Suppression of Jahn–Teller Distortions and Origin of Piezochromism

Article pubs.acs.org/IC

Suppression of Jahn−Teller Distortions and Origin of Piezochromism and Thermochromism in Cu−Cl Hybrid Perovskite Sunny Gupta, Tribhuwan Pandey, and Abhishek Kumar Singh* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: While pressure-induced changes in the electronic, magnetic, and optical properties of Cu−Cl hybrid perovskites have been studied intensively, the correlation between these properties and pressure-induced structural changes is still vaguely understood. Here, by first-principles calculations on a model system (EDBE)[CuCl4] (EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)) (a Cu−Cl hybrid perovskite), we correlate the evolution of a series of exciting physical properties with pressure while resolving some of the long-standing debates on the fundamental electronic nature of this important class of material. The material shows two structural phase transitions and an anisotropy in compressibility with increasing pressure. After a critical pressure of 17 GPa, the structure becomes highly symmetric, thereby suppressing the Jahn− Teller distortions. At zero pressure, mapping the optical transitions with the Laporte selection rules, lower and higher energy excitations are found to be of Mott−Hubbard (MH) and charge transfer (CT) type, respectively, signifying the material to be a Mott insulator. The material shows a red shift in the charge transfer band edge with increasing pressure and temperature, demonstrating the piezochromism and the thermochromism, respectively. Piezochromism originates from the changes in mixing of Cl−Cu p−d states, while thermochromism is due to broadening of conduction band states, thereby showing different electronic and structural evolution with pressure and temperature. Furthermore, the magnetic ordering in the material was found to be stable up to higher pressures, making pressure a tool to tune the electronic property without perturbing the magnetic property.

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INTRODUCTION Hybrid perovskites are layered materials with alternate stacking of organic and inorganic molecules. While the presence of skeleton of inorganic elements enables excellent electronic properties and mechanical and thermal stability, the organic components ensures structural flexibility and low cost. Twodimensional layered Cu−Cl hybrid perovskites (with structure similar to K2CuF41), in particular with the antiferrodistortive (AFD) arrangement (Figure 1a) and strongly Jahn−Teller (J− T) active Cu−Cl atoms,2−4 have shown remarkable physical phenomena. Due to their unique properties these materials offer applications in optoelectronics,5−8multiferroics,9,10 photovoltaics,11−13and ferromagnetic14,15 and thermoelectrics16 devices. Owing to their similar structure with other transition-metal perovskites, they are also used as model systems to study the interplay between pressure-induced structural changes and various phenomena associated with it like high Tc superconductivity, ferro- to anitferromagnetic transformation, colossal magnetoresistance, and structural phase transition. Moreover, pressure also has been found to change the crystal field (CF) and CT band energies in the J−T active material,17 which directly depends on its electronic structure and are responsible for its optical properties. Cu atoms in Cu−Cl perovskites are J−T active,2−4 which engenders octahedral © XXXX American Chemical Society

tilting and elongation in the axial Cu−Cl bonds. The two magnetic Cu ions in the structure are also connected by a nonmagnetic Cl ion. Thus, changes in the Cu−Cl−Cu bond lengths with pressure can change the magnetic character of the material. Therefore, the study of structural, optical, magnetic, and electronic properties with pressure is very important. The fundamental understanding of the structure−property relation will give insight into ways to design these materials for various applications. In a recent report, the structural and electronic properties of (EDBE)[CuCl 4 ] (EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)) (a Cu−Cl hybrid perovskite) were shown to be sensitive to the pressure.18 Under pressure it exhibits yellow to black piezochromism, insulator to semiconductor transition, increase in electrical conductivity, and structural phase transition.18 Although these properties have also been studied for other perovskites, a correlation between these properties and pressure-induced structural changes around the transition-metal ion (which are responsible for these phenomena) are not clearly understood. In particular, the structural evolutions leading to (i) anisotropic compressibility in these compounds,18−21 (ii) the distant optical and magnetic behavior Received: May 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b01186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Crystal structure of (EDBE)[CuCl4] showing the polyhedral arrangement of the inorganic (Cu−Cl) layers. The organic layer is sandwiched between the inorganic layers. (Inset) Antiferrodistortive arrangement of Cu and Cl atoms. (b) Schematic representation of the Mott and charge transfer (CT) insulators based on the Zaanen−Sawatzky−Allen scheme. “U” and “Δ” are the d−d Coulomb and exchange interaction and charge transfer energy, respectively.

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of different structural phases with pressure,18 (iii) piezochromism and thermochromism,18 and (iv) critical pressure for J−T distortion suppression17−19,22,23 are still vaguely understood. Furthermore, the origin of the insulating nature of Cu−Cl hybrid perovskites is still debatable. The absorption spectra measurement of (C2H5NH3)2CuCl424,25 shows it to be a CT insulator based on the Zaanen−Sawatzky−Allen (ZSA) scheme26 as shown in Figure 1b. Other studies on absorption spectra of (C 6 H 5 CH 2 CH 2 NH 3 ) 2 CuCl 4 (PEACuCl), 27 (C3H7NH3)2CuCl4,17 and (EDBE)[CuCl4]18 show that a significant peak corresponding to the d−d transition occurs at a lower energy than the CT peak. In spite of a d−d transition peak at lower energies, all cupric halide hybrids were described as a CT insulator in the literature.18,22,27 Thus, the reason behind the insulating nature of Cu−Cl-based hybrid pervoskite remains an open question. Moreover, Cu−Cl hybrid perovskite also has a distinctive magnetic property. They show hole− orbital ordering19 and behave as two-dimensional ferromagnets (Tc ≈ 10 K).28 To the best of our knowledge, the effect of pressure on the magnetic ordering in this material has not been reported. It is also important to examine the effect of J−T distortion on the magnetic ordering in the lattice. Using first-principles calculations, we studied the interplay between pressure-induced structural changes and physical properties of (EDBE)[CuCl4]. Pressure-induced structural changes around the transition metal atom (Cu) are found to be responsible for changes in its electrical, optical, and magnetic properties. The effect of structural distortions on the electronic structure, leading to piezochromism and thermochromism with pressure and temperature, respectively, are found to be completely different. Thermochromism is observed due to broadening of conduction bands, while piezochromism originates from changes in mixing of Cl−Cu p−d states. Moreover, on mapping the electronic excitation observed in the optical spectra with the electronic structure at zero pressure, we found (EDBE)[CuCl4] to be a Mott insulator rather than a widely believed conventional CT insulator. Our work correlates the evolution of physical properties with pressure-induced structural changes and provides the rationale behind the vaguely understood phenomena in (EDBE)[CuCl4].

METHOD

Theoretical calculations were performed within first-principles density functional theory (DFT)29 using the Vienna ab initio simulation package (VASP).30,31 Ion−electron interactions were represented by all-electron projector augmented wave potentials.32,33 The Perdew− Burke−Ernzerhof34 generalized gradient approximation (GGA) was used to account for the electronic exchange and correlation. A highenergy cutoff of 300 eV with the Monkhorst−Pack37 k-grid of 2 × 5 × 5 was used in all calculations. All structures were fully relaxed by employing a conjugate gradient scheme until the forces on every atom were less than 0.005 eV/Å. In order to evaluate the absorption coefficient, the frequency-dependent dielectric response was calculated within the independent particle approximation38 without including the local field effects. To include the magnetic ordering in our calculations, we assumed three different magnetic configurations as shown in Figure S1 (Supporting Information): (i) all spins in the Cu−Cl plane to be aligned along the same direction giving both interplane and intraplane ferromagnetic behavior, (ii) all spins in the alternate Cu−Cl plane to be aligned along the opposite direction giving intraplane ferromagnetic behavior and interplane antiferromagnetic behavior, and (iii) all spins on adjacent Cu sites to be arranged in opposite direction giving both inter- and intraplane antiferromagnetic behavior. Both structures with (i) and (ii) configurations showed minimum energy and similar behavior with pressure. The materials of these families have been found to behave as ferromagnets (Tc ≈ 10 K);28 thus, we have taken configuration (i) for all calculations. The electronic structure within GGA suffers from an underestimation of the band gap as it does not correctly account for the self-interactions and derivative discontinuity in the exchange-correlation energy.35 In order to improve the value of the band gap, the hybrid exchange-correlation functional of Heyd− Scuseria−Ernzerhof (HSE06)36 was used only for electronic structure calculation. The Hartree−Fock exchange (α) was optimized to 0.08 to reproduce the experimental band gap.

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RESULTS AND DISCUSSION Structural Transition under Pressure. (EDBE)[CuCl4] hybrid perovskite (with structure similar to K2CuF41) crystallizes in the orthorhombic structure with space group Pccn.39 It is a layered structure with alternate stacking of twodimensional Cu−Cl inorganic layer and EDBE organic layer, where the organic cations adopt a zigzag conformation as shown in Figure 1a. In order to study structural evolution under pressure, a hydrostatic pressure was applied from 0 to ∼42 GPa, and all the bond lengths and bond angles were measured (in the calculated structure) as a function of pressure. Similar to other Cu−Cl hybrid perovskites19,22 of this family, (EDBE)B

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Figure 2. (a) Evolution of crystal volume with pressure showing two-phase transition α, β, and γ. (b) Evolution of Cu−Cl axial and equatorial bonds with pressure. The Cu−Cl eq(2) bond first increases and then decreases with pressure (as shown in inset). (c) Change of octahedral and unit cell volume with pressure (V0 is the volume at 0 GPa). (d) Change of lattice parameter (R − R0)/R0 (R0 is the lattice parameter at 0 GPa) with pressure showing anisotropy in the compression.

octahedron compresses less than the crystal volume and leads to octahedral tilting. On calculating the percentage change in the octahedral volume and the unit cell volume for (EDBE)[CuCl4] (Figure 2c), we found that the crystal compresses much more than the octahedra and thus should increase the octahedral tilting (the octahedra tilt angle (θ) should decrease). However, on measuring the Cu−Cl−Cu octahedral tilt angle (Figure 3a), we observe an opposite trend as shown in Figure 3b. In an ideal perovskite with no tilt, the change in bond length with pressure should follow the relation δRCu−Cu ≈ δR(Cu−Cl)axial + δR(Cu−Cl)eq (where δR is the change in the bond length with pressure).40 The Cu−Cu and (Cu−Cl)axial + (Cu−Cl)eq bond lengths and their relative variation with pressure (δR) are shown in Figure 3c and 3d, respectively. For (EDBE)[CuCl4], we observe that δRCu−Cu ≥ δR(Cu−Cl)axial + δR(Cu−Cl)eq as shown in Figure 3d, which corresponds to a increase in the octahedral tilt. The above two results are contradictory, but both can reconcile if there is an internal distortion in the octahedron. The Cl−Cu−Cl bond angle inside the octahedra decreases from an ideal value ∼90° at 0 GPa to ∼80° at 41.5 GPa as shown in Figure S2 (Supporting Information). This corresponds to a rotation about the a axis in contrast to the rotation in the b−c (inorganic) plane, which leads to the tilt in the octahedra. This shows that compressions can internally distort the octahedron without causing a tilt, signifying that octahedral tilt is not the sole reason for J−T distortion suppression with pressure. Evolution of Electronic Structure with Pressure. Cu− Cl hybrid perovskite has long been considered as a charge transfer insulator, where the charge transfer is from the metal to

[CuCl4] shows two structural phase transition and an anisotropy in compressibility with increasing pressure. The volume of the unit cell exhibits a discontinuous change with the pressure signifying first phase transition from α−β at ∼3−4 GPa and a second phase transition from β−γ at ∼8−9 GPa as shown in Figure 2a, which is in good agreement with the previous experimental pressure-dependent study of this compound.18 Cu in (EDBE)[CuCl4] is octahedrally coordinated by six Cl, forming two axial Cu−Cl (Cu−Claxial) and four equatorial Cu− Cl (Cu−Cleq) bonds. The J−T active nature of Cu ion in CuCl6 octahedra elongates the Cu−Claxial bonds, thereby distorting the structure. The respective Cu−Cl axial (R(Cu−Cl)axial) and equatorial (R(Cu−Cl)eq) bond lengths at 0 GPa are shown in Figure 2b. J−T distortion as reported in previous studies is found to be suppressed by the pressure; however, the value of critical pressure (Pc) at which complete J−T distortion suppression occurs is highly debated.17−19,22,23 In order to understand the origin of J−T distortion suppression with pressure, we measured R(Cu−Cl)axial and R(Cu−Cl)eq at different pressures. J−T distortion suppression occurs at the pressure at which R(Cu−Cl)axial becomes equal to R(Cu−Cl)eq. On calculating the equatorial and axial Cu−Cl bond lengths, we found that the bond lengths approach similar values at pressure P > 17 GPa (Figure 2b), which is in good agreement with the value of Pc = 16.7 GPa reported by a recent experimental study.18 The concept of evaluating the critical pressure was first developed by Aguado et al.23 by comparing the bulk compressibility of the lattice and the local compressibility of the octahedron. They observed in (C3H7NH3)2CuCl4 that the C

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Figure 3. (a) Schematic showing the tilt between the two Cu−Cl octahedra. (b) Octahedral tilt angle as a function of pressure. (c) Variation of Cu1−Cu2 bond length and Cu1−Cl1−Cu2, i.e., (Cu1−Cl1) + (Cl1−Cu2) with pressure. (d) Variation of δR(Cu−Cu), and δR(Cu−Cl)axial + δR(Cu−Cl)eq with pressure (δR denotes the change in bond length with pressure). (Inset in c) Various bond lengths, where Cu1−Cl1 is the equatorial (eq) bond and Cl1−Cu2 is the axial bond.

Figure 4. (a) GGA density of states of zero pressure structure with ferromagnetic ordering. Partial DOS shows contribution from Cu-d and Cl-p atoms. (b) HSE06 DOS showing improved band gap (Eg).

the ligand. According to the ZSA scheme,26 the band gap in 3d transition-metal compounds can be characterized by the relative strength of “U” and “Δ”, where U is the d−d Coulomb and exchange interaction and Δ is the charge transfer energy. In a Mott insulator U < Δ and the band gap is described only by d− d interactions, while in a charge transfer insulator U > Δ as shown in Figure 1b. Cu in CuCl4 is J−T active, and the d−d interactions in these materials are due to the crystal field (CF) splitting between eg and t2g levels and the J−T splitting between the eg levels.17 The relative strength between these d−d interactions and the charge transfer between ligand and metal

decide the nature of the band gap in these materials. The calculated optical spectrum for (EDBE)[CuCl4] shows a weak d−d transition to occur at lower energies than the CT energies as shown in Figure S3 (Supporting Information), similar to that observed in other materials of this family.1,18,27,41 These d−d transitions correspond to a MH- and not CT-type transition (Figure 1b). Thus, in order to confirm the insulating nature of (EDBE)[CuCl4], we next calculated the electronic structure. The spin-polarized density of states (DOS) shows a band gap of 0.36 eV (Figure 4a). This semiconducting behavior is in D

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Figure 5. (a) Orbital energy diagram showing the three different possible optical transition. First, within spin-splitted bands of Cu dx2−y2 character, which is Laporte spin forbidden (LSF), (ii) from other d orbitals of Cu, dz2, dyz, dxy, and dxz → dx2−y2, which is Laporte orbital forbidden (LOF), and (iii) between Cl p states to Cu dx2−y2, which is the charge transfer (CT) between Cu and Cl. Both the valence band maxima and the conduction band minima originate from the spin-splitted states of Cu dx2−y2. (b and c) ELF plot of (EDBE)[CuCl4] at 0 and 40 GPa, respectively. Blue and green atoms are Cu and Cl, respectively. Complete charge depletion near the Cu atom indicates a complete charge transfer from Cu to Cl showing ionic character of the bond. The arrow above the Cu atom in part b and c points to a region of localized and delocalized electron density, respectively, which corresponds to the unpaired electron in the dx2−y2 orbital. Scale bar in the left denotes no (0) to complete localization (1) of electrons.

agreement with the previous experimental report.18 The states near the Fermi level have more contributions from d levels of Cu than p levels of Cl as shown in Figure 4a. In order to find the orbital nature of these states, orbital-decomposed DOS (LDOS) was calculated (Figure 5a and Figure S4, Supporting Information). The d states near the Fermi level have a major contribution from dx2−y2 and dz2 orbitals of Cu, which are split eg orbitals under Jahn−Teller distortion. Moreover, the t2g orbital of Cu is also found to be split into dyz and degenerate dxy and dxz orbitals. This splitting of eg and t2g orbitals is in good agreement with previous reports on similar compounds under Jahn−Teller distortion.1,13,18,42 Furthermore, the Cl p states lies much deeper in energy than the Cu d states (Figure 4a), suggesting that the state near the Fermi level (both valence band and conduction band) is dominated by Cu d orbitals. This behavior is also observed in other transition metals in octahedral symmetry.43,44 The LDOS analysis also suggests three different optical transitions (Figure 5a and Figure S4, Supporting Information): (i) within spin-splitted bands of Cu dx2−y2 character, (ii) from other d orbitals of Cu, dz2, dyz, dxy, and dxz to dx2−y2, and (iii) between Cl p states to Cu dx2−y2. In order to confirm these possible optical transitions, we calculated the absorption spectra of (EDBE)[CuCl4]. The optical spectra show two optical transitions: (i) a weak d−d transition (i.e., between Cu dz2, dyz, dxy, and dxz → dx2−y2) near ∼1.5 eV and (ii) a strong CT transition (i.e., between Cl p →Cu dx2−y2) near ∼3 eV as shown in Figure S3 (Supporting Information). These optical transitions are in good agreement with a recent experimentally reported transmission spectra of this compound,18 thereby validating our results. However, the transition within spinsplitted bands of Cu dx2−y2 character is not observed in our calculated optical spectra. This transition can be seen as a small dip near 0.6−0.7 eV in the experimentally reported transmission spectra18 of this compound and was not observed in our calculated optical spectra (Figure S3, Supporting Information) because it violates the Laporte spin selection (ΔS = 0) rules45,46 for optical transition in centrosymmetric solids and is extremely weak. Moreover, the transition between Cu dz2, dyz, dxy, and dxz → dx2−y2 is also observed to be weak

because it is between two d levels of Cu and violates the Laporte orbital rule (Δl = ±1). However, the transition between Cl p and Cu dx2−y2 states is strong because it follows the Laporte selection rules45,46 and originates from the charge transfer between metal and ligand. The first two transitions with their weak strength suggest that these excitations are only between split Cu d states and are of Mott−Hubbard character. Since the optical transitions between Mott−Hubbard states (U) occur at lower energy than charge transfer between metal and ligand (Δ), thereby signifying (EDBE)[CuCl4] to be a Mott insulator (U < Δ) rather than a CT insulator. Further, the consideration of (EDBE)[CuCl4] as a Mott insulator correctly explains the anisotropy observed in the calculated absorption spectra. The absorption spectra show anisotropy along the three crystal directions as shown in Figure 6a−c. In the spectra along the a axis Figure 6a (which correspond to out of plane direction from the Cu−Cl plane),

Figure 6. Absorption coefficient of (EDBE)[CuCl4] at ambient pressure along the (a) x axis, (b) y axis, and (c) z axis. Anisotropy in the strength of the d−d transition and CT band edge can be seen along three directions. (d) Comparison ofd−d transition strength with pressure. E

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al.20 hypothesized that the anisotropic compressibility is due to changes in orbital occupation of the material, i.e., the shrinkage of the c-axis lattice parameter occurs due to filling of the upper Hubbard band. In order to find a reason behind the anisotropic compressibility in these compounds and also the influence of orbital occupation on the same, we studied structural changes and orbital occupancies with pressure. Our calculations show that pressure rotates the structure along the a axis, leading to alignment of the longer Cu−Cl axial bonds (RCu−Cllong) along the c axis as shown in Figure S5 (Supporting Information). This also turns out to be the highly compressible direction. Moreover, RCu−Cllong bonds as discussed earlier shorten under compression due to suppression of J−T distortion with pressure. Thus, the alignment of RCu−Cllong bond along the c axis and its enhanced shrinkage with pressure (as compared to other Cu−Cl bonds) induces larger contraction along the c axis and leads to more compressibility. Furthermore, with this alignment, the Cu−Cl−Cu bond tilts and the bond angle changes from ∼90° at 0 GPa to ∼80° at 41.5 GPa as shown in Figure S2 (Supporting Information). This tilt increases the hopping of electron from one Cu site to another Cu site and leads to increased population of the upper Hubbard band, thereby enhancing the d−d transition strength with pressure (as discussed in the previous section). Therefore, the alignment of the RCu−Cllong bond along the c axis enhances the d−d transition with pressure, giving rise to the anisotropy in compressibility and ascertains the contribution of orbital occupation on the same. Piezochromism in (EDBE)[CuCl4]. A yellow to black piezochromism was reported experimentally in (EDBE)[CuCl4],18 which is common to this class of materials.27,40 Jaffe et al.18 with their absorption spectra measurements ascribed the piezochromism to the charge transfer from Cl p → Cu d. With pressure they found a shift in the CT band edge from ∼2.6 eV at 0 GPa to ∼1 eV at 40 GPa. This shift of the CT band edge from the visible to the IR range corresponds to the yellow to black piezochromism in (EDBE)[CuCl4]. As shown in Figure 7, our calculated absorption spectra show a strong peak around ∼3 eV at 0 GPa, which is due to a charge transfer between Cu and Cl. Specifically, this charge transfer is between the Cl p and the Cu dx2−y2 orbitals (Figure 5a). On the

the d−d transition peak is weak compared to the CT peak, which is in good agreement with the previous reports.18 However, the spectra along the b and c axes (i.e., in the Cu−Cl plane) are quite different (Figure 6b and 6c) because the intensity of the d−d transition is more than that of the CT peak. The d−d transition involves hopping of electrons from one Cu atom to another. The interplane Cu−Cu hopping is weak due to the separation of the inorganic layers along the a axis, while it is strong in the b−c (Cu−Cl) plane. This anisotropy in the hopping strength leads to anisotropy in the absorption spectra. The strength of the intraplane d−d transition of a MH-type compared with the CT peak also increases with increasing pressure as shown in Figure 6 d. Moreover, with pressure we observed that the Cu−Cl−Cu bond angle decreases from the ideal 90° (Figure S2 (Supporting Information)), which signifies more overlap between the half-filled dx2−y2 orbitals of the neighboring Cu atoms. This increased overlap increases the strength of electron hopping between the adjacent Cu sites, thus leading to increased MH-type transition strength. This increased hopping transfer with pressure can also be seen from the ELF plots at 0 and 40 GPa (Figure 5b and 5c). The ELF plot at 0 GPa shows strong electron localization of the Cu (3dx2−y2) lone pair, which delocalizes at 40 GPa, signifying increased hopping transfer with pressure and further confirming increased MH-type transition strength with pressure. The complete assignment of the optical spectra peak for low-energy excitations within the MH model gives enough evidence to conclude that (EDBE)[CuCl4] can effectively be considered as a Mott insulator. The electronic structure within GGA completely describes the electronic excitations of the material. However, the transition energy between the spin-splitted dx2−y2 orbitals of Cu, which is also the band gap of the material, is slightly underestimated. In order to get a better estimate of the band gap, we included exact Hartree−Fock exchange α = 0.08 in our HSE06 calculations. The HSE band gap as shown in Figure 4b is in good agreement with the observed experimental transmission spectra.18 Structure−Property Relationship in (EDBE)[CuCl4]. A complete understanding of the evolution of structural and electronic properties with pressure allows us to provide a fundamental insight into the structure−property relationship (i.e., how structural changes in the material affect the physical properties) in Cu−Cl hybrid perovskite. Using inputs from the previous section and examining the structure−property relationship we will now discuss the rationale behind the vaguely understood phenomena in these compounds like (i) anisotropic compressibility, (ii) the distant optical behavior of different structural phases with pressure, (iii) piezochromism and thermochromism, and (iv) magnetic ordering with pressure. Anisotropic Compressibility in (EDBE)[CuCl4]. An anisotropy in the rate of compression can be seen along the three crystal directions in (EDBE)[CuCl4]. a and c directions are found to be more compressible than b (Figure 2d). In this structure, c and b are equivalent directions in the plane of Cu− Cl sheet (Figure 1a) and ideally the compressions should be the same along these two directions. Such anisotropic compressibility has been reported in other materials of this family, e.g., in (C2H5NH3)2CuCl4 (EA2CuCl4),19 and also in transition-metal complex of type MX6 (where M is the transition metal and X is the ligand), like in LaSrMnO420 and LaMnO3.21 Gössling et

Figure 7. Tauc plot50 (in units of 103 cm−4) at different pressure showing a red shift of the charge transfer (CT) peak. (Inset) Weak d− d transition. Here α, h, and ν are the absorption coefficient, Planck’s constant, and frequency, respectively. F

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the electronic structure without disturbing the magnetic property in the pressure range of 0−11 GPa. The material also showed intralayer ferromagnetic coupling, while the interlayer coupling was not clear. Both structures with interlayer antiferromagnetic and ferromagnetic coupling have lowest energy. This signifies that one can externally tune the interlayer magnetic character of the material easily. Furthermore, the varied nature of the interlayer coupling will give different resistance depending on whether the coupling is ferromagnetic or antiferromagnetic and can be of considerable use for magnetoresistive property.51,52

application of pressure this peak shows a red shift (Figure 7) and the CT band edge decreases from ∼3.15 eV at 0 GPa to ∼2.87 eV at 7.68 GPa (CT band edge calculation details are given in SE2, Figure S6, Supporting Information). This CT red shift is in good agreement with an experimental report,18 with a slight overestimation (∼14%) because of limitation of the independent electron approximation method.47−49 Thus, the red shift of the charge transfer band edge due to changes in mixing of Cl−Cu p−d orbitals expounds the CT band transitions giving rise to piezochromism. The decrease of the CT band edge can also be related to the suppression of J−T distortion with increasing pressure. Previous reports 17 on A 2 CuCl 4 structures found 27% contribution of the J−T distortion on the CT band energy. It was reported in a recent experimental study of this compound18 that the rate of decrease of the CT band energy increases on going from the α−β phase. Our results show that this shift in the rate of decrease can be corroborated to the different extent of J−T distortion suppression in the two phases. In the α phase, one of the two Cu−Claxial bond length increases with pressure (Figure 2b), which is opposite to that observed in the β phase. The difference in the variation of the Cu−Cl axial and equatorial bond lengths in the α and β phase correspond to a different J−T effect in these two phases. In the former phase, the weak J−T distortion suppression results in a weaker effect on the CT band energy. This leads to less variation of the CT band edge with pressure in the α phase in comparison to the β phase. Thus, the J−T contribution to the CT red shift is very significant. Thermochromism in (EDBE)[CuCl4]. A yellow to red-brown transition was also observed experimentally with increasing temperature from 100 to 350 K, signifying thermochromism in (EDBE)[CuCl4].18 In order to study the origin of thermochromism, we calculated the electronic structure at different temperatures. For better comparison of results, the temperature-dependent structures proposed by Jaffe et al.18 were used in our simulations. We found that the octahedral tilt angle shows nominal variation; however, the Cu−Claxial bond length increases with temperature, which signifies increased J−T distortion (Figure S7 and ST1, Supporting Information). This structural evolution with temperature is contrary to the one observed with pressure (Figure 2b), where the Cu−Claxial bond length decreases, indicating decreased J−T distortion. J−T distortion suppression as mentioned earlier has a significant contribution to the CT red shift; however, the trend with pressure and temperature is very different. The DOS of all four structures is similar, and the only difference comes from the band widths of the CBM states, which increases from 0.25 (at 100 K) to 0.35 eV (at 350 K), as shown in Figure S7 (Supporting Information). Thus, one of the possible reason (neglecting any electron−phonon effects) for thermochromism in (EDBE)[CuCl4] is due the broadening of states which increases the band widths of the CBM states. This behavior is different from the changes in mixing of Cl−Cu p−d states, which gives piezochromism. Similar broadening of states showing thermochromism was also observed in the PEACuCl structure.27 Magnetic Ordering in (EDBE)[CuCl4]. (EDBE)[CuCl4] as mentioned earlier also has magnetic ordering in the lattice. We found that the magnetic moment assuming ferromagnetic interaction remains constant up to a pressure of 11 GPa as shown in Figure S8 (Supporting Information), signifying magnetic ordering with pressure. This can be useful in tuning

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CONCLUSION In summary, we study the evolution of electronic, optical, as well as magnetic properties of (EDBE)[CuCl4] with pressure. The material shows two structural phase transitions and an anisotropy in compressibility with increasing pressure. The anisotropic compressibility has been correlated to the alignment of the RCu−Cllong bond along the highly compressible c axis with pressure. At a critical pressure of 17 GPa the structure approaches a symmetric state, thereby suppressing the J−T distortion. Further, mapping the respective transition strengths in optical spectra to the electronic structure we find excitations at lower and higher energies to be of MH and CT type, respectively, signifying the material to be a Mott insulator. This further explains the anisotropy in the absorption spectra. A CT red shift in optical absorption spectra of (EDBE)[CuCl4] was observed with both temperature and pressure leading to thermochromism and piezochromism, respectively. The structural changes causing the red shift are found to be completely different for pressure and temperature. Thermochromism is due to broadening of conduction band states, while piezochromism is due to changes in mixing of Cl−Cu p−d states. Moreover, the material shows a tunability in the interlayer ferromagnetic ordering, which can find use for magnetoresistive devices. The magnetic ordering remains stable until a pressure of 11 GPa, signifying pressure as a selective tool to tune the electronic property without disturbing the magnetic property until this pressure. This fundamental understanding of evolution of structure−property relation with pressure can lead to an unprecedented controlled tuning of functionalities of this material.

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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.inorgchem.6b01186.

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Further details about the absorption coefficient, CT band edge calculation, orbital-decomposed DOS (LDOS), structural evolution with pressure (PDF) CIFs for (EDBE)[CuCl4] at various pressures (ZIP)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.6b01186 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by DST nanomission. The authors thank the Materials Research Centre and Supercomputer Education and Research Centre, Indian Institute of Science, for providing required computational facilities.

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DOI: 10.1021/acs.inorgchem.6b01186 Inorg. Chem. XXXX, XXX, XXX−XXX