Polarity and Ferromagnetism in Two-Dimensional Hybrid
Copper Perovskites with Chlorinated Aromatic Spacers
Two-dimensional (2D) organic–inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium copper(II) chloride perovskites A2CuCl4. 2D polar ferromagnets (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4 (ClbaH+ = chlorobenzylammonium) are successfully obtained. They both adopt a polar monoclinic space group Cc at room temperature, displaying significant differences in crystal structures. In contrast, (2-ClbaH)2CuCl4 adopts a centrosymmetric space group P21/c at room temperature. This associated structural evolution successfully enhances the physical properties of the two polar compounds with high thermal stability, discernible second harmonic generation (SHG) signals, ferromagnetism, and narrow optical band gaps. These findings demonstrate that the introduction of chlorine atoms into the interlayer organic species is a powerful tool to tune crystal symmetries and physical properties, and this inspires further exploration of designing high-performance multifunctional copper-based materials.
Introduction<!>Materials<!>Synthesis<!>Single-Crystal
X-ray Diffraction<!>Powder X-ray Diffraction (PXRD)<!>Thermogravimetric
Analyses (TGA)<!>Second
Harmonic Generation (SHG) Measurements<!>Dielectric Measurements<!>Magnetic
Measurements<!>UV–Vis Absorption Spectral Measurements<!>Crystal
Structures<!><!>Crystal
Structures<!><!>Second Harmonic Generation
Effect<!><!>Dielectric Properties<!><!>Magnetic Properties<!><!>Magnetic Properties<!>Optical Properties<!><!>Optical Properties<!><!>Conclusions<!><!>Notes
<p>Organic–inorganic
hybrid halide perovskites have recently
received tremendous interest owing to their extraordinary photovoltaic
and optoelectronic properties.1−3 Among them, two-dimensional (2D)
hybrid perovskites can be regarded as derived by slicing the three-dimensional
(3D) cubic perovskite aristotype ABX3 along vertices of
the BX6 octahedra and inserting additional organic moieties
between these layers.4 Among the known
families of 2D layered perovskites, two conventional families are
the Dion–Jacobson (DJ)5 and Ruddlesden–Popper
(RP)6 phases, which are commonly defined
in terms of their generic stoichiometries ABX4 and A2BX4, respectively, for examples with single octahedral
layers. The tolerance factor constraint that occurs in 3D perovskites
can be considerably relaxed in 2D hybrid perovskites, endowing them
with striking structural flexibility and novel functionalities.7−9 As a result, the crystal structures and physical properties of 2D
hybrid perovskites can be modulated by much larger and more complex
organic molecules with various sizes and functional groups.10−12 A halogen substitution strategy, especially introducing fluorine
atoms into the organic molecules, has been proved to be a powerful
approach to enhance the ferroelectric performance in 2D lead-based
halide perovskites.13,14</p><p>Copper-based halide perovskites
have recently been studied intensively
for developing new multifunctional materials because of their interesting
thermochromism, ferromagnetism, and ferroelectricity.15−17 The Cu-based perovskites possess the inherent benefits of lower
toxicity and greater light and humidity stability, in comparison to
Pb-, Cd-, Sn-, or Bi-based perovskites.18,19 Structurally,
the Jahn–Teller (J–T) distortion of the 3d9 ion Cu2+ results in the elongation of octahedral coordination,
introducing additional flexibility into copper-based systems.20 Consequently, more complex organic moieties
can be included in copper halide perovskites to adjust the physical
and chemical properties. To date, a few copper-based halide perovskites
with halogen-substituted organic molecule spacers have been prepared.
The introduction of a single fluorine atom at the meta-position in the benzylamine structure leads to a polar ferromagnet
(3-FbaH)2CuCl4 (3-FbaH+ = 3-fluorobenzylammonium).21 A multiaxial ferroelectric (DF-CBA)2CuCl4 (DF-CBA = 3,3-difluorocyclobutylammonium) with a
ferroelectric phase transition temperature of 380 K is successfully
obtained by incorporating two fluorine substituents in the centrosymmetric
(CBA)2CuCl4 (CBA = cyclobutylammonium) structure.22 Heavier halogen substitutions, such as chlorine
or bromine substitution in ethylamine, influence the reversible and
irreversible thermochromism in 2D layered perovskites (CEA)2CuCl4 (CEA = 2-chloroethylammonium) and (BEA)2CuCl4 (BEA = 2-bromoethylammonium).23</p><p>However, there is no report on multifunctional 2D
copper-layered
perovskites by introducing chlorine atoms into isomeric organic molecules
at various positions. Herein, for the first time, we report the influence
of positional isomerism on the crystal structures of chlorobenzylammonium
copper(II) chloride perovskites A2CuCl4 by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure. The monochlorine
substitution at various positions in the benzylamine structure changes
the crystal symmetries and physical properties. We present 2D polar
ferromagnets (3-ClbaH)2CuCl4 (3-ClbaH+ = 3-chlorobenzylammonium) and (4-ClbaH)2CuCl4 (4-ClbaH+ = 4-chlorobenzylammonium). They both crystallize
in a polar monoclinic space group Cc at room temperature,
displaying significant differences in crystal structures. However,
their isomeric analogue (2-ClbaH)2CuCl4 (2-ClbaH+ = 2-chlorobenzylammonium) crystallizes in a centrosymmetric
space group P21/c at room temperature. This associated
structural evolution successfully enhances the physical properties
of the two polar compounds, which exhibit high thermal stability,
discernible second harmonic generation (SHG) signals, polar-to-nonpolar
phase transition temperatures up to 433 K, ferromagnetism, and narrow
optical band gaps.</p><!><p>Anhydrous copper(II) chloride (CuCl2, 98%), hydrochloric
acid (HCl, 36%, w/w aqueous solution), and absolute
ethanol (C2H5OH, 99.99%) were purchased from
Alfa Aesar. 2-Chlorobenzylamine (C7H8NCl, 95%),
3-chlorobenzylamine (C7H8NCl, 98%), and 4-chlorobenzylamine
(C7H8NCl, 98%) were purchased from Fluorochem.
All chemicals were directly used without further purification.</p><!><p>The compounds (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4 were crystallized by a slow evaporation method.</p><p>For (2-ClbaH)2CuCl4 (C14H18N2CuCl6), CuCl2 (268.9 mg,
2 mmol) was dissolved in concentrated HCl (5 mL) with moderate heating.
2-Chlorobenzylamine (0.5 mL, 4 mmol) was added. Once fully dissolved,
the solution was allowed to cool. By cooling for several hours, green,
platelet-shaped crystals were obtained. Elemental anal. calcd (%)
for (2-ClbaH)2CuCl4: C, 34.28; H, 3.70; N, 5.71.
Found: C, 34.58; H, 3.56; N, 5.52.</p><p>For (3-ClbaH)2CuCl4 (C14H18N2CuCl6), CuCl2 (134.45
mg, 1 mmol) was dissolved in concentrated HCl (20 mL) with slow stirring
and moderate heating. Once fully dissolved, 3-chlorobenzylamine (0.24
mL, 2 mmol) was added. The produced precipitates were dissolved by
adding excess concentrated HCl and ethanol to get a clear solution.
By naturally cooling the solvent for a few hours, yellow, platelet-shaped
crystals were obtained. Elemental anal. calcd (%) for (3-ClbaH)2CuCl4: C, 34.28; H, 3.70; N, 5.71. Found: C, 34.55;
H, 3.52; N, 5.54.</p><p>For (4-ClbaH)2CuCl4 (C14H18N2CuCl6), CuCl2 (134.45
mg, 1 mmol) was dissolved in concentrated HCl (20 mL) with moderate
heating. Once fully dissolved, 4-chlorobenzylamine (0.24 mL, 2 mmol)
was added. The produced precipitates were dissolved by adding excess
concentrated HCl and ethanol to get a clear solution. By cooling for
a few hours, yellow, platelet-shaped crystals were obtained. Elemental
anal. calcd (%) for (4-ClbaH)2CuCl4: C, 34.28;
H, 3.70; N, 5.71. Found: C, 34.53; H, 3.49; N, 5.54.</p><!><p>Single-crystal X-ray
diffraction data were collected on a Rigaku SCX Mini diffractometer
at 173 and 298 K using Mo Kα radiation (λ = 0.71075 Å).
The data were processed using Rigaku CrystalClear software.24 Absorption corrections were
conducted empirically from equivalent reflections according to multiscans
using CrystalClear.24 Crystal
structures were solved using structure solution program SHELXT,25 and full-matrix least-squares refinements
on F2 were carried out using SHELXL-2018/325 incorporated in
the WinGX program.26 All of the hydrogen
atoms were treated as riding atoms, and all non-H atoms were refined
anisotropically.</p><!><p>Powder X-ray diffraction
data were measured on a PANalytical EMPYREAN diffractometer using
Cu Kα1 (λ = 1.5406 Å) radiation at ambient
temperature. The data were collected in the range of 3–70°
for 1 h to confirm the phase purity of each sample.</p><!><p>TGA data were collected
on an STA-780 instrument between 293 and 523 K at a heating rate of
5 K min–1 under a flowing N2.</p><!><p>Samples
were filled in fused silica tubes with an outer diameter of 4 mm.
Relevant comparisons with known SHG material, α-SiO2, were made at the same condition. A 1064 nm pulsed Nd:YAG laser
(Quantel Laser, Ultra 50) generated the fundamental light, and the
SHG intensity was recorded at room temperature on an oscilloscope
(Tektronix, TDS3032).</p><!><p>Dielectric
measurements were
made on pellets ca. 1 mm thick and 10 mm in diameter formed by uniaxially
pressing powder under a load of 2 tonnes. Silver conductive paste
was applied to the opposing pellet faces and allowed to dry at 373
K. The data were recorded over the frequency range 100 Hz and 10 MHz
at a heating/cooling rate of 3 K min–1 for (2-ClbaH)2CuCl4 and 1 K min–1 for (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4 with the furnace working temperature between 298 and 470 K.</p><!><p>The magnetic measurements were
carried out on a Quantum Design (MPMS XL) SQUID magnetometer. Data
were collected by cooling a known mass of material at 100 Oe field
between 300 and 2 K. Measurements of magnetization versus applied
field were carried out between −5000 and 5000 Oe at different
temperatures.</p><!><p>Ambient
temperature solid UV–vis absorbance spectra of powder samples
were collected on a JASCO-V550 ultraviolet–visible spectrophotometer
with a wavelength range of 200–900 nm.</p><!><p>The single-crystal X-ray structures
suggest no phase transitions in the regime 173 < T < 298 K, so the crystallographic details will be discussed based
on the structures at 298 K. Details of the structures at 173 K are
provided in Supporting Information (SI).
Crystallographic parameters for all three compounds at 298 K are given
in Table 1, and selected
geometrical parameters are given in Table 2.</p><p>The powder X-ray diffraction (PXRD) patterns
of (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4 all show similar
characteristics to those
calculated from their single-crystal structures (Figure S1). The phase purity of the powder samples was confirmed
by elemental analysis and supported by Rietveld refinements (Figure S2), although a strong preferred orientation
effect is observed due to the platelet morphology of the crystals.
Thermogravimetric analysis (TGA) data reveal that these compounds
show good thermal stability, with similar onset decomposition temperatures
reaching 440, 443, and 453 K for (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4, respectively (Figure S3).</p><p>The crystal structures of (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4 are shown in Figure 1. All three structures exhibit the same generic
2D
layered perovskite structure type, with a single [CuCl4]∞ layer of corner-shared CuCl6 octahedra
separated by a double layer of the protonated chlorobenzylammonium
cations, similar to their parent structure (baH)2CuCl4 (baH+ = benzylammonium) and fluorinated counterparts
(2/3/4-FbaH)2CuCl4 (FbaH+ = fluorobenzylammonium).21,27 Looking in more detail at the unit cell metrics, it can be determined
that (2-ClbaH)2CuCl4, which crystallizes in
the centrosymmetric monoclinic system, P21/c, has a supercell
described as c × √2a × √2a relative to the parent DJ-like
structure (i.e., just a single [CuCl4]∞ layer repeat per cell). In fact, this is an example of the most
common structure type among layered hybrid perovskites, having the
simple Glazer tilt system a–a–c. At each temperature
studied, both (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4 crystallize in more complex supercells, having
two [CuCl4]∞ layers per cell. Despite
having the same polar monoclinic Cc space group with
a doubled unit cell along the layer direction, the apparent similarity
is coincidental, with (3-ClbaH)2CuCl4 displaying
unit cell metrics 2a × 2a × c, relative to the RP-like parent, and (4-ClbaH)2CuCl4 adopting unit cell metrics c × a × 2a, relative to the RP-like parent.
(3-ClbaH)2CuCl4 displays a complex octahedral
tilt system not easily described in the Glazer scheme, whereas (4-ClbaH)2CuCl4 has no octahedral tilt modes. Both of these
structure types are rare, but analogues have been seen in the lead
halide family (see Table 3 of ref (7)). A fuller and more systematic discussion of
these types of structural features can be found in our recent review.7 In the (4-ClbaH)2CuCl4 structure,
the [CuCl4]∞ sheets display an additional
J–T disorder, similar to the known chiral ferromagnets (R-MPEA)2CuCl4 and (S-MPEA)2CuCl4.28 In
contrast, (3-ClbaH)2CuCl4 displays a well-behaved
structure with two different Cu sites. Figure 2 shows that neighboring [CuCl4]∞ layers in all three structures are staggered
relative to each other in an RP style, with a greater degree of layer
shift in the (2-ClbaH)2CuCl4 structure, described
by the larger β angle.</p><!><p>Crystal structures of (a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and
(c) (4-ClbaH)2CuCl4 at 298 K parallel to the
layer direction.</p><p>Crystal structures of
(a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and (c) (4-ClbaH)2CuCl4 at
298 K perpendicular to the layer direction,
emphasizing the similarities and differences between the [CuCl4]∞ layers.</p><!><p>The Cu2+ ions in halide perovskite structures exhibit
a substantial J–T distortion, resulting in significant variations
in the Cu–Cl bond lengths and therefore structural distortions
of the CuCl6 octahedra, with the shorter in-plane bond RS, the longer one RL, and the out-of-plane bond RZ (Table 2). To learn more details
of the distortions, the degree of octahedral distortion is measured
quantitatively using equation .29 Among the
three compounds, (2-ClbaH)2CuCl4 displays the
largest octahedral and interoctahedral distortions (Cu–Cl–Cu
angles).</p><p>In all three structures, as shown in Figure 3, the protonated chlorobenzylammonium
moieties
are ordered and form hydrogen bonds (N–H···Cl)
with the inorganic layers [CuCl4]∞. In
the centrosymmetric structure (2-ClbaH)2CuCl4, the two organic moieties are correlated by an inversion center
with stronger hydrogen bonds (N–H···Cl <
2.89 Å), which can be seen as a single crystallographically distinct
organic molecule. In the two polar structures, there are four and
two distinct organic moieties for (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4, respectively.</p><!><p>Hydrogen-bonding
interactions for (a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and (c) (4-ClbaH)2CuCl4 at 298 K.</p><!><p>To verify the presence
or absence of inversion symmetry in the ortho-, meta-, and para-Cl-substituted benzylammonium
crystal structures, we conducted second harmonic generation (SHG)
experiments. Since the SHG response can only exist in noncentrosymmetric
crystal structures, this is a sensitive method to probe the inversion
symmetry in the crystal structure. As depicted in Figure 4, (2-Clba)2CuCl4 is SHG-inactive, while both (3-Clba)2CuCl4 and (4-Clba)2CuCl4 show clear SHG signals
at room temperature, consistent with the centrosymmetric P21/c and noncentrosymmetric Cc space groups, respectively.</p><!><p>SHG signals
of (2-Clba)2CuCl4, (3-Clba)2CuCl4, and (4-Clba)2CuCl4.</p><!><p>Powder-pressed pellets were measured
at various frequencies to investigate the dielectric properties of
all three compounds. The dielectric permittivity usually displays
large anomalies in the vicinity of the phase transition temperature.30 As shown in Figure 5, upon heating, the relative permittivity
(εr) data show anomalies at 379, 433, and 420 K for
(2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4, respectively,
suggesting the existence of phase transitions in the three compounds.
The peak temperatures of the dielectric peaks show no apparent shift
under various frequencies, suggesting that this dispersion is not
due to dielectric relaxation. In the case of (2-ClbaH)2CuCl4, it is likely that the phase transition involves
a change of the octahedral tilt system to a higher-symmetry centrosymmetric
polymorph. This phenomenon has been observed in hybrid lead systems,
for example, [4-fluorobenzylammonium]2PbCl4 displays
a centrosymmetric-to-centrosymmetric phase transition with significant
dielectric anomalies.31 However, the sharp
dielectric peaks of the two polar compounds support the occurrence
of polar-to-nonpolar phase transitions, which may be induced by the
ordered–disordered dynamic motions of organic moieties in hybrid
perovskites.31,32 The variation of εr on cooling for the two polar compounds also shows prominent
peaks under different frequencies, strongly supporting their reversible
polar-to-nonpolar phase transitions (Figure S4). To the best of our knowledge, such a high polar-to-nonpolar phase
transition temperature 433 K of (3-ClbaH)2CuCl4 is the highest in 2D copper(II) perovskites and significantly larger
than [3,3-difluorocyclobutylammonium]2CuCl4 (380
K),22 (C6H5CH2CH2NH3)2CuCl4 (340
K),33 and [C6H5(CH2)4NH3]2CuCl4 (143
K).34</p><!><p>Relative permittivity, εr, as a function of temperature
at different frequencies in heating runs of (a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and
(c) (4-ClbaH)2CuCl4.</p><!><p>SQUID magnetometry was used to
explore the magnetic properties of the three compounds. As shown in Figure 6 a–c, the
magnetic susceptibility (χ) is displayed as a function of temperature T during field cooling. χ shows a gradual increase
in cooling from 300 K to around 10 K, below which it increases very
steeply, with a curvature that suggests a tendency toward saturation
as the temperature is further reduced. This leads to a peak in the
product χT shown in Figure 6d–f, which otherwise has a very slight
upward trend over much of the range as the temperature falls. These
observations are all strongly indicative of a system dominated by
ferromagnetic interactions with a transition to an ordered ferromagnetic
state at low temperature, with no indication of moment compensation
due to antiferromagnetic correlations at any temperature. This is
further supported by the magnetization M vs T plots taken at low temperature shown in Figure 7a–c. At the lowest temperatures
measured, these display very soft ferromagnetic behavior, with near-saturation
values at the maximum field measured of approximately 1.00, 1.02,
and 1.03 Î¼B for (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4, respectively. This is comparable to the expected
pure spin value (1 Î¼B) for an S =
1/2 system using g = 2.35,36 This recovery of the full possible moment confirms the onset of
ferromagnetic order at low temperature. The insets of Figure 7 indicate a small amount of
hysteresis present in the curves. Although we have not attempted to
quantify the coercivity, it is clear that these are very soft magnetically.</p><!><p>Magnetic
susceptibility (χ) and its inverse 1/χ with
the Curie–Weiss fit (red line) in the region 30–300
K for (a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and (c) (4-ClbaH)2CuCl4. χT versus T for (d) (2-ClbaH)2CuCl4, (e) (3-ClbaH)2CuCl4, and (f) (4-ClbaH)2CuCl4, respectively.</p><p>Ferromagnetic hysteresis loops of (a) (2-ClbaH)2CuCl4, (b) (3-ClbaH)2CuCl4, and
(c) (4-ClbaH)2CuCl4 at 2, 5, and 200 K. (d)
The susceptibility
χ in the vicinity of the transition.</p><!><p>The inverse susceptibility (1/χ) plots (Figure 6) have an approximately linear
dependence on high temperature and have been fitted to the Curie–Weiss
law in the range of 30–300 K. The extracted Curie constants
(C) for (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4, are 0.50, 0.53, and 0.54 cm3 mol–1 K, respectively, and the corresponding Weiss constants (θ)
are 6.8, 12.9, and 3.3 K, the latter again being consistent with ferromagnetic
correlations. The derived effective moments (Î¼eff) are 2.004, 2.059, and 2.078 Î¼B, which, by making
use of the moment per Cu site M taken from the saturation
curves, leads to ratios Î¼eff/M of
2.004, 2.02, and 2.02. In an idealized measurement for a pure S = 1/2 spin system, this ratio should be , under the assumption of a temperature-independent
Curie constant. The interpretation of experimental values of Î¼eff should, however, always be treated with some caution37 and may overlook potential temperature-dependent
contributions, including subtle modifications to the ligand field,
spin–orbit interactions, and exchange anisotropy. The moment
obtained from the low-temperature saturated ordered ferromagnetic
state is a more reliable measure of the ground state, which indeed
agrees very well with that expected for the S = 1/2
system. The naive application of the Curie–Weiss law leads
to values of the Landé factors (g) of 2.314,
2.379, and 2.400 for (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4, respectively, which although are within the predicted range for
one Cu2+ ion with S = 1/2,35,36,38 likely to be an overestimate
given the slightly inflated values of Î¼eff.</p><p>To compare the influence of the structure on the ferromagnetic
ordering temperature Tc, we plot in Figure 7d the susceptibility
χ in the vicinity of transition. (4-ClbaH)2CuCl4 shows the onset of order at the highest temperature around
10 K, with (3-ClbaH)2CuCl4 ordering at a slightly
lower temperature. By contrast, (2-ClbaH)2CuCl4 orders at a significantly lower temperature around 6 K. Clearly,
the Curie temperature is determined mainly by the inorganic framework
[CuCl4]∞: of the three compounds, (2-ClbaH)2CuCl4 possesses the largest octahedral and interoctahedral
distortions (Table 2) and has the lowest Tc. This may be
the most significant factor, but differences in the Jahn–Teller
orbital ordering may also have an effect.</p><p>The measurements clearly
demonstrate that all three compounds possess
dominant ferromagnetic interactions within the [CuCl4]∞ layers, which are similar to previously reported 2D
layered copper(II) perovskites.35,36,38 The relationship between the ferromagnetic order and the structure
in inorganic–organic hybrid perovskites containing [CuCl4]∞ layers has recently been explored using
DFT calculations.39 Although the organic
spacer layers considered are different from those studied here, the
systems share the common motif of [CuCl4]∞ layers, with J–T distortions leading to alternating in-plane
bond lengths at nearest-neighbor Cu sites. These calculations considered
both the full structures and also a single [CuCl4]∞ layer sandwiched between the organic cations. Perhaps
not surprisingly, it was found that exchange constants that were only
slightly different in the single-layer systems compared to those in
the bulk and all materials (3D and 2D) were found to display strong
in-plane ferromagnetic correlations arising from the near-180°
superexchange pathway coupled with the alternating nearest-neighbor
bond length. The out-of-plane exchange coupling was very small and
generally antiferromagnetic in nature. A moment of around 1 Î¼B per Cu was predicted, comparable to that found in our data,
though interestingly, the apparent strong covalency suggested that
around a third of this could be associated with the Cl sites.</p><p>It is interesting to consider the driver for ferromagnetic order
in these materials since for an isotropic Heisenberg model the occurrence
of long-range order is forbidden by the Merin–Wagner theorem
in 2D rotationally invariant systems.40 The breaking of this symmetry is unlikely to come from the exchange
anisotropy due to the relatively small out-of-plane coupling. In the
DFT calculations, the inclusion of spin–orbit coupling suggested
the existence of a single ion anisotropy capable of precipitating
2D ferromagnetic order,39 giving rise to
an additional large unquenched moment of 0.1 Î¼B at
the Cu site. Calculations further suggested that an in-plane orientation
of the moment would be preferred. To explore further, the moment orientation
using magnetometry will require the growth of single crystals of the
materials, but probing the anisotropy in these materials is likely
to significantly enhance our understanding of the origins of magnetic
order. The importance of these materials as tunable 2D ferromagnets
can be viewed in the context of the significant interest in the monolayer
limit of materials such as CrI341 and Cr2Ge2Te642 and the present highly 2D systems might be expected to
have a good potential for cleavage if suitable single crystals could
be grown.</p><!><p>The optical properties of the three
materials were explored by ultraviolet–visible (UV–vis)
absorption spectra in the solid state. In the UV–vis absorption
spectra at ambient temperature (Figure 8), they display similar absorption behavior with a
sharp absorption edge at about 540, 570, and 580 nm for (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and
(4-ClbaH)2CuCl4, respectively. The strongest
absorption peaks in these spectra at around 390 nm can be attributed
to the excitation of an electron from the valence to the conduction
band, as previously studied for [CuCl4]∞ perovskites,11 suggesting a direct band-gap
characteristic.</p><!><p>UV–vis absorption spectra of (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4.</p><!><p>Although the band-gap values determined from Tauc plots have a
significant error, we can roughly predict their optical band gaps.43 The band gaps obtained from Tauc plots (Figure 9) are about 2.43
eV for (2-ClbaH)2CuCl4, 2.28 eV for (3-ClbaH)2CuCl4, and 2.25 eV for (4-ClbaH)2CuCl4, accompanied by a small regulative magnitude of 0.18 eV,
which are comparable to that observed in other perovskites, e.g.,
2.48 eV for (CH3NH3)2CuCl411 and 2.16 eV for (BED)2CuCl6.16 The interoctahedral distortions
of the inorganic framework play an important role in determining the
band gap of layered hybrid halide perovskites.44 The two polar compounds (3-ClbaH)2CuCl and (4-ClbaH)2CuCl4 possess narrower band gaps, in agreement
with their smaller interoctahedral distortions (larger Cu–Cl–Cu
angles; Table 2). As
shown in Figure 9,
the colors of the three compounds change from bright green ((2-ClbaH)2CuCl4) to green-yellow ((3-ClbaH)2CuCl4) and then to yellow ((4-ClbaH)2CuCl4), perfectly corresponding to their band gaps.</p><!><p>Tauc plots and the corresponding
crystals of (2-ClbaH)2CuCl4, (3-ClbaH)2CuCl4, and (4-ClbaH)2CuCl4.</p><!><p>In summary, we have
explored the use of chlorine-substituted benzylamine
isomers in templating differing structural features in layered copper
chloride perovskites. While the (2-ClbaH)2CuCl4 isomer displays a common, centrosymmetric variant of this generic
structure type, both the 3- and 4-substituted analogues display more
complex and distinct noncentrosymmetric structures. SHG measurements
confirm the noncentrosymmetric nature of the latter isomers, while
dielectric and magnetic measurements confirm unusually high-temperature
transitions to assume higher-symmetry phases and ferromagnetism at
low temperatures, respectively. The trends in magnetic behavior and
optical properties can be rationalized based on trends in the distortions
of the [CuCl4]∞ layers. The introduction
of chlorine atoms in the benzylamine structure is therefore shown
to be a useful strategy to modify crystal structures and physical
properties in hybrid copper perovskites. This research further highlights
2D hybrid copper halide perovskites as an excellent platform for the
development of innovative multifunctional materials.</p><!><p>Rietveld refinements
of PXRD data, crystallographic
data at 173 K, thermogravimetric analysis data, relative permittivity
data in cooling runs, and hydrogen-bonding details (PDF)</p><p>Crystallographic data (CIF)</p><p>Crystallographic
data (CIF)</p><p>cm2c00107_si_001.pdf</p><p>cm2c00107_si_002.cif</p><p>cm2c00107_si_003.cif</p><p>The authors
declare no
competing financial interest.</p><!><p>The research data supporting
this publication
can be accessed at https://doi.org/10.17630/50f4f521-80ec-4027-8676-d82c46ccc7ca.</p>