | V = 361.17 (1) Å3 |
Orthorhombic, Pca21 | Z = 4 |
Hall symbol: P 2c -2ac | Synchrotron radiation, λ = 0.696800 Å |
a = 7.55254 (13) Å | T = 298 K |
b = 6.30507 (10) Å | white |
Data collection top
Pilatus 300K-W diffractometer | Data collection mode: transmission |
Radiation source: synchrotron | Scan method: Stationary detector |
Refinement top
Rp = 3.441 | Profile function: pseudo-Voigt |
Rwp = 5.683 | 52 parameters |
Rexp = 2.847 | 10 restraints |
RBragg = 4.696 | (Δ/σ)max = 0.02 |
6161 data points | Background function: manual |
Special details top
Refinement. The N-H disstances have been constrained to 1.0 A.
Bizo of hydroden atoms were increased by 2.0 compared to N atoms and refined
simultaneously. The Bizo of F and O atoms were refined independently.
The Ti atoms were refined anisotropically. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
TI1 | 0.4985 (7) | 0.5012 (11) | 0.3773 (18) | 0.0613 (7) | |
N2 | 0.21999 (11) | 0.02308 (13) | 0.11828 (12) | 0.0632 (14)* | |
H2A | 0.3208 (12) | 0.060 (2) | 0.1970 (15) | 0.0632 (14)* | |
H2B | 0.2535 (17) | −0.1061 (13) | 0.0506 (16) | 0.0632 (14)* | |
H2C | 0.1098 (10) | −0.009 (2) | 0.1852 (16) | 0.0632 (14)* | |
H2D | 0.1990 (18) | 0.1458 (14) | 0.0384 (15) | 0.0632 (14)* | |
F3 | 0.4658 (11) | 0.7776 (17) | 0.327 (2) | 0.093 (3)* | |
F4 | 0.5085 (10) | 0.4564 (9) | 0.126 (3) | 0.1015 (16)* | |
F5 | 0.5533 (8) | 0.2138 (13) | 0.393 (2) | 0.0548 (17)* | |
O6 | 0.754 (2) | 0.5660 (6) | 0.359 (2) | 0.0442 (14)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
TI1 | 0.0705 (8) | 0.0589 (6) | 0.0544 (6) | 0.0037 (10) | −0.002 (2) | 0.0011 (16) |
Geometric parameters (Å, º) top
TI1—F3 | 1.801 (13) | TI1—O6ii | 1.901 (16) |
TI1—F4 | 1.93 (3) | N2—H2A | 0.995 (10) |
TI1—F4i | 1.91 (3) | N2—H2B | 0.995 (10) |
TI1—F5 | 1.863 (11) | N2—H2C | 0.996 (9) |
TI1—O6 | 1.976 (16) | N2—H2D | 0.995 (10) |
| | | |
F3—TI1—F4 | 86.4 (12) | F4i—TI1—O6ii | 94.4 (14) |
F3—TI1—F4i | 94.0 (14) | F5—TI1—O6 | 89.4 (8) |
F3—TI1—F5 | 170.2 (10) | F5—TI1—O6ii | 90.2 (9) |
F3—TI1—O6 | 85.3 (9) | O6—TI1—O6ii | 171.7 (14) |
F3—TI1—O6ii | 93.8 (9) | H2A—N2—H2B | 107.8 (17) |
F4—TI1—F4i | 179 (2) | H2A—N2—H2C | 112.4 (14) |
F4—TI1—F5 | 84.9 (12) | H2A—N2—H2D | 107.8 (17) |
F4—TI1—O6 | 85.6 (13) | H2B—N2—H2C | 108.0 (17) |
F4—TI1—O6ii | 86.2 (13) | H2B—N2—H2D | 111.3 (14) |
F4i—TI1—F5 | 94.6 (12) | H2C—N2—H2D | 109.6 (17) |
F4i—TI1—O6 | 93.9 (13) | | |
Symmetry codes: (i) −x+1, −y+1, z+1/2; (ii) x−1/2, −y+1, z. |
Crystal data top
F3OTi·H4N | c = 3.80544 (9) Å |
Mr = 138.9 | V = 184.21 (1) Å3 |
Orthorhombic, Pma2 | Z = 2 |
Hall symbol: P 2 -2a | Synchrotron radiation, λ = 0.522600 Å |
a = 7.58642 (17) Å | T = 498 K |
b = 6.38074 (14) Å | white |
Data collection top
Pilatus 300K-W diffractometer | Data collection mode: transmission |
Radiation source: synchrotron | Scan method: Stationary detector |
Refinement top
Rp = 2.838 | Profile function: pseudo-Voigt |
Rwp = 4.646 | 61 parameters |
Rexp = 1.607 | 7 restraints |
RBragg = 3.999 | (Δ/σ)max = 0.02 |
2322 data points | Background function: linear interpolation |
Special details top
Refinement. The N-H disstances have been constrained to 1.0 A.
Bizo of hydroden atoms were increased by 2.0 compared to N atoms and refined
sumaltiniously. The Bizo of F and O atoms were refined independently.
The Ti atoms were refined anisotropically. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
TI1 | 0.50000 | 0.50000 | 0.753 (9) | 0.0429 (17) | |
N2 | 0.25000 | 0.0244 (10) | 0.253 (3) | 0.065 (2)* | |
H2A | 0.3580 (17) | 0.064 (4) | 0.390 (5) | 0.065 (2)* | |
H2B | 0.25000 | −0.1241 (18) | 0.177 (8) | 0.065 (2)* | |
H2D | 0.25000 | 0.127 (4) | 0.052 (6) | 0.065 (2)* | |
F4 | 0.50000 | 0.50000 | 0.236 (11) | 0.090 (3)* | |
F5 | 0.5305 (6) | 0.2169 (4) | 0.787 (10) | 0.073 (2)* | |
O6 | 0.75000 | 0.5594 (11) | 0.764 (10) | 0.037 (3)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
TI1 | 0.0418 (19) | 0.0381 (12) | 0.049 (2) | −0.007 (2) | 0.00000 | 0.00000 |
Geometric parameters (Å, º) top
TI1—F4 | 1.97 (5) | N2—H2D | 1.01 (3) |
TI1—F4i | 1.84 (5) | F4—F5ii | 2.50 (4) |
TI1—F5 | 1.826 (4) | F4—O6ii | 2.64 (4) |
TI1—O6 | 1.9346 (18) | O6—F5 | 2.749 (7) |
N2—H2A | 1.003 (17) | O6—F5iii | 2.564 (6) |
N2—H2B | 0.991 (16) | | |
| | | |
F4—TI1—F4i | 180 (4) | F5vii—F4—O6viii | 64.6 (12) |
F4—TI1—F5 | 94 (2) | O6ii—F4—O6viii | 94.2 (12) |
F4—TI1—O6 | 91 (2) | TI1—F5—F4i | 47.2 (15) |
F4i—TI1—F5 | 86 (2) | TI1—F5—O6v | 48.8 (2) |
F4i—TI1—O6 | 89 (2) | F4i—F5—O6 | 60.2 (12) |
F5—TI1—F5iv | 171.9 (3) | F4i—F5—O6v | 62.9 (13) |
F5—TI1—O6 | 93.9 (3) | O6—F5—O6v | 93.4 (2) |
F5—TI1—O6v | 85.9 (3) | F4i—O6—F4ix | 91.9 (12) |
F5iv—TI1—O6v | 93.9 (3) | F4i—O6—F5 | 55.2 (11) |
O6—TI1—O6v | 177.52 (14) | F4i—O6—F5x | 107.4 (11) |
H2A—N2—H2Avi | 109 (2) | F4i—O6—F5iii | 130.8 (12) |
H2A—N2—H2B | 113 (3) | F4i—O6—F5iv | 57.3 (12) |
H2A—N2—H2D | 103 (3) | F5—O6—F5x | 74.6 (3) |
H2B—N2—H2D | 114 (3) | F5—O6—F5iii | 160.8 (3) |
F5ii—F4—F5vii | 93.7 (13) | F5—O6—F5iv | 86.5 (2) |
F5ii—F4—O6ii | 64.6 (12) | F5iii—O6—F5iv | 112.2 (3) |
F5ii—F4—O6viii | 59.8 (12) | | |
Symmetry codes: (i) x, y, z+1; (ii) x, y, z−1; (iii) x+1/2, −y+1, z; (iv) −x+1, −y+1, z; (v) x−1/2, −y+1, z; (vi) −x+1/2, y, z; (vii) −x+1, −y+1, z−1; (viii) x−1/2, −y+1, z−1; (ix) −x+3/2, y, z+1; (x) −x+3/2, y, z. |