Methylphosphonium Tin Bromide: A 3D Perovskite Molecular Ferroelectric Semiconductor

Article abstract of DOI:10.1002/adma.202005213

3D ABX₃ organic–inorganic halide perovskite (OIHP) semiconductors like [CH₃NH₃]PbI₃ have received great attention because of their various properties for wide applications. However, although a number of low-dimensional lead-based OIHP ferroelectric semiconductors have been documented, obtaining 3D ABX₃ OIHP ferroelectric semiconductors is challenging. Herein, an A-site cation [CH₃PH₃]⁺ (methylphosphonium, MP) is employed to successfully obtain a lead-free 3D ABX₃ OIHP ferroelectric semiconductor MPSnBr₃, which shows clear above-room-temperature ferroelectricity and a direct bandgap of 2.62 eV. It is emphasized that MPSnBr₃ is a multiaxial molecular ferroelectric with the number of ferroelectric polar axes being as many as 12, which is far more than those of the other OIHP ferroelectric semiconductors and even the classical inorganic perovskite ferroelectric semiconductors BiFeO₃ (4 polar axes) and BaTiO₃ (3 polar axes). MPSnBr₃ is the first MP-based 3D ABX₃ OIHP ferroelectric semiconductor. This finding throws light on the exploration of other excellent 3D ABX₃ OIHP ferroelectric semiconductors with great application prospects.

Full text of DOI:10.1002/adma.202005213

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Methylphosphonium Tin Bromide: A 3D Perovskite
Molecular Ferroelectric Semiconductor
Han-Yue Zhang,* Xiao-Gang Chen, Zhi-Xu Zhang, Xian-Jiang Song, Tie Zhang,
Qiang Pan, Yi Zhang, and Ren-Gen Xiong*
the superior photovoltaic performance of
3D [CH3NH3]PbI3 might benefit from its
ferroelectric polarization.[5] [CH3NH3]PbI3
has been shown to exhibit a polar crystal
structure at room temperature by Breter-
nitz et al.[6a] and the ferroelectric domain
shifting by Röhm et al.[6b] However,
3D OIHP ferroelectric semiconductors
remain very rare. To date, OIHP ferroelec-
tric semiconductors are mainly the lower-
dimensional lead-based ones,[2,4,7] such as
the 1D [trimethyliodomethylammonium]
3D ABX3 organic–inorganic halide perovskite (OIHP) semiconductors like
[CH3NH3]PbI3 have received great attention because of their various proper-
ties for wide applications. However, although a number of low-dimensional
lead-based OIHP ferroelectric semiconductors have been documented,
obtaining 3D ABX3 OIHP ferroelectric semiconductors is challenging.
Herein, an A-site cation [CH3PH3]+ (methylphosphonium, MP) is employed
to successfully obtain a lead-free 3D ABX3 OIHP ferroelectric semiconductor
MPSnBr3, which shows clear above-room-temperature ferroelectricity and
a direct bandgap of 2.62 eV. It is emphasized that MPSnBr3 is a multiaxial
molecular ferroelectric with the number of ferroelectric polar axes being as
many as 12, which is far more than those of the other OIHP ferroelectric
semiconductors and even the classical inorganic perovskite ferroelectric
semiconductors BiFeO3 (4 polar axes) and BaTiO3 (3 polar axes). MPSnBr3 is
the first MP-based 3D ABX3 OIHP ferroelectric semiconductor. This finding
throws light on the exploration of other excellent 3D ABX3 OIHP ferroelectric
semiconductors with great application prospects.
[7a]
PbI3 and the 2D [4,4-diflorocyclohexyla
mmonium]2PbI4.[4b] The 3D OIHP ferroe-
lectric semiconductors are highly desirable
for the development of high-performance
optoelectronic devices because high struc-
tural dimensionality is in favor of excel-
lent optoelectronic properties and device
performances.[8] For instance, in contrast
to the low-dimensional counterparts, the
3D OIHP semiconductors represented by
Organic–inorganic halide perovskite (OIHP) semiconductors
exampled by the 3D [CH3NH3]PbI3 have gained tremendous
interests in the past decade owing to their intriguing opto-
electronic properties for wide applications in solar cells, light
emitting diodes, photodetectors, etc.[1] Among them, the OIHP
semiconductors with ferroelectricity, which enables the spon-
taneous polarization (Ps) to be switched by electric field, have
recently attracted considerable attention as candidates for next-
generation optoelectronic applications.[2] In ferroelectric semi-
conductors, the Ps induced built-in electric field is beneficial
for the dissociation of photoexcited carriers.[3] The incorpora-
tion of ferroelectricity also endows ferroelectric semiconduc-
tors with some unique features from the ordinary counterparts
such as the switchable photovoltaic effect, polarization-induced
photovoltaic effect, directionally tunable photovoltaic effect, and
polarization-sensitive photodetection.[4] It has been deemed that
[CH3NH3]PbI3 show lower bandgap, better conductivity, and
higher charge carrier mobility, making them promising solar-
cell absorbers,[8b] and the solar cells based on 3D OIHPs also
show much higher power conversion efficiency.[8c]
3D OIHP semiconductors like [CH3NH3]PbI3 adopt the
ABX3 perovskite structure, where A is the organic cation, B
is the Pb(II), Sn(II), or Ge(II) ion, and X is the halogen ion.[1a]
The A site cation resides in the cavity of the 3D BX3 framework
formed by corner-sharing BX6 octahedra. To maintain the 3D
BX3 framework, the A site is limited to some small size cat-
ions like the classical [CH3NH3]+ (methylammonium, MA) and
[NH2(CH)NH2]+ (formamidinium, FA) according to the Gold-
schmidt tolerance factor rule.[9] Despite great efforts have been
made to broad the 3D ABX3 OIHP family, the resulting fruits
were minimal. Recently, the [F-CH2NH3]+ and [CH3NH2NH2]+
cations were utilized to form the 3D ABX3 lead-based OIHP
[10]
[F-CH2NH3]PbBr3 and [CH3NH2NH2]PbX3 (X = Br or Cl),[11]
respectively, but none of them shows the ferroelectricity. Dis-
covering a new 3D ABX3 OIHP semiconductor has been a
great challenge, while endowing it with ferroelectricity is more
arduous. From a structural perspective, in OIHP system, it is the
reorientation and ordered alignment of organic cation within
an inorganic framework that contributes the possibility of fer-
roelectricity.[12] Very recently, we constructed a 3D lead chloride
perovskite-related ferroelectric [2-trimethylammonioethylam-
monium]Pb2Cl6 in which the reorientation of organic cation
occurs during the ferroelectric phase transition.[13] However, the
Dr. H.-Y. Zhang, Dr. X.-G. Chen, Dr. Z.-X. Zhang, Dr. X.-J. Song,
Dr. T. Zhang, Dr. Q. Pan, Prof. Y. Zhang, Prof. R.-G. Xiong
Jiangsu Key Laboratory for Science and Applications of Molecular
Ferroelectrics
Southeast University
Nanjing 211189, P. R. China
E-mail: zhanghanyue@seu.edu.cn; xiongrg@seu.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202005213.
DOI: 10.1002/adma.202005213
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environmental toxicity of lead in [2-trimethylammonioethylam-
monium]Pb2Cl6 and the low-dimensional OIHP ferroelectric
semiconductors is an obstacle for future applications. More-
over, [2-trimethylammonioethylammonium]Pb2Cl6 is a uni-
axial ferroelectric, similar to the cases of most low-dimensional
OIHP ferroelectric semiconductors,[2,4,7] which also impedes
the further applications since the polarization in uniaxial fer-
roelectrics can only be switched between two opposite polari-
zation directions.[14] The widely used ferroelectrics are mainly
the multiaxial ones like BaTiO3 and Pb(Zr,Ti)O3, of which the
polarization can be switched between multiple directions to
obtain high ferroelectric performance.[14]
crystallizes in the P21/m space group at 223 K (Table S2, Sup-
porting Information), of which the crystal structure is com-
posed of MP cations and Br anions (Scheme 1). MPSnBr3 was
then prepared as crystals from a hydrobromic acid solution with
equimolar amounts of MPBr and SnBr2. Single-crystal structure
determinations based on the single-crystal X-ray diffraction data
reveal that MPSnBr3 adopts the 3D ABX3 OIHP structure, where
A, B, and X is MP, Sn (II), and Br−, respectively, and the organic
MP cation occupies the void of inorganic 3D [SnBr3]− framework
of corner-sharing SnBr6 octahedron (Figure 1 and Figure S1, Sup-
porting Information), isostructural to the 3D OIHP MASnBr3.[16]
At 293, 333, and 383 K, MPSnBr3 adopts the polar Pc (m point
group), polar Pna21 (mm2 point group), and centrosymmetric
Pm3m (m3m point group) space group, respectively (Table S2,
Supporting Information), which correspond to the room-tem-
perature ferroelectric phase (RFP), high-temperature ferro-
electric phase (HFP), and high-temperature paraelectric phase
(HPP), respectively. In the RFP, the MP molecule in the crystal
structure occupies a general symmetry position and shows an
ordered state. The atomic coordinates of C and P atoms can be
[CH3PH3]+ (methylphosphonium, MP) is a small size cation,
which was theoretically predicted to be a choice of A site for
constructing 3D ABX3 OIHPs.[9b] However, MP-based OIHPs
have not been experimentally explored. Here, we successfully
synthesized a MP-based lead-free 3D ABX3 OIHP, MPSnBr3.
We found that MPSnBr3 exhibits two distinct transitions from
ferroelectric m to ferroelectric mm2 to paraelectric m3m point
group at 314 and 357 K, respectively, accompanied by the reori-
entation of MP cation. MPSnBr3 shows clear above-room-
temperature ferroelectricity and a direct bandgap of 2.62 eV.
Very importantly, MPSnBr3 is a multiaxial molecular ferro-
electric with the number of ferroelectric polar axes as many as
12, which is outstanding in molecular ferroelectrics,[14] and is
much more than those of the other OIHP ferroelectric semi-
conductors (Table S1, Supporting Information),[2,4,7,13] most
of which are uniaxial or biaxial ferroelectrics, and the clas-
sical inorganic perovskite ferroelectric semiconductors BiFeO3
(4 polar axes) and BaTiO3 (3 polar axes).[14] Strikingly, in terms
of the ferroelectric-to-paraelectric phase transition, the mecha-
nism in MPSnBr3 is originated not only from the commonly
clearly identified. The MP molecule thus shows a single orien-
−

tation (Figure 1a). The [SnBr3] framework shows both Sn Br
bonds and Sn···Br interactions with short (average 2.722(3)
Å) and long (average 3.361(3) Å) SnBr interatomic distances,
respectively (Figure 1a and Table S3, Supporting Information).
In the HFP, the MP cation also lies on a general symmetry posi-
tion and keeps ordered with a single orientation (Figure 1b). For
−

the [SnBr3] framework, the Sn Br bonds and Sn···Br interac-
tions shows no obvious difference of average SnBr interatomic
distances from those in the RFP (Figure 1b and Table S3, Sup-

porting Information), while the average angle of Sn Br···Sn
increases from 156.5° in the RFP to 170.6° (Figure S1, Sup-
porting Information). In the HPP, the MP molecule occupies
a special symmetry position of high symmetry m3m. Conse-
quently, the MP cation becomes highly orientational disorder
with many orientations (Figure 1c). In the [SnBr3]− framework,
seen cationic order−disorder dynamics but also from the reor-
−

ganization of Sn Br bonds in [SnBr3] framework, which is
unprecedented in molecular ferroelectrics.[15] Besides the ferro-
electricity and the semiconducting property commonly found
in OIHP ferroelectric semiconductors, MPSnBr3 also shows
interesting thermochromism. To our knowledge, MPSnBr3
is the first example of MP-based 3D ABX3 OIHP ferroelectric
semiconductor. This work offers a promising A-site cation for
constructing 3D ABX3 OIHP ferroelectric semiconductors and
should inspire further exploration of more excellent 3D ABX3
OIHP ferroelectric semiconductors.

there are only Sn Br bonds with SnBr interatomic distance
of 3.041(1) Å (Figure 1c). This indicates the reorganization of

Sn Br bonds. Thus, besides the order-disorder transition of MP
cation accompanied by its orientation change, the reorganization
−

of Sn Br bonds in [SnBr3] framework also contributes to the
ferroelectric-to-paraelectric phase transition from HFP to HPP.
We also confirm the structural phase transition of MPSnBr3 by
the variable-temperature powder X-ray diffraction (PXRD) exper-
iments (Figure S2, Supporting Information).
We first synthesized methylphosphonium bromide (MPBr)
through the synthetic route shown in Scheme 1. MPBr
Scheme 1. Synthesis of MPBr.
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Figure 1. a–d) The basic structures for MPSnBr3 at 293 K (a), 333 K (b), and 383 K (c,d) with selected interatomic distances (Å) labeled. The yellow
dashed line denotes the Sn···Br interactions.
Differential scanning calorimetry (DSC) curves of MPSnBr3
present two above-room-temperature phase transitions at
of conductivity values of semiconductor materials, revealing the
semiconducting characteristic. This conductivity value is larger
than that of the 2D OIHP ferroelectric (benzylammonium)2PbCl4
(about 10−6.8 S m−1).[7d] Thermogravimetric analysis reveals that
MPSnBr3 is thermally stable up to 450 K, much higher than the
T
= 314 K and T = 357 K, respectively (Figure 2a). It is
c1
c2
worth noting that the 3D ABX3 OIHP analogue MASnBr3
also show several phase transitions, but the T is far below
c
room temperature, with the highest T being of 229.4 K.[17]
T (Figure S5, Supporting Information).
c
c
The higher T of MPSnBr3 than that of the MASnBr3 is attrib-
We further investigated the phase transitions of MPSnBr3
by temperature-dependent second harmonic generation (SHG)
signal, which is only active in non-centrosymmetric com-
pounds. MPSnBr3 exhibits clear SHG signals with a non-zero
intensity in the RFP (Figure 2d), corresponding to the polar
m crystal symmetry. The SHG intensity keeps non-zero in
the HFP, and becomes almost zero in the HPP (Figure 2d).
This coincides with the polar mm2 and centrosymmetric
m3m crystal symmetry in the HFP and HPP, respectively.
c
uted to the larger molecular volume and heavier molecular
mass of MP cation, of which the rotational energy barrier
is higher than that of MA cation (Figures S3 and S4, Sup-
porting Information). The ferroelectric-to-paraelectric phase
transition temperature of 357 K of MPSnBr3 is also higher than
those of some other OIHP ferroelectric semiconductors such as
(C4H11IN)PbI3 (312 K),[7a] (C5H14N)2(CH6N)2Pb3Br10 (305 K),[2a]
and(C4H12N)2(CH5N2)Pb2Br7 (322K),[7f ] andiscomparabletothat
of (C6H14N)2PbBr4 (363 K) (Table S3, Supporting Information).[7c]
We also confirm the phase transitions of MPSnBr3 by the die-
The changes of SHG intensity around T also verify the phase
c
transitions. We then calculated the number ferroelectric polar
axes of MPSnBr3 based on the change of symmetry elements
from paraelectric phase to ferroelectric phase.[14] For the two
sequential phase transition from m3m to mm2 to m phase in
MPSnBr3, m3m is the paraelectric phase, and mm2 and m are
two ferroelectric phase. The number of polar axes for the two
ferroelectric phase should be determined based on the m3m
to mm2 and m3m to m phase transitions, similar to the cases
lectric anomalies around T in the temperature-dependent die-
c
lectric real part (ε′) (Figure 2b). The giant variation of ε′ around
T
c2
reveals the ferroelectric-to-paraelectric phase transition
nature. The AC conductivity of MPSnBr3 also displays anoma-
lies around T and T , corresponding to the phase transitions
c1
c2
(Figure 2c). The conductivity of MPSnBr3 at room temperature
is about 10−4.7 S m−1, which falls in the range (10−8 to 103 S cm−1)
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Figure 2. Phase transitions of MPSnBr3. a) DSC curves. b) Temperature-dependent ε′ at 1 MHz. c) Temperature-dependent AC conductivity at 1 MHz.
d) Temperature-dependent SHG intensity.
of [quinuclidinium]ReO4.[14] The number of total symmetry
m3m
HFP, and m point group in RFP is 48, 4, and 2, respectively.
Thus, the paraelectric m3m to ferroelectric mm2 phase and
because the polarization directions in two domains do not inter-
sect at 180°, demonstrating the multiaxial nature of MPSnBr3.
Meanwhile, we also observed the vertical and lateral PFM
images in the HFP. As Figure S7, Supporting Information,
shows, the vertical and lateral phase images exhibit different
patterns, which resemble those in the RFP and thus demon-
strate the existence of non-180° domains. As Figure 3f shows,
the vertical PFM phase and amplitude signals as a function of
the dc voltage at RFP appear as quasi-square hysteretic loops
and well-defined butterfly-shaped curves, which are typical for
the ferroelectric switching and demonstrate the ferroelectric
property of the single crystal of MPSnBr3.
To visualize the domain switching process, point polariza-
tion study was carried out at RFP. First, we scanned the vertical
PFM signals of the initial state (Figure 4a), where the phase
image shows a clear contrast in the right region. The corre-
sponding lateral phase in Figure S8, Supporting Information,
is uniform for the right region, like that in Figure 3, and the
striking 180° contrast appears in the left region, which suggests
the emergence of another polarization direction and further
confirms the multiaxial nature of MPSnBr3. When a tip under
−150 V bias was used to pole the crystal surface for 160 s, the
polarization direction of the region marked by a red dashed
ellipse is switched (Figure 4b). Once the opposite +150 V bias
was applied to the tip for 160 s, the polarization direction of
two corresponding regions can also be switched (Figure 4c).
Notably, the switched domains can remain unchanged 2 h
later (Figure 4d). Moreover, by superimposing the vertical PFM
phase and amplitude signals into the 3D topography image,
it can be seen that there is no obvious correlation between
the piezoelectric signals and the morphology (Figure S9,
elements of
point group in HPP, mm2 point group in
m3m
paraelectric
to ferroelectric m phase transitions give
12 and 24 polarization directions,[14] as shown in Figure S6, Sup-
porting Information. Since for both paraelectric-to-ferroelectric
phase transition, the polarization vector in ferroelectric phase is
reversible, the number of polar axes is half that of the polariza-
tion directions, thus the transitions from paraelectric m3m to
ferroelectric mm2 phase and paraelectric m3m to ferroelectric m
phase reveals 6 and 12 equivalent ferroelectric polar axes, respec-
tively.[14,18] This indicates that MPSnBr3 is a multiaxial molecular
ferroelectric, facilitating its future applications.[14] Such many
polar axes are rarely seen in molecular ferroelectrics.[14] The ferro-
electric polar axes of MPSnBr3 are much more than those of the
other OIHP ferroelectric semiconductors (Table S3, Supporting
Information), most of which are uniaxial or biaxial ferroelectrics,
and the classical inorganic perovskite ferroelectric semiconduc-
tors BiFeO3 (4 polar axes) and BaTiO3 (3 polar axes).[2,7,13,14]
Piezoresponse force microscopy (PFM) is a powerful tool
for the study of ferroelectric and piezoelectric phenomena,
which enables non-destructive visualization and control of fer-
roelectric domains.[19] Figure 3 presents the vertical and lateral
PFM phase and amplitude images at RFP on the single crystal
of MPSnBr3 without any thermal treatment. The striking 180°
vertical phase contrast and the uniform lateral phase signals
indicate that the polarizations in two domains have the oppo-
site and same directions in the out-of-plane and in-plane com-
ponents, respectively. The domain walls emerge as darker lines
both in two amplitude images, which should be non-180° ones
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Figure 3. a–e) Vertical PFM phase (a) and amplitude (c), lateral phase (b) and amplitude (d), and topography (e) images at RFP for the single crystal
of MPSnBr3. f) Vertical PFM phase and amplitude signals versus the tip voltage at RFP, displaying local piezoelectric hysteresis loops.
Supporting Information). These results provide a solid evidence
for the stable and switchable polarization of MPSnBr3, which
is an intrinsic characteristic of ferroelectrics. We then carried
out the macroscopic ferroelectric measurements of polariza-
tion−electric field (P−E) curves on the compressed pellet sam-
ples because of the lack of large-size single crystals of MPSnBr3
at different phases (Figure S10, Supporting Information). At
303 K, the P−E curve is linear due to the large coercive field,
which then opens at 333 K with a certain remnant polariza-
tion at zero field because of the contribution of polarization. At
368 K, the P−E curve becomes round owing to the paraelectric
nature and the large electric leakage.
polarization change path lies in selecting polarization quantum
properly to avoid the wrong estimation of polarization value. A
dynamic path between two ferroelectric states is constructed
based on the crystal structure obtained from the single-crystal
X-ray diffraction. While the other states are obtained from the
matrix transformation of the coordinates considering both the
rotational and displacive motion of MP cations in the anionic
[SnBr3]− framework. The variation of polarization as a function
of the dynamic path is shown in Figure 5, from which the ferro-
electric polarization with 4.5 µC cm−2 can be extracted from two
equivalent ferroelectric configurations (λ = 1). During the fer-
roelectric switching process (−1 < λ < 1), the polarization value
changes smoothly, and turns to zero at λ = 0, which indicates
a reference phase with zero polarization. We also calculated
the polarization value of MPSnBr3 by using the point charge
model (Figure S11, Supporting Information). The obtained
We also performed density functional theory (DFT) calcula-
tion to evaluate the origin of polarization to gain deep insight
into the ferroelectric polarization reversal.[20] According to the
modern theory of polarization,[21] the necessity of constructing
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Figure 4. Topographic (top), vertical PFM amplitude (middle), and phase (bottom) images at RFP of the crystal surface for MPSnBr3. a) Initial state.
b) After the first switching on the red dot in (a) with −150 V for 160 s. The domain inside the red dashed ellipse is switched. c) After the succeeding
switching on the blue dots in (b) with +150 V for 160 s. The domains inside the blue dashed ellipses are switched. d) After 2 h.
polarization of 4.28 µC cm−2 is comparable to the calculated
value (4.5 µC cm−2) from DFT calculation.
MPSnBr3 is thus a 3D OIHP ferroelectric semiconductor,
having potential applications in future optoelectronics.[4] Both
the conduction band (CB) minimum and the valence band (VB)
maximum are localized at the same k-vector in the Brillouin
zone (Figure 6b), revealing a direct-bandgap semiconductor as
well. The calculated bandgap is 2.58 eV, which agrees well with
the experimental value of 2.62 eV. From partial density of states
(PDOS) (Figure 6c), it is obvious that for the organic part, the
H-1s states overlap fully with the C-2p and P-3p states in the
−8 to −5 eV region, indicating the strong covalent interactions
in C H and P H bonds. For the inorganic part, strong inter-
actions between Sn and Br atoms are also found in almost the
whole energy region as Sn-s/p and Br-s/p states overlap obvi-
ously. The bands at the VB top originate from the nonbonding
states of Br-4p, and those at the CB bottom are mainly from the
unoccupied Sn-5p orbitals. Clearly, both VB maximum and CB
minimum are from the electronic states of Sn and Br atoms,
thus the inorganic [SnBr3]− framework determines the bandgap
of MPSnBr3. Interestingly, as the temperature increases,
the absorption edge of the MPSnBr3 shows a clear redshift
(Figure 6a and Figure S13, Supporting Information), accom-
panied by the color changes to orange. The absorption edge
within each solid phase has almost no obvious change, while
the absorption edge between different phases has shifted sig-
nificantly (Figure S13, Supporting Information). Therefore, the
shift of the absorption edge can be attributed to the structural
phase transitions rather than thermally induced continuous
changes. The bandgaps of HFP and HPP are 2.48 and 2.42 eV,
respectively. Therefore, MPSnBr3 is also a thermochromic
ferroelectric semiconductor, having the potential application in
smart photovoltaic window.[22]
The optical property of MPSnBr3 was investigated by the
solid-state UV−vis absorbance spectrum (Figure 6a), which dis-
plays an intense absorption at the band edge onset 500 nm at
room temperature, corresponding well with its yellow appear-
ance of bulk crystal. The power law for the variation of absorp-
tion coefficient versus photoenergy shows the features of a
direct-bandgap semiconductor with optical bandgap of 2.62 eV
(Figure S12, Supporting Information), smaller than that of the
2D bromide OIHP ferroelectric (C6H14N)2PbBr4 (3.05 eV).[7c]


Figure 5. Evolution path of polarization with structural distortion of
MPSnBr3.
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Figure 6. a) Solid-state variable-temperature UV−vis absorption spectra, b) energy band structure, and c) PDOS of MPSnBr3.
Simulation Package (VASP).[24] The energy cut-off for the expansion of
the wave functions was fixed to 550 eV and the exchange−correlation
interactions were treated within the generalized gradient approximation
of the Perdew−Burke−Ernzerhof type.[25] For the integrations over the
k-space we used a 4 × 2 × 2 k-point mesh. The experimental crystal
structure was used as the ground state for evaluating the ferroelectric
polarization.
In summary, we have successfully demonstrated a MP-based
3D ABX3 OIHP ferroelectric semiconductor MPSnBr3, which
undergoes two phase transitions at 314 and 357 K, respectively,
and shows a multiaxial nature with the number of ferroelec-
tric polar axes up to 12. Both the order-disorder transition of
−

MP cation and the reorganization of Sn Br bonds in [SnBr3]
framework contribute to the ferroelectric-to-paraelectric phase
transition at 357 K. The ferroelectric domains and polarization
switching confirm its ferroelectricity. MPSnBr3 also shows a
direct bandgap of 2.62 eV and the interesting thermochromism
with the decrease of bandgap upon heating. This work high-
lights the potential of MP as A site for constructing high-perfor-
mance 3D ABX3 OIHP ferroelectric semiconductors with great
application prospects. Work on other MP-based OIHPs such as
MPPbBr3, MPSnI3, and MPPbI3 is currently underway.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (21991142 and 21831004).
Experimental Section
Synthesis of MPBr: Anhydrous aluminum chloride (0.57 mol) and
phosphorus trichloride (0.5 mol) were added into a three-necked flask.
Then the mixture was heated at 70 °C for 1 h. After cooling, methyl
iodide (0.5 mol) was added dropwise over 0.5 h at −10 °C. The reaction
solution solidified gradually over the next 3–5 h. Potassium chloride
(0.57 mol) and iron powder (30 g) were added to the above to distill
collecting the fraction at 80–165 °C. The mixture was then fractionated
to give pure dichloro(methyl)phosphine as a colorless liquid, yield: 30%,
b.p. 80–82 °C. Dichloro(methyl)phosphine (4.2 g) was added to 33 mL
of dichloromethane. Then, ethylene glycol (3 g) was added dropwise
at room temperature. Colorless crystals continuously appeared on the
sublimator, which is methylphosphonium chloride,[23] which was then
dissolved in hydrobromic acid solution and stored in the refrigerator,
giving crystals of MPBr, with a yield of 12%.
Crystals of MPSnBr3: Yellow plate crystals of MPSnBr3 were collected
from a hydrobromic acid solution with equimolar amounts of MPBr
and SnBr2 by the temperature cooling method in an inert atmosphere.
The results of elemental analysis (Table S4, Supporting Information)
and PXRD (Figure S2, Supporting Information), confirm the purity of
samples.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
3D hybrid perovskites, methylphosphonium, molecular ferroelectrics,
multiaxial ferroelectric nature, semiconductors
Received: July 31, 2020
Revised: September 19, 2020
Published online:
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