Target Designing Phase Transition Materials through Halogen Substitution

Article abstract of DOI:10.1002/cphc.202100040

Crystalline materials have received extensive attention due to their extraordinary physical and chemical properties. Among them, phase transition materials have attracted great attention in the fields of photovoltaic, switchable dielectric devices, and ferroelectric memories, etc. However, many of them suffer from low phase transition temperatures, which limits their practical application. In this work, we systematically designed crystalline materials, (TMXM)₂PtCl₆ (X=F, Cl, Br, I) through halogen substitution on the cations, aiming to improving phase transition temperature. The resulting phase transition of (TMXM)₂PtCl₆ (X=F, Cl, Br, I) get a significant enhancement, compared to the parent compound [(CH₃)₄N]₂PtCl₆ ((TM)₂PtCl₆). Such phase transition temperature enhancement can be attributed to the introduction of halogen atoms that increase the potential energy barrier of the cation rotation. In addition, (TMBM)₂PtCl₆ and (TMIM)₂PtCl₆ have a low symmetry and crystallize in the space group C₂/c and P2₁2₁2₁, respectively. This work highlights the halogen substitution in designing crystal materials with high phase transition temperature.

Full text of DOI:10.1002/cphc.202100040

Articles
doi.org/10.1002/cphc.202100040
ChemPhysChem
Target Designing Phase Transition Materials through
Halogen Substitution
Hao Cheng+,[a] Meng-Juan Yang+,[a] Yu-Qiu Xu,[a] Meng-Zhen Li,[a] and Yong Ai*[a]
In Memoriam: Prof. Jean-Michel Savéant.
Crystalline materials have received extensive attention due to
their extraordinary physical and chemical properties. Among
them, phase transition materials have attracted great attention
in the fields of photovoltaic, switchable dielectric devices, and
ferroelectric memories, etc. However, many of them suffer from
low phase transition temperatures, which limits their practical
application. In this work, we systematically designed crystalline
materials, (TMXM)2PtCl6 (X=F, Cl, Br, I) through halogen
substitution on the cations, aiming to improving phase
transition temperature. The resulting phase transition of
(TMXM)2PtCl6 (X=F, Cl, Br, I) get a significant enhancement,
compared to the parent compound [(CH3)4N]2PtCl6 ((TM)2PtCl6).
Such phase transition temperature enhancement can be
attributed to the introduction of halogen atoms that increase
the potential energy barrier of the cation rotation. In addition,
(TMBM)2PtCl6 and (TMIM)2PtCl6 have a low symmetry and
crystallize in the space group C2/c and P212121, respectively. This
work highlights the halogen substitution in designing crystal
materials with high phase transition temperature.
1. Introduction
a lower symmetry of crystal packing. This important semi-
empirical method is quite powerful to guide the designing of
molecular crystals with low symmetry. When functional groups
are introduced on the cations, the dipole moment of the
molecules can be tuned and polar crystal structures with low
symmetry are expected. For instance, the chloride substitution
lead to the symmetry lowered from space group, P63/m for
[Me4N]CdCl3 to the polar space group, Cc for [Me3NCH2Cl]
CdCl3.[24] The symmetry reducing by halogen substitution also
occurred in the [Me3NCH2X]MnX3 and [Me3NCH2X]PbI3 (X=H, F,
Cl, Br, I), hybrid crystal materials.[23,24,31]
Inspired by those work in designing halogen-substituted
hybrid crystal materials, we synthesized crystalline materials
(TMBM)2PtCl6 and (TMIM)2PtCl6 with high transition temper-
atures. As shown in scheme 1, the [Me4N]2PtCl6 was selected as
the parent crystal structure, which was crystallized in the
centrosymmetric space group Fd3c (NO. 228) at room temper-
ature and has a phase transition at 193 K.[32] With halogen
substitution, the phase transition temperature was enhanced by
Phase transition crystalline materials, such as organic-inorganic
hybrid perovskites, ferroelectric materials, usually have the
capabilities in the conversion of physical or chemical properties
when subjected to external inputs.[1–5] Those phase transition
crystalline materials have broad applications in the fields of the
photoelectric switch,[6–9] switchable dielectric equipment,[10–12]
ferroelectric memory devices,[13–15] sensors,[16] etc. How to chemi-
cally synthesize and obtain crystalline materials with high phase
transition temperature is crucial concerning the practical
applications.[17–19] From the point view of crystallography, the
symmetry of the crystal structure is usually described using 32
point groups and 230 space groups,[20,21] and the solid-solid
phase transition is always accompanied by the symmetry
breaking,[22] which reveals the physical and/or mathematical
characteristics of the phase transition. The symmetry breaking is
essential for understanding phase transition behaviors of
crystalline materials, especially for the hybrid organic and
inorganic materials and electrolyte materials.[23–25] Reducing the
symmetry of the crystals through chemical modification is an
efficient way to obtain phase transition materials.[22,26,27] It was
found that the globular molecules usually have phase
transitions[28–30] and substitution on such molecules may lead to
[a] H. Cheng,+ M.-J. Yang,+ Y.-Q. Xu, M.-Z. Li, Prof. Y. Ai
College of Chemistry
Nanchang University
Nanchang 330031, P. R. China
E-mail: yong.ai@ncu.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cphc.202100040
Scheme 1. Design of (TMXM)2PtCl6 (X=F, Cl, Br and I) through halogen
substitution.
An invited contribution to a joint Special Collection in memory of Prof.
Jean-Michel Savéant
ChemPhysChem 2021, 22, 752–756
752
© 2021 Wiley-VCH GmbH

Articles
doi.org/10.1002/cphc.202100040
ChemPhysChem
the halogen atoms reached up to 317 K for (TMBM)2PtCl6 and
341 K for (TMIM)2PtCl6. Moreover, the halogen substitution
effect on the crystal symmetry is worth to investigate. This work
highlights the halogen substitution for design phase transition
materials with high phase transition point.
ature range below the phase transition point was labelled as
the low temperature phase (LTP), and the temperature range
above the phase transition point as the high temperature phase
(HTP). Prior to dielectric constant measurement, these com-
pounds were ground into powder and pressed into thin sheets.
Figure 2 shows the temperature dependence of the real part of
the complex relative permittivity (ɛ’) measured at 1 MHz. There
are step-like anomalies that could be observed for (TMXM)2PtCl6
(X=F, Cl, Br, and I). The dominant abnormal behaviours of the
dielectric constant suggest that the phase transition occurs at
195, 269, 317, and 341 K for (TMXM)2PtCl6 (X=F, Cl, Br, and I),
respectively. The phase transition points are matched well with
that deduced from the DSC measurement.
2. Results and Discussion
2.1. Phase Transitions
Differential Scanning Calorimeter (DSC) measurements were
carried out to investigate the phase transition behaviours of
(TMXM)2PtCl6 (X=F, Cl, Br, and I). As shown in Figure 1, there
are significant endothermic and exothermic peaks during the
heating and cooling process for (TMXM)2PtCl6 (X=F, Cl, Br, and
I). According to the peaks in the heating runs, the phase
transition temperatures are deduced as 195/214 K, 269, 317,
and 341 K for (TMFM)2PtCl6, (TMCM)2PtCl6, (TMBM)2PtCl6 and
(TMIM)2PtCl6, respectively. There is a strong phase transition
temperature enhanced tendency as the halogen atoms varied
from F to I. This phase transition temperature enhanced
phenomenon is similar to the halogen-substituted [Me3NCH2X]
PbI3 (X=H, F, Cl, Br, I).[27] The entropy change (ΔS) is an
important factor to understand the phase transition. According
to the formula, ΔS=ΔH/T, the ΔS of (TMXM)2PtCl6 are 13.651/
3.812, 19.460, 4.013, and 3.583 JmolÀ 1 KÀ 1 for F, Cl, Br, and I
substituted (TMXM)2PtCl6, respectively. The Boltzmann equation,
ΔS=R ln(N), where N represents the possible coordination
number, and R represents the gas constant, gives the disorder
level of the cations when the phase transition occurs.[33] It is
calculated that N values are 5.165/1.582, 10.387, 1.620 and
1.539 for (TMFM)2PtCl6, (TMCM)2PtCl6, (TMBM)2PtCl6 and
(TMIM)2PtCl6, respectively. These larger N values indicate that
ordered to the disordered type of phase transition occurred in
the (TMXM)2PtCl6 (X=F, Cl, Br, and I) crystals.
2.2. Single-crystal Structures
To get a deep insight into the phase transition behaviours,
single-crystal X-ray diffraction (XRD) was carried out to charac-
terize the crystal structures. Due to the weak diffraction caused
by the highly disordered organic cations, it was difficult to
obtain the accurate crystal structures of the halogen-substituted
(TMXM)2PtCl6 (X=F, Cl, Br, and I) at HTP. Only cubic system
parameters of (TMFM)2PtCl6, (TMCM)2PtCl6, and (TMBM)2PtCl6
were obtained at HTP, see Table S1, supporting information.
Luckily, (TMBM)2PtCl6 and (TMIM)2PtCl6 crystal data was
obtained at LTP. (TMBM)2PtCl6 crystal was refined in the space
group, C2/c (No. 15), monoclinic crystal system. unit cell
parameters were a=15.1319 (13) Å, b=19.2792(12) Å, c=
3
°
°
15.6803(16) Å, α=γ=90 , β=111.963(11) , V=4242.4(7) Å .
(TMIM)2PtCl6 was refined in the space group P212121 (No. 19),
orthogonal system. The unit cell parameters were a=9.1438 (3)
°
Å, b=14.0615(4) Å, c=16.6790(6) Å, α=β=γ=90 , V=
2144.51(12) Å3, see Table S2 in the supporting information. As
shown in Figure 3a, the asymmetric unit of (TMBM)2PtCl6
contains two [TMBM]+ and one [PtCl6]2À . For the inorganic
anion, the Pt(II) center has a octahedral geometry with six
chlorine atoms as ligands. As shown in the packing diagrams in
Figure 3b, there are no intermolecular interactions (for instance,
Besides DSC, dielectric constant measurement is widely
performed to determine the phase transition temperature of
the solid electrolyte materials. For convenience, the temper-
Figure 1. DSC curves acquired in a heating-cooling cycle of (TMXM)2PtCl6
(X=F, Cl, Br and I).
Figure 2. Temperature dependence of the real part (ɛ’) of the dielectric
constant of (TMXM)2PtCl6 (X=F, Cl, Br and I) at 1 MHz in the heating run.
ChemPhysChem 2021, 22, 752–756
www.chemphyschem.org
753
© 2021 Wiley-VCH GmbH

Articles
doi.org/10.1002/cphc.202100040
ChemPhysChem
supporting information), which reveals the high purity. Figure 4
shows the PXRD curves of (TMBM)2PtCl6. Below phase transition
temperature (317 K), the number of peaks remained the same
as the initial state, indicating no phase transition occurring.
When the temperature was greater than phase transition
temperature, the PXRD pattern changed significantly with the
fact that diffraction peaks at 11.47, 12.13, 12.56, 16.5, 17.9,
18.38, 19.81, 20.49, 22.33, and 22.57 disappeared, and three
new diffraction peaks were observed at 11.74, 22.73 and 35.86,
respectively. The reduction of the number of peaks at high
temperature suggests a highly symmetric lattice. Since the
phase transition of (TMIM)2PtCl6 occurred from the orthorhom-
bic system to the orthorhombic system, no significant changes
were observed in the variable temperature PXRD pattern, see
Figure S1, supporting information.
It is worth to mention that the halogen bond has a strong
influence on crystal packing and crystal designing.[34] The
halogen bond, RÀ X···YÀ Z, occurs when there is a net attractive
interaction between an electrophilic region on a halogen atom
X belonging to a molecule or a molecular fragment RÀ X (where
R can be another atom, including X, or a group of atoms) and a
nucleophilic region of a molecule, or molecular fragment,
YÀ Z.[35] In terms of crystal design, halogen bonds offer a unique
opportunity in the strength, atom size, and interaction
gradation.[34] For instance, in the crystal system of TMXMPbI3
(X=F, Cl, Br and I) and TMCMCdX3 (X=Cl, Br), the halogen
°
bonds tend to form at approximately 180 angles. It was found
that the strength of the halogen bond increases as the
substitution of halogen atoms varied from F to I.[24,31,36] Here,
one could believe that the halogen bonding effect may have a
great influence on (TMIM)2PtCl6, where halogen bond energy of
[(CH3)3NCH2I]-I may assist the [TMIM]+cations to persist its order
state at high temperature. Moreover, halogen bond energy is
prone to possess the high potential energy barrier of the cation
rotation, which is responsible for the phase transition temper-
ature enhancement. What’s more, halogen substitution have a
great chance to tune the dipole moment of the molecules and
Figure 3. Crystal structures. a) asymmetric unit of (TMBM)2PtCl6, b) packing
view of (TMBM)2PtCl6 along ac plane, c) asymmetric unit of (TMIM)2PtCl6, d)
packing view of (TMIM)2PtCl6 along bc plane.
NÀ H···Br hydrogen bonding) between organic [TMBM]+ cations
and inorganic [PtCl6]2À anions. In addition, the cations and
anions are in a cross-arrangement. The structure of (TMIM)2PtCl6
is similar to (TMBM)2PtCl6, the asymmetric unit contains two
[TMIM]+ and one [PtCl6]2À , as shown in Figure 3c, (TMIM)2PtCl6
has a significant intermolecular interactions between organic
[TMIM]+ cations and inorganic [PtCl6]2À anions. In Figure 3d, Cl
atoms locate close to the anionic chains (nucleophilic regions)
forming CÀ I···ClÀ Pt weak bonds.
To get a further understanding of the structural phase
transition, we carried out the variable temperature powder X-
ray diffraction (PXRD) measurements. The PXRD patterns of
(TMBM)2PtCl6 and (TMIM)2PtCl6 at room temperature were in
good agreement with the simulation results (see Figure S2,
Figure 4. Variable-temperature PXRD pattern of (TMBM)2PtCl6 obtained in
the temperature range 298–343 K.
ChemPhysChem 2021, 22, 752–756
www.chemphyschem.org
754
© 2021 Wiley-VCH GmbH

Articles
doi.org/10.1002/cphc.202100040
ChemPhysChem
thus affect symmetry of the molecular crystals. Halogen Conflict of Interest
substitution on (TM)2PtCl6 was an success for designing crystal
materials with high phase transition temperatures.
The authors declare no conflict of interest.
Keywords: crystalline materials · crystal design · halogen
substitution · phase transition
3. Conclusions
In summary, we systematically designed crystalline materials
(TMBM)2PtCl6 and (TMIM)2PtCl6. Starting from the parent
compound [(CH3)4N]2PtCl6, the symmetry of the (TMXM)2PtCl6
was reduced by introducing the single halogen atom on the
cations. The phase transition temperatures were increased from
193 K for [(CH3)4N]2PtCl6 to 317 K for (TMBM)2PtCl6 and 341 K for
(TMIM)2PtCl6. Moreover, (TMBM)2PtCl6 and (TMIM)2PtCl6 have a
low symmetry and crystallizes in space group C2/c (No. 15),
monoclinic system, and space group P212121 (No. 19), orthogo-
nal system, respectively. The succusses of these accurate
molecular modifications towards high phase transition temper-
ature could be attributed to the increase of potential energy
barrier of the cation rotation. Hence, the introduction of
halogens on the cations is an effcient way to explore and
design crystal materials with high phase transition temperature
and low symmetry.
[1] M. Wuttig, N. Yamada, Nat. Mater. 2007, 6, 824–832.
[2] D. W. Fu, H. L. Cai, Y. Liu, Q. Ye, W. Zhang, Y. Zhang, X. Y. Chen, G.
Giovannetti, M. Capone, J. Li, R. G. Xiong, Science 2013, 339, 425–428.
[3] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M.
Graetzel, T. J. White, J. Mater. Chem. A 2013, 1, 5628–5641.
[4] B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geertsema, Chem. Rev.
2000, 100, 1789–1816.
[5] H. Zheng, J. B. Rivest, T. A. Miller, B. Sadtler, A. Lindenberg, M. F. Toney,
L. W. Wang, C. Kisielowski, A. P. Alivisatos, Science 2011, 333, 206–209.
[6] Z. Sun, J. Luo, S. Zhang, C. Ji, L. Zhou, S. Li, F. Deng, M. Hong, Adv.
Mater. 2013, 25, 4159–4163.
[7] M. Samoc, N. Gauthier, M. P. Cifuentes, F. Paul, C. Lapinte, M. G.
Humphrey, Angew. Chem. Int. Ed. 2006, 45, 7376–7379; Angew. Chem.
2006, 118, 7536–7539.
[8] D. Y. Fu, J. R. Yuan, S. C. Wu, Y. P. Yao, X. Y. Zhang, X. M. Zhang, Inorg.
Chem. Front. 2020, 7, 1394–1399.
[9] C. Ji, Z. Sun, S. Zhang, S. Zhao, T. Chen, Y. Tang, J. Luo, Chem. Commun.
2015, 51, 2298–2300.
[10] Y. Du, H. Hao, Q. Zhang, H. Zhao, L. Long, R. Huang, L. Zheng, Sci. China
Chem. 2013, 56, 917–922.
[11] Z. X. Zhang, T. Zhang, W. Y. Zhang, P. P. Shi, Q. Ye, D. W. Fu, Inorg.
Chem. 2019, 58, 4600–4608.
[12] C. Ji, Z. Sun, S. Q. Zhang, T. Chen, P. Zhou, Y. Tang, S. Zhao, J. Luo, J.
Mater. Chem. C 2014, 2, 6134–6139.
[13] Y. Zhang, H. Y. Ye, H. L. Cai, D. W. Fu, Q. Ye, W. Zhang, Q. Zhou, J. Wang,
G. L. Yuan, R. G. Xiong, Adv. Mater. 2014, 26, 4515–4520.
[14] W. J. Xu, P. F. Li, Y. Y. Tang, W. X. Zhang, R. G. Xiong, X. M. Chen, J. Am.
Chem. Soc. 2017, 139, 6369–6375.
[15] H. Y. Ye, Q. Zhou, X. Niu, W. Q. Liao, D. W. Fu, Y. Zhang, Y. M. You, J.
Wang, Z. N. Chen, R. G. Xiong, J. Am. Chem. Soc. 2015, 137, 13148–
13154.
[16] S. Hannah, A. Davidson, I. Glesk, D. Uttamchandani, R. Dahiya, H.
Gleskova, Org. Electron. 2018, 56, 170–177.
[17] Y. Ai, X. G. Chen, P.-P. Shi, Y. Y. Tang, P. F. Li, W. Q. Liao, R. G. Xiong, J.
Am. Chem. Soc. 2019, 141, 4474–4479.
[18] Y. Ai, D. J. Wu, M. J. Yang, P. Wang, W. H. He, W. Q. Liao, Chem. Commun.
2020, 56, 7033–7036.
[19] Y. Y. Tang, Y. Ai, W. Q. Liao, P. F. Li, Z. X. Wang, R. G. Xiong, Adv. Mater.
2019, 31, 1902163.
Experimental Section
Synthesis
Trimethylfluoromethylammonium chloride (TMFM) was synthesized
by the reaction of equimolar amounts of trimethylamine (30 wt.%
in water) and chlorofluoromethane dissolved in ethanol at room
temperature. After one week, the solvent was removed under
reduced pressure to obtain TMFM as colourless solid. Trimeth-
ylchloromethylammonium chloride (TMCM), Trimethylbromometh-
ylammonium bromide (TMBM·Br) and Trimethyliodometh-
ylammonium iodide (TMIM·I) were synthesized by a similar recipe.
Briefly, mixing the equimolar amounts of dichloromethane or
dibromomethane and diiodomethane with trimethylamine
(30 wt.% in water) at room temperature, the quaternary ammonium
salt were obrained by removing the solvant. The halogen
substituted methylammonium then were mixed with PtCl4 at a
molar mass ratio of 2:1 in the concentrated HCl solution.
(TMFM)2PtCl6, (TMCM)2PtCl6, (TMBM)2PtCl6, and (TMIM)2PtCl6 were
obtained as orange crystals after one week.
[20] R. R. Birss, Symmetry and Magnetism, North-Holland, Amsterdam, 1964.
[21] G. Burns, A. M. Glazer, Space Groups for Solid State Scientists, Elsevier,
Amsterdam, 2013.
[22] P. P. Shi, Y. Y. Tang, P. F. Li, W. Q. Liao, Z. X. Wang, Q. Ye, R. G. Xiong,
Chem. Soc. Rev. 2016, 45, 3811–3827.
[23] W. Q. Liao, Y. Y. Tang, P. F. Li, Y. M. You, R. G. Xiong, J. Am. Chem. Soc.
2017, 139, 18071–18077.
[24] Y. M. You, W. Q. Liao, D. Zhao, H. Y. Ye, Y. Zhang, Q. Zhou, X. Niu, J.
Wang, P. F. Li, D. W. Fu, Z. Wang, S. Gao, K. Yang, J. M. Liu, J. Li, Y. Yan,
R.-G. Xiong, Science 2017, 357, 306–309.
[25] H. Y. Ye, Y. Y. Tang, P. F. Li, W. Q. Liao, J. X. Gao, X. N. Hua, H. Cai, P. P.
Shi, Y. M. You, R. G. Xiong, Science 2018, 361, 151–155.
[26] L. He, L. Zhou, P. P. Shi, Q. Ye, D. W. Fu, Chem. Mater. 2019, 31, 10236–
10242.
Measurements
Differential Scanning Calorimeter (DSC), dielectric, powder X-ray
diffraction (PXRD) measurements and X-ray diffraction (XRD)
measurements were described elsewhere.[37,38]
[27] L. He, P. P. Shi, M. M. Zhao, C. M. Liu, W. Zhang, Q. Ye, Chem. Mater.
Acknowledgements
2021, doi:10.1021/acs.chemmater.0c04735.
[28] D. Chandra, W. Ding, R. A. Lynch, J. J. Tomilinson, J. Less-Common Met.
1991, 168, 159–167.
[29] Y. Kaneko, M. Sorai, Phase Transitions. 2007, 80, 517–528.
[30] M. F. Daniel, A. J. Leadbetter, M. A. Mazid, J. Chem. Soc. Faraday Trans. 2
1981, 77, 1837–1849.
The authors thank the supporting by the National Natural Science
Foundation of China (21901099).
[31] X. N. Hua, W. Q. Liao, Y. Y. Tang, P. F. Li, P.-P. Shi, D. Zhao, R. G. Xiong, J.
Am. Chem. Soc. 2018, 140, 12296–12302.
[32] M. Autillo, R. E. Wilson, Eur. J. Inorg. Chem. 2017, 41, 4834–4839.
ChemPhysChem 2021, 22, 752–756
www.chemphyschem.org
755
© 2021 Wiley-VCH GmbH

Articles
doi.org/10.1002/cphc.202100040
ChemPhysChem
[33] P. Jain, V. Ramachandran, R. J. Clark, H. D. Zhou, B. H. Toby, N. S. Dalal,
H. W. Kroto, A. K. Cheetham, J. Am. Chem. Soc. 2009, 131, 13625–13627.
[34] A. Mukherjee, S. Tothadi, G. R. Desiraju, Acc. Chem. Res. 2014, 47, 2514–
2524.
[38] Q. Ye, P. P. Shi, X. Q. Fu, T. Akutagawa, T. Nakamura, CrystEngComm.
2013, 15, 5307–5313.
[35] G. R. Desiraju, P. S. Ho, L. Kloo, A. C. Legon, R. Marquardt, P. Metrangolo,
P. Politzer, G. Resnati, K. Rissanen, Pure Appl. Chem. 2013, 85, 1711–
1713.
[36] W. Q. Liao, Y. Y. Tang, P. F. Li, Y. M. You, R. G. Xiong, J. Am. Chem. Soc.
2018, 140, 3975–3980.
[37] W. Y. Zhang, Y. Y. Tang, P. F. Li, P. P. Shi, W. Q. Liao, D. W. Fu, H. Y. Ye, Y.
Zhang, R. G. Xiong, J. Am. Chem. Soc. 2017, 139, 10897–10902.
Manuscript received: January 22, 2021
Revised manuscript received: February 15, 2021
Accepted manuscript online: February 15, 2021
Version of record online: March 15, 2021
ChemPhysChem 2021, 22, 752–756
www.chemphyschem.org
756
© 2021 Wiley-VCH GmbH