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M A T E R I A L S S C I E N C EMagnetizing lead-free halide double perovskites
Weihua Ning1, Jinke Bao2, Yuttapoom Puttisong1, Fabrizo Moro1, Libor Kobera3, Seiya Shimono4,
Linqin Wang5, Fuxiang Ji1, Maria Cuartero5, Shogo Kawaguchi6, Sabina Abbrent3,
Hiroki Ishibashi7, Roland De Marco8, Irina A. Bouianova1, Gaston A. Crespo5,
Yoshiki Kubota7, Jiri Brus3, Duck Young Chung2, Licheng Sun5,9, Weimin M. Chen1,
Mercouri G. Kanatzidis2,10, Feng Gao1*
Spintronics holds great potential for next-generation high-speed and low–power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show inter-esting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite,
Cs2Ag(Bi:Fe)Br6, Fe3+ replaces Bi3+ and forms FeBr6 clusters that homogenously distribute throughout the double
perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites.
INTRODUCTION
As a new generation of solution-processed semiconductors, lead halide perovskites (LHPs) have taken a dominant position within the portfolio of compounds due to their outstanding optoelectronic properties, including high carrier mobility, long carrier lifetime, low trap density, and high absorption coefficient. These properties result in high-performance optoelectronic devices, including solar cells (1, 2), light-emitting diodes (3, 4), lasers (5), and photodetectors (6, 7). Very recently, LHPs also show interesting spin-related prop-erties due to spin-orbit coupling (SOC) (8) of Pb. Thus, they exhibit spin-dependent optical selection rules (9, 10), large-scale Rashba and Dresselhaus splitting (11–13), magneto-optical effects (14, 15), and polarized light-related effects (16). These observations make LHPs interesting for spintronic devices (17). However, the magnetic properties resulting from the SOC are weak, therefore limiting their further development in the field.
In addition to SOC enhancement, a straightforward yet more efficient approach to achieving spintronic materials is magnetic doping/alloying into nonmagnetic semiconductors. This approach has played an increasingly prominent role to revolutionize electronics and spintronics in conventional semiconductors and has also been very recently examined in LHPs, showing optically switched mag-netism at 5 K (18). Since the magnetic properties in this approach
mainly originate from magnetic dopants, it is more desired to turn to other optoelectronic lead-free halide perovskites (e.g., double perovskites), which provide much more room for doping/alloying (19), in addition to being environmentally friendly and stable.
Here, we focus on the benchmark double perovskite Cs2AgBiBr6,
which is a promising lead-free and stable optoelectronic material. Through magnetic element iron alloying, we provide new strategies to develop a new generation of materials that can potentially couple optoelectronics with spintronics. Combining near edge x-ray absorp-tion fine structure (NEXAFS) and solid-state nuclear magnetic res-onance (ssNMR) measurements, we reveal that iron is in the form
of Fe3+, replacing Bi3+ and forming FeBr6 clusters, which
homoge-nously distribute throughout the double perovskite crystals. Our
double perovskite alloy, Cs2Ag(Bi:Fe)Br6, exhibits a structural phase
transition at ~120 K and a strongly temperature-dependent magnetic response at temperatures below 30 K.
RESULTS
Basic characterizations of double perovskite Cs2Ag(Bi:Fe)Br6
Double perovskites Cs2Ag(Bi:Fe)Br6 were prepared by using a
pro-cedure similar to that reported for Cs2AgBiBr6 (20, 21). CsBr, AgBr,
BiBr3, and FeBr3 precursors were mixed into an HBr solution in a
hydrothermal autoclave, which was heated at 120°C for 24 hours and then slowly cooled down, resulting in 2- to 5-mm black octa-hedral crystals as the final products. Figure 1A presents
photo-graphs of the Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6 single crystals. The
crystal color of Cs2Ag(Bi:Fe)Br6 changes from red to black after
alloying, indicating enhanced absorption (fig. S1). Electron spin
resonance (ESR) measurements of the Cs2Ag(Bi:Fe)Br6 crystal show
a strong resonance located at g ~ 2.032 at room temperature, point-ing out the existence of paramagnetic Fe (Fig. 1B). The
concen-tration of Fe in Cs2Ag(Bi:Fe)Br6 is ~11.4%, as confirmed by both
energy dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements (fig. S2 and table S1).
1Department of Physics, Chemistry and Biology (IFM), Linköping University,
Linköping SE-581 83, Sweden. 2Materials Science Division, Argonne National
Labo-ratory, Argonne, IL 60439, USA. 3Institute of Macromolecular Chemistry of the
Czech Academy of Sciences, Heyrovskeho nam. 2, 162 06 Prague 6, Czech Republic.
4Department of Materials Science and Engineering, National Defense Academy,
Yokosuka, Kanagawa 239-8686, Japan. 5Department of Chemistry, KTH Royal Institute
of Technology, SE-10044 Stockholm, Sweden. 6Japan Synchrotron Radiation Research
Institute (JASRI), SPring-8, Sayo, Hyogo 679-5198, Japan. 7Department of Physical
Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan. 8Faculty of Science,
Health, Education and Engineering, University of the Sunshine Coast, 90 Sippy Downs Drive, Sippy Downs, QLD 4556, Australia. 9Center of Artificial Photosynthesis for
Solar Fuels, School of Science, Westlake University, Hangzhou 310024, China.
10Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.
*Corresponding author. Email: feng.gao@liu.se
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Both single-crystal x-ray diffraction (SC-XRD) and power XRD
(PXRD) measurements indicate that Cs2Ag(Bi:Fe)Br6 retains the
cubic phase of Cs2AgBiBr6. On the basis of the SC-XRD
measure-ments, Cs2AgBiBr6 crystallizes in the cubic Fm-3m space group
with the lattice parameter of a = 11.27 Å (table S2), manifesting a three-dimensional classic perovskite structure (Fig. 1A, bottom).
When 11.4% Fe is alloyed into Cs2AgBiBr6, the structure retains its
original cubic phase with lattice parameter slightly decreasing to
a = 11.25 Å (table S2). This lattice shrink phenomenon results from
smaller radius of Fe3+ (0.65 Å)/Fe2+ (0.78 Å) relative to Bi3+ (1.03 Å)/Ag+
(1.15 Å) (22), consistent with the Vegard’s law (23). The PXRD patterns further confirm the cubic double perovskite structure of
Cs2Ag(Bi:Fe)Br6, since all the diffraction peaks are almost the same
as the simulated and experimental Cs2AgBiBr6 (Fig. 1C). The highly
intense reflections (220) and (400) shift toward high 2 angles together with symmetric to asymmetric transformation after Fe alloying (Fig. 1, D and E), which is consistent with the lattice shrink observed from SC-XRD.
We additionally notice that Fe exhibits multiple valence states in
compounds (e.g., FeO, Fe2O3, and Fe3O4) and subsequently perform
NEXAFS, x-ray photoelectron spectroscopy (XPS), and synchrotron radiation–XPS (SR-XPS) measurements so as to understand the valence state and surface distribution of Fe in our alloyed double
perovskites. The NEXAFS Fe 2p3/2 edge spectrum for Cs2Ag(Bi:Fe)
Br6 (Fig. 1F) shows a double peak in the range of 705 to 713 eV. Peak
deconvolution indicates an intensity ratio of approximately 1:3
between the Fe 2p3/2 signal at 708.1 eV and that at 709.9 eV, pointing
out that the Fe is in the state of trivalence (24). The valence state of
Fe is further confirmed by the XPS Fe 2p3/2 core level spectrum
(fig. S3), as evidenced by the small atomic shake-up satellite as a
high binding energy shoulder at 712.5 eV on the major Fe 2p3/2 peak
at 707.5 eV that is characteristic of the trivalent state (25). It is important to note that SR-XPS of the Fe 2p level at a low photoelectron kinetic energy of 50 eV (electrons escaping from a shallow depth of one monolayer) does not reveal the presence of Fe in the sampled region, while laboratory XPS using deeper originating Fe 2p photo-electrons at a kinetic energy of 777 eV (compared with an Auger electron energy of 650 eV in the NEXAFS measurement, a compa-rable kinetic energy and photoelectron inelastic mean free path) gives a clearly discernable Fe 2p XPS signal. The laboratory XPS, SR-XPS, and NEXAFS results confirm that the alloyed Fe is not at the surface of the crystal but is instead buried beneath the surface monolayer.
Temperature-dependent structure of double perovskite Cs2Ag(Bi:Fe)Br6
We further investigate the effect of Fe alloying on the crystal structures at low temperatures. We perform specific heat capacity (Cp)
mea-surement to examine the phase transition behavior of Cs2Ag(Bi:Fe)
Br6 (Fig. 2A). A thermal anomaly is observed at ~120 K, which is
probably attributed to a weak first-order or second-order phase transition. To deeply understand the thermal anomaly observed from the Cp measurements, we characterize the crystal structures
of Cs2Ag(Bi:Fe)Br6 by using temperature-dependent synchrotron
powder diffraction (SPD). Figure 2B and fig. S4 show the SPD
patterns of Cs2Ag(Bi:Fe)Br6 in 300 to 30 K, where the cubic Bragg
reflection (4 0 0) split into two peaks at ~120 K, indicating that the phase transition at this temperature is a symmetric breaking structural Fig. 1. Basic characterizations of the double perovskite Cs2Ag(Bi:Fe)Br6. (A) Photographs and crystal structures of double perovskites Cs2AgBiBr6 and
Cs2Ag(Bi:Fe)Br6. (B) ESR signals from Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6 at room temperature (RT). a.u., arbitrary units. (C) Powder XRD patterns for Cs2AgBiBr6 and
Cs2Ag(Bi:Fe)Br6, and the simulated XRD pattern of Cs2AgBiBr6 as reference, an expansion of highly intense reflections (220) (D) and (400) (E), illustrating shift toward
higher 2 angles and the symmetric to asymmetric transformation after Fe3+ alloying. (F) NEXAFS Fe 2p3/2 edge spectrum for Cs2Ag(Bi:Fe)Br6, in which the baseline
has been corrected.
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phase transition. In contrast, the reflection peaks of (4 4 4) only regularly shift toward the high-angle side, possibly due to lattice shrink during the cooling process.
To determine the low-temperature crystal structure of Cs2Ag(Bi:Fe)
Br6 accurately, we use the JANA2006 soft package (26) to do the Rietveld
refinements. The Rietveld refinement shows a perfect single-phased
material; thus, Cs2Ag(Bi:Fe)Br6 sample is free of CsBr, BiBr3, AgBr,
and Fe clusters or any other impurity (fig. S5). Table S3 lists the lattice parameters at different temperatures. A cubic to tetragonal (c > a) first-order structural transition is observed at 120 K. This temperature agrees well with thermal anomaly temperature (Tc) observed in the temperature-dependent Cp (Fig. 2A). With
decreas-ing temperature, Cs2Ag(Bi:Fe)Br6 experiences a symmetry
break-ing phase transition with the space group changbreak-ing from Fm-3m to I4/m. Meanwhile, an abnormal change is observed from the temperature-dependent lattice constant and rotation angle between
AgBr6 and (Bi:Fe)Br6 octahedron (Fig. 2, C and D), further
con-firming the structural phase transition at 120 K. As the temperature gradually decreases, the crystal phase retains I4/m with the lattice volume slightly decreasing at 30 K, which correlates well with the
temperature dependence of Cp. As for pristine Cs2AgBiBr6, the
structural behaviors are almost the same as Cs2Ag(Bi:Fe)Br6,
in-dicating that the structural phase transition is intrinsic, without
any influence from Fe3+ alloying (27). The lattice parameters of
Cs2Ag(Bi:Fe)Br6 are observed to be smaller than those of Cs2AgBiBr6,
consistent with SC-XRD results (22).
ssNMR of double perovskites Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6
Next, we proceed to investigate how Fe ions distribute within the
Cs2AgBiBr6 matrix. We analyze miscellaneous 133Cs and 209Bi ssNMR
[magic angle spinning (MAS) ssNMR)] experiments to gain deep
insight in the structure for both Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6
systems. As shown in Fig. 3A, only one symmetric peak located at
iso = 80.7 ± 0.5 parts per million (ppm) in 133Cs MAS NMR spectra
of Cs2AgBiBr6, confirming the existence of one crystallographic
position of the Cs+ ions in pristine Cs
2AgBiBr6. In contrast, besides
the peak at iso = 80.7 ± 0.5 ppm, an additional weak peak appears at
iso = 83.4 ± 0.5 ppm (Fig. 3B) in Cs2Ag(Bi:Fe)Br6, indicating the
presence of paramagnetic ions (Fe3+) in close proximity to Cs+ ions
(28–30). The presence of paramagnetic metal ions (e.g., Fe3+ and
Cu2+) usually causes extremely rapid longitudinal and transverse
relaxation of the nearby nuclei due to electron spin couplings. This behavior allows us to probe a more precise distribution of the
para-magnetic species in the perovskite matrix by using 133Cs NMR T
1
relaxation measurements (31).
The detected 133Cs NMR signal contains two partially overlapped
peaks (components), and determination of their relaxation
param-eters (T1 relaxation times and amounts of individual components)
is performed in two steps. First, the saturation recovery buildup
curves of the detected 133Cs NMR signal are analyzed using a double-
exponential saturation recovery function and preliminary values of
T1(133Cs) relaxation times, and the corresponding fractions of
indi-vidual components are obtained. Second, each 133Cs NMR spectrum
recorded in T1(133Cs) saturation recovery experiment is fitted. Then,
the buildup curve of individual components is separately analyzed by using the single-exponential saturation recovery functions (Fig. 3, C and D). This way, both peaks are analyzed, and the values
of T1 relaxation times and the amounts of individual phases are
re-fined. Figure 3 (C and D) shows the resulting 133Cs T1 relaxation
curves for Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6. We obtain a slow T1
relaxation time of 306 s in Cs2AgBiBr6 and one slow- and one
fast-relaxing phase corresponding to signals at iso = 80.7 ± 0.5 ppm
Fig. 2. Temperature-dependent specific heat capacity and SPD. Specific heat capacity (A) and SPD patterns (B) of Cs2Ag(Bi:Fe)Br6 at different temperatures. The lattice
constant (C) and rotation angle (D) in 293 to 30 K for Cs2Ag(Bi:Fe)Br6. The crystal structures of Cs2Ag(Bi:Fe)Br6 at high temperature (HT, 300 K) (E) and low temperature (LT,
30 K) (F), red lines representing unit cells. Inset plot in (A) represents a magnification of the Cp peak at 120 K.
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(31 s) and iso = 83.4 ± 0.5 ppm (4 s) in Cs2Ag(Bi:Fe)Br6. The
shortened 133Cs T
1 relaxation times indicate that the
paramag-netic ions (Fe3+) are alloyed into the perovskite framework. Since
we now know the amount of Fe3+ and also the amount of Cs+, which
is in close proximity to Fe3+, simple calculations lead us to
con-clude that Fe3+ is homogenously distributed as small FeBr
6 clusters
throughout the lattice (more details in the Supplementary Ma-terials) (31).
The remaining doubt is whether Fe3+ ions replace Bi3+. Hence,
we perform the 209Bi ssNMR experiment on both Cs
2AgBiBr6 and
Cs2Ag(Bi:Fe)Br6 systems. As shown in Fig. 3E, a very broad signal
locates at about iso = 5500 ppm for both Cs2AgBiBr6 and Cs2Ag(Bi:Fe)
Br6. The 209Bi ssNMR spectrum of Cs2AgBiBr6 provides relatively
symmetric signal with evident J-coupling (JX−Bi = 2002 Hz, where X
corresponds to Br). In the case of Cs2Ag(Bi:Fe)Br6, the 209Bi ssNMR
spectrum shows broadened spectral line with J-coupling completely
missing. This phenomenon is caused by the fast T1 relaxation times
of 209Bi nuclei induced by the paramagnetic Fe3+, which is
homoge-nously distributed in the matrix, replacing Bi3+ ions.
Magnetic properties characterization of double perovskite Cs2Ag(Bi:Fe)Br6
Encouraged by the fact that Fe3+ ions are incorporated into the
lattice, we perform temperature-dependent magnetic susceptibility
for Cs2Ag(Bi:Fe)Br6 by superconducting quantum interference
device (SQUID). The results exhibit diamagnetism at room tempera-ture and weak temperatempera-ture dependence of magnetic susceptibility above 30 K (Fig. 4A), indicating that diamagnetism from core
elec-trons in this compound dominates over paramagnetic contribution
from the alloyed magnetic ion Fe3+. The magnetic susceptibility
in-creases rapidly below 20 K, indicating that a certain kind of magnetic response appears. The magnetic susceptibilities under zero-field– cooled (ZFC) and field-cooled (FC) procedures basically overlap with each other in the whole temperature range (Fig. 4A) and exhibit very tiny difference below 30 K (fig. S7). Field-dependent magnetization M(H) is linear with a negative slope above 50 K, which is consistent with temperature-dependent magnetic suscepti-bility (Fig. 4B). M(H) at 2 K has a positive component at low fields
and saturates to ~0.15 B per Fe after subtracting the diamagnetic
component. The saturated magnetic moment is much smaller than
both high spin (5 B per Fe) and low spin (1 B per Fe) state of Fe3+.
In addition, there is no obvious magnetic hysteresis in the M(H) loop in the low fields (see the inset of Fig. 4B).
To gain further insights into the observed temperature dependence of the magnetic response probed by SQUID, we perform ESR mea-surements. ESR is well-known to be capable of providing microscopic information associated with the chemical origins of the spin (mag-netic) species, being hence probed. In our case, this has enabled us to identify the paramagnetic centers or possible magnetic resonance signals arising from magnetic coupling such as ferromagnetic or antiferromagnetic resonance (FMR or AFMR) signals that are in-volved in the total ESR spectra as well as their contributions to the overall magnetic response evolving with temperature. Figure 4C
shows ESR spectra from Cs2Ag(Bi:Fe)Br6 crystal powder (the red
curves) obtained at 7.2 and 40 K, respectively. Strong multiline ESR signals are observed throughout the temperature range of 5 to 300 K: Fig. 3. ssNMR of Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6. 133Cs-133Cs SD/MAS NMR and 133Cs Hahn-echo MAS NMR spectra of Cs2AgBiBr6 (A) and Cs2Ag(Bi:Fe)Br6 (B). 133Cs T1
saturation recovery buildup curves of Cs2AgBiBr6 (C) and Cs2Ag(Bi:Fe)Br6 (D). (E) 209Bi ssNMR of Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6 conducted at static conditions. (F)
Sche-matic representation of possible scenarios for Fe3+ distribution inside the perovskite lattice: parent perovskite lattice, isolated Fe3+ ions, small Fe3+ clusters, and large Fe3+
clusters. The percentages are the ideally calculated values for Cs+ exhibiting fast relaxation in different scenarios.
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These are introduced upon the incorporation of Fe in Cs2AgBiBr6,
as they are absent in the ESR spectra from the pristine Cs2AgBiBr6
(shown as the gray curves in Fig. 4C). As shown in Fig. 4C (right),
the ESR spectrum at 40 K from the Cs2Ag(Bi:Fe)Br6 crystal powder
is best described by a sum of two paramagnetic species: (i) isolated
Fe3+ of tetragonal symmetry with S = 5/2 and g = 2.032 and (ii) a
defect center of unknown origin with S = 1/2 and g = 2.032. The assignment is supported by the good agreement between the simu-lated ESR spectra of these two centers by the spin Hamiltonian and the experimental spectrum (the spin Hamiltonian analysis is described in the Supplementary Materials). Upon cooling down below 35 K, in addition to the two aforementioned paramagnetic centers, an extra, broad component emerges and becomes dominant in the ESR spectrum at 7.2 K.
To find a correlation with the temperature-dependent magnetic response from the SQUID and additionally identify the relative importance of various spin species, we carefully investigate and analyze the ESR signal intensity (directly proportional to ) as a function of temperature. The results are summarized in Fig. 4D, where the temperature dependences of the ESR intensities of the individual ESR components and the total ESR signal intensity are displayed. Evidently, the total ESR signal intensity exhibits a similar temperature-dependent trend as the SQIUD data shown in Fig. 4A, confirming a direct correlation in measuring by the two
tech-niques. Using the protocol of spectral deconvolution of the three
different ESR contributions, the intensities of the isolated Fe3+ and
the S = 1/2 center are found to gradually and monotonically increase with deceasing temperature, as expected according to the paramagnetic centers. Our analysis proves that the sharp rising of the magnetic response at T < 35 K is not related to these two paramagnetic species but rather due to the third and broad component that becomes dominant at T < 35 K, as shown in Fig. 4D.
Combining the results from the ESR and SQUID studies, the fol-lowing conclusions can be drawn. First, judging from the fact that
the Fe3+ ions remain magnetically isolated throughout the entire
temperature range of 5 to 300 K, the Cs2Ag(Bi:Fe)Br6 crystal is not
globally magnetic. Otherwise, the isolated Fe3+ ESR signal would
have decreased due to their strong magnetic coupling occurring below the magnetic phase transition temperature, which would have directly correlated with a concomitant increase of an FMR- or AFMR-like signal. Second, although the origin of the broad ESR component that is responsible for the steep rising of magnetic re-sponse at low temperatures is unknown, it is induced by the Fe in-corporation in the crystal and it resembles that of an FMR or AFMR signal. As no obvious shift in the resonance field of the broad ESR signal can be resolved, which should otherwise reflect the internal magnetic field induced by magnetization, it is not possible at pres-ent to single out whether or not the broad ESR signal corresponds to an FMR or AFMR signal. If it is induced by FMR or AFMR coupling, the corresponding magnetic response should then be (i) local, only representing a small part of the total sample volume, and (ii) rather
weak, to account for the negligible reduction of the isolated Fe3+ ESR
signal intensity and the small saturated magnetic moment detected in SQUID. A possible origin of the corresponding magnetic response
is ferromagnetic coupling from localized regions with high Fe3+
con-centrations due to a strong composition fluctuation. If this is true,
then it could hint that it may be possible to make the Cs2Ag(Bi:Fe)
Br6 crystal a magnetic semiconductor provided that a higher Fe
composition could be achieved. Thus, this work points toward some possible directions for further studies aiming to explore lead-free double perovskites for future spintronic applications. We cannot either rule out the possibility of magnetic inclusions of phase separated Fe clusters/composites beyond the detection limit of our characterization techniques in this work. The origin of this magnetic response needs further investigations on more samples with different iron concentrations, which is beyond the scope of this study at this stage.
DISCUSSION
In summary, we alloy magnetic element Fe3+ into optoelectronic
double perovskite Cs2AgBiBr6, providing a new class of halide double
perovskite alloys, which can be potentially used for spintronics.
The magnetic Fe3+ ions homogenously distribute with small FeBr
6
clusters in the crystal lattice. The temperature-dependent specific
heat and SPD of Cs2Ag(Bi:Fe)Br6 demonstrate a cubic-tetragonal
structural transition at ~120 K. Combined SQUID and ESR mea-surements reveal a strongly temperatudependent magnetic re-sponse at temperatures below 30 K, which is tentatively attributed a weak ferromagnetic or antiferromagnetic response from localized regions containing, e.g., phase separated magnetic clusters/composites or high Fe compositions due to alloy fluctuations. We believe that halide double perovskites hold great potential for a next generation Fig. 4. Magnetic characterizations of Cs2Ag(Bi:Fe)Br6. (A) Temperature-dependent
magnetic susceptibility of Cs2Ag(Bi:Fe)Br6 under zero-field-cooled (ZFC) and
field-cooled (FC) procedures with a magnetic field H = 1000 Oe. (B) Isothermal field-dependent magnetization of Cs2Ag(Bi:Fe)Br6. The inset plot presents the
low–magnetic field magnetization from −0.1 to 0.1 T. (C) ESR spectrum from Cs2AgBiBr6 (the gray curves) and Cs2Ag(Bi:Fe)Br6 (the red curves) crystal powder,
measured at 7.2 and 40 K, respectively. The simulated ESR spectra (the blue curves) are obtained from a spin Hamiltonian analysis, which consist of the contributions from Fe3+, with S = 5/2 and g = 2.032, and a defect center of unknown origin, with S = 1/2 and g = 2.032. The discrepancy between the simulated and experimental
ESR spectra indicates the existence of a third ESR signal—a broad background signal, which becomes more pronounced at low temperatures. (D) Temperature dependence of total microwave absorption (the black spheres), together with the contributions from the three individual ESR components revealed from the spin Hamiltonian analysis.
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of spintronic devices that have been widely studied in oxide-based perovskites and merit further study to realize their full potential. MATERIALS AND METHODS
Preparation of Cs2AgBiBr6 crystals
Solid CsBr (213 mg, 1.00 mmol) and BiBr3 (225 mg, 0.5 mmol) were
dissolved in 3 ml of 47% HBr. Solid AgBr (94 mg, 0.5 mmol) was then added to the solution, and the mixture was transferred into a
25 cm3 Teflon-lined digestion autoclave. The autoclave was sealed
and placed in the oven where it was heated at 120°C for 24 hours. After being slowly cooled to room temperature, red octahedral single crystals were achieved (yield ca. 79% based on Ag).
Preparation of Cs2Ag(Bi:Fe)Br6 crystals
Solid CsBr (213 mg, 1.00 mmol), BiBr3 (135 mg, 0.3 mmol), and
FeBr3 (59.1 mg, 0.2 mmol) were dissolved in 4 ml of 47% HBr. Solid
AgBr (94 mg, 0.5 mmol) was then added to the solution, and the
mixture was transferred into a 25-cm3 Teflon-lined digestion
auto-clave. The autoclave was sealed and placed in the oven where it was heated at 120°C for 24 hours. After being slowly cooled to room temperature, black octahedral single crystals were achieved (yield ca. 67% based on Ag).
Basic characterizations of Cs2AgBiBr6 and
Cs2Ag(Bi:Fe)Br6 crystals
The optical absorption measurements were carried out using a PE Lambda 950 ultraviolet-visible spectrophotometer. XRD patterns of the products were recorded with a X’Pert PRO x-ray diffractometer using Cu K1 irradiation ( = 1.5406 Å). Energy-dispersive x-ray spectroscopy analysis was performed using an LEO 1550 scan-ning electron microscope operated at 20-kV accelerating voltage,
with an Oxford Instruments X-Max 80-mm2 Silicon Drift Detector.
ICP-OES measurements were carried out using a PerkinElmer Avio
500 spectrometer with the Cs2Ag(Bi:Fe)Br6 crystal powder
dis-solved in aqua regia. XPS of Cs2Ag(Bi:Fe)Br6 crystals was performed
using a Scienta ESCA200 spectrometer with a base pressure of
2 × 10−10 mbar (ultrahigh vacuum) and monochromatized Al (K)
radiation (h = 1486 eV). NEXAFS spectroscopy and SR-XPS measurements were performed at the Elettra synchrotron radiation facility in Trieste, Italy. Specific heat (Cp) measurement was per-formed using physical property measurement system (PPMS-9 T)
Quantum Design Co. with Cs2Ag(Bi:Fe)Br6 crystals.
Temperature-dependent SPD
The SPD experiments were carried out at beamline BL02B2 at
SPring-8, Japan (32). The Cs2AgBiBr6 and Cs2Ag(Bi:Fe)Br6 crystal
samples were crushed into a fine powder and filled into a borosilicate glass capillary with a diameter of 0.1 mm. The incident x-ray beam was monochromatized to 24.8 keV (0.5 Å) using Si(111) double crystals. The detail of the wavelength was 0.50018 Å determined
by using standard reference materials CeO2.The temperature of the
powder sample was controlled using a nitrogen and helium gas blower. Rietveld refinements were performed using the JANA2006 software package (26).
Solid-state nuclear magnetic resonance
The ssNMR spectra were recorded at 11.7 T using a Bruker Avance III HD spectrometer. The 4-mm cross-polarization MAS (CP/
MAS) probe was used for 133Cs and 209Bi experiments at Larmor
frequency of (133Cs) = 65.611 MHz and (209Bi) = 80.858 MHz,
respectively. 133Cs MAS NMR experiments were collected at 10-kHz
spinning speed without 1H decoupling. The recycle delay was 4 s for
all ssNMR experiments. The 133Cs chemical shift was calibrated
using solid CsCl (133Cs, 228.1 ppm) (33). The pulse length was set to
2.4 s at 100 W for maximal signal intensity. The 133Cs rotor-
synchronized (1 loop) spin-echo MAS NMR experiments (90°-nR
-180°-nR-acq.) (34) were performed. The 133Cs-133Cs correlation
MAS NMR spectra were recorded using nuclear Overhauser effect spectroscopy–type three-pulse sequence. Duration of the spin- exchange period between the second and third pulse was 10 s. Spectral width in both frequency dimensions was rotor-synchronized
to be 10 kHz. The indirect detection period t1 consisted of 128
in-crements each made of 48 scans. The 209Bi chemical shift was
cali-brated using a saturated solution of Bi(NO3)3·5H2O in concentrated
HNO3 (209Bi, 0.0 ppm) (35). The pulse length was 9.0 s at 100 W at
maximal signal intensity. 209Bi NMR experiments were collected at
static conditions using spin-echo NMR experiments (90°--180°-acq.) (34), and the delay between pulses was 20 s. To compensate fric-tional heating of the spinning samples, all NMR experiments were measured under active cooling. The sample temperature was main-tained at 298 K, and the temperature calibration was performed on
Pb(NO3)2 using a calibration procedure described in the literature
(36). Dried sample was packed into ZrO2 rotors and subsequently
stored at room temperature. All NMR spectra were processed using the TopSpin 3.5 pl2 software package.
Magnetic characterization
Magnetic susceptibility measurements were performed in the com-mercial Quantum Design magnetic property measurement system
(MPMS3). Lots of small Cs2Ag(Bi:Fe)Br6 crystals with a total mass
of 29.9 mg were loaded into a polycrystalline sample holder, which was fixed in a brass sample rod. Both ZFC and FC procedures were used to check possible magnetic transition in this compound. ESR was performed with a Bruker Elexsys E500 spectrometer operating
at about 9.3 GHz. ESR spectra were recorded in dark. Cs2Ag(Bi:Fe)
Br6 and Cs2AgBiBr6 crystal powder samples were prepared via
col-lecting many randomly oriented small crystals into sealed and evac-uated quartz tubes and placed in an He-flow cryostat.
X-ray single crystallography
The SC-XRD data for Cs2Ag(Bi:Fe)Br6 and Cs2AgBiBr6 were
col-lected at 298 K with graphite monochromated Mo K ( = 0.71073 Å) on a charge-coupled device area detector (Bruker D8 VENTURE and Bruker-SMART), respectively. Data reductions and absorption corrections were performed with the APEX3 suite, SAINT and SADABS software packages, respectively. Structures were solved by a direct method using the SHELXL-97 software package. The non-hydrogen atoms were anisotropically refined using the full-matrix
least-squares method on F2. The details about data collection,
struc-ture refinement, and crystallography are summarized in table S2. SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/ content/full/6/45/eabb5381/DC1
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Acknowledgments: We thank J. Klarbring, S. I. Simak, I. A. Abrikosov, and A. Wildes for the helpful discussions. Funding: This work was financially supported by Knut and Alice Wallenberg Foundation (Dnr KAW 2019.0082), the Swedish Energy Agency (2018-004357 and P43288-1), the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU no. 2009-00971), and the Grant Agency of the Czech Republic (grant GA19-05259S). We acknowledge the Diamond Light Source for access to beamline I19 (MT20805). We also thank L. Saunders and D. Allan for the technical support. L.W. acknowledges the China Scholarship Council (CSC) for the financial support. Work at Argonne (magnetic susceptibility measurements) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Synchrotron radiation experiment was performed at the BL02B2 of SPring-8 with approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2017B0074). We are grateful for the support of the European Community’s Seventh Framework Programme (FP7/2007–2013, grant agreement no. 312284) for the research at the Materials Science Beamline at the Elettra Synchrotron and the CERIC-ERIC Consortium for access to experimental facilities and financial support of the Czech Ministry of Education (LM2015057). G.A.C., M.C., and R.D.M. pay special thanks to N. Tsud and K. C. Prince at Elettra Synchrotron for assistance with the experiments. We also thank J. Bradley for assistance with measurements at the Elettra Synchrotron. M.C. and G.A.C. acknowledge the CERIC users’ grant for travel funding to visit the Elettra Synchrotron, while R.D.M. thanks the International Synchrotron Access Program of the Australian Synchrotron for travel funding to attend the experiment at the Elettra Synchrotron. F.G. is a Wallenberg Academy Fellow. Author contributions: W.N. and F.G. conceived the idea for the manuscript and designed the experiments. W.N. and F.J. developed the synthesis procedures and performed the basic chemical and physical characterization. J.B. performed the SQUID measurements and analyzed the results under the supervision of D.Y.C. and M.G.K. Y.P. and F.M. measured and analyzed the ESR results under the supervision of W.M.C. and I.A.B. L.K., S.A., and J.B. measured and
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analyzed the ssNMR results. S.S., S.K., H.I., and Y.K. helped with the specific heat and SPD measurements. L.W. performed the SC-XRD measurement and analysis under the supervision of L.S. M.C., R.D.M., and G.A.C. performed the NEXAFS measurements. W.N. wrote the paper with the contributions from the coauthors. F.G. supervised the project. All authors commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
Submitted 3 March 2020 Accepted 16 September 2020 Published 6 November 2020 10.1126/sciadv.abb5381
Citation: W. Ning, J. Bao, Y. Puttisong, F. Moro, L. Kobera, S. Shimono, L. Wang, F. Ji, M. Cuartero, S. Kawaguchi, S. Abbrent, H. Ishibashi, R. De Marco, I. A. Bouianova, G. A. Crespo, Y. Kubota, J. Brus, D. Y. Chung, L. Sun, W. M. Chen, M. G. Kanatzidis, F. Gao, Magnetizing lead-free halide double perovskites. Sci. Adv. 6, eabb5381 (2020).
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Magnetizing lead-free halide double perovskites
Yoshiki Kubota, Jiri Brus, Duck Young Chung, Licheng Sun, Weimin M. Chen, Mercouri G. Kanatzidis and Feng Gao Cuartero, Shogo Kawaguchi, Sabina Abbrent, Hiroki Ishibashi, Roland De Marco, Irina A. Bouianova, Gaston A. Crespo, Weihua Ning, Jinke Bao, Yuttapoom Puttisong, Fabrizo Moro, Libor Kobera, Seiya Shimono, Linqin Wang, Fuxiang Ji, Maria
DOI: 10.1126/sciadv.abb5381 (45), eabb5381.
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