Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH3NH3)2CdX4 (X = Cl, Br, I)

Article abstract of DOI:10.1021/acs.inorgchem.7b01986

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MA₂CdX₄ (MA = CH₃NH₃; X = Cl, Br, I). MA₂CdI₄ is a new compound, whereas, for MA₂CdCl₄ and MA₂CdBr₄, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MA₂CdX₄ solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MA₂CdCl₄ forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MA₂CdBr₄ and MA₂CdI₄ adopt 0D K₂SO₄-derived crystal structures based on isolated CdX₄ tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MA₂CdX₄ family impact their air stabilities, investigated for the first time in this work; MA₂CdCl₄ is air-stable, whereas MA₂CdBr₄ and MA₂CdI₄ partially decompose when left in air. Optical absorption measurements suggest that MA₂CdX₄ have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA₂CdX₄ yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MA₂CdX₄ following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MA₂CdX₄. On the basis of our combined experimental and computational results, MA₂CdX₄ and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.

Full text of DOI:10.1021/acs.inorgchem.7b01986

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Synthesis, Crystal and Electronic Structures, and Optical Properties
of (CH3NH3)2CdX4 (X = Cl, Br, I)
Rachel Roccanova,† Wenmei Ming,§ Vincent R. Whiteside,‡ Michael A. McGuire,§ Ian R. Sellers,‡
Mao-Hua Du,§ and Bayrammurad Saparov*,†
†Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United
States
‡Homer L. Dodge Department of Physics & Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, Oklahoma 73019,
United States
§Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: We report the synthesis, crystal and electronic
structures, as well as optical properties of the hybrid organic−
inorganic compounds MA2CdX4 (MA = CH3NH3; X = Cl, Br, I).
MA2CdI4 is a new compound, whereas, for MA2CdCl4 and
MA2CdBr4, structural investigations have already been conducted
but electronic structures and optical properties are reported here
for the first time. Single crystals were grown through slow
evaporation of MA2CdX4 solutions with optimized conditions
yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I)
crystals. Single crystal and variable temperature powder X-ray
diffraction measurements suggest that MA2CdCl4 forms a 2D
layered perovskite structure and has two structural transitions at
283 and 173 K. In contrast, MA2CdBr4 and MA2CdI4 adopt 0D
K2SO4-derived crystal structures based on isolated CdX4
tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the
MA2CdX4 family impact their air stabilities, investigated for the first time in this work; MA2CdCl4 is air-stable, whereas
MA2CdBr4 and MA2CdI4 partially decompose when left in air. Optical absorption measurements suggest that MA2CdX4 have
large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA2CdX4 yield broad peaks in the 375−
955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped
excitons in MA2CdX4 following the crystal and electronic structure considerations. The bands around the Fermi level have small
dispersions, which is indicative of high charge localization with significant exciton binding energies in MA2CdX4. On the basis of
our combined experimental and computational results, MA2CdX4 and related compounds may be of interest for white-light-
emitting phosphors and scintillator applications.
connectivity (e.g., edge-sharing, face-sharing), among others.5
1. INTRODUCTION
These future goals are also relevant for solar cell research due to
the fact that the state-of-the-art photovoltaic (PV) material,
In recent years, hybrid organic−inorganic halide materials have
emerged as excellent candidates for optoelectronic applications
including solar cells,1 light-emitting diodes (LED), lasers,2 and
MAPbI3, suffers from Pb toxicity and poor stability in air, and
their Sn-based alternatives have also proven to be air-sensitive.6
Therefore, studies of non-Pb/Sn hybrid halide compositions
are primarily fueled by the desire to make air-stable, Pb-free
solar cells, and so far has mostly focused on Sb- and Bi-based
compositions.7−10
photodetectors.3−5 In addition to their outstanding optoelec-
tronic properties, low temperature solution processability and
broad tunability of their chemical compositions, crystal, and
electronic structures5 make hybrid halides the focus of global
attention. Among hybrid materials, the perovskite MAPbI3
(MA = CH3NH3) and its other Pb and Sn derivatives are the
most extensively studied types due to their performance in high
efficiency solar cells.1 The grand challenges of hybrid organic−
inorganic materials research5 include the exploration of
compositions beyond the dominant Pb- and Sn-based systems,
the systematic study of compounds that are based on different
types of polyhedra (e.g., tetrahedral building blocks), and
Considering the compositional and structural tunability of
hybrid halide A−B−X materials (A = organic cation, B = metal
cation, X = halide anion), in principle, a variety of substitutions
can be carried out targeting each of A, B, and X sites. Variation
Received: August 3, 2017
© XXXX American Chemical Society
A
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Article
of the organic cation size via A-site substitution allows control
of the material’s dimensionality, which can be freely varied from
0D cluster compounds to 3D networks.5 In addition to the size
effects, substitutions of the B and X sites allow fine-tuning of
electronic structure and optoelectronic properties since the B
and X element states are the dominant contributors to the
states around the Fermi level.5 Through control of structural
dimensionality and chemical compositions, one can design
organic−inorganic halides for specific applications including
materials for lighting11 and scintillator applications.12−16 For
impurity-doped scintillators, the design of which are targeted in
this work, large band gaps (e.g., > 5 eV for NaI:Tl+ and
LaBr3:Ce3+) are desirable to accommodate dopant levels and to
avoid thermal quenching at room temperature.17
Large band gaps and highly localized charges with remarkably
high exciton binding energies and stable excitonic emission can
be realized in low-dimensional hybrid organic−inorganic
materials.5,18 Briefly, excitons (a bound pair of an excited
electron and a hole) in inorganic semiconductors, called
Wannier excitons, typically have large radii (30−100 Å) and
low binding energies (10−30 meV) on the order of kBT at
room temperature (∼26 meV). Charge transport and emission
in such systems are typically nonexcitonic; i.e., excitons are
broken apart at room temperature into free charge carriers and
the emission from the recombination of the delocalized charges
produces lower light output.17 In contrast, by selectively
combining organic and inorganic components, exciton binding
energies (60−545 meV) and radii (22.9−6.2 Å) can be tuned
over a broad range in hybrid materials.5,18 Decay of stable
excitons via recombination of the electron−hole pair results in
spontaneous light emission, which can be used for lighting and
high-light-yield scintillator applications. To obtain large band
gap materials with high exciton binding energies and localized
charges, a low electronegativity B metal element, such as Cd
(1.69),19 can be paired with halogen elements in hybrid
organic−inorganic halides.20
Here, we report a systematic study of prospective scintillator
materials MA2CdX4 (X = Cl, Br, I) including their low
temperature solution synthesis, variable temperature single
crystal and powder X-ray diffraction studies, band structure
calculations, and optical properties. Several studies focusing on
crystal structures of MA2CdCl4 and MA2CdBr4 have been
previously reported,21−23 whereas MA2CdI4 is a new compound
reported here for the first time. Despite the compositional
similarity, the crystal structures of MA2CdX4 differ from each
other with MA2CdCl4 adopting a ⟨100⟩-oriented 2D layered
perovskite structure, while MA2CdBr4 and MA2CdI4 crystallize
in 0D K2SO4-derived structures built upon isolated CdX4
tetrahedra. MA2CdCl4 undergoes two structural transitions
upon cooling from room temperature down to 20 K, consistent
with the earlier reports,21,22 whereas MA2CdBr4 and MA2CdI4
do not have phase transitions in the measured range. According
to density functional theory (DFT) band structure calculations,
MA2CdX4 have direct band gaps. The measured optical
absorption data suggest band gaps larger than 3.9 eV with
onsets of absorption at 5.29 eV for MA2CdCl4, 4.92 eV for
MA2CdBr4, and 3.94 eV for MA2CdI4. The valence and
conduction bands feature flat bands, suggesting a high degree of
charge localization that allows for the formation of stable
excitons in MA2CdX4. Room temperature photoluminescence
(PL) measurements give broad PL peaks within the 375−955
nm range with full width at half-maximum (fwhm) values up to
208 nm. We discuss the potential of MA2CdX4 for optical
applications based on our combined experimental and
theoretical work.
2. EXPERIMENTAL SECTION
2.1. Reactants. Chemicals used in this study were either used as
purchased or synthesized from the starting materials listed: (i)
methylammonium hydrochloride, 99%, Sigma-Aldrich; (ii) methyl-
amine solution, 40 wt % in H2O, Sigma-Aldrich; (iii) hydrobromic
acid, 48 wt % in H2O; (iv) hydroiodic acid, 57 wt % in H2O; (v)
cadmium chloride, 99.99%, Sigma-Aldrich; (vi) cadmium bromide,
98%, Alfa Aesar; (vii) cadmium iodide, 99.999%, Alfa Aesar; (viii)
absolute ethanol, 200 proof; (ix) diethyl ether, 98%, Alfa Aesar; (x)
benzene, >99.9% HPLC grade, Aldrich; (xi) N,N-dimethylformamide
(DMF), 99%, Fisher; (xii) methanol, reagent grade, Aldrich.
2.2. MABr and MAI Synthesis. MABr and MAI were synthesized
using the methods previously reported in the literature.24 Briefly,
stoichiometric amounts of HBr(aq) and HI(aq) were added dropwise
to cooled (0 °C) methylamine solutions under vigorous stirring and
allowed to mix for up to 1.5 h. After removing excess solvent under
vacuum at 60 °C, the powder product was redissolved in ethanol to
make a slurry from which white powder products of MABr and MAI
were precipitated by adding excess diethyl ether. The resulting crystals
were washed with 200 proof ethanol and benzene, then dried under
vacuum overnight under an inert atmosphere. Typical yields of these
reactions ranged from 45% to 65%.
2.3. Bulk MA2CdX4 Synthesis and Single Crystal Growth.
MA2CdCl4, MA2CdBr4, and MA2CdI4 crystals were all grown through
slow evaporation of their stoichiometric (2:1 molar ratio of MAX to
CdX2) solutions. All solution synthesis and crystal growth experiments
were carried out in small vials (2−20 mL) at around room temperature
(20−65 °C). Initially, various solvent systems including pentane,
dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dimeth-
yl sulfoxide (DMSO), water, and methanol were tried for crystal
growth. The best quality crystals resulted from a 1 M MA2CdCl4 water
solution, and 1 M MA2CdBr4 and MA2CdI4 methanol solutions. The
MA2CdCl4 solution readily formed colorless plates, whereas the
MA2CdBr4 and MA2CdI4 formed aggregations of small colorless and
pale yellow blocks, respectively (sub-mm crystal sizes). Crystals were
also grown at an accelerated rate by evaporating saturated solutions on
low heat (between 40 and 65 °C) in ambient air; however, the quality
of crystals produced from fast growth were generally poorer compared
to the ambient temperature growth described above. Large single
crystals of MA2CdCl4 were grown using 1 M DMF solutions under
ambient conditions in crystallization dishes and by applying low
vacuum to a solution enclosed in a 5 mL vial over 2 weeks. In this
method, the crystal sizes were limited by the container dimensions
with the largest crystal size of 4 × 3 × 2 mm3 (Figure S1).
Elemental analysis for (CH3NH3)2CdI4: calculated C 3.51, H 1.77,
N 4.09, I 74.19%; found C 3.55, H 1.69, N 4.03, I 73.97%.
2.4. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD)
measurements were performed on a Rigaku MiniFlex600 system with a
D/tex detector using Ni-filtered Cu−Kα radiation. Typical scans were
done to determine phase identity and purity and were performed in
the 3−90° (2θ) range, with a step size of 0.02°. Data analysis was
performed using Rigaku’s PDXL software package. The collected data
were fitted using the decomposition method (also called Pawley
fitting) embedded in the PDXL package. Noticeable wetting of the
MA2CdI4 powdered samples occurred when samples were left out in
the air for more than 1 day, whereas moisture instability of the other
analogues was not observed over the same period. However, as a
precaution, powder samples were stored in nitrogen flushed containers
or in a N2-filled glovebox.
For controlled air stability studies, MA2CdX4 thin films prepared by
spin-coating 50 μL of 1 M methanol solutions were left in ambient air
on a laboratory bench for a period of 2 weeks. During this time, PXRD
measurements were regularly performed using the conditions
described above. In order to distinguish between oxygen and moisture
stability of MA2CdX4, powder and thin-film samples were also left in a
dry air chamber fabricated from a desiccator containing Drierite
B
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Table 1. Selected Room Temperature Single Crystal Data Collection and Refinement Parameters for (CH3NH3)2CdCl4,
(CH3NH3)2CdBr4, and (CH3NH3)2CdI4
(CH3NH3)2CdBr4
496.18
(CH3NH3)2CdI4
684.14
(CH3NH3)2CdCl4
318.34
(CH3NH3)2CdBr4
496.18
formula weight (g/mol)
temperature (K)
298 (2)
Mo Kα, 0.71073
orthorhombic
100 (2)
Mo Kα, 0.71073
radiation, wavelength (Å)
crystal system
monoclinic
P21/c, 4
monoclinic
P21/c, 2
space group, Z
Pbca, 8
P21/c, 4
unit cell parameters
a = 8.1257(5) Å
b = 13.4317(8) Å
c = 11.4182(7) Å
β = 96.1840(10)°
1238.95(13)
2.660
a = 10.9692(14) Å
b = 12.1583(15) Å
c = 20.861(3) Å
a = 10.375(4) Å
b = 7.424(3) Å
c = 7.349(3) Å
β = 110.829(5)°
529.1(4)
a = 8.0734(8) Å
b = 13.2215(13) Å
c = 11.3649(11) Å
β = 96.082(2)°
1206.3(2)
volume (Å3)
2782.2(6)
3.267
density (ρcalc) (g/cm3)
absorption coefficient (μ) (mm−1
θmin−θmax (deg)
1.998
2.732
)
14.609
10.400
3.010
15.005
2.3491−26.9605
27253
1.95−26.44
20954
4.04−27.25
13540
2.3708−30.8818
25703
reflns collected
independent reflns
7755
2408
1190
2777
a
R
indices (I > 2σ(I))
R1 = 0.0293
wR2 = 0.0653
1.031
R1 = 0.0524
wR2 = 0.1488
1.041
R1 = 0.0726
wR2 = 0.1750
1.063
R1 = 0.0208
wR2 = 0.0481
1.076
goodness-of-fit on F2
largest diff. peak and hole (e−/Å3)
1.995 and −1.625
1.778 and −1.821
2.519 and −2.035
0.764 and −0.940
a
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = |∑|w(F2o − Fc2)2|/∑|w(Fo2)2||1/2, where w = 1/|σ2Fo2 + (AP)2 + BP|, with P = (Fo2 + 2F2c)/3 and weight coefficients A
and B.
Figure 1. Polyhedral views of the crystal structures of (a) (CH3NH3)2CdCl4, (b) (CH3NH3)2CdBr4, and (c) (CH3NH3)2CdI4. Orange, blue, red,
gray, and black spheres represent Cd, halogen (Cl, Br or I), N, H, and C atoms, respectively. For clarity, only a fraction of methylammonium ions are
shown in (b) and (c).
desiccant, and periodically tested for O2 stability. For moisture stability
studies, a vacuum desiccator was used to remove ambient air and
introduce dry nitrogen gas into the chamber with 2 mL of distilled
water to induce a humid N2 environment.
empirical method based on equivalents, and the structures were solved
by direct methods using the SHELXTL program and refined by full
matrix least-squares on F2 by use of all reflections.25,26 All non-
hydrogen atoms were refined with anisotropic displacement
parameters and all occupancies within two standard deviations,
whereas all hydrogen atom positions were determined by geometry.
Details of the crystallographic results are given in Table 1. Atomic
coordinates, equivalent isotropic displacement parameters, and
selected interatomic distances and bond angles are provided in Tables
S1−S8. Further details of the crystal structure studies are summarized
in the CIF (Crystallographic Information File) files and have been
deposited in The Cambridge Crystallographic Data Centre (CCDC)
and can be found under deposition numbers 1563635−1563638.
2.6. UV−vis Absorbance Measurements. Optical absorption
measurements were carried out on an HP 8452A Diode Array UV Vis
Visible Spectrophotometer over a 190−820 nm range. For absorption
measurements, thin films of MA2CdX4 were prepared by spin-coating
50 μL of 1 M methanol solutions of the respective MA2CdX4 on
cleaned UV quartz slides for 30 s at 2000 rpm using the Laurell model
WS-650MZ-23NPPB spin processor.
Variable temperature powder X-ray diffraction measurements were
carried out using an Oxford PheniX closed-cycle cryostat on a
PANalytical X’pert Pro MPD diffractometer using Cu Kα1 radiation
with an X’celerator position sensitive detector. Partial decomposition
of the MA2CdBr4 and MA2CdI4 samples used for these measurements
was observed. Therefore, low temperature diffraction results for these
compounds were analyzed by fitting the target phase component only.
2.5. Single Crystal X-ray Diffraction. The X-ray intensity data
for MA2CdX4 were collected on a Bruker Apex CCD area detector
diffractometer with graphite-monochromated Mo−Kα (λ = 0.71073
Å) radiation. For low temperature measurements, crystals were cut to
suitable sizes in a DOW Chemical vacuum grease and cooled under a
stream of nitrogen to 100(2) K. For room temperature measurements,
crystals were selected from their mother liquors, covered with Super
Glue, and affixed to the goniometer head. Once dry, the measurements
were conducted at room temperature, 298(2) K. The crystals
structures for all analogues were determined from a nonlinear least-
squares fit. The data were corrected for absorption by the semi-
2.7. Photoluminescence Measurements. Photoluminescence
(PL) measurements were conducted over a spectral range from 360 to
C
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Figure 2. Powder X-ray diffraction (PXRD) patterns (black dots) for (CH3NH3)2CdCl4 at (a) 300 K, (b) 200 K, and (c) 100 K, showing successive
phase transitions upon cooling. The Pawley fits and difference plots are shown as red and blue dotted lines, respectively.
factors5,35−37 as many of the compositions cited above fall
1025 nm using a He−Cd laser set at an excitation wavelength of 325
nm using a silicon CCD detector. Samples were spin-coated on
within the acceptable tolerance and octahedral factor ranges.
cleaned UV quartz slides and allowed to dry in ambient conditions as
For example, tolerance factors of MACdCl3, MACdBr3, and
detailed above. PXRD was performed on the samples to verify phase
MACdI3 compositions are 1.02, 1.00, and 0.98, respectively,
purity both before and after the PL measurements.
and their octahedral factors are 0.50, 0.48, and 0.43,
respectively. A recent comprehensive analysis of the formability
2.8. Electronic Structure Calculations. Our calculations were
based on density functional theory (DFT) with the Perdew−Burke−
Ernzerhof (PBE) exchange-correlation functional27 as implemented in
the plane-wave basis VASP code.28 The projector augmented wave
method was used to describe the interaction between ions and
electrons.29 The kinetic energy cutoff was 520 eV. The lattice
parameters were fixed at the experimentally measured values while the
atomic positions were optimized until the force on each atom was less
than 0.01 eV/Å. The k-point meshes of 4 × 4 × 1, 3 × 2 × 2, and 3 ×
2 × 1 were used for structural optimization of MA2CdCl4, MA2CdBr4,
and MA2CdI4, respectively, while denser k-point meshes of 8 × 8 × 1,
8 × 4 × 4, and 6 × 4 × 1 were used for the calculations of density of
states (DOS). Spin−orbit coupling (SOC) was also included in these
calculations.
of perovskite structures suggest that acceptable ranges of the
tolerance and octahedral factors for the formation of 3D
perovskites are 0.8−1.1 (t) and 0.44−0.895 (μ).5,37 Therefore,
in principle, 3D perovskites MACdCl3, MACdBr3, and MACdI3
should be accessible; however, MACdCl3 and MACdI3 have
not been reported yet, whereas MACdBr3 crystallizes in the
BaNiO3-derived 1D chain structure.38 It should be noted that
the calculated tolerance and octahedral factors for NH4CdCl3
are also within the acceptable range with t = 0.84 and μ = 0.50.
This analysis shows the limitations of the crystal structure
predictions based on the tolerance and octahedral factors alone.
Notwithstanding the fact that cadmium halides with 3D
perovskite structures are yet to be reported, A2CdCl4 (A = Rb,
Cs) compounds are known to adopt layered perovskite derived
K2NiF4-type structures.38,39 In contrast, Cs2CdBr4 and Cs2CdI4
crystallize in the K2SO4-type structure featuring 0D structures
3. RESULTS AND DISCUSSION
3.1. Room Temperature Crystal Structures. Given the
fact that MA2CdCl4 and MA2CdBr4 structures have already
been reported in the literature,2,3 the major focus of our
structural work was on the new compound MA2CdI4; however,
during our studies, we also complemented the literature
reported results for MA2CdCl4 and MA2CdBr4 with further
work (including variable temperature X-ray diffraction studies,
see below). A summary of the room temperature single crystal
X-ray diffraction studies of these compounds is provided in
Table 1 and Tables S1−S4, and the corresponding crystal
structures are depicted in Figure 1. Although the compounds in
this family are isoelectronic, their structures are markedly
different. The crystal structure of MA2CdCl4 features 2D
inorganic [CdCl4]2− layers built upon corner-sharing CdCl6
octahedra separated by organic cation layers. Consequently,
MA2CdCl4 belongs to the large family of ⟨100⟩-oriented
layered hybrid organic−inorganic perovskites.5 In contrast, the
0D crystal structures of MA2CdBr4 and MA2CdI4 feature
2−
with isolated tetrahedral CdX4 anions and cations filling the
voids in between.30 Similarly, in the MA2CdX4 series,
MA2CdCl4 is a perovskite derivative, whereas MA2CdBr4 and
MA2CdI4 structures can be envisioned as derived from the
K2SO4 structure through replacing K+ with MA+. The exact
2−
arrangement of CdX4
anions and organic cations in
MA2CdBr4 and MA2CdI4 differ, leading to different space
groups for the two compounds. Thus, MA2CdBr4 is
40−42
isostructural with MA2HgBr4
and crystallizes in the
monoclinic space group P21/c, whereas MA2CdI4 crystallizes
in the orthorhombic space group Pbca.
Structural distortions of hybrid materials not only impact
their formability and stability but also have a major influence on
their optoelectronic and semiconducting properties.5 Since
divalent Cd2+ in MA2CdX4 has a 5s0 4d10 electronic
configuration, distortions of the B-metal halide polyhedra due
to a stereochemically active lone pair, which regularly occurs for
Pb- and Sn-based compounds,5 is not expected. Indeed, the
CdBr4 and CdI4 tetrahedra are almost regular with the Br−Cd−
Br and I−Cd−I tetrahedral angles ranging from 105.17(3)° to
112.16(2)° and 106.78(5)° to 115.88(5)° (Tables S2, S5, S7,
S8). The narrow deviation from the ideal tetrahedral angle can
be attributed to the distortions of the CdX4 units caused by
hydrogen bonding interactions, another major source of
structural distortions in hybrid organic−inorganic perovskites.5
In the case of MA2CdCl4, the reported room temperature
structure (space group Cmca) also features nearly regular
2−
2−
isolated CdBr4 and CdI4 anions separated by MA+ cations.
Interestingly, to the best of our knowledge, there is no report
of a parent 3D hybrid organic−inorganic perovskite compound
based on cadmium halides. The reported structures of inorganic
and hybrid organic−inorganic halides based on cadmium are
predominantly of the NH4CdCl3-type, which features 1D
double chains made of edge-sharing CdCl6 octahedra.30
Examples of cadmium halides adopting the NH4CdCl3-type
structure include ACdCl3 (A = K, Rb, Tl, NH4) and ACdBr3 (A
= Rb, In, NH4).24,31−34 These observations cannot be simply
explained using either the tolerance (t) or octahedral (μ)
D
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CdCl6 octahedra with the Cl−Cd−Cl angles of 89.54° and
90.46°.21
7.424(3) Å compared to the interlayer parameter c = 19.37(1)
Å in the previous report (Figure S2).22
3.2. Variable Temperature X-ray Diffraction Studies.
Hybrid organic−inorganic materials notoriously undergo
successive phase transitions, especially upon cooling, which
are often associated with the motion of organic cations,
hydrogen bonding interactions, and subsequent distortions of
the inorganic substructure.5,22,43 Since structural distortions and
phase transitions have a strong influence on the resultant
semiconducting, optoelectronic, and excitonic properties of the
hybrid materials,5 in this section, we describe the results of our
variable temperature X-ray diffraction experiments on
MA2CdX4.
MA2CdCl4. Variable temperature structural studies have
already been conducted on MA2CdCl4 by Chapuis et al.21,22
MA2CdCl4 undergoes three phase transitions: (1) at 484 K
from the high temperature tetragonal I4/mmm phase to the
orthorhombic Cmca phase, (2) at 283 K from the Cmca phase
to the tetragonal P42/ncm phase, and (3) at 173 K from the
P42/ncm phase to the monoclinic P21/a phase.2,3 Our variable
temperature powder X-ray diffraction measurements (Figures 2
and 3) on MA2CdCl4 have largely confirmed these findings,
MA2CdBr4 and MA2CdI4. Several structural studies have been
previously reported for MA2CdBr4 utilizing X-ray diffraction,
differential scanning calorimetry (DSC), infrared spectroscopy,
1H nuclear magnetic resonance (NMR), and nuclear quadru-
pole resonance (NQR).23,44−46 The study conducted by Rao et
al. suggests that structural transitions exist at 167 and 400 K
based on DSC and infrared spectroscopy measurements.44
However, Nakayama et al. reported that there is no evidence of
a phase transition from 4.2 to 300 K.45 Given the discrepancy in
literature regarding the low temperature phase transitions in
MA2CdBr4, we performed variable temperature PXRD and
SXRD experiments (Table 1, Figures 4a and 5a). Our results
Figure 4. Room temperature PXRD results for (a) (CH3NH3)2CdBr4
and (b) (CH3NH3)2CdI4. Pawley fits are shown in red, which overlap
the gray observed spectra, and the difference maps are in blue.
Figure 3. Variation in cell volume with temperature based on variable
temperature PXRD and SXRD results shown in black squares and red
dots, respectively. The observed phase changes are denoted by a
dashed line with each colored region corresponding to the
orthorhombic (light yellow), tetragonal (light green), and monoclinic
(light blue) phases of (CH3NH3)2CdCl4. In this plot, the monoclinic
phase volumes are doubled to bring them to scale with the tetragonal
and orthorhombic phases of (CH3NH3)2CdCl4.
Figure 5. Temperature dependence of unit cell volume for (a)
(CH3NH3)2CdBr4 and (b) (CH3NH3)2CdI4 based on variable
temperature PXRD and SXRD results, which are shown in black
and gold, respectively.
including the transition temperatures. The transition temper-
atures were determined based upon the appearance of splitting
peaks, especially in the 20−25° range, as shown in the Figure 2
insets. Importantly, the temperature dependence of the unit cell
volume (Figure 3) indicates some deviation from the normal
linear cell contraction with cooling at ∼100 K. This could
indicate a possible missed phase transition at this temperature,
especially considering the structural transitions in the hybrid
materials are often subtle due to slight changes in the atomic
positions of the organic moieties. Furthermore, upon closer
inspection, we noticed that the interlayer distance in the
reported low temperature monoclinic P21/a phase22 is
unusually and unphysically large (Figure S2). To determine
the correct structure of the low temperature form of
MA2CdCl4, we carried out single crystal X-ray diffraction
experiments for MA2CdCl4 and MA2CdBr4 at 100 K, which are
summarized in Tables 1, S3 and S6. As expected, the correct
structure refined in the space group P21/c has a much smaller
interlayer distance with the interlayer unit cell parameter of b =
further support the findings of Nakayama et al., demonstrating
that there is no evidence of a phase transition in the 20−295 K
range.45 Upon cooling, a regular contraction of the unit cell
volume is observed for MA2CdBr4 (Figure 5a). Interestingly,
Nakayama et al.45 also reported the observation of an anomaly
at 120 K in their NQR readings, which they attributed to
anisotropic thermal expansion of the unit cell. In contrast, we
were not able to confirm any structural anomalies at this
temperature (Figure 5).
Variable temperature measurements were also performed on
MA2CdI4 to probe for possible phase transitions. On the basis
of our measurements, as shown in Figure 5b, MA2CdI4 exhibits
a normal contraction of unit cell volume from 295 to 100 K and
then begins to deviate from 100 to 20 K with an abnormal
increase in cell volume as the temperature is decreased. This
unexpected deviation is not accompanied by structural
transitions at these temperatures based on our PXRD results.
In the literature, hybrid organic−inorganic compounds have
been shown to exhibit anisotropic thermal expansion, zero
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thermal expansion, and negative thermal expansion effects that
deviate from the normal contraction of the cell volume with
cooling.47−49
absorption measurements in the 190−820 nm (1.5−6.5 eV)
range (Figure 6), which narrowly captures the UV region where
3.3. Air and Moisture Stability. Given their highly ionic
nature, some hybrid organic−inorganic halides are unstable in
ambient air, especially in a humid environment.5 In fact, this is a
major shortcoming of the high performance photovoltaic
material MAPbI3,6,7 making air stability studies very important
for the evaluation of prospective optoelectronic applications of
hybrid halides. In this study, air stabilities of spin-coated
MA2CdX4 films, some of which show high preferential
orientation, were studied by exposing them to ambient air for
over 2 weeks (Figure S3). For MA2CdCl4, no observable
change was recorded during the two-week period, whereas
MA2CdBr4 and MA2CdI4 show signs of degradation. In the case
of MA2CdBr4, impurity peaks are clearly noticeable after leaving
the thin film sample out for 1 day in ambient air. These
impurity peaks have successfully been assigned to MACdBr3.50
Interestingly, after a quick partial decomposition, degradation
of MA2CdBr4 slows and no further change has been observed
after exposing the sample to ambient air for 2 weeks. In the case
of MA2CdI4, its thin film shows a higher degree of preferential
orientation after 1 day in air. Note here that powder MA2CdI4
samples show noticeable wetting when left in air overnight;
therefore, the observed change in the degree of preferential
orientation of the MA2CdI4 can be explained by the wetting of
the film and the crystallites reorienting to form a highly
oriented film. Beyond the change in preferential orientation,
after 2 weeks in air, a strong impurity peak is also observed.
However, we were not able to assign this impurity peak to any
known compound in this system.
Figure 6. Absorbance vs energy plots for (CH3NH3)2CdCl4 (black),
(CH3NH3)2CdBr4 (red), and (CH3NH3)2CdI4 (blue). From the linear
fits, the onsets of optical absorption are 5.29 eV for (CH3NH3)2CdCl4,
4.92 eV for (CH3NH3)2CdBr4, and 3.94 eV for (CH3NH3)2CdI4. The
absorbance spectrum of the UV quartz substrate (green) is also
included for comparison.
onsets of absorption appear. From the linear fits of the
absorbance data, the onsets of optical absorption were
determined to be 5.29 eV for MA2CdCl4, 4.92 eV for
MA2CdBr4, and 3.94 eV for MA2CdI4. This trend in the onsets
of optical absorption for halides going from chloride to iodide is
consistent with the trend in electronegativity and calculated
band structures (see below); i.e., with decreasing electro-
negativity of halogen element, the band gap is expected to
decrease.
In order to distinguish between the impacts of oxygen and
water, we also monitored the stabilities of MA2CdX4 samples in
dry air and wet N2 environments (Figures S4 and S5).
MA2CdCl4 and MA2CdI4 exhibited no observable change over
the two-week period in our dry air chamber, whereas
MA2CdBr4 showed clear signs of degradation after 2 weeks as
evidenced by the emergence of MACdBr3 peaks (Figure S4). In
the wet N2 environment, the MA2CdCl4 sample showed a
higher degree of preferential orientation due to moisture
absorption, but as expected, the sample remained largely
unchanged. In contrast, MA2CdBr4 and MA2CdI4 demon-
strated signs of complete degradation and loss of crystallinity
after 1 day within the chamber.
However, we note that the onsets of optical absorption for
MA2CdX4 cannot be directly used to extract band gap
information for hybrid organic−inorganic materials with strong
excitonic features. Indeed, most low-dimensional hybrid
materials exhibit room temperature excitonic features in their
absorption spectra appearing before the features that can be
ascribed to band-to-band transitions.5,53−56 In fact, MA2CdX4
are expected to have a strong charge localization in the
2−
inorganic CdX4 substructures due to their large band gaps
and low dimensional crystal structures (see DFT results
below); therefore, the exact determination of the optical band
gaps of MA2CdX4 cannot be done solely based on our optical
absorbance results.
On the basis of these results, MA2CdX4 compounds have
contrasting air and moisture stabilities. MA2CdCl4 is air- and
moisture-stable, MA2CdBr4 is unstable both in dry air and wet
N2 environments, and the instability of MA2CdI4 is primarily
caused by water. The marked difference in the air stabilities of
these compounds can be attributed to the fact that chlorides are
generally more stable in air compared to bromides and
iodides.51 Additionally, stability has also been shown to be
affected by structural dimensionality;52 the 2D layered structure
of MA2CdCl4 featuring protective organic cation layers could
prevent fast degradation of the material as opposed to the 0D
cluster structures of MA2CdBr4 and MA2CdI4.
3.4. UV−vis Spectroscopy and Photoluminescence.
Cadmium halides are large band gap materials; for example,
CdCl2, CdBr2, and CdI2 are reported to have band gaps of 6.4,
5.4, and 4.3 eV.53 Optical absorption and photoluminescence
measurements for such large band gap materials require
specialized instrumentation that extends into the deep UV
region. For MA2CdX4, we were able to carry out optical
Further inspection of the optical absorption data for
MA2CdI4 reveals a noticeable subgap absorption, which is
reproducible in two different UV−vis spectrometers. The
presence of such weak subgap absorption may be indicative of
defect states in MA2CdI4. Given the fact that the MA2CdI4
band gap is larger than 3.9 eV, MA2CdI4 crystals should be
colorless instead of their actual pale yellow color. Therefore, the
yellow color of MA2CdI4 crystals may be due to the defect-
induced states, which are also responsible for the sub-band gap
absorption.
In the presented absorption data in Figure 6, we were not
able to observe excitonic fine structures or band-to-band
transitions. However, comparisons can be made with the
literature reported data for the parent CdX2 and hybrid
organic−inorganic Cd-based halides. Pollini et al.53 reported
reflectivity and deep UV absorption spectra that show multiple
subgap excitonic features for CdX2 that have been assigned to
excitons formed from halogen p-band holes and Cd 5s band
electrons. The lowest excitonic features are located at 6.05 eV
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Figure 7. Normalized room temperature photoluminescence (PL) spectra of (a) (CH3NH3)2CdCl4, (b) (CH3NH3)2CdBr4, and (c)
(CH3NH3)2CdI4.
for CdCl2, 4.90 eV for CdBr2, and 3.68 eV for CdI2, which
suggest exciton binding energies of 0.35−0.62 eV for CdX2.
Using similar exciton binding energy values for MA2CdX4, the
optical band gaps can be estimated to be 5.3−5.9 eV for
MA2CdCl4, 4.9−5.5 eV for MA2CdBr4, and 3.9−4.5 eV for
MA2CdI4. In the literature, there have been only a few optical
studies of hybrid organic−inorganic Cd-based halides.57,58
Interestingly, one of the few papers reporting optical properties
of hybrid Cd-based halides suggests that the optical band gap of
the layered perovskite compound (C2H5NH3)2CdCl4 is 1.866
eV.57 In another report,58 the same authors report a larger band
gap value of 3.11 eV for the same compound. The discrepancy
and the very low optical band gap values reported in these
studies given the colorless nature of Cd-halides, especially the
chlorides, are likely due to the noticeably poor quality optical
absorption data. Ohnishi et al. reported a more comprehensive
study of (C2H5NH3)2CdCl4 including low temperature (7 K)
optical reflection spectra on single crystals that show the lowest
exciton absorption band at 6.19 eV and optical band gap of 6.8
eV,59 which are values much closer to that observed for our
samples.
Photoluminescence (PL) experiments were performed to
further study the optical properties of MA2CdX4. Each sample
was subjected to an excitation wavelength of 325 nm (3.81 eV),
and the resulting PL spectra are summarized in Figure 7. Since
our source energy is below the band gaps of MA2CdX4, such
measurements are considered subgap excited PL measure-
ments.60−62 On the basis of these preliminary studies, the PL
spectra for MA2CdX4 feature broad peaks centered at ∼2.2 eV
(560 nm) for MA2CdCl4, ∼3 eV (415 nm) for MA2CdBr4, and
∼2.5 eV (500 nm) for MA2CdI4. These broad peaks with full
width at half-maximum (fwhm) values of 0.81 eV (208 nm) for
MA2CdCl4, 0.61 eV (94 nm) for MA2CdBr4, and 0.96 eV (203
nm) for MA2CdI4, are likely composed of several components,
as evidenced by the fine structure clearly noticeable in the
MA2CdCl4 and MA2CdI4 PL spectra. These PL peaks,
especially for the chloride analogue, are reminiscent of the
broadband white-light-emitting hybrid organic−inorganic per-
ovskites including α-(DMEN)PbBr4 (DMEN = 2-
(dimethylamino)ethylamine), which has an ∼550 nm peak
with an fwhm of 183 nm,63 (N-MEDA)[PbBr4] (N-MEDA =
N1-methylethane-1,2-diammonium), which has a PL peak
maximum at 558 nm and an fwhm of 165 nm,64 and
(EDBE)[PbX4] (EDBE = 2,2′-(ethylenedioxy)bis-
(ethylammonium); X = Cl, Br), which also have PL peaks at
538−578 nm and fwhm values of 208−215 nm.65 For these
reported white-light emitters, the photoluminescence quantum
efficiencies (PLQE) remain low (<10%). We were also able to
observe blue and white-green emission from MA2CdBr4 and
MA2CdI4, respectively, when excited using a 325 nm laser
source at 4 K (Figure S6). However, no visible emission is
observed at room temperature, consistent with the weaker PL
peaks, especially for MA2CdBr. This is attributed to a significant
thermal quenching at room temperature, also observed for
other known cadmium halide materials (see below).
In the literature, extensive luminescence studies of pure
CdX2 and their halide solid solutions (e.g., CdCl2−CdBr2) have
been carried out, however, mostly at low (liquid nitrogen and
liquid helium) temperatures due to the fact that emissions are
much weaker at room temperature because of thermal
quenching effects.66−72 According to these reports, CdX2
exhibit strong self-trapped exciton (STE) luminescence
observed in the 2−3.8 eV range, with exact peak positions
dependent on the measurement temperature and excitation
source. Self-trapped excitons in ionic crystals form due to lattice
distortions caused by optically created excitons, which are
subsequently trapped at the distortions they create.69 Decay
time studies indicate that the observed STE’s in CdX2 can be
both spin-singlet and spin-triplet in nature.71,72 Interestingly,
the STE luminescence in CdX2 halides has been observed both
using above the gap excitation and subgap excitation. For
example, when excited with 3.53 eV light, melt grown single
crystals of CdI2 showed 3 emission bands at 2.12, 2.43, and 3.36
eV at 2 K.68 Emission spectra of hybrid Cd-based halides are
largely similar to that of CdX2; for example, (C2H5NH3)2CdCl4
has an emission at 2.50 eV with an fwhm value of 0.66 eV when
excited into the excitonic absorption range at 11 K.73
Temperature dependence of emission intensity studies showed
that the 2.50 eV band decays rapidly above 50 K due to thermal
quenching.73 Drawing parallels to these studies, we tentatively
assign the observed PL peaks for our MA2CdX4 compounds to
self-trapped excitons. The exact mechanism and nature of
emission in MA2CdX4, however, warrant further scrutiny
through time-resolved, temperature- and power-dependent PL
studies.
3.5. Band Structure Calculations. Figure 8 shows the
calculated band structures and DOS of MA2CdCl4 (RT phase),
MA2CdBr4, and MA2CdI4. All three halides have direct band
gaps at the Γ point. The calculated band gaps of MA2CdCl4,
MA2CdBr4, and MA2CdI4 are 3.41, 3.46, and 2.87 eV,
respectively, which are expected to be underestimated due to
the well-known band gap error in DFT. Typically, the band gap
of the chloride should be significantly larger than that of the
bromide with common cations due to the higher electro-
negativity of Cl. Here, the small band gap difference between
MA2CdCl4 and MA2CdBr4 is due to their different crystal
structures, namely, the 2D layered structure of MA2CdCl4 as
compared to the 0D cluster structure of MA2CdBr4. The
significant intralayer coupling of electronic states in the layered
MA2CdCl4 leads to relatively dispersive conduction and valence
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dispersion leads to a narrow Cd-5s-derived conduction band,
which is completely separated from the higher Cd-5p-derived
band, as shown in Figure 8b,c. Similar isolated narrow
conduction bands have been found in double-perovskite halide
materials.74−76 The highly localized electronic states near the
conduction band minimum (CBM) and the valence band
maximum (VBM) promote charge localization, leading to the
formation of electron and hole polarons and self-trapped
excitons (STEs). The formation of STEs with sufficient binding
energies is important for stabilizing excitons and enabling
exciton emission. The PL peaks of MA2CdBr4 and MA2CdI4
(Figure 7), therefore, are likely due to the STE emission. High
light output due to STE emission has been reported in double-
perovskite halides, which also have highly localized states near
the CBM and the VBM.77 Compared to MA2CdBr4 and
MA2CdI4, MA2CdCl4 with its layered structure has relatively
more dispersive electronic bands on the ab plane and flat bands
along the c direction. The localization level of the VBM and the
CBM states is likely still sufficient to stabilize STE and enable
STE emission. This is supported by the strong STE emission
observed in other 2D halide perovskites.63,64,74,78−80
The formation of STEs in 0D MA2CdBr4 or MA2CdI4 may
simply involve the elongation of the Cd−halogen bonds within
an isolated CdBr4 or CdI4 tetrahedral cluster, respectively. As
such, the hole is localized on the four halogen ions while the
electron is localized on the Cd ion within a tetrahedral cluster.
On the other hand, an STE in MA2CdCl4 likely involves the
formation of a Vk center (a commonly observed hole polaron in
halides, which can trap an electron at the nearby cation). The
formation of a Vk center requires strong lattice relaxation, i.e.,
the formation of a Cl2 molecule,74 which should result in a
−
Figure 8. Band structures (left) and density-of-states (DOS) (right)
plots of (a) (CH3NH3)2CdCl4, (b) (CH3NH3)2CdBr4, and (c)
(CH3NH3)2CdI4 calculated using PBE functionals with SOC. Note
that the band gaps are underestimated in PBE calculations.
large Stokes shift and may explain the lower emission energy in
MA2CdCl4 than in MA2CdBr4 and MA2CdI4 (see Figure 7).
4. CONCLUSIONS
bands (see Figure 8a). On the other hand, the 0D compounds
MA2CdBr4 and MA2CdI4 have weak intercluster coupling and
consequently very small band dispersion (see Figure 8b,c),
which widens their band gaps. This observation is consistent
with the reported trend for hybrid perovskites, for which it has
been shown that dimensional reduction leads to larger band
gaps.5
The conduction and the valence bands of the three halides
are dominated by the Cd−halogen hybridization; their primary
features are described below using MA2CdBr4 as an example. In
Figure 8b, the narrow bands near −3 and 4 eV are mainly
derived from the bonding and the antibonding states between
Br 4p and Cd 5s, respectively. The bonding and the
antibonding states between Br 4p and Cd 5p contribute
strongly to the bands between −2 and −1 eV and those above 5
eV, respectively. The MA-related bands are far away from the
band gap, having negligible contribution to chemical bonding.
The above electronic structure features also exist for MA2CdCl4
and MA2CdI4 as shown in Figure 8a,c.
Despite the similarities described above, there are distinct
differences in electronic structures between the 0D compounds
(MA2CdBr4 and MA2CdI4) and 2D MA2CdCl4. It can be seen
in Figure 8 that both the conduction and the valence bands in
0D MA2CdBr4 and MA2CdI4 are much less dispersive than
those in 2D MA2CdCl4 because the electronic states originating
from the Cd−Br(I) bonds in MA2CdBr(I)4 are highly localized
within the CdBr(I)4 clusters, which are spatially separated from
each other by MA molecular cations. The lack of sufficient band
In summary, we report low temperature solution synthesis,
crystal and electronic structures, and optical properties of the
hybrid organic−inorganic MA2CdX4 (X = Cl, Br, I). High
quality single crystals were obtained through slow evaporation
of DMF, water, and methanol solutions of MA2CdX4 with
optimized conditions, i.e., slow evaporation at room temper-
ature, yielding mm-sized colorless (X = Cl, Br) and pale yellow
(X = I) crystals. In future studies, large single crystal growth
(>1 cm in all dimensions) will be necessary to accurately assess
scintillation properties of MA2CdX4. Single crystal and variable
temperature powder X-ray diffraction measurements confirm
the literature reported 2D layered perovskite structure for
MA2CdCl4, which undergoes two phase transitions at 283 and
173 K. In contrast, MA2CdBr4 and MA2CdI4 adopt 0D K2SO4-
derived crystal structures based on isolated CdBr(I)4
tetrahedra, and show no phase transition down to 20 K. The
contrasting crystal structures and chemical compositions in the
MA2CdX4 family impact their air stabilities; the 2D layered
compound MA2CdCl4 is air-stable, whereas the 0D cluster
compounds MA2CdBr4 and MA2CdI4 partially decompose
when left in air for a two-week period.
Our band structure calculations show that the states around
the Fermi level are dominated by the bands from Cd and
halogen elements in MA2CdX4, consistent with the simple
charge counting according to (MA+)2(Cd2+)(X−)4. Thus, the
upper valence band (VB) is primarily composed of halogen p
states with a small contribution from the full Cd 4d states,
whereas the bottom of the conduction band (CB) is
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predominantly made of Cd 5s states. Because of the large
differences in the electronegativities of Cd and halogen
elements, MA2CdX4 have large optical band gaps above 3.9
eV based on optical absorption experiments.
ACKNOWLEDGMENTS
■
We thank Dr. Douglas R. Powell for his help with the single
crystal X-ray diffraction measurements. B.S. thanks Oak Ridge
Associated Universities for the Ralph E. Powe Junior Faculty
Enhancement Award. Additional financial support for this work
was provided by the University of Oklahoma startup funds and
by a grant from the Research Council of the University of
Oklahoma Norman Campus. Low temperature powder
diffraction (M.A. McGuire) and theoretical calculations (W.
Ming and M.-H. Du) at ORNL were supported by the US
Department of Energy, Office of Science, Basic Energy
Sciences, Materials Sciences and Engineering Division.
Interestingly, MA2CdX4 are potential broadband white-light-
emitting materials with photoluminescence (PL) peaks
centered at ∼560 nm with fwhm = 208 nm for MA2CdCl4,
415 nm with fwhm = 94 nm for MA2CdBr4, and 500 nm with
fwhm = 203 nm for MA2CdI4. These PL peaks are tentatively
assigned to self-trapped excitons (STEs) in MA2CdX4 using an
analogy to the related CdX2 and (C2H5NH3)2CdCl4. This view
is further supported by the results of the DFT calculations,
which indicate that the bands in VB and CB have small
dispersions, suggesting high charge localization with significant
exciton binding energies in MA2CdX4. The degree of charge
localization is also impacted by the structural dimensionality;
i.e., the band structure of the 2D layered compound MA2CdCl4
features flat bands in the interlayer direction but more
dispersive bands along the intralayer direction, whereas the
0D cluster compounds MA2CdBr4 and MA2CdI4 have a much
stronger charge localization. On the basis of both experimental
and computational results, therefore, the 0D compounds
MA2CdBr4 and MA2CdI4 could be promising candidates as
self-activated scintillators, whereas the 2D layered perovskite
MA2CdCl4 may require doping with an activator such as Ce3+
or Eu2+. In conclusion, the presented results in this work
including the ease of synthesis using inexpensive low
temperature solution methods, single crystal growth, large
band gaps, and highly localized charges indicate that MA2CdX4
and related compounds may be of interest for white-light-
emitting phosphors and scintillator applications.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01986.
Tables with atomic coordinates, interatomic distances,
and bond angles; crystal structure figures; and powder X-
ray diffraction patterns (PDF)
Accession Codes
CCDC 1563635−1563638 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
̈
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: saparov@ou.edu. Phone: 405-325-4836. Fax: 405-325-
6111.
ORCID
Michael A. McGuire: 0000-0003-1762-9406
Mao-Hua Du: 0000-0001-8796-167X
Bayrammurad Saparov: 0000-0003-0190-9585
Notes
The authors declare no competing financial interest.
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