Page 3 of 11
Chemistry of Materials
another class of true perovskites in which A is a molecular
cation. Various amines may be placed on the A-site, such as
Our synthetic procedure is a modification of a reported meth-
1
2
3
4
5
6
7
8
od [15] and gives no observable impurities while it allows for
easier manipulation of the materials. Self-organization is a
process where some form of global order or coordination aris-
es out of the local interactions between the components of an
initially disordered system. This process is spontaneous: it is
not directed or controlled by any agent or subsystem inside or
outside of the system; however, the laws followed by the pro-
cess and its initial conditions may have been chosen or caused
by an agent. Self-organization is also relevant in chemistry,
where it has often been taken as being synonymous with “self-
assembly” (SA).
+
+
ammonium NH4 [23], methylammonium, CH3NH3 [14, 15,
24, 25], and formamidinium NH2CH=NH2+ [26, 27]. These are
particularly common in compounds based around lead and tin
halides. In these compounds, the perovskite aristotype is a
cubic Pm3m framework structure of composition ABX3,
where A (Wyckoff position 1a) is commonly a large cation
coordinated to 12 X (3c) anions, B (1b) is a smaller metal
bonded to six X anions, and BX6 octahedra are corner-
connected to form a three-dimensional framework. The hinged
octahedra allow for wide adjustment of the B–X–B bond an-
gle, and several sets of cooperative rotations, known as tilt
transitions, promote symmetry reduction of the aristotype [19
and references therein]. Rapid reorientation motions of MA+
were detected by variable temperature NMR that revealed two
phase transitions as the temperature decreases, as a result of
progressive ordering of the MA+ ions [28, 29]. It is expected
that these structural distortions strongly affect the relevant PV
properties but there is a need of further systematic investiga-
tion to clarify the involved structure-properties relationships.
In the Br-containing plumbate, the crystalline reflections in-
dicate the formation of a cubic structure at room temperature
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Self-assembly in the classic sense can be defined as the
spontaneous and reversible organization of molecular units
into ordered structures by non-covalent interactions. The main
property of a self-assembled system that this definition sug-
gests is the spontaneity of the self-assembly process: the inter-
actions responsible for the formation of the self-assembled
system act on a strictly local level—in other words, the
nanostructure builds itself. At this point, one may argue that
any chemical reaction driving atoms and molecules to assem-
ble into larger structures, such as precipitation, could fall into
the category of SA. However, there are at least three distinc-
tive features that make SA a distinct concept. These are: order,
interactions, building blocks. First, the self-assembled struc-
ture must have a higher order than the isolated components, be
it a shape or a particular task that the self-assembled entity
may perform. This is generally not true in chemical reactions,
where an ordered state may proceed towards a disordered state
de-pending on thermodynamic parameters. The second im-
portant aspect of SA is the key role of slack interactions (e.g.
Van der Waals, capillary, π-π, hydrogen bonds) with respect to
more "traditional" covalent, ionic, or metallic bonds. Although
typically less energetic, these weak interactions play an im-
portant role in materials synthesis. For instance, they deter-
mine the physical properties of liquids, the solubility of solids,
and the organization of molecules in biological membranes.
The third distinctive feature of SA is that the building blocks
are not only atoms and molecules, but span a wide range of
nano- and mesoscopic structures, with different chemical
compositions, shapes and functionalities [35]. Recent exam-
ples of novel building blocks include “polyhedra” and “patchy
particles” [36]. Important examples of SA in materials science
include the formation of molecular crystals, colloids, lipid bi-
layers, phase-separated polymers, and self-assembled mono-
layers [37, 38].
(space group Pm 3m) as shown in Figure 1b. Our XRD data
are supported by the investigations in literature [15, 30-34].
Poglitsch and Weber [15], in the case of MAPbBr3, showed
there existed four phases which they labelled α - δ; but which
subsequent workers have generally referred to as I–IV. NMR,
calorimetry, and infrared spectroscopic were performed by
Knop et al. [24] who examined the states of disorder for the
phases of MAPX3 as a function of temperature. The authors
found that transition temperatures for protonated material are
I–II 235.1(2)K and II–III 154.2(2)K and III–IV 148.35(5)K. In
the case of MAPbBr3 the structure can be rationalized as be-
ing, to first order, a framework-driven tilt instability, in which
deformation of octahedra are not required, but are allowed.
These secondary distortions of the octahedral in MAPbBr3 are
in fact quite large [24].
The XRD data of the Cl-containing system (shown in Figure
1c) revealed the presence of MAPbCl3 as major crystalline
phase; however crystalline reflections (indicated with * in
Figure 1c) revealed the formation of CH3NH3Cl as secondary
phase. The inorganic sub-lattice of MAPbCl3 is built up by a
three-dimensional array of PbCl6 octahedra and the me-
thylammonium (MA+) cations (with C3v molecular symmetry)
are situated in the cavities between the octahedra; to satisfy the
site symmetry, these cations have to execute complex rota-
tional and orientational motions and possess almost spherical
statistical symmetry. In this structure the MA+ cations and the
Pb atom (one of each per unit cell) occupy Oh sites and the
three chlorine atoms lie on D4h sites [33]. Due to this unusual
structure, there is also an alternating long and short Pb-Cl
bond along a, due to an off- center displacement of Pb within
the octahedron. This suggests that the most rigid unit is actual-
ly the methylammonium cation, rather than the PbCl6 octahe-
dra, in agreement with existing spectroscopic data and conse-
quently the size of the distortion of the PbCl6 octahedra is sub-
stantial for the physical properties of the material.
The results of the morphological investigation on the
MAPbX3 systems are shown in Figure 2a-c. The single crys-
tals of MAPbI3 generally formed as dodecahedra, with some
examples exhibiting faceting consistent with rhombo-
hexagonal dodecahedra [19], which is a typical crystal habit of
a body centered tetragonal lattice, in agreement with the re-
ported structure at room temperature (space group I4/mcm)
(Figure 2a). A very similar behavior has been observed in
MAPbBr3 as shown in Figure 2b. The crystals of the obtained
materials have a well-defined habit which can even be modi-
fied depending on the temperature profile of the precipitation.
Namely, the products vary from discrete polyhedral crystals
To obtain pure materials with exact stoichiometries and
lowest number of defects, the solution method is very suitable.
ACS Paragon Plus Environment
Dimesso
