Influence of hidden halogen mobility on local structure of CsSn(Cl1?xBrx)3mixed-halide perovskites by solid-state NMR

Article abstract of DOI:10.1039/d0sc05614f

Tin halide perovskites are promising candidates for lead-free photovoltaic and optoelectronic materials, but not all of them have been well characterized. It is essential to determine how the bulk photophysical properties are correlated with their structures at both short and long ranges. Although CsSnCl₃is normally stable in the cubic perovskite structure only above 379 K, it was prepared as a metastable phase at room temperature. The transition from the cubic to the monoclinic phase, which is the stable form at room temperature, was tracked by solid-state¹³³Cs NMR spectroscopy and shown to take place through a first-order kinetics process. The complete solid solution CsSn(Cl_1?xBr_x)₃(0 ≤x≤ 1) was successfully prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl₃and stable CsSnBr₃end-members. The NMR spectra of CsSnBr₃samples obtained by three routes (high-temperature, mechanochemical, and solvent-assisted reactions) show distinct chemical shift ranges, spin-lattice relaxation parameters and peak widths, indicative of differences in local structure, defects and degree of crystallinity within these samples. Variable-temperature¹¹⁹Sn spin-lattice relaxation measurements reveal spontaneous mobility of Br atoms in CsSnBr₃. The degradation of CsSnBr₃, exposed to an ambient atmosphere for nearly a year, was monitored by NMR spectroscopy and powder X-ray diffraction, as well as by optical absorption spectroscopy.

Full text of DOI:10.1039/d0sc05614f

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Influence of hidden halogen mobility on local
structure of CsSn(Cl Br ) mixed-halide
perovskites by solid-state NMR†
1
ꢀx x 3
Cite this: Chem. Sci., 2021, 12, 3253
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Abhoy Karmakar,
Amit Bhattacharya, Diganta Sarkar,
Guy M. Bernard,
Arthur Mar and Vladimir K. Michaelis
*
Tin halide perovskites are promising candidates for lead-free photovoltaic and optoelectronic materials, but
not all of them have been well characterized. It is essential to determine how the bulk photophysical
3
properties are correlated with their structures at both short and long ranges. Although CsSnCl is
normally stable in the cubic perovskite structure only above 379 K, it was prepared as a metastable phase
at room temperature. The transition from the cubic to the monoclinic phase, which is the stable form at
133
room temperature, was tracked by solid-state Cs NMR spectroscopy and shown to take place through
a first-order kinetics process. The complete solid solution CsSn(Cl1ꢀxBr (0 # x # 1) was successfully
prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl and stable
CsSnBr end-members. The NMR spectra of CsSnBr samples obtained by three routes (high-
x 3
)
3
3
3
temperature, mechanochemical, and solvent-assisted reactions) show distinct chemical shift ranges,
spin-lattice relaxation parameters and peak widths, indicative of differences in local structure, defects
119
Received 11th October 2020
Accepted 29th December 2020
and degree of crystallinity within these samples. Variable-temperature
measurements reveal spontaneous mobility of Br atoms in CsSnBr
Sn spin-lattice relaxation
3 3
. The degradation of CsSnBr ,
DOI: 10.1039/d0sc05614f
exposed to an ambient atmosphere for nearly a year, was monitored by NMR spectroscopy and powder
rsc.li/chemical-science
X-ray diffraction, as well as by optical absorption spectroscopy.
+
imparted by the Cs , than CH
within an hour.
3
NH
3
SnBr
3
, which degrades in air
do not
Lead halide perovskites APbX (A ¼ Cs , CH NH , CH(NH ) ; have to be encapsulated, lasting for hours with diode charac-
1
. Introduction
13,15
Solar devices fabricated with CsSnBr
3
+
+
+
3
3
3
2 2
ꢀ
ꢀ
ꢀ
13
X ¼ Cl , Br , I ) show attractive optical and electrical properties teristics being retained. The band gap of CsSnBr
3
(1.75–1.80
13,16,17
for solar cells, lasers, light-emitting diodes, X-ray detectors, and eV) lies in the optimal region for tandem solar cell
1
–6
18
other applications, with power conversion efficiencies now technology.
7
exceeding 25%, but they face signicant challenges for
Recently, the mixed-halide perovskites CsSn(Cl1–xBr ) have
commercialization because of poor chemical stability and risk been prepared in the form of nanocrystals or thin lms, in
of lead toxicity.
Among lead-free alternatives, the tin- which the band gap can be adjusted with composition, making
substituted analogues have been highly promising, showing them suitable for optoelectronic applications such as light
x 3
8
9,10
11
19,20
power conversion efficiencies of up to 10%. Most of these emitting diodes and lasers.
However, some of the structural
compounds are iodides such as CH NH SnI , CH(NH ) SnI , details of these mixed-halide perovskites are unclear because
3
3
3
2 2
3
and CsSnI
optimum value (1.34 eV) to maximize efficiency.
organic–inorganic compounds CH NH SnI and CH(NH
are more efficient but suffer from poor thermal stability because standardized as P2
of the presence of the volatile organic cations; the purely inor- structure (space group Pm3m) above 379 K, and CsSnBr
3
, which have band gaps (1.2–1.4 eV) close to the the end-members are known to exhibit multiple phase transi-
12,13
The hybrid tions: CsSnCl
SnI
ature (own type; space group originally reported as P2
/c) and transforms to the cubic perovskite
adopts a monoclinic structure at room temper-
3
)
2
3
1
/n, but
3
3
3
2
1
21
ꢀ
3
ganic compound CsSnI is more thermally stable but oxidizes undergoes complicated phase transitions at low temperature,
3
13,14
22
rapidly in air (Sn(+2) / Sn(+4)).
Among the bromide but ultimately attains the cubic structure above 292 K. Because
analogues, CsSnBr exhibits greater thermal and air stability, the physical properties depend sensitively on the structure and
3
stability of these mixed-halide perovskites, it is essential to
determine the local and long-range atomic arrangement, to
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
unravel the dynamics of halogen mobility, and to evaluate
E-mail: vladimir.michaelis@ualberta.ca
changes entailed by different synthetic methods and exposure
†
Electronic supplementary information (ESI) available: Experimental section,
to ambient conditions.
Tables S1–S8 and Fig. S1–S14. See DOI: 10.1039/d0sc05614f
©
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To date, halide perovskites have been extensively character- CsSnCl
3
transforms to the cubic perovskite structure (space
ꢀ
ized by X-ray diffraction (XRD), which gives information about group Pm3m) containing a network of corner-sharing ideal
4
ꢀ
21,50
˚
the average long-range structure. However, solid-state nuclear [SnCl6] octahedra with Sn–Cl bonds of 2.78 A (Fig. 1b).
magnetic resonance (NMR) spectroscopy is an invaluable However, as described later, the cubic form can be obtained at
method to investigate local structure (e.g., halide distribution, room temperature as a metastable phase. From le Bail tting of
2
3–38
domain structure) and dynamics in perovskites.
In partic- the powder XRD patterns (Fig. 1d), the cell parameters were
1
33
119
˚
˚
˚
ular, Cs (I ¼ 7/2, 100% abundance) and Sn (I ¼ 1/2, 8.59% rened to be a ¼ 5.7286(6) A, b ¼ 7.6936(11) A, c ¼ 16.9175(15) A,
ꢁ
˚
abundance) are ideal NMR-sensitive nuclei, useful for probing and b ¼ 106.505(4) for the monoclinic phase and a ¼ 5.5894(5) A
the local structure of the A and B sites in perovskites for the room temperature cubic phase, in good agreement with
30,37,39–45
46–49
50,51
ABX3,
as well as of sites in other types of compounds.
previously reported values.
The room temperature UV-visible
Here we target the preparation of CsSn(Cl1 Br ) to ascer- absorption spectra show that the monoclinic phase has a larger
ꢀx x 3
tain if a complete solid solution with the cubic perovskite optical band gap (3.33 eV) than the cubic phase (2.83 eV) (Fig. 1c),
structure can be attained for the entire range. Powder XRD and consistent with the less delocalized electronic structure associ-
ꢀ
NMR spectroscopy were carried out to determine the long-range ated with the discrete [SnCl
structure and the local coordination around the Cs and Sn sites
by Cl and Br atoms, which could be ordered or disordered. and cubic structures of CsSnCl
3
]
units in the former.
The lack of a simple relationship between the monoclinic
implies a reconstructive phase
3
Optical band gaps were measured by UV-visible absorption transition involving considerable mobility of atoms, because
spectroscopy and correlated with composition. Given the the process takes place at a relatively low temperature. In
ambiguity about the nature of CsSnBr , we evaluate whether preparation for a detailed examination of the dynamics of this
3
samples prepared by various methods show important struc- process, the local environments around the Cs and Sn atoms
tural differences which could inuence their optical properties. within the monoclinic and cubic phases were rst probed by
The activation energy for rapid halogen dynamics in CsSnBr
3
solid-state NMR spectroscopy to establish a frame of reference.
1
19
133
was quantied by variable-temperature
copy. Finally, the stability of CsSnBr
was assessed by examining the products and pathways of its of the Cs resonances, as measured by their isotropic chemical
Sn NMR spectros-
The
Cs NMR characteristics depend strongly on local
3
under ambient conditions electronic and chemical environments. The position and widths
1
33
degradation.
shi (diso) and full-width-at-half-maximum (fwhm) values, are
quite different within the monoclinic (diso ¼ 168.5(1) ppm,
fwhm ¼ 320 Hz) and cubic structures (d ¼ 64.7(1) ppm, fwhm
iso
2. Results and discussion
1
33
¼
138 Hz) (Fig. 2a). The
Cs spin-lattice relaxation time
2.1. Monoclinic and cubic phases of CsSnCl
3
decreases signicantly from the monoclinic (T
1
¼ 252 s) to the
At room temperature, CsSnCl
space group P2
3
adopts a monoclinic structure
ꢀ
(
1
/c) containing isolated [SnCl
3
] ions in trigonal
˚
pyramidal geometry with Sn–Cl bonds of 2.50–2.55 A (Fig. 1a). If
˚
three much more distant Cl atoms at 3.21–3.77 A are included,
the coordination geometry around the Sn atoms (coordination
number (CN) with 3 shorter and 3 longer bonds, CN3 + 3) could
21
also be described as distorted octahedral. Above 379 K,
1
33
119
Fig.
temperature for monoclinic and cubic CsSnCl
spectra were acquired at 11.75 T with a magic-angle spinning
2
(a)
Cs and (b)
Sn NMR spectra collected at room
133
3
. The
Cs NMR
1
19
Fig. 1 CsSnCl
cubic perovskite structure above 379 K, (c) UV-visible absorption for monoclinic and 11.75 T for cubic CsSnCl
spectra collected at room temperature, and (d) Rietveld refinements of sample conditions at 293 K. In (a), the asterisks (*) indicate spinning
powder XRD patterns. side bands.
3
: (a) monoclinic structure at room temperature, (b) frequency of 13 kHz. The Sn NMR spectra were acquired at 7.05 T
3
under non-spinning
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cubic structure (T
1
¼ 15 s), which may be related to greater
21
mobility of Cl atoms in the latter. The local environment of Cs
atoms surrounded by Cl atoms is less symmetrical within the
monoclinic structure (CN10, bicapped square prismatic), giving
rise to a manifold of spinning sidebands due to a small quad-
rupole coupling constant (C
symmetrical within the cubic structure (CN12, cuboctahedral)
for which C
Q
) of 190 kHz, whereas it is highly
Q
is ꢂ0 kHz. At a magic-angle spinning (MAS)
frequency of 13 kHz, a single low-intensity spinning side band is
observed in cubic CsSnCl (Fig. S1†), which is most likely due to
3
52
intrinsic defects (e.g., Cl vacancies). The distinctly different
1
33
Cs NMR spectra for these two forms of CsSnCl can then be
3
exploited to quantitatively determine their relative amounts in
more complex samples obtained by various synthetic
procedures.
1
19
Similarly, the
Sn NMR features serve as helpful diagnos-
(Fig. 2b). The lower symmetry
tics of these forms of CsSnCl
3
monoclinic form exhibits a broad pattern characteristic of
chemical shi anisotropy (diso ¼ ꢀ295(2) ppm, span U ¼
8
70(5) ppm, skew k ¼ 0.95(2)), whereas the higher symmetry
cubic form exhibits a symmetric resonance (diso ¼ ꢀ560(1) ppm,
fwhm ¼ 1.8 kHz) with no evidence for chemical shi anisotropy.
133
Fig. 3 (a) Time-dependent Cs NMR spectra (B
3 kHz MAS; a short-tip angle and an optimized 300 s recycle delay
were used for quantification) of a sample of CsSnCl prepared by
0
¼ 11.75 T, u
r
/2p ¼
2
.2. Trapping cubic CsSnCl
According to recent reports, cubic CsSnCl
a metastable phase at room temperature by ball milling, or by
3
at room temperature
1
3
can be obtained as
3
53
quenching in ice-water. The spectra are normalized to the highest
intensity. The asterisks (*(m)) indicate positions of spinning side bands
4
4
“briey heating” the monoclinic form to 380 K. However,
133
for the monoclinic phase. (b) Plot of
Cs NMR peak area (ꢃ2%) as
questions still remain about how long this cubic phase persists a function of time for the cubic phase.
under ambient conditions, and to our knowledge, there has
been no systematic study on its stability upon exposure to air.
Two separate samples of CsSnCl
3
were prepared by reaction
2
.3. Cubic CsSn(Cl1ꢀxBr
x
)
3
solid solution: synthesis, long-
of CsCl and SnCl , which were loaded into sealed and evacuated
2
range structure and optical properties
fused-silica tubes, and heated at 673(10) K for 15 h in a box
furnace (see Experimental section in ESI†). For the rst sample,
The multiple phase transitions encountered for both CsSnCl
and CsSnBr
halide solid solution CsSn(Cl1ꢀxBr
skite structure over the entire range for x at room temperature.
Moreover, the parent end-members only adopt the stable cubic
structure above room temperature (>379 K for CsSnCl and >292
K for CsSnBr3).
solution methods were reported to form monoclinic phases for
Cl-rich compositions (x < 0.50) and cubic phases for Br-rich
compositions (x > 0.50) at room temperature. The retention
of cubic CsSnCl as a metastable phase at room temperature,
presented above, suggests that it may yet be possible to prepare
a complete solid solution with the cubic structure through
3
ꢀ
1
3
make it challenging to prepare a complete mixed-
having the cubic perov-
the tubes were cooled slowly at 5 K min to room temperature.
This sample contained a mixture of cubic and monoclinic
phases with the latter >50%, as revealed by both powder XRD
x 3
)
1
33
and
Cs NMR spectroscopy performed within 24 hours of
3
synthesis (Fig. S2†). Aer 6 days at room temperature, the
sample transformed to mostly the monoclinic phase, with <10%
of the cubic phase remaining. For the second sample, the tubes
21,22
Samples of CsSn(Cl Br ) prepared by
1ꢀx x 3
ꢀ1
were cooled at 5 K min to 573 K, followed by quenching in an
ice-water bath. Although the powder XRD data suggest a phase-
pure sample consisting of only the cubic phase (lower part of
16
3
1
33
Fig. 1d), the Cs NMR spectrum shows a small amount (6%) of
1
33
the monoclinic phase (Fig. 3a). Time-dependent
Cs NMR
similar high-temperature reactions of CsX and SnX (X ¼ Cl, Br).
2
spectra were collected on the sample, kept at room temperature,
at various intervals up to 72 days. The proportion of the cubic
phase decreases monotonically in an exponential manner, with
As detailed in the experimental section, the critical step is rapid
quenching (see ESI†).
Freshly synthesized samples of CsSn(Cl1ꢀxBr
starting from faint yellow (CsSnCl ) and gradually evolving with
increasing Br content to orange, red, and nally black
CsSnBr ), consistent with a decrease in the band gap (Fig. 4a).
x 3
) show colours
6
5% remaining aer 10 days and 14% aer 72 days (Fig. 3b and
3
S2c†). This process is much slower than a similar one that we
have recently reported for the transformation of metastable g-
CsPbI perovskite to d-CsPbI non-perovskite at room temper-
(
3
3
3
1
33
The samples are polycrystalline with micron-sized particles
having uniform distribution of all elements (Cs, Sn, Cl, Br)
ature, which is complete within 2 hours, as monitored by Cs
41
NMR spectroscopy.
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Fig. 4 (a) Photographs of freshly synthesized CsSn(Cl1ꢀxBr
x
)
3
(top to bottom, x ¼ 0, 0.10, 0.33, 0.50, 0.67, 0.90, 1), (b) scanning electron
micrograph and elemental maps for CsSn(Cl0.50Br0.50
3
) , (c) powder XRD patterns, (d) plots of unit cell parameters and band gaps, and (e) UV-
visible absorption spectra.
1
33
down to this scale, as seen in elemental mapping images halogen atoms (Fig. 5a). For the end-members, the
Cs
collected on a eld-emission scanning electron microscope chemical shis are similar but distinguishable: d ¼ 64.7 ppm
iso
4
4
(
Fig. 4b and S3†). The chemical compositions, as determined by for CsSnCl
3
and diso ¼ 64.0 ppm for CsSnBr
Cs resonances are quite sharp for all members of
, with fwhm ranging from 70 to 150 Hz (or 1.1 to
2.3 ppm) (Fig. 5b). This result is in contrast to the lead-
, where much broader
Cs resonances (fwhm of 2 kHz) were observed and attributed
members of the solid solution (Fig. 4c and S4†). The rened unit to inhomogeneities caused by the numerous possible arrange-
3
. Interestingly, the
1
33
energy-dispersive X-ray analysis, agree well with the nominal
compositions CsSn(Cl1ꢀxBr
S1†).
x
)
3
x 3
loaded in the reactions (Table CsSn(Cl1ꢀxBr )
Powder XRD patterns collected at room temperature conrm containing analogues CsPb(Cl0.50Br0.50)
3
1
33
ꢀ
the cubic perovskite structure (space group Pm3m) for all
41
133
cell parameter for CsSn(Cl1 Br ) increases linearly from ments of Cl and Br atoms. The sharper Cs NMR spectra for
ꢀx x 3
to 5.8031(3) A for CsSnBr
˚
˚
5
.5894(5) A for CsSnCl
3
3
, in accor- CsSn(Cl1ꢀxBr
x 3
) imply that rapid halogen dynamics must be
dance with Vegard's law, with no deviations being shown taking place even at room temperature, such that the effects of
4
4
(
Fig. 4d and Table S2†). Strict adherence to Vegard's law the 12 surrounding halogen atoms are averaged.
1
33
behaviour has also been observed in lead-containing mixed-
halide perovskites.
Most surprisingly, the
Cs chemical shi does not vary
monotonically between the end-members; rather, it is displaced
35,36
UV-visible absorption spectra, which were converted from to higher frequency relative to the end-members and reaches
diffuse reectance spectra using the Kubelka–Munk function, a maximum of diso ¼ 74.7 ppm in CsSn(Cl0.50Br ) . The rela-
0.50 3
show gradual shis in the absorption edge to lower energy with tionship between the chemical shi d and the Br content x
iso
increasing Br content in CsSn(Cl1ꢀxBr
x
)
3
(Fig. 4e). With the can be tted to a parabolic curve, following the quadratic
1
33
2
assumption of a direct band gap, the linear regions in the Tauc function diso
(
Cs)/ppm ¼ 64.8 + 40.6x ꢀ 41.5x (Fig. 5c). A
2
89
119
plots of (ahn) vs. E were extrapolated to extract optical band gap comparable nonlinear change in Y and
Sn NMR chemical
54,55
values (Fig. S5†). The band gap in CsSn(Cl1ꢀxBr
linearly with increasing Br content from 2.83 eV for CsSnCl
.79 eV for CsSnBr (Fig. 4d), similar to the behaviour seen in mixed-halide double perovskites Cs AgBi(Cl Br ) due to
x
)
3
decreases shis has been observed in pyrochlores.
A similar nonlinear
1
33
3
to change in Cs NMR chemical shis was recently reported for
1
3
2
1ꢀx x 6
3
5,36,41
lead-containing mixed-halide perovskites.
a non-additive effect of nearest and next-nearest neighbours on
1
33
the chemical shi for Cs nuclei and the associated extended
56
halide environment.
2
.4. Cubic CsSn(Cl Br ) solid solution: local structure
1ꢀx x 3
A mixture of Cl and Br atoms in the cuboctahedral envi-
ronment around the Cs atoms would be expected to enhance
quadrupolar coupling interactions because of the lowering of
symmetry and correspondingly to increase the manifold of
1
33
119
using Cs and Sn NMR spectroscopy
The local environments of Cl and Br atoms around the Cs and
Sn atoms in CsSn(Cl1ꢀxBr
x 3
) were probed by NMR spectroscopy.
The Cs atoms reside in a cuboctahedral site surrounded by 12
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The Sn atoms in CsSn(Cl1ꢀxBr
x
)
3
reside in an octahedral
site surrounded by six halogen atoms (Fig. 6a). For the end-
1
19
members, the
shapes with clear differences in isotropic chemical shis for
CsSnCl (diso ¼ ꢀ289 ppm)
(diso ¼ ꢀ562 ppm) and CsSnBr
Fig. 6b). There is no evidence of spinning side bands,
consistent with a highly symmetric chemical environment.
Sn NMR spectra show symmetric-like line-
3
3
(
1
19
As the Br content increases, the
Sn chemical shi gradu-
ally displaces to higher frequency (Fig. 6b and Table S4†) and
the linewidth increases from CsSnCl (fwhm ¼ 1.8 kHz) to
3
1
19
CsSnBr (fwhm ¼ 19.0 kHz) (Fig. 6c). For CsSnBr , the
Sn
3
3
NMR linewidth and lineshape are unchanged whether the
magnetic eld (B ) is 7.05 or 11.75 T (Fig. S6†). Furthermore,
0
the linewidth (fwhm ¼ 21 kHz) only decreases slightly when
magic angle spinning is applied (fwhm ¼ 19 kHz) (Fig. S7†);
the small decrease is attributed to a reduced heteronuclear
1
19
79/81
dipolar coupling between
Sn and
Br. The observed
Fig. 5 (a) Local cuboctahedral environment of 12 halogen atoms
1
33
around the Cs atom. (b) Room temperature
CsSn(Cl1ꢀxBr , acquired at 11.75 T with a magic-angle spinning
frequency of 13 kHz. (c) Plots of diso
a function of Br content. The diso plot was fit to the equation diso
Cs NMR spectra for
x 3
)
133
133
(
Cs) and
1
Cs T values as
133
(
Cs)/
2
2
ppm ¼ 64.8 + 40.6x ꢀ 41.5x (R ¼ 0.999).
1
33
Cs spinning side bands, as was previously observed in the
41
lead-containing analogues CsPb(Cl0.50Br0.50
a MAS frequency of 13 kHz, only a single spinning side band of
low intensity was observed for all members of CsSn(Cl1ꢀxBr
Fig. S1†), which could occur if the environment contains
)
3
.
However, at
x 3
)
(
identical halogen atoms (not possible here) or if the halogen
dynamics are sufficiently rapid (i.e., much shorter than the
correlation times) such that only average values are measured
1
33
for the relevant interactions. The
Cs spin-lattice relaxation
times T vary nonlinearly between 10 to 35 s with increasing Br
1
content (Fig. 5c and Table S3†). These short relaxation times, on
the order of seconds, are consistent with the absence of
signicant covalent bonding interactions between Cs and
Fig. 6 (a) Local octahedral environment of 6 halogen atoms around
119
the Sn atom. (b) Room-temperature
Sn NMR spectra for
x 3
halogen atoms, similar to previous observations for CsSn(Cl1ꢀxBr )
, acquired at 11.75 T with a magic-angle spinning
119
119
42
frequency of 10 kHz. (c) Plots of Sn T and Sn linewidth values as
CsPb(Cl0.50Br ) .
1
0.50 3
a function of Br content.
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linewidths may also be inuenced by indirect spin–spin solvent-synthesis (fwhm ¼ 15 Hz), and growing increasingly
1
19
coupling between
Sn and six surrounding quadrupolar broader for those prepared by high-temperature reaction (fwhm
3
5/37
79/81
nuclei (I ¼ 3/2 for
Cl and
Br) as recently reported in ¼ 70 Hz) and ball-milling (fwhm ¼ 120 Hz) (Fig. 7a). For these
2
4,28
CH NH PbCl and CsPbCl .
For the Br-rich samples, the spectra acquired under slow magic-angle spinning (5 kHz), the
3
3
3
3
1
19
Sn signal-to-noise ratio drastically deteriorates upon spinning side band manifold increases along the same order.
increasing the echo delays from 24 to 94 ms, due to fast T These observations indicate that the solvent-synthesized
relaxation (Fig. S8†). These results are similar to previous sample has the most well-ordered structure containing fewer
observations on the lead-containing analogues.
The occurrence of single symmetric-like
and the absence of spinning side bands imply that the chemical ments and more defects.
2
4
4
2
4,28,31,35,36,41
defects, whereas the ball-milled sample has the most locally
1
19
Sn resonances disordered structure exhibiting a wide distribution of environ-
27,35
The higher defect concentrations
environment around the Sn atoms is uniform. If a distribution in our ball-milled sample are maintained because no further
of several types of SnCl6 Br (n ¼ 0–6) octahedra were present, post-grinding annealing treatment was performed. It is worth
ꢀn
n
they would give rise to multiple Sn resonances and possibly noting that one may minimize or circumvent the formation of
spinning side bands due to magnetic shielding anisotropy, but grinding induced defects by performing a subsequent heat
6
1,62
neither of these are observed. As before, the most probable treatment aer ball milling.
explanation is that rapid halogen dynamics is taking place in
1
19
The Sn NMR spectra for these samples (Fig. 7b) show even
CsSn(Cl1ꢀxBr on the timescale of the NMR experiment, greater differences in chemical shis, which span a range of
x 3
)
resulting in averaged chemical environments around the Sn almost 100 ppm, and linewidths: the sharpest at ꢀ386 ppm
atoms. This situation contrasts with the lead-containing (fwhm ¼ 7.5 kHz) for the solvent-synthesized sample, an
analogues CsPb(Cl1 Br ) , where multiple chemical environ- intermediate one at ꢀ284 ppm (fwhm ¼ 21 kHz) for the sample
ꢀx x 3
41
119
ments around Pb atoms are observed. The Sn NMR chem- prepared at high temperature, and the broadest at ꢀ295 ppm
1
19
ical shi can be further correlated with the band gap in (fwhm ¼ 34 kHz) for the ball-milled sample. The
CsSn(Cl1ꢀxBr (Fig. S9†). The decrease in band gap is accom- lattice relaxation times depend greatly on the synthetic route:
panied by a displacement of the
higher frequency, suggesting a dominating paramagnetic high-temperature sample, and 3 ms for the ball-milled sample
Sn spin-
x 3
)
1
19
Sn NMR chemical shi to
T
1
¼ 25 ms for the solvent-synthesized sample, 7 ms for the
shielding contribution.
(Table S5†). A faster relaxation process implies greater local
1
19
The
Sn spin-lattice relaxation times decrease dramati- surface disorder/defects in the ball-milled sample, consistent
1
33
119
cally from CsSnCl3 (T1 ¼ 3.5 s) to CsSnBr (T1 ¼ 0.006 s), with the conclusions from the Cs NMR spectra and the Sn
3
a difference of three orders of magnitude which is compa- NMR linewidths. This greater disorder is also reected in
4
4
rable to recent observations on CH
is monotonic with increasing Br content (Fig. 6c and Table sample compared to the other samples (Fig. S10†); all samples
S4†). It has been shown previously that the T relaxation exhibit micron sized crystallites. However, the optical band gaps
3 3 3
NH SnX . The decrease broader peaks in the powder XRD patterns for the ball-milled
1
mechanism is dominated by the scalar coupling strength extracted from the UV-visible absorption spectra appear to be
1
119
35/37
79/81
44,57,58
J( Sn, X) (X ¼
Cl or
Br) for tin halides.
virtually identical (within experimental error), with values
The smaller T1 value for CsSnBr compared to CsSnCl3 ranging from 1.77 eV for the solution-synthesized sample to
3
1
119
79/81
is consistent with J( Sn,
Br) being larger than 1.81 eV for the ball-milled sample. We note this disorder may
1
119
35/37
J( Sn,
Cl). In addition to J-coupling, chemical also be related to the UV-vis absorption prole being different
exchange processes due to the rapid halogen dynamics at for the ball-milled sample (Fig. S11†).
room temperature may also play a signicant role in the
1
19
1
Sn T values for the Br-rich samples, as discussed later.
2.6. Spontaneous halogen mobility in CsSnBr
3
The presence of point defects or vacancies enables ionic migration
within perovskites and plays an important role in photovoltaic
efficiency and material stability, while also being responsible for
2
.5. Local structure of CsSnBr prepared by solvent, high-
3
temperature, and mechanochemical routes
63–65
There have been many reports of halide perovskites being photocurrent hysteresis characteristics.
Metal halide perovskites
prepared by nonconventional routes, either in the presence or ABX can exhibit a vacancy transport hopping mechanism through
3
66
absence of solvent. For example, solvent-free “mechanochemical A-, B- or X-site migration, but X-site (halogen) diffusion is most
synthesis” routes involving ball-milling have been proposed to be probable as the halogen vacancy formation energy is comparable to
advantageous for large-scale production of perovskite photovol- the halogen ionic diffusion activation energy barrier (ca. 0.30 eV) in
5
9,60
52,67,68
133
taic materials.
prepared by these nonconventional routes are really identical to and Sn NMR spectra above suggests that halogen atoms may be
more traditional solution-based methods on a local atomic-level. highly mobile in CsSn(Cl1ꢀxBr even at room temperature. To
To understand this, samples of CsSnBr were prepared by three deconvolute the halogen dynamics, variable-temperature
routes (solvent synthesis, high-temperature reaction, and ball- NMR spectroscopy was performed on the parent CsSnBr3
However, it is unclear whether samples organic–inorganic hybrid perovskites.
The analysis of the Cs
119
x 3
)
119
3
Sn
1
33
119
milling) and analyzed by Cs and Sn NMR spectroscopy.
compound, prepared by solution synthesis. As the temperature
1
33
119
The
Cs NMR spectra show resonances appearing in the increases from 230 to 418 K, the
Sn resonances shi mono-
same position, being sharpest for the sample prepared by tonically to higher frequency by about 75 ppm (Fig. 8 and Table S6†),
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133
119
Fig. 7 Room temperature (a) Cs NMR spectra with magic-angle spinning (5 kHz) and (b) Sn NMR spectra with no magic-angle spinning,
acquired at 11.75 T, of CsSnBr samples prepared by solvent synthesis (SS), high-temperature (HT) reactions, and ball-milling (MCS). In (a), the
asterisks (*) indicate spinning side bands and the daggers (†) mark peaks attributed to Cs SnBr
3
2
6
.
1
19
following the cubic dependence disoð SnÞ=ppm ¼ 364:25 (T ) decreases drastically from 0.635 to 0.0009 s, a difference of three
1
ꢀ
ꢁ
ꢀ
ꢁ
03
103
2
10
T ðKÞ
3
3
1
orders of magnitude (Table S6†). In accordance with an Arrhenius
ꢀ
497:09 ꢄ
119
þ 107:16 ꢄ
ꢀ 7:56 ꢄ
:
119
T ðKÞ
T ðKÞ
relationship, a plot of log10[T
1
(
Sn)/s] vs. [1000/T (K)] yields a linear
dependence (Fig. 8b) from which an activation energy of 28.9 ꢃ
The Sn NMR linewidth narrows from 14.7 to 4.5 kHz, the effective
ꢀ
1
*
2
1.2 kJ mol (0.30 ꢃ 0.01 eV) for the halogen dynamics was extracted
see ESI Note 1†). Although this result is in contrast to the metallic
behaviour previously suggested from electrical conductivity
spin–spin relaxation time ðT Þ increases from 68 to 222 ms (Fig. S12
(
and Table S6†), and most notably, the spin-lattice relaxation time
67
measurements, it is in excellent agreement with those for other Sn-
and Pb-based perovskites and metal halide solid ionic conductors
4
4,67,68
including CH
3
NH
3
SnBr
3
(0.30–0.37 eV),
CH
3
NH
3
PbI
(0.2 eV,
and PbBr
3
(0.29
69
70
70
eV), CsPbCl
3
(0.29 eV), CsPbBr
3
(0.25 eV), CsSnCl
(0.20–0.30 eV),
3
71
72
70,73
theoretical), a-SnI
2
(0.29 eV), PbCl
2
2
70,74
(0.23–0.25 eV).
These observations support the conclusion that
behaves like an ionic conductor and Br mobility is
responsible for the fast relaxation process of Sn nuclei in CsSnBr
as observed recently in the organic–inorganic hybrid CH NH SnBr
3
ꢀ
CsSnBr
3
119
3
,
3
3
44
perovskite.
The above NMR results suggest that the replacement of Pb with
Sn in perovskite appears to enable faster halogen mobility, which
causes an averaging of the relevant interactions through halide
dynamics. The faster halogen dynamics alter the electrical properties
for tin halide perovskites. For example, cubic CsSnCl
have high ionic conductivity >10 S cm at 313 K, fueling its
3
was shown to
ꢀ
4
ꢀ1
71
potential as a solid electrolyte candidate for chloride ion batteries.
We further note that a higher concentration of B site vacancies (ca.
7
1
2
0
.9%) was reported in CsSnCl
3
than that for CH
3 3 3
NH PbI (ca.
7
5
.4%). The higher B site vacancies in tin halide perovskites is most
likely associated with Sn(+2) oxidation (Sn(+2) / Sn(+4) vs. Pb(+2)),
which may cause an increase in the rate of halogen dynamics.
2.7. Air stability and degradation pathways for CsSnBr3
CsSnBr shows higher thermal and moisture stability than the
3
1
19
organic–inorganic hybrid CH NH SnBr . Nevertheless, CsSnBr
3
3
3
3
Fig. 8 (a) Variable temperature
Sn NMR non-spinning spectra
119
slowly degrades over time under ambient conditions, becoming
partially oxidized to Cs SnBr , and the UV-visible absorption
acquired at 11.75 T and (b) plots of Sn T
1
and NMR chemical shift vs.
inverse of absolute temperature for solvent-synthesized CsSnBr
3
.
2
6
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spectra change drastically (Fig. S13†). Powder XRD and solid- extending over four orders of magnitude (Table S8†). Hence,
1
33
119
119
state
Cs and
Sn NMR spectroscopy were performed to the
Sn NMR acquisition parameters were optimized
elucidate the decomposition pathways and the nature of the within different spectral regions (Table S8†). Region 1 shows
1
19
degraded products. A sample of CsSnBr3 was freshly synthe- that a signicant amount of CsSnBr is present. The
Sn
3
sized by high-temperature reactions and stored in a paralm- resonance intensity for CsSnBr
3
is highly sensitive to the
sealed glass vial under ambient laboratory conditions (298 K magic-angle spinning frequency; because these spectra were
and 50–70% humidity) over 300 days. The powder XRD pattern acquired with a spin echo, for which the delay depends on
of the degraded sample was compared to those for various the spinning frequency, the signal is weakened due to the
reference standards (pristine CsSnBr
3
, Cs
2
SnBr
6
, CsBr, SnO
2
,
2
fast T relaxation process when slower MAS frequencies are
1
19
SnBr , and SnBr ) (Fig. 9a). In addition to CsSnBr , which still used. Region 2 shows a
Sn resonance at ꢀ600 ppm, which
4
2
3
constituted about 40% of the sample, signicant amounts of corresponds to SnO , and a broad underlying component,
2
Cs SnBr and CsBr, and smaller amounts of SnBr were formed, which most likely corresponds to SnBr , indicating that
2
6
4
4
1
19
but no SnO
also shows an increased background, suggestive of amorphous signal for SnO
contributions.
(fwhm ¼ 3.7 kHz) than in the bulk SnO
Complementary information about crystalline and amor- 0.2 kHz). Although crystalline SnO was not detected in the
phous components can be obtained from the solid-state NMR powder XRD pattern, the broadness of the
spectra. For quantitative analysis, the NMR parameters were suggests that it may either be amorphous or in the form of
optimized to achieve maximum sensitivity (Tables S7 and S8†). nanodomains (<10 nm) embedded within other phases in
The Cs NMR spectrum of the degraded sample was compared the sample. Region 3 conrms the presence of Cs SnBr , as
to those for three reference standards (pristine CsSnBr
SnBr
NMR resonance areas, the sample consisted of 40% CsSnBr
ppm), 45% Cs SnBr (113 ppm), 8% CsBr (260 ppm) and 7% of was detected only aer two million scans, implying only
an unidentied Cs-containing phase (230 ppm). The
resonances are slightly broader for Cs SnBr (fwhm of 320 Hz, in the starting material SnBr2 used in the synthesis, as
2
or SnBr
2
were present. The powder XRD pattern oxidation from Sn(+2) to Sn(+4) has taken place. The
in the degradation product is much broader
standard (fwhm ¼
Sn
2
2
2
1
19
Sn NMR signal
7
6
1
33
2
6
1
19
3
, Cs
2
-
seen by the
Sn resonance at ꢀ1964 ppm. Region 4 reveals
1
33
119
6
, and CsBr) (Fig. 9b). Based on assignment of the
Cs a weak
Sn signal that is assigned to metallic b-Sn and is
7
7
3
(65 Knight-shied to higher frequency; however, this signal
2
6
1
33
Cs a trace amount of b-Sn that likely originated as an impurity
2
6
compared to 170 Hz for the solvent-synthesized standard) and shown in Fig. S14.† Based on the XRD and NMR results, two
for CsBr (fwhm of 370 Hz, compared to 270 Hz for the standard). simultaneous pathways are proposed for the room-
This broadening implies that Cs
2 6 3
SnBr and CsBr formed during temperature oxidation of CsSnBr :
the slow degradation process have smaller crystallite sizes than
in the reference standards. The powder XRD pattern also
shows broadening of diffraction peaks for these phases
44
(1) 2 CsSnBr + O (air) / Cs SnBr (crystalline) + SnO
3
2
2
6
2
(
amorphous or nano-sized)
(
Fig. 9a).
In the
1
19
(2) 2 CsSnBr
3
+ O
2
(air) / SnBr
4
(crystalline) + 2 CsBr
Sn NMR spectrum of the degraded sample, the
(
crystalline) + SnO
2
(amorphous or nano-sized).
resonances span about 8800 ppm (1.6 MHz at B ¼ 11.75 T,
0
Fig. 9c and S14†), with spin-lattice relaxation times
Fig. 9 Degraded CsSnBr
3
sample that was exposed to ambient laboratory conditions over 300 days. (a) Powder XRD pattern compared to
(p), Cs SnBr (5), CsBr (A), and SnBr (#). The blue lines indicate the absence of peaks due to
. (b) Cs NMR spectrum, acquired at 11.75 T with a magic-angle spinning frequency of 5 kHz, shows the presence of CsSnBr
, and CsBr. The asterisks (*) indicate spinning side bands and the dagger (†) indicates an unidentified Cs-containing degradation product.
c) Sn NMR spectrum, acquired at 11.75 T with magic-angle spinning frequencies of 0 to 13 kHz.
reference standards shows presence of CsSnBr
3
2
6
4
133
SnO
2
and SnBr
2
3
,
Cs SnBr
(
2
6
1
19
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3
4
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. Conclusions
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Author's contribution
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All authors have given approval to the nal version of the
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18 D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon,
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19 T. C. Jellicoe, J. M. Richter, H. F. J. Glass, M. Tabachnyk,
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Acknowledgements
This research is supported by the Natural Sciences and Engi-
neering Research Council (NSERC) of Canada, Canada Foun-
dation for Innovation, NSERC CREATE through ATUMS, Alberta
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