Polar and non-polar structures of NH₄TiOF₃

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.

Computing details

For both structures, program(s) used to refine structure: FULLPROF.

(NH4TiOF3)

Crystal data

F3OTi·H4N	c = 7.58450 (12) Å
Mr = 138.9	V = 361.17 (1) Å^3
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

Pilatus 300K-W diffractometer	Data collection mode: transmission
Radiation source: synchrotron	Scan method: Stationary detector

Refinement

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

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 (Ų)

	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 (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
TI1	0.0705 (8)	0.0589 (6)	0.0544 (6)	0.0037 (10)	−0.002 (2)	0.0011 (16)

Geometric parameters (Å, º)

TI1—F3	1.801 (13)	TI1—O6^ii	1.901 (16)
TI1—F4	1.93 (3)	N2—H2A	0.995 (10)
TI1—F4^i	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)	F4^i—TI1—O6^ii	94.4 (14)
F3—TI1—F4^i	94.0 (14)	F5—TI1—O6	89.4 (8)
F3—TI1—F5	170.2 (10)	F5—TI1—O6^ii	90.2 (9)
F3—TI1—O6	85.3 (9)	O6—TI1—O6^ii	171.7 (14)
F3—TI1—O6^ii	93.8 (9)	H2A—N2—H2B	107.8 (17)
F4—TI1—F4^i	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—O6^ii	86.2 (13)	H2B—N2—H2D	111.3 (14)
F4^i—TI1—F5	94.6 (12)	H2C—N2—H2D	109.6 (17)
F4^i—TI1—O6	93.9 (13)

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

(NH4TiOF3_HT1)

Crystal data

F3OTi·H4N	c = 3.80544 (9) Å
Mr = 138.9	V = 184.21 (1) Å^3
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

Pilatus 300K-W diffractometer	Data collection mode: transmission
Radiation source: synchrotron	Scan method: Stationary detector

Refinement

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

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 (Ų)

	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 (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
TI1	0.0418 (19)	0.0381 (12)	0.049 (2)	−0.007 (2)	0.00000	0.00000

Geometric parameters (Å, º)

TI1—F4	1.97 (5)	N2—H2D	1.01 (3)
TI1—F4^i	1.84 (5)	F4—F5^ii	2.50 (4)
TI1—F5	1.826 (4)	F4—O6^ii	2.64 (4)
TI1—O6	1.9346 (18)	O6—F5	2.749 (7)
N2—H2A	1.003 (17)	O6—F5^iii	2.564 (6)
N2—H2B	0.991 (16)
F4—TI1—F4^i	180 (4)	F5^vii—F4—O6^viii	64.6 (12)
F4—TI1—F5	94 (2)	O6^ii—F4—O6^viii	94.2 (12)
F4—TI1—O6	91 (2)	TI1—F5—F4^i	47.2 (15)
F4^i—TI1—F5	86 (2)	TI1—F5—O6^v	48.8 (2)
F4^i—TI1—O6	89 (2)	F4^i—F5—O6	60.2 (12)
F5—TI1—F5^iv	171.9 (3)	F4^i—F5—O6^v	62.9 (13)
F5—TI1—O6	93.9 (3)	O6—F5—O6^v	93.4 (2)
F5—TI1—O6^v	85.9 (3)	F4^i—O6—F4^ix	91.9 (12)
F5^iv—TI1—O6^v	93.9 (3)	F4^i—O6—F5	55.2 (11)
O6—TI1—O6^v	177.52 (14)	F4^i—O6—F5^x	107.4 (11)
H2A—N2—H2A^vi	109 (2)	F4^i—O6—F5^iii	130.8 (12)
H2A—N2—H2B	113 (3)	F4^i—O6—F5^iv	57.3 (12)
H2A—N2—H2D	103 (3)	F5—O6—F5^x	74.6 (3)
H2B—N2—H2D	114 (3)	F5—O6—F5^iii	160.8 (3)
F5^ii—F4—F5^vii	93.7 (13)	F5—O6—F5^iv	86.5 (2)
F5^ii—F4—O6^ii	64.6 (12)	F5^iii—O6—F5^iv	112.2 (3)
F5^ii—F4—O6^viii	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.