Supporting Information

Ferroelectricity and Dipole- Dipole Interactions in NH4TiOF3 Mesocrystals

Yanqiong Liu, Amit Kumar, Zhen Fan, Yu Zhang, Qingqing Ke, Kaiyang Zeng, John Wang, David J. Singh, Khuong P. Ong

Experimental Section

Sample preparation. Surfactant Brij 58 (Aldrich, 3 g) was dissolved in DI water (10g) first. Fluoride scavenger H3BO3 (99.8%, Merck, 123.7 mg) was then dissolved. Lastly, (NH4)2TiF6 (99.99%, Aldrich, 198.0 mg) was added. When (NH4)2TiF6 was well dissolved, a piece of fluorine-doped tin oxide (SnO2:F) (FTO) coated glass (TEC15, Dyesol) was inserted into the solution. NH4TiOF3 mesocrystals were allowed to grow under static condition in a PPE tube. After 3 h, the glass was removed from solution and dipped in water to remove inorganic impurities and subsequently ultrasonicated for 5 s in ethanol to remove Brij 58 molecules and the mesocrystals which were loosely attached to the substrate. Drying was done by purging with N2. The solid particles in the solution were then recovered by centrifugation and washing successively for three times. The solvents used for washing were water, acetone and ethanol. Drying of powder was carried out at 60 ˚C for 12 h.

To experimentally observe ferroelectricity ofNH4TiOF3 mesocrystals, we first tried the conventional P-E loop measurement [1, 2] which turned out to be problematic. Firstly, the disk samples could not be densified by calcination due to the decomposition of NH4TiOF3 above 200 oC. Secondly, as a consequence, the gold-dots-sputtered disk samples turned out to be fatally leaky and no macroscopic P-E polarization hysteresis loops were measured. Therefore, we prepared samples for piezoresponse force microscopy (PFM)measurement, a technique suitable for local P-E loop and imaging. Since PFM operates in contact mode, the NH4TiOF3 mesocrystals must be adhered strongly to the substrate so that they do not move during measurement. We placed different substrates in the reaction solution and found NH4TiOF3 mesocrystals could grow and adhere strongly to fluorine-doped tin oxide (FTO) film coated glass substrates during PFM measurement. To prevent the aggregation of mesocrystal NH4TiOF3,[3] surfactant Brij 58 (23 wt%) has been introduced. [4, 5]Growth time was limited to 3 h for a smaller particle size so that the canteliver tip can keep contact with particle surface better at the edges of particles during PFM measurement.

Characterization. Powder and thin film X-ray diffraction (XRD, Bruker AXS D8 Advance, Germany) measurement was carried out to identify the crystalline phases present in each of the samples. Field emission gun scanning electron microscopy (SEM, Philips XL30) was performed to characterize the particle morphology. Transmission Electron Microscopy and selected area diffraction pattern (SAED) (TEM, JEOL 2010F, 200 kV) were employed to determine the morphology, crystalline phase, and crystallographic directions of the mesocrystals.

PFM study.Vertical and lateral Piezoresponse Force Microscopy (PFM, MFP-3D, Asylum Research, USA) were used to investigate the piezoelectric and ferroelectric responses, domain structure and local hysteresis loops for the mesocrystals. A Pt-coated silicon cantilever tip (OLYMPUS OMCL-AC240TM) (Electric-lever, Olympus, Japan) was used.

Computational methods. All the first principles calculations were done using the PBE-GGA[6, 7]as implemented in the Vienna Ab initio Simulation Package (VASP),[8] employing the Projected Augmented Wave (PAW) method [9, 10]. A 4×4×2 Monkhorst-Pack k-mesh and an energy cut-off of 400eV for plane waves were used. Within our calculations, all atoms in NH4TiOF3were fully relaxed until the Hellmann-Feynman forces were less than 20 meV/Å. The electronic polarization was calculated using the Born effective charge method[11, 12].

Figure S1. SEM of the powder synthesized at 4 ℃ and 3 days in aqueous solution: various additives (a) no additive; (b) ethanol (10 wt%); (c) polyethylene glycol (PEG, MW 200 (10 wt%); (d) ethylene glycol (EG) (10 wt%); (e) ethylene glycol (EG) (30 wt%); (f) diethylene glycol (DEG) (30 wt%). When pure water or mixture solutions of water and these miscible organic liquids (ethanol, ethylene glycol, diethylene glycol, and polyethylene glycol 200) are used instead of surfactant solutions, NH4TiOF3 mesocrystals are still formed. The scale bars in panel (a) apply to the rest. The individual NH4TiOF3 nanocrystals are the most obvious in panels (c) and (f).

  • Figure S2. Possible structures of NH4TiOF3, A-type antiferroelectric: (a) A1, (b) A2, (c) A3; C-type antiferroelectric: (d) C1, (e) C2, (f) C3; G-type antiferroelectric: (g) G; Ferroelectric (h) Fc, (i) Aa, (j) Ab; (k) Mi-ii, (l) F, and (m) Fzig. The red circles are O atoms and the white circles are F atoms. Ti atoms are in the octahedral. [NH4]+ tetrahedra are omitted for simplicity of figures.
  • Table S1. Possible crystal structures of NH4TiOF3 within the first principle calculations. Fzig is the most stable structure

Type / Space group / Energy (eV/f.u.) / Polarization (μC/cm2)
Pa Pb Pc
A1
A2
A3
C1
C2
C3
G
Fc
Aa
Ab
Mi-ii
F
Fzig / P1
P1
P1
P1
P21
P21
P21
P21
P212121
P212121
P1
P1
P1 / -58.59858
-58.59687
-58.58842
-58.59772
-58.60211
-58.60009
-58.59953
-58.60133
-59.03144
-59.01927
-58.83635
-59.11410
-59.11816 / 0
0
0
0
0
0
0
0
0
0
8.9
8.6
8.2 / 0
0
0
0
0
0
0
0
0
0
0.2
5.4
0.3 / 0
0
0
0
0
0
0
6.9
0
0
2.6
0.1
1.6

Figure S3. (a) SEM of NH4TiOF3 mesocrystals on FTO glass prepared at 23 ℃ for 3 h, Some platelet particles more or less lie flat on the substrate while some attach to the substrate from the sides.; (b) XRD spectra of FTO glass, NH4TiOF3 mesocrystals on FTO glass, and NH4TiOF3 powder; the powder obtained is NH4TiOF3 which corresponds to the peaks of PDF card number 52-1674 very well, the peaks for the NH4TiOF3 deposited FTO glass can be indexed to NH4TiOF3 and FTO. (c) TEM image of a microscale NH4TiOF3 mesocrystal from powder synthesized at 23 ℃ for 3 h; (d)-(f) selected area electron diffraction (SAED) patterns for different spots (center and edges) of the microscale mesocrystal shown in panel (c), the particles from collected powder are NH4TiOF3 mesocrystals as different spots of the same particle (center and edges) show the same single-crystal-like selected area electron diffraction (SAED) patterns which correspond to the [001] zone of NH4TiOF3. The top surface diagonals along the slightly truncated corners of the particle correspond to the [010] and [100] crystallographic directions of NH4TiOF3.

FigureS4. Total and partial density of states of Fzig - NH4TiOF3 within the PBE approximation using the Full-potential Linearized Augmented Plane Wave (FP-LAPW) code, WIEN2K [13]. The energy band gap is 3.05eV. The O1, O2, O3 are oxygen atoms as depicted in Fig.S5.

FigureS5. The position of oxygen atoms in (TiOF3)- octahedra in the Fzig NH4TiOF3. The numbers in the figure are in Å unit. The distance between Ti and O atoms are d(Ti-O1) ~1.72Å, d(Ti-O2) ~2.22Å,and d(Ti-O3) ~1.98Å.

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