Pressure-dependent polymorphism and band-gap tuning of methylammonium lead iodide perovskite

Article abstract of DOI:10.1002/anie.201601788

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI₃ (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO₃-type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction. Band aid: Through the determination of methylammonium lead iodide perovskite crystallographic transitions under pressure, in situ laser-excited photoluminescence suggests a reduction in band gap in the tetragonal phase on increasing the pressure up to about 0.3 GPa and then an increase in band gap in ReO₃-type cubic phase up to a pressure of 2.7 GPa, a result that is supported by DFT calculations.

Full text of DOI:10.1002/anie.201601788

Angewandte
Communications
Chemie
>
International Edition: DOI: 10.1002/anie.201601788
German Edition: DOI: 10.1002/ange.201601788
Halide Perovskites
Pressure-Dependent Polymorphism and Band-Gap Tuning of
Methylammonium Lead Iodide Perovskite
Shaojie Jiang, Yanan Fang, Ruipeng Li, Hai Xiao, Jason Crowley, Chenyu Wang,
Timothy J. White, William A. Goddard III,* Zhongwu Wang, Tom Baikie,* and Jiye Fang*
Abstract: We report the pressure-induced crystallographic
transitions and optical behavior of MAPbI3 (MA = methyl-
ammonium) using in situ synchrotron X-ray diffraction and
laser-excited photoluminescence spectroscopy, supported by
density functional theory (DFT) calculations using the hybrid
functional B3PW91 with spin-orbit coupling. The tetragonal
polymorph determined at ambient pressure transforms to
a ReO3-type cubic phase at 0.3 GPa. Upon continuous
compression to 2.7 GPa this cubic polymorph converts into
a putative orthorhombic structure. Beyond 4.7 GPa it separates
into crystalline and amorphous fractions. During decompres-
sion, this phase-mixed material undergoes distinct restoration
pathways depending on the peak pressure. In situ pressure
photoluminescence investigation suggests a reduction in band
gap with increasing pressure up to ꢀ 0.3 GPa and then an
increase in band gap up to a pressure of 2.7 GPa, in excellent
agreement with our DFT calculation prediction.
mobilities and electron/hole diffusion lengths of hundreds of
nanometers.[1–3] The power conversion efficiency (PCE) of
perovskite PV cells has rapidly increased from about 6% in
2011 towards approximately 20% presently, although device
reproducibility is questionable.[3–8] The PCE can be enhanced
by modifying charge-carrier mobility and the direct band gap
through tuning crystal chemistry.[9,10] Nonetheless, to date,
extensive synthesis campaigns have revealed a limited range
of materials. Alternatively, pressurization is a straightforward
and robust way to transform crystal structures, where changes
in bond strength and geometry are induced without the
complication of chemical adjustments. By monitoring changes
in photovoltaic properties, strategies for selecting ions of
appropriate size and charge can be devised.[11,12] To this end,
pressure-dependent structural behaviors of stable and meta-
stable polymorphs can guide design strategies.[13]
The perovskite prototype has the general formula of
ABX3, where corner-sharing BX6 octahedra create a 3D
framework with an A component providing charge compen-
O
rganic–inorganic metal halide perovskite-based photo-
voltaic (PV) cells show potential for the direct conversion of
sunlight into electricity, as a result of their high charge-carrier
ꢀ
sation if necessary (Figure 1a). The aristotype is cubic Pm3m
(space group, No. 221), in which all the BX6 octahedra show
B-X-B angles of 908. This arrangement of untilted octahedra
can be described in Glazer notation[14,15] as a0a0a0. Among the
Pb-based methylammonium (MA) halide perovskites (ionic
[*] S. Jiang, Prof. J. Fang
Materials Science and Engineering Program
State University of New York at Binghamton
Binghamton, NY 13902 (USA)
+
A = CH3NH3 or MA; X = ClÀ, BrÀ, IÀ; B = Pb2+), the band
gaps of MAPbCl3, MAPbBr3, and MAPbI3 are reported as
around 3.1, 2.3, and 1.5 eV, respectively.[10,16] For single
junction PV cells a band gap in the range between 1.1 and
E-mail: jfang@binghamton.edu
Dr. Y. Fang, Dr. T. Baikie
Energy Research Institute@NTU (ERI@N)
Nanyang Technological University
50 Nanyang Drive, Singapore 637553 (Republic of Singapore)
E-mail: tbaikie@ntu.edu.sg
Dr. R. Li, Dr. Z. Wang
Cornell High Energy Synchrotron Source, Cornell University
Ithaca, NY 14853 (USA)
Dr. H. Xiao, J. Crowley, Prof. W. A. Goddard III
Materials and Process Simulation Center (MSC) and Joint Center for
Artificial Photosynthesis (JCAP)
California Institute of Technology
Pasadena, CA 91125 (USA)
E-mail: wag@wag.caltech.edu
Dr. C. Wang, Prof. J. Fang
Department of Chemistry, State University of New York at Bing-
hamton
Binghamton, NY 13902 (USA)
Prof. T. J. White
School of Materials Science and Engineering, Nanyang Technological
University
Nanyang Avenue, Singapore 639798 (Republic of Singapore)
ꢀ
Figure 1. a) MAPbI3 perovskite Pm3m polymorph (left, polyhedral
À
model; right, with Pb I bonds emphasized); b) Diamond anvil cells
holder system with balanced compression screws (left: side view;
right: top-down view).
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601788.
6540
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6540 –6544

Angewandte
Communications
Chemie
1.4 eV is desired.[17] There-
fore, MAPbI3 is the pre-
ferred material. Previous
studies of pressure-induced
transformations in hybrid
perovskites illustrate the
crystallographic richness
and band-gap changes.
Although
induced
pressure-
synchrotron
powder X-ray diffraction
(XRD) of MAPbI3 was
reported
recently,[18]
extraction of high-pressure
lattice parameters, the
unambiguous determina-
tion of crystal symmetry
and the refinement of
atomic fractional coordi-
nate are challenging.
Herein we synthesize
MAPbI3 as reported previ-
ously.[19] In brief, polycrys-
talline MAPbI3 was pre-
cipitated by 6 h cooling
from 100 to 468C after
mixing PbI2 and MA-I sol-
utions in a Pyrex test tube.
The PbI2 solution was pre-
pared by dissolving 2.5 g of
lead(II) acetate (Chemical
Reagents, Sigma–Aldrich)
in 10 mL of aqueous HI solution (57 wt%), whereas the MA-I
solution was formed by dissolving 0.597 g of aqueous CH3NH2
solution (40%, Merck) in 2 mL of HI solution. The diamond
anvil cells (DACs) with a size of 500 mm were aligned,
assembled, and pressurized before conducting XRD (Fig-
ure 1b). A stainless-steel gasket was indented before drilling
a roughly 200 mm diameter hole that served as the compaction
chamber. Ruby fluorescence was used to calibrate pressure[20]
and ruby chips were distributed on the sample surface before
closing the DAC. At each pressure, an XRD pattern (l =
0.485946 Š) was collected at the B1 station of the Cornell
High Energy Synchrotron Source (CHESS),[21] with a data
accumulation time of 10 mins at each pressure, after main-
taining the pressure for approximate 20 min.
Figure 2. Conventional XRD patterns of MAPbI3 during a) compression and b) decompression. a) The ReO3-
type cubic supercell (as) appears at 0.3 GPa (highlighted by broken-lined boxes) and becomes more abundant
to 2.3 GPa, before conversion into the putative orthorhombic polymorph at 2.7 GPa, and finally approaches X-
ray amorphicity above 3.5 GPa. b) Decompression from the amorphous mass restores the tetragonal phase at
0.58 GPa. c) and d) Representative 2D XRD patterns at specific pressures.
is vacant.[23,24] No evidence was found for the Pm3m
ꢀ
aristotype identified in a previous variable-temperature
study.[19] Although the Pm3m reflections may substantially
ꢀ
ꢀ
overlap with those of the Im3 supercell, the conventional
primitive unit cell was not observed as a separate phase
(Figure S1). For comparison, temperature-dependent XRD
measurements were collected to 608C (Figure S2) to demon-
ꢀ
strate reflections of the conventional Pm3m polymorph
persist at elevated temperature (Figure 2a and Figure S2).
Beyond 0.4 GPa, MAPbI3 completely transforms to the
+
+ +
ꢀ
Im3 phase with as = 12.4076(8) Š and tilt character a a a ,
with octahedral tilt of equal magnitude about all three axes
(Figure S1). For this polymorph, the cell volume contracts and
the Bragg reflections broaden as a function of the applied
pressure until 2.3 GPa. Prior to amorphization at 4.7 GPa, an
additional phase appearing at 2.7 GPa was identified, with 00l
reflections becoming triply degenerate (Figure S3). Such
splitting would be consistent with orthorhombic symmetry,
Synchrotron XRD patterns of MAPbI3 collected at room
temperature (Figure 2a) and ambient pressure confirms
a tetragonal polymorph (I4/mcm, No. 140) with lattice con-
stants of at = 8.8648(6) Š and ct = 12.6746(8) Š. For this
phase, octahedral tilting is about the unique c axis only and
[25]
and by analogy with MASnI3 may be an Immm (No. 71)
the Glazer notation is a0a0cÀ. For the tetragonal (t) heterotype
polymorph with a+b+c+ PbI6 tilting. Reports of an ortho-
rhombic high-pressure form of MAPbBr3 remain ambigu-
ous.[22,26] Beyond 2.7 GPa MAPbI3 continues to amorphize to
near completion at 4.7 GPa, with no further change in X-ray
scattering character up to 6.4 GPa (also refer to 2D diffrac-
tion patterns in Figure 2c). Upon a gradual release of
pressure, the sample remains partially amorphous until
0.58 GPa without the appearance of the putative Immm
pffiffi
derived from the cubic (c) aristotype, at ¼ ac 2 and ct = 2ac.
At approximately 0.3 GPa, additional reflections appear at 2q
ꢀ 7.18 and 8.48, which Pawley fitting (Figure S1 in the
Supporting Information) shows to be consistent with a cubic
ꢀ
bilayer supercell (s) (Im3, No. 204), where as = 2ac as
previously observed for the MAPbBr3 analogue,[22] and
a high pressure polymorph of ReO3, where A site in ABX3
Angew. Chem. Int. Ed. 2016, 55, 6540 –6544
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6541

Angewandte
Communications
Chemie
ꢀ
polymorph (Figure 2b). However, the Im3 phase appeared at
The MAPbI3 polymorphs show discrete band gaps that
can be calculated from in situ pressure-photoluminescence
(PL) spectra during a compression (Figure 4a and Table S3)
0.58 GPa, and the I4/mcm was restored at 0.55 GPa (Fig-
ure 2b,d).
To determine the influence of the maximum applied
pressure and microstructural evolution during decompres-
sion, a series of experiments with different peak-pressures
were conducted. For example, using 3.5 GPa as the peak
pressure (Figure S4) the Immm structure was favored at
ꢀ
2.8 GPa, and the Im3 polymorph was recovered when the
pressure was reduced to 0.9 GPa. This confirms the recovered
structure during decompression is controlled by the peak
pressure. The decompression pattern collected at 1.37 GPa
ꢀ
was modelled using both Immm and Im3 polymorphs (Fig-
ure S5). However, the poor Pawley fit of the 004 reflection (at
2q ꢀ 98) for the cubic polymorph indicates this phase(s)
resides on the borderline of the orthorhombic–cubic transi-
tion. This evidence supports the existence of the Immm
polymorph, but suggests it is metastable with respect to the
ꢀ
Im3 and I4/mcm polymorphs. The tetragonal structure forms
reversibly when the applied pressure does not exceed
8.0 GPa. Taken alone, Pawley fitted cell metrics do not
show a clear discontinuity as the symmetry varies (Figure S6),
but optical microscopy does show some distinct changes
(Figure S7).
Figure 4. a) Measured photoluminescence spectra (right panel) and
calculated band gaps (left panel) of MAPbI3 as a function of compres-
sion pressure at room temperature. b) The band gaps determined by
measurement and B3PW91+SOCPBE calculations (also refer to
Table S2 in supporting information).
Using the refined atomic fractional coordinates for Pb and
ꢀ
I in the ReO3-type Im3 cubic supercell as a basis, the
octahedral topologies in the cubic (c), tetragonal (t), and
ReO3-type cubic supercell (s) of the MAPbI3 can be derived
(Figure 3).[24] It should be noted that only the MA groups are
dynamic[27] and not included. The octahedral tilt angle in the
cubic supercells approaches around 158 in advance of the
appearance of the orthorhombic polymorph (refer to
Table S1).[24] The CIFs generated from the density functional
theory (DFT) calculations (see below) also show all the
relevant coordinates (Table S2).
but are typically larger than those determined by absorp-
tion,[28] as the Tauc plot[29] was not calculated. At ambient
conditions, the PL spectrum shows a broad band at 764 nm,
equivalent to the band gap of 1.62 eV and in agreement with
previous reports.[29–32] The PL band of tetragonal perovskite is
believed to originate from a near-band-edge transition.[30–31,33]
Up to 0.3 GPa, PL of the I4/mcm structure shows a progres-
sive red shift (1.62 to 1.61 eV). In this pressure regime, the
DFT calculations find the band gap to decrease from 1.67 to
1.62 eV, in accord with the PL (Figure 4b).
With increasing PbI6 octahedral tilt angles, the transition
ꢀ
of the Im3 polymorph to the suggested orthorhombic Immm
structure at 0.4 GPa leads to a blue-shifted PL spectra
accompanying an increase of the band gap from 1.64 to
1.69 eV. The pressure-dependent PL variations are similar to
those in the MAPbBr3 system reported recently.[26] The PL
band weakens substantially beyond 2.0 GPa, and vanishes at
ꢀ
2.7 GPa with the destabilization of the Im3 supercell. The PL
spectra during decompression are also instructive. For
example, when a peak-pressure of 6.4 GPa was applied, the
PL was annihilated upon release of pressure. In contrast, from
a peak-pressure of 3.5 GPa, weak PL band appears at
1.7 GPa, near the incipient recovery of the cubic supercell
(Figures S4,S5).
As PL correlates directly with polymorphism, conversion
ꢀ
from the low-pressure I4/mcm phase into the cubic Im3
supercell (< 0.4 GPa) offers a chemically independent means
of reducing MAPbI3 band gaps, or directing the design of
novel organic–inorganic hybrid chemistries. When these two
strategies are applied in tandem, the judicious selection of
ionic ratios near polymorphic boundaries and the application
Figure 3. Characteristic PbI6 octahedral and Glazer symbols of some
typical pressure-related polymorphs. The image for Immm polymorph
should be similar to that of Im3 with slightly different lattice
ꢀ
parameters along each axis and an octahedral tilt change (Glazer
symbol: a+b+c+).
6542
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6540 –6544

Angewandte
Communications
Chemie
of relatively low pressures could be deployed to extend the
absorption range over the long wavelength region for the
future photovoltaic device design.
orthorhombic (Immm) form (2.7 GPa), through to an ReO3-
ꢀ
type (Im3) cubic supercell (0.4–2.7 GPa), and finally amorph-
ꢀ
ization above 2.7 GPa. The transition from I4/mcm to Im3
To better understand the atomistic origin of the PL in
MAPbI3, DFT calculations using the Perdew-Burke-Ernzer-
hof (PBE)[34] approach were undertaken to optimize atomic
positions, including London dispersion (van der Waals
attraction) using the empirical D3 correction of Grimme
et al.[35] to account for the strong hydrogen bonding between
MA cations and the tilted octahedra in MAPbI3.[36] The band
gaps were then calculated using the B3PW91 flavor of DFT,
which has been validated to yield band gaps accurate to
approximately 0.09 eV.[37] Spin-orbit coupling (SOC) makes
a significant contribution in the MAPbI3 system and was also
included.[38] Owing to the rotational disorder of MA, the high
symmetry tetragonal and cubic phases were modelled using
a 2 ” 2 ” 2 supercell, with two sets of MA cation configurations
that are energetically close (Figure S8), and the band gap
results were averaged according to Boltzmann distribution at
room temperature (Table S4). The predicted pressure-depen-
dent band gaps (Figure 4b), decreasing from 1.67 to 1.62 eV
up to 0.3 GPa, while increasing from 1.58 to 1.74 eV between
0.4 to 2.0 GPa, are in excellent agreement with the exper-
imental values (1.62 to 1.61 eV, 1.64 to 1.69 eV, respectively).
The success of our prediction from averaging over config-
urations is consistent with the fact that both tetragonal and
cubic phases are of thermally averaged structures.
leads to a smaller band gap energy, whereas conversion from
the Im3 polymorph into the suggested Immm polymorph
ꢀ
reverses this trend. During decompression, crystallographic
restoration was dependent on the peak-pressure achieved.
ꢀ
Recovery from 6.4 GPa restored the Im3 polymorph from the
partially amorphized state at around 0.5 GPa. However, from
a peak pressure of 3.5 GPa the same structure was recovered
at 0.9 GPa owing to better preservation of long-range order at
the 3.5 GPa compared to 6.4 GPa. These studies provide
insights into the correlations between structure and band gap,
uncompromised by changes in chemistry. Changes in power
conversion efficiency (PCE) through the application of
external pressure can be mimicked through chemical tailor-
ing, where the introduction of ions in different sizes can
effectively modify internal pressure and stabilize new poly-
morphs. By chemically adjusting hybrid perovskite composi-
tions near to polymorphic boundaries, the application of
modest pressure can be sufficient to initiate phase changes
and band gap adjustments. This combined chemical–pressure
strategy may prove valuable in the design of new perovskites
for photovoltaic applications.
Acknowledgements
In summary, the pressure-induced polymorphism and
band-gap tuning of MAPbI3 were investigated using synchro-
tron XRD, and the trends in band gap extracted from laser-
excited photoluminescence (PL) spectra compared with the
DFT calculations. As illustrated in Scheme 1, MAPbI3 under-
goes two high-pressure phase transformations from a tetrag-
onal polymorph (I4/mcm) at ambient pressure, to a suggested
This work is partially supported by NRF-CRP14-2014-03,
Custom Electronics, Inc., and the Joint Center for Artificial
Photosynthesis, a DOE Energy Innovation Hub, supported
through the Office of Science of the U.S. Department of
Energy under Award No. DE-SC0004993. CHESS is sup-
ported by the NSF award DMR-1332208. S.J. acknowledges
the support by Binghamton University.
Keywords: band gap · halide perovskite ·
high-pressure chemistry · phase transitions ·
photoluminescence
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6540–6544
Angew. Chem. 2016, 128, 6650–6654
[1] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J.
Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith,
Science 2013, 342, 341 – 344.
[2] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S.
Mhaisalkar, T. C. Sum, Science 2013, 342, 344 – 347.
[3] N.-G. Park, Mater. Today 2015, 18, 65 – 72.
[4] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao,
M. K. Nazeeruddin, M. Grätzel, Nature 2013, 499, 316 – 319.
[5] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z.
Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542 – 546.
[6] N.-G. Park, J. Phys. Chem. Lett. 2013, 4, 2423 – 2429.
[7] H. J. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623 – 3630.
[8] T.-B. Song, Q. Chen, H. Zhou, C. Jiang, H.-H. Wang, Y. M. Yang,
Y. Liu, J. You, Y. Yang, J. Mater. Chem. A 2015, 3, 9032 – 9050.
[9] Y. Zhao, K. Zhu, J. Am. Chem. Soc. 2014, 136, 12241 – 12244.
[10] J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano
Lett. 2013, 13, 1764 – 1769.
Scheme 1. A schematic illustration of structure transition of MAPbI3 as
a function of pressure (not on scale). “PL” represents photolumines-
cence observation. In the phases the top shading indicates with or
without PL, the bottom shadings indicate different phases.
Angew. Chem. Int. Ed. 2016, 55, 6540 –6544
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6543

Angewandte
Communications
Chemie
[11] Z. Quan, Y. Wang, I.-T. Bae, W. S. Loc, C. Wang, Z. Wang, J.
Fang, Nano Lett. 2011, 11, 5531 – 5536.
[12] Y. Ma, M. Eremets, A. R. Oganov, Y. Xie, I. Trojan, S.
Medvedev, A. O. Lyakhov, M. Valle, V. Prakapenka, Nature
2009, 458, 182 – 185.
[27] T. Baikie, N. S. Barrow, Y. Fang, P. J. Keenan, P. R. Slater, R. O.
Piltz, M. Gutmann, S. G. Mhaisalkar, T. J. White, J. Mater. Chem.
A 2015, 3, 9298 – 9307.
[28] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem.
2013, 52, 9019 – 9038.
[13] T. Wang, R. Li, Z. Quan, W. S. Loc, W. A. Bassett, Y. C. Cao, J.
Fang, Z. Wang, Adv. Mater. 2015, 27, 4544 – 4549.
[29] S. Colella, E. Mosconi, P. Fedeli, A. Listorti, F. Gazza, F. Orlandi,
P. Ferro, T. Besagni, A. Rizzo, G. Calestani, Chem. Mater. 2013,
25, 4613 – 4618.
[30] Y. Yamada, T. Nakamura, M. Endo, A. Wakamiya, Y. Kane-
mitsu, Appl. Phys. Express 2014, 7, 032302.
[31] W. Kong, Z. Ye, Z. Qi, B. Zhang, M. Wang, A. Rahimi-Iman, H.
Wu, Phys. Chem. Chem. Phys. 2015, 17, 16405 – 16411.
[32] J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan, S. Yang,
Nanoscale 2013, 5, 3245 – 3248.
[33] C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston, L. M.
Herz, J. Phys. Chem. Lett. 2014, 5, 1300 – 1306.
[34] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77,
3865 – 3868.
[35] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010,
132, 154104.
[36] J.-H. Lee, N. C. Bristowe, P. D. Bristowe, A. K. Cheetham,
Chem. Commun. 2015, 51, 6434 – 6437.
[37] H. Xiao, J. Tahir-Kheli, W. A. Goddard III, J. Phys. Chem. Lett.
2011, 2, 212 – 217.
[38] P. Umari, E. Mosconi, F. D. Angelis, Sci. Rep. 2014, 4, 4467.
[14] A. M. Glazer, Acta Crystallogr. Sect. B 1972, 28, 3384 – 3392.
[15] A. M. Glazer, Acta Crystallogr. Sect. A 1975, 31, 756 – 762.
[16] B. Wang, X. Xiao, T. Chen, Nanoscale 2014, 6, 12287 – 12297.
[17] F. Meillaud, A. Shah, C. Droz, E. Vallat-Sauvain, C. Miazza, Sol.
Energy Mater. Sol. Cells 2006, 90, 2952 – 2959.
[18] T. Ou, J. Yan, C. Xiao, W. Shen, C. Liu, X. Liu, Y. Han, Y. Ma, C.
Gao, Nanoscale 2016, 8, http://dx.doi.org/10.1039/C5NR07842C.
[19] 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.
[20] H. K. Mao, J. Xu, P. M. Bell, J. Geophys. Res. 1986, 91, 4673 –
4676.
[21] Z. Wang, O. Chen, C. Y. Cao, K. Finkelstein, D.-M. Smilgies, X.
Lu, W. A. Bassett, Rev. Sci. Instrum. 2010, 81, 093902.
[22] I. P. Swainson, M. G. Tucker, D. J. Wilson, B. Winkler, V.
Milman, Chem. Mater. 2007, 19, 2401 – 2405.
[23] E. Suzuki, Y. Kobayashi, S. Endo, T. Kikegawa, J. Phys. Condens.
Matter 2002, 14, 10589 – 10593.
[24] J.-E. Jorgensen, J. D. Jorgensen, B. Batlogg, J. P. Remeika, J. D.
Axe, Phys. Rev. B 1986, 33, 4793 – 4798.
[25] Y. Lee, D. B. Mitzi, P. W. Barnes, T. Vogt, Phys. Rev. B 2003, 68,
020103.
[26] Y. Wang, X. Lü, W. Yang, T. Wen, L. Yang, X. Ren, L. Wang, Z.
Lin, Y. Zhao, J. Am. Chem. Soc. 2015, 137, 11144 – 11149.
Received: February 19, 2016
Published online: April 21, 2016
6544 www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6540 –6544