Hydrogen Bonding Controls the Structural Evolution in Perovskite-Related Hybrid Platinum(IV) Iodides

Article abstract of DOI:10.1021/acs.inorgchem.8b01597

We describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A₂PtI₆, where A is one of the following amines: ammonium, NH₄⁺ methylammonium, CH₃NH₃⁺ formamidinium, CH(NH₂)₂⁺ guanidinium, C(NH₂)₃⁺. These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A₂PtI₆ ( indicates a vacant site) structure in the K₂PtCl₆ archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA)₂PtI₆, crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl₃ structure: the K₂MnF₆ structure. A combination of solid-state ¹⁹⁵Pt and ¹H NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. The calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I-Pt-I connectivity.

Full text of DOI:10.1021/acs.inorgchem.8b01597

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydrogen Bonding Controls the Structural Evolution in Perovskite-
Related Hybrid Platinum(IV) Iodides
Hayden A. Evans,†,‡ Douglas H. Fabini,‡,§ Jessica L. Andrews,† Mitchell Koerner,⊥
Molleigh B. Preefer,†,‡ Guang Wu,† Fred Wudl,§ Anthony K. Cheetham,‡,∥ and Ram Seshadri*,†,‡,§
‡
§
⊥
†Department of Chemistry and Biochemistry, Materials Research Laboratory, Materials Department, and Physics Program,
College of Creative Studies, University of CaliforniaSanta Barbara (UCSB), Santa Barbara, California 93106, United States
∥Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
S
* Supporting Information
ABSTRACT: We describe the solid-state structural evolution in four
hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly
important role that extended hydrogen bonding plays in directing the
structure across the series. The compounds are A2PtI6, where A is one of the
+
+
following amines: ammonium, NH4 ; methylammonium, CH3NH3 ;
+
+
formamidinium, CH(NH2)2 ; guanidinium, C(NH2)3 . These are closely
related in structure and properties to the hybrid halide perovskites of
lead(II) that have recently established their prowess in optoelectronics. The
first three of these compounds crystallize in the vacancy-ordered double
perovskite A2Pt□I6 (□ indicates a vacant site) structure in the K2PtCl6
archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA)2PtI6, crystallizes in a
1
vacancy-ordered variant of the hexagonal CsNiCl3 structure: the K2MnF6 structure. A combination of solid-state 195Pt and H
NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the
hydrogen-bonding tendencies. The calculations and measured optical properties support the surprising observation in these
iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I−Pt−
I connectivity.
atoms and vacancies □ (usually) display 3D rock-salt-like
INTRODUCTION
■
ordering. The aristotype for this structure type is K2PtCl6 in
The AMX3 perovskite crystal structure is unique in that it is the
simplest and only ternary structure type that has near-180°
extended M−X−M interactions in all three dimensions. The
extended interactions result in disperse bands that can enable a
host of interesting functional properties in perovskites. Halide
perovskites with the formula AMX3, where A is an alkali-metal
or small organic cation, M is a divalent main group metal, and
X are halides, are one such material family combining
impressive optoelectronic properties with the remarkable
ease of preparation.1−4 Thin-film, single-junction photovoltaic
devices of these perovskite materials have exhibited upward of
22% power conversion efficiency, owing to a direct-band-gap
electronic structure, inherently long-lived charge carriers, and
high defect tolerance of the perovskite active material.5
Halide perovskites, like their oxide counterparts, are a
diverse class of materials that can tolerate many different kinds
of chemical substitution.6,7 This includes A-site substitution by
different ions to obtain layered variants.8,9 The substitution of
divalent M by a trivalent ion such as BiIII can result in a
vacancy-ordered perovskite such as K3Bi2□I9, where every
third M is missing, indicated by a vacancy □.10 Forking the
charge on the divalent M site through substitution with mono-
and trivalent cations results in double perovskites or
elpasolites.11−15 Finally, substitution on the M site with a
tetravalent ion creates vacancy-ordered perovskites where M
the Fm3
̅
m space group.16 It is interesting to note that M-site
vacancy-ordered perovskites are rare or nonexistent in the
oxide perovskite world, even when permitted by the perovskite
tolerance factor (for example, the hypothetical compound
La2W□O6), as a consequence of the cost of oxide−oxide
repulsions and of stabilizing high cation charge states in
isolated species.
Halide compounds with the K2PtCl6 composition have been
known for a long time. They have been examined as
diamagnetic hosts (A2PtI6) of paramagnetic ions (Ir4+, Re4+,
and Os4+)17 and studied for their interesting lattice dynamics
associated with the free rotation of isolated MX6 octahedra.18
A comprehensive review by Armstrong on A2MX6 materials (A
= NH4, K, Rb, Cs; M = Pt, Ir, Os, Re, W; X = Cl, Br, I)
provides an overview.17 The structure type is also associated
with charge disproportionation of the M-site metal, as seen for
Cs2SbCl6, which displays mixed-valence SbIII and SbV atoms
instead of what would appear to be SbIV.19 Hybrid A2MCl6 and
A2MBr6 compounds were previously studied for how organic
cations, predominantly methylammonium in (MA)2MX6
compounds (M = Sn, Pt, Te; X = Cl, Br, I)20−24 or
Received: June 8, 2018
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
the reducing stabilizer was not used) is thought to be responsible for
the oxidation of Pt2+ to Pt4+.
dimethylammonium in (DMA)2MX6 compounds (M = Sn, Pt,
Te, Se; X = Cl, Br),25−27 impact structural phase transitions
through molecular reorientation.
(MA)2PtI6 was prepared by combining 71.70 mg of CH3NH3I
(0.451 mmol) with 58.9 mg (0.225 mmol) PtCl2 (Strem, 99.9%) in
15.5 g of 57% (w/w) hydroiodic acid. This solution was brought to
gentle reflux for 25 min, slowly cooled to room temperature, and
placed in a freezer overnight. The solution was then vacuum-filtered
cold, and the isolated crystals were washed with diethyl ether. Isolated
crystals were vacuum-dried overnight. Dark-bronze-colored, hexago-
nal-plate-habit crystals were isolated.
(FA)2PtI6 was prepared by combining 130 mg (0.760 mmol) of
FAI with 100 mg (0.375 mmol) of PtCl2 (Strem, 99.9%) in 15.5 g of
57% (w/w) hydroiodic acid. This solution was brought to a
temperature no greater than 70 °C and stirred for 25 min. The
solution was slowly cooled to room temperature (no precipitate forms
at this point) and placed in a freezer overnight. Bronze, metallic-
looking needles of (FA)2PtI6 were vacuum-filtered cold, washed with
diethyl ether, and vacuum-dried overnight. Exceeding 70 °C results in
degradation of the FA cation, producing free ammonium.
(GUA)2PtI6 was prepared by combining 282 mg (1.5 mmol) of
C(NH2)3I (Sigma-Aldrich) and 200 mg (0.75 mmol) of PtCl2 (Strem,
99.9%) in 15.5 g of 57% (w/w) hydroiodic acid. The solution
temperature was raised to gentle reflux for 25 min, slowly cooled to
room temperature, and placed overnight in a freezer. This solution
was vacuum-filtered cold and washed with diethyl ether. The isolated
crystals were vacuum-dried overnight. Large, dark-gray, block-habit
crystals were isolated. We note that if a more concentrated reaction
mixture is employed, an unidentified Pt-containing contaminant
coforms with the desired compound. Careful control of the
concentration allows the pure compounds to be obtained.
Single-crystal X-ray diffraction (XRD) data were collected on a
Bruker KAPPA APEX II diffractometer equipped with an APEX II
CCD detector using a TRIUMPH monochromator with a Mo Kα X-
ray source (λ = 0.71073 Å). The crystals were mounted on a cryoloop
under Paratone-N oil and kept under nitrogen. Absorption correction
of the data was carried out using the multiscan method, as
implemented in SADABS.35 Subsequent calculations were carried
out using SHELXTL.36 Structure determination was done using
intrinsic methods. All H-atom positions were omitted. Structure
solution, refinement, and creation of publication data were performed
using SHELXTL. Crystal structures were visualized using the VESTA
software suite.37
The 2014 demonstration that the molecular iodostannate
salt Cs2SnI6 is an effective hole conductordespite not
possessing extended I−Sn−I connectivitythat can be
employed as the hole-transport layer in dye-sensitized solar
cells has been an important landmark in the way in which these
materials are viewed.28 Related work that has appeared
includes a study of the role of defects in determining the
29
properties of Cs2SnI6 and Cs2TeI6 and of the role of
anharmonicity in dictating the properties of A2SnI6 when A =
CH3NH3 or CH(NH2)2.31
In the present work, we study the series of compounds
+
A2PtI6, where A = ammonium (NH4 ), methylammonium
+
(CH3NH3 , abbreviated as MA), formamidinium [CH-
+
+
(NH2)2 , abbreviated as FA], and guanidinium [C(NH2)3 ,
abbreviated as GUA]. The synthesis and composition of
several A2PtI6 compounds were already reported by Datta in
1913,32 albeit with no descriptions of the structure.
Furthermore, Datta refers back to some literature on these
from the first half of the 19th century. The crystal structures of
33,34
A2PtI6 at room temperature for A = NH4
and MA20 have
been previously reported. We demonstrate that hydrogen
bonding becomes especially important in retaining the
vacancy-ordered perovskite in the compounds with the larger
A cations. Specifically, if the organic cation is small, I−I van der
Waals interactions guide atomic packing, and if the organic
cation is large, hydrogen-bonding forces (N−H···I) guide the
structure. Consequences of the hydrogen-bonding tendencies
1
manifest in solid-state H NMR experiments and in density
functional theory (DFT)-based electronic structure calcula-
tions. Just as in the case of Cs2SnI6,28 we find that, for the
smaller A cations, the conduction bands of A2PtI6 display
surprisingly disperse electronic bands. The measured optical
properties are compared with the results of DFT calculations.
EXPERIMENTAL SECTION
■
Powder XRD was performed on a Panalytical Empyrean powder
diffractometer (Bragg−Brentano HD module; no monochromator)
equipped with a Cu source (λ = 1.5418 Å). Rietveld refinements were
The starting material CH3NH3I (MAI) was prepared by adding a
solution of CH3NH2 [1.00 equiv, 33% (w/w) in absolute ethanol,
Spectrum] and HI [1.05 equiv, 57% (w/w) in aqueous solution,
Spectrum] held at 0 °C. After stirring for 2 h at 0 °C, excess solvent
was removed using a rotary evaporator. The precipitate was
recrystallized twice from a hot ethanol/water mixture, washed with
ether, and dried under vacuum overnight to give white crystals with a
plate-like habit.
performed in the TOPAS software suite.38 Solid-state NMR: H and
1
195Pt single-pulse, 1H inversion−recovery, and 14N spin−echo
experiments were conducted on a 500 MHz (11.7 T) Bruker Avance
NMR spectrometer with a Bruker 4 mm H/X/Y triple-resonance
magic-angle-spinning (MAS) probe. Powdered samples were packed
into a 4 mm zirconia MAS rotor and capped with a Kel-F drive cap.
The 1H chemical shift was referenced to adamantane (1.71 ppm
relative to tetramethylsilane at 0 ppm). The 195Pt chemical shift was
referenced to a 1 M solution of Na2PtCl6 in D2O (0 ppm).
Longitudinal relaxation (T1) experiments utilized the inversion−
recovery method. Chemical shielding anisotropy (CSA) parameters
for 195Pt were extracted using the SOLA line-shape analysis module
within Bruker Topspin. Solid-state UV−vis spectra were obtained
using an integrating-sphere-equipped Shimadzu UV3600 UV−near-IR
spectrometer in diffuse-reflectance mode. The title compounds tested
were suspended in a BaSO4 medium (via grinding). The reflectance
spectra were Kubelka−Munk-transformed for relative absorbance
spectra. Thermogravimetric analysis (TGA) on all compounds was
conducted using a TA Instruments Discovery instrument. A rate of 25
cm3 min−1 dry nitrogen purge was employed with a temperature ramp
rate of 10 °C min−1. The maximum temperature of the experiment
was 900 °C. Differential scanning calorimetry (DSC) measurements
were performed using a TA Q2000 calorimeter. Samples (2−10 mg
each) were hermetically sealed inside TZero aluminum pans. Samples
CH(NH2)2I (FAI) was prepared by metathesis, by adding solid
formamidinium acetate (1.00 equiv, 99%, Sigma-Aldrich) to room
temperature HI [2.0 equiv, 57% (w/w) in aqueous solution,
Spectrum]. After stirring for 2 h, excess solvent was removed using
a rotary evaporator. The residue was washed with boiling toluene to
remove any potential triazine contaminant, recrystallized from
ethanol, vacuum-filtered, and dried to give white crystals with a
needle-like habit.
(NH4)2PtI6 was prepared by combining 46.0 mg of NH4I (0.300
mmol, Sigma Aldrich, 99.5%) and 40 mg (0.150 mmol) of PtCl2
(Strem, 99.9%) in 6.0 g of 57% (w/w) HI, which was brought to
gentle reflux for 15 min. The solution was slowly cooled to room
temperature and placed in a refrigerator overnight. The solution was
vacuum-filtered cold and washed with diethyl ether. The isolated
crystals were vacuum-dried overnight. Small, dark-bronze, cubic-habit
crystals were isolated. The hydroiodic acid used for this and all other
iodoplatinate reactions did not contain the reducing stabilizer
hypophosphorous acid. The iodine contained in solution (because
B
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Article
Figure 1. Crystal structure evolution at room temperature of the title compounds (a) (NH4)2PtI6, (b) (MA)2PtI6, (c) (FA)2PtI6, and (d)
(GUA)2PtI6 displaying the evolution from a cubic to a distorted K2PtI6 type to eventually the hexagonal polar K2MnF6 structure type. The shading
of the [PtI6]2− octahedra reflects the depth perpendicular to the display plane. (NH4)2PtI6, (MA)2PtI6, and (FA)2PtI6 are viewed as if looking down
[001] of the cubic parent.
were first cooled to −150 °C and then heated to almost 180 °C at 10
°C min−1. This was repeated for three cycles.
Ab initio calculations of the crystal structures, electronic structures,
and optical properties were performed with the Vienna Ab initio
Simulation Package (VASP),39−42 which implements the Kohn−Sham
formulation of DFT using a plane-wave basis set in conjunction with
projector-augmented waves.43,44 The generalized gradient approx-
imation was employed using the exchange and correlation functional
of Perdew, Burke, and Ernzerhof (GGA-PBE).45 The plane-wave basis
set cutoff energy (800 eV) and k-point mesh density (≈1500 k points
per reciprocal atom for convergence of the charge density and
≈40000 k points per reciprocal atom for computing the density of
states and dielectric function; all Γ-centered Monkhorst−Pack
samplings46 were chosen based on convergence of the total energy).
Structure relaxations were performed, including van der Waals
corrections (the DFT-D3 method of Grimme),47 to a force tolerance
of 4 meV Å−1. All relaxed structures preserved the original space
group symmetry, with the exception of (MA)2PtI6, which appeared to
drift slightly from the original R3
that was found within a 0.001 Å lattice parameter tolerance using the
FINDSYM program48 to actually retain R3
m. This symmetrized cell
was used for subsequent calculations to minimize computational
expense. Brillouin zone paths for band-structure calculations were
taken from Setyawan and Curtarolo.49 Frequency-dependent
̅
m high-symmetry space group, but
Figure 2. Evolution of (a) volumes per formula unit and (b) the
perovskite tolerance factors, as a function of the A effective ionic
radius for octahedral coordination (CN = 12). Radii values employed
̅
are from Kieslich, Sun, and Cheetham54 (for NH4 , MA, FA, and
+
GUA) and from Shannon52,53 for Rb+ and Cs+. The symbols for A =
MA and GUA are distinct to indicate that they do not follow the same
volume/size trend as the other compounds.
dielectric functions were calculated using the method described by
50
̌
Gajdos et al., as implemented in VASP. The dielectric tensors were
converted to isotropic equivalents and subsequently to isotropic
absorption coefficients via the custom python code. Band structures
and densities of states, including orbital projections, were visualized
using the custom python code.
effective radii employed for Rb+ and Cs+ are the 12-coordinate
Shannon values.52,53 The values for other A cations are from
Kieslich, Sun, and Cheetham.54 Twelve-coordinate radii were
employed because these appear to be consistent with the
methods employed to estimate the radii of the more complex
ions.
RESULTS AND DISCUSSION
■
All of the samples were prepared by standard methods
(precipitation from acid) and their structures solved by single-
crystal XRD; details of the methods employed are provided in
the Experimental Section. Figure 1 depicts the A2PtI6 series
beginning with the smallest organic-cation-containing com-
pound, (NH4)2PtI6, and ending with the largest, (GUA)2PtI6.
Panel a of Figure 2 shows that all of the compounds except
for A = FA and GUA follow a simple trend, which suggests that
the volume per formula unit of A2PtI6 depends solely on the A
cation separating out the molecular [PtI6]2− octahedra in the
solid state. The A = FA and GUA compounds do not follow
this trend, pointing to a distinct structural role, which we will
presently implicate with hydrogen-bonding tendencies. Panel b
of Figure 2 displays the evolution of the tolerance factors t of
these compounds. While the tolerance factor is frequently
employed to understand perovskites, including ones with more
complex ammonium cations,54 its utility in vacancy-ordered
compounds of the K2PtCl6 type has recently been noted.55 We
observe that the A = MA and FA compounds have tolerance
factors that are well in excess of the range usually expected for
compounds with perovskite-like arrangements of their
octahedra. The other surprise is that (FA)2PtI6 appears to
display tilting and rotation, which is contrary to the
+
The A = NH4 compound crystallizes in an undistorted cubic
K2PtCl6 structure (space group Fm3
a rhombohedrally distorted variant structure (space group
R3m), and the FA compound in a monoclinic variant (space
̅
m), the MA compound in
̅
group P21/n). The A = GUA compound crystallizes in a
distinct structure type of K2MnF6 in the polar space group
P63mc.51 All compounds could be prepared as pure powders, as
confirmed by the Rietveld refinement of powder XRD data
(fits are displayed in the Supporting Information).
The panels of Figure 2 display the evolution with an effective
ionic radius of (a) the volume per formula unit and (b) the
perovskite tolerance factor for the A2PtI6 compounds reported
here and the data on all-inorganic Rb2PtI6 and Cs2PtI6.34 The
C
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expectation for t > 1. While t should be interpreted with care
because of the absence of extended I−Pt−I connectivity in the
structure, it appears to be insightful here. In particular, when t
is significantly greater than unity, we see the kind of structural
of Bloembergen, Purcell, and Pound (BPP)57 has been applied
to establish molecular correlation times in similar com-
pounds,58−62 because in these plastic crystals, longitudinal
1
relaxation is usually mediated by H dipolar interactions that
̅
transformation, from the Fm3m K2PtCl6 type to the P63mc
fluctuate due to molecular motion. In the “fast motion” limit
(ω0τC ≪ 1, where ω0 is the nuclear Larmor frequency and τC is
the autocorrelation time for molecular motion) which is valid
for similar plastic crystals at ambient temperature, the
expression for longitudinal relaxation time in BPP theory
reduces τC ∝ T−1 1. While there are small differences in the
proportionality prefactor related to dipole−dipole separation,
the substantially longer T1 values for the ammonium and
methylammonium phases imply more rapid molecular motion
(short τC) compared to the FA and GUA compounds, which
may be expected simply on the basis of molecular size.
However, it is well-known that strong hydrogen bonding of
cations like FA and GUA can produce rigid structures, as seen
in formate based perovskites,63,64 or in the recently reported
h i g h - t e m p e r a t u r e f e r r o e l e c t r i c m a t e r i a l ( 3 -
ammoniopyrrolidinium)RbBr3.65 Furthermore, when com-
pared to MA and FA lead iodide perovskites (APbI3) where
the molecular cations are known to reorient at room
temperature (and below),62 these two compounds only
undergo phase transitions to structures with cation disorder
when brought to well above room temperature (Supporting
Information).
K2MnF6 type, that is reminiscent of what happens with
perovskites when the tolerance factor becomes large, for
example, the transformation from the structure of SrRuO3 to
BaRuO3.56
To complement the crystallography and to better under-
stand the role of the A cation, we have employed room
temperature solid-state 1H and 195Pt NMR spectroscopy.
1
Solid-state H NMR confirmed the identity of each molecular
cation and probed aspects of each cation’s motion, and solid-
state 195Pt NMR examined the local Pt environments to
confirm our crystallographic models. Single-pulse 195Pt NMR
spectra with 10 kHz MAS, as well as the relationship between
the isotropic chemical shift and unit cell volumes, are
presented in Figure 3. For all members of the A2PtI6 series,
To better understand if hydrogen bonding can explain the
suspected reduced molecular motion of the cations in
(FA)2PtI6 and (GUA)2PtI6, we examined how hydrogen
bonds arrange in ab initio relaxed structures. Figure 4 shows
crystallographic depictions of (FA)2PtI6 and (GUA)2PtI6
(including hydrogen bonds). It can be seen, in Figure 4a,
that FA cations hydrogen bond with iodides on [PtI6]2−
Figure 3. Room temperature 10 kHz MAS 195Pt NMR spectra of
A2PtI6 showing the influence of the countercation on the chemical
shift and line shape. Asterisks indicate spinning sidebands, shifts are
referenced to a 1 M solution of Na2PtCl6 in D2O (0 ppm), and the
field is 11.7 T (500 MHz for 1H). Inset: Isotropic 195Pt chemical shift,
δiso, correlated with the unit cell volume per formula unit, V/FU. A
dashed line is drawn as a visual guide.
only a single Pt site was observed, in agreement with the
crystallographic studies. A clear monotonic upfield shift with
increasing countercation size, as well as significant differences
in the CSA and line widths, is evident. Line-shape and CSA
parameters are given in Table S4 and correlate well with the
Pt-site symmetries from XRD.
1H inversion−recovery experiments were conducted to
assess the degree of molecular motion in the A2PtI6 phases
(Figure S6), and the resulting longitudinal relaxation times, T1,
are given in Table 1. We observe that T1 is substantially longer
in the ammonium and methylammonium phases. The theory
Figure 4. Crystallographic depictions of ab initio relaxed structures of
(FA)2PtI6 (a and b) and (GUA)2PtI6 (c and d) illustrating their
hydrogen-bonding networks. (a) View of the 3D-connected network
of hydrogen bonds linking [PtI6]2− octahedra in (FA)2PtI6. The
hydrogen bond length of all bonds shown in panel a is 2.68 Å. (b)
View of a representative [PtI6]2− octahedron in (FA)2PtI6 with FA
cations that hydrogen bond to it. (c) View of the 2D-connected
network of hydrogen bonds linking [PtI6]2− octahedra in (GUA)2PtI6.
(d) View along the layering direction of GUA cations hydrogen
bonding to nearby [PtI6]2−. Each GUA cation bonds to a total of six
iodides on three separate [PtI6]2− octahedra.
1
Table 1. H Longitudinal Relaxation Times, T1, for A2PtI6
Phases, with Confidence Intervals Expressed as 1σ
A
1H T1 (s)
A
1H T1 (s)
NH4
MA
6.18 0.07
5.66 0.24
FA
GUA
1.35 0.01
0.598 0.004
D
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octahedra to form a connected network, and, in Figure 4b, how
a representative [PtI6]2− octahedron is surrounded by the FA
cations. Each FA cation forms a total of four hydrogen bonds
(N−H···I), two to two iodides on one [PtI6]2− octahedron
(with bond lengths of 2.71 and 2.84 Å) and two “bridging
bonds” to two other iodides on two separate neighboring
[PtI6]2− octahedra (bond lengths of 2.68 and 2.80 Å). These
bridging bonds have been labeled in Figure 4a,b where visible
and are responsible for linking the discrete [PtI6]2− octahedra
together. All of these bonds create a 3D-connected hydrogen-
bonding network for (FA)2PtI6, suggesting that hydrogen
bonding may contribute substantially to the suspected reduced
molecular motion of the formaminidinium cation. Figure 4c
shows the 2D hydrogen-bonding network formed in
(GUA)2PtI6, and Figure 4d illustrates a top-down view
emphasizing the orientation of the GUA cations in relation
to nearby [PtI6]2− octahedra. Figure 4c displays a 2D-
connected network of hydrogen-bonded GUA cations and
[PtI6]2− octahedra, where each GUA cation hydrogen bonds to
a total of six iodides on three separate [PtI6]2− octahedra. In
Figure 4d, one can see how there are two guanidinium layers
per one layer of [PtI6]2− octahedra. Similar to the hydrogen
bonding seen in (FA)2PtI6, we find that the location of the
bonds in (GUA)2PtI6 help to explain the suspected reduced
molecular motion of the organic cation because each hydrogen
on the guanidinium cations aligns directly with a nearby
[PtI6]2− iodide. Indeed, we believe it is not a coincidence that
the space group symmetry of (GUA)2PtI6 is compatible with
the shape of the GUA cation. From this examination, we
believe that the hydrogen bonds formed by these large organic
cations actually dictate the observed structural trends in
(FA)2PtI6 and (GUA)2PtI6.
Figure 5. Electronic band structures (GGA-PBE) for the A2PtI6
phases, illustrating decreasing bandwidths (and increasing band gaps)
with increasing countercation size. Orbital projections of Pt d and I p
character are presented as colored markers of variable size. In all cases,
the valence bands derive primarily from the I 5p orbitals, while the
conduction bands derive from the unoccupied Pt 5d orbitals with
irrep eg. The effect of spin−orbit coupling is shown to be modest
(Figure S9). Energies are referenced to the valence-band maximum
for each phase.
extended interactions being nonbonded, the conduction
bandwidth (≈1 eV) is significant, and despite the central
cation being different, it is similar to those reported for
isostructural Cs2SnI6 and Cs2TeI6, where delocalized states
similarly emerge because of iodine close-packing.28−30 In all
cases, the band gaps are indirect, but there are direct
transitions that are only slightly higher in energy, as discussed
in the Supporting Information. Additionally, the electronic
band gaps and line-effective masses for electrons and holes are
given in Tables S5 and S6, together with the cell volumes from
ab initio structure relaxation. The computed band structures
also offer an explanation for the downfield 195Pt chemical shift
with decreasing countercation size. The increasingly delocal-
ized electronic states from closer intermolecular contact draw
some charge density away from the Pt nucleus and into the
intermolecular regions, deshielding the Pt nucleus.
Furthermore, the hypothesis of reduced motion in the FA
and GUA compounds is consistent with analysis of the
hydrogen-bonding geometries. Steiner has reported average
hydrogen-bond donor···acceptor and H···acceptor distances
based on a crystallographic database analysis of the crystal
structures determined by neutron diffraction.66 The donor···
2
acceptor distances reported here are very similar for −Nsp H2
and −N+H3 donors and iodide acceptors (≈3.7 Å). The
donor···acceptor distances in the X-ray crystal structures of the
A2PtI6 phases presented are similar for FA and GUA (3.63 and
3.76 Å, respectively), while those for ammonium (3.95 Å) fall
outside the reported distribution, and the donor−H···acceptor
bond angles for methylammonium (139°) are outside the
range considered by Steiner. This analysis would suggest that
molecular motion could be uninhibited in the ammonium
phase, while hydrogen bonding to the neighboring anions
significantly restricts molecular motion in the FA and GUA
phases.
The electronic band structures of the A2PtI6 phases,
calculated within the framework of DFT, are presented in
Figure 5 (details are provided in the Supporting Information).
In all cases, the band gap is between filled I p orbitals and
empty Pt eg orbitals, as expected for a low-spin d6 [PtI6]2−
polyanion. The valence and conduction bands across the series
of compounds display diminished dispersion as one traverses
across from the smaller A = NH4+ cation to MA and FA. In the
hexagonal structure of (GUA)2PtI6, the valence and con-
duction bands are nearly fully localized. For the smaller A
cations, the iodine sublattice is essentially close-packed, leading
to overlap of the large 5p orbitals between neighboring
[PtI6]2− anions. For the undistorted (NH4)2PtI6, despite the
The ab initio optical absorption coefficients for the A2PtI6
series are displayed in Figure 6. Values for weakly absorbing
crystalline silicon (c-Si) and strongly absorbing CuSbS2,67
calculated similarly, are given for comparison. Additionally, the
spectral dependence for solar irradiance (direct + diffuse at
global tilt, ASTM G173-03) is included for reference.68 For all
of the platinum(IV) iodides, the absorption onset is rather
strong despite their indirect band gaps, reflecting a relatively
high joint density of states near the band gap due to the
partially localized nature of the electronic structure. The A =
NH+4 compound, with the most disperse bands, displays the
lowest-energy absorption edge, as expected from the band
structure. The A = MA and FA compounds display very similar
calculated absorption edges, and the A = GUA compound has
the highest-energy absorption edge, in keeping with its more
molecular nature. Despite slight differences in the near-IR and
red due to the modestly differing bandwidths, the A2PtI6
phases exhibit very similar absorption around 2 eV, reflecting
the similar underlying molecular character. Absorption is weak
for these phases in the violet and near-UV, a consequence of
the wide gap between the Pt eg conduction bands and the next
higher-lying excited states.
Experimental optical absorption spectra are displayed in
Figure 7. The observed absorption onsets across the different
E
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Article
structures undergo progressive distortion from the parent cubic
vacancy-ordered double perovskite K2PtCl6-type structure,
until converting to the K2MnF6 structure type with
(GUA)2PtI6. Analysis of the solid-state 1H NMR T1 relaxation
times and ab initio relaxed structures of (FA)2PtI6 and
(GUA)2PtI6 suggests that FA and GUA cations engage in
strong hydrogen-bonding interactions with nearby iodide
atoms on separate [PtI6]2− octahedra, causing reduced cation
molecular motion and distinct structural signatures at room
temperature. The hydrogen bonds appear to guide the final
structure formation for the FA and GUA compounds, with
structures that can be best described as hydrogen-bonded
networks. From DFT calculations, it is found that the
intermolecular contact through close-packed iodide anions
leads to dispersive frontier bands for the phases with smaller
molecular ions.
Figure 6. Ab initio optical absorption coefficients (GGA-PBE) for the
A2PtI6 phases, with band gaps indicated by vertical gray lines. Indirect-
gap c-Si (dashed gray line) and the strongly absorbing direct-gap
chalcogenide CuSbS2 (solid gray line) are included for comparison.
Absorption in the violet/near-UV is poor for the A2PtI6 phases
because of the paucity of available states immediately above the
unoccupied Pt eg states.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01597.
■
S
Details of the crystallography, powder XRD attesting to
the purity of powders, details of the NMR studies,
further details on the electronic structure, TGA, and
DSC data (PDF)
Accession Codes
CCDC 1848508−1848513 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing 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: seshadri@mrl.ucsb.edu.
■
ORCID
Douglas H. Fabini: 0000-0002-2391-1507
Anthony K. Cheetham: 0000-0003-1518-4845
Ram Seshadri: 0000-0001-5858-4027
Figure 7. Normalized optical absorption spectra of the title
compounds recorded on powders, obtained from Kubelka−Munk
transformation of UV−vis diffuse-reflectance powder spectra.
Notes
The authors declare no competing financial interest.
compounds appear to be quite similar, exhibiting a peak in the
absorption about 1 eV above the absorption edge, similar to
what was calculated for these compounds. The red shift of the
peak in the calculated spectra compared with the experiment is
consistent with the functionals employed here in the DFT
calculation. The more disperse valence and conduction bands
calculated for the A = NH4+ compound give rise to the slightly
sharper absorption edge, at lower energy than that seen for the
other compounds.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, under Award DE-SC-
0012541. Extensive use of the Shared Experimental Facilities of
the Materials Research Science and Engineering Center
(MRSEC) at UCSB [National Science Foundation (NSF)
Grant DMR 1720256] is gratefully acknowledged. The
MRSEC at UCSB is a member of the NSF-supported Materials
Research Facilities Network (www.mrfn.org). We additionally
acknowledge support from the Center for Scientific Comput-
ing at UCSB, supported by NSF Grants CNS-1725797 and
DMR-1720256. D.H.F. thanks the National Science Founda-
tion Graduate Research Fellowship Program for support under
CONCLUSION
A study of the evolution of the structure and properties of the
compound A2PtI6 series, where A = NH4 , MA, FA, and GUA,
■
+
suggests the importance of hydrogen bonding for stabilizing
some of the structures with the larger A cations, and the role of
nonbonded I−−I− interactions in establishing relatively
disperse electronic bands for the compounds with the smaller
A cations. We describe how as the cation size increases, the
́
̈
Grant DGE 1144085 and Raphaele Clement for helpful
discussions. A.K.C. thanks the Ras al Khaimah Centre for
Advanced Materials for financial support.
F
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Article
(18) Krupski, M. Rigid Sphere Model Applied to the Phase
Transitions of Complex Compounds of the Type K2PtCl6. Phys. Status
Solidi 1983, 78, 751−758.
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