Structural correlations in Cs₂CuCl₄: Pressure dependence of electronicstructures

 

E. Jara,¹ J. A. Barreda-Argueso,¹ J. Gonzalez,1 R. Valiente,² F. Rodrguez¹

¹Nanomaterials Group-IDIVAL, Dpto. Física Aplicada,
²*E-mail: fernando.rodriguez@unican.es
³MALTA TEAM, DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain.
⁴Universidad de Cantabria, 39005 Santander, Spain.

Received: 19 November 2018, Accepted: 10 May 2019  DOI: http://dx.doi.org/10.4279/PIP.110004

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Abstract

We have investigated the crystal structure of Cs₂CuCl₄ in the 0-20 GPa range as a function of pressure and how pressure affects its electronic properties by means of optical absorption spectroscopy. In particular, we focused on the electronic properties in the low-pressure Pnma phase, which are mainly related to the tetrahedral CuClp units distorted by the Jahn-Teller effect. This study provides a complete characterization of the electronic structure of Cs₂CuCl₄ in the Pmna phase as a function of the cell volume and the Cu-Cl bond length, Rc-u-ci. Interestingly, the opposite shift of the charge-transfer band-gap and the Cu^{1 4}+ d-d crystal-field band shift with pressure are responsible for the strong piezochromism of Cs₂CuCl₄. We have also explored the high-pressure structure of Cs₂CuCl₄ above 4.9 GPa yielding structural transformations that are probably associated with a change of coordination around Cu²+. Since the high-pressure phase appears largely amorphized, any structural information from X-ray diffraction is ruled out. We use electronic probes to get structural information of the high-pressure phase.

Keywords: Cs2CuCl4; electronic structure; high pressure; band gap

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I. Introduction

Cs2CuCl₄ (orthorhombic Pnma at ambient pressure) is a wide-band-gap Charge-Transfer (CT) semiconductor (E_g = 2.52 eV), which exhibits a puzzling optical behaviour under pressure, associated with the Cu¹+ absorption and its structural changes 1. Both Cl^-^Cu²⁺ CT and d-d absorption bands undergo unusually large pressure shifts and intensity changes showing abrupt jumps at about 5 GPa. This crystal exhibits a yellow-orange color at ambient conditions and below 5 GPa, which is mainly defined by the tail of the CT band (band gap) placed around 450 nm 2. The isolated

CuCl;)^- tetrahedra in the Pnma phase show a flat-tened (D_2d) distortion by the Jahn-Teller (JT) effect, which is responsible for the low-lying CT band gap, and thus its yellow-orange color, in compari-son to other transitionmetal ion (M) isomorphous compounds Cs₂MCl₄ (M = Co, Zn) 3. Unlike Cs₂CoCl₄, the d-d bands of Cu¹+ (3d⁹), which are split by the JT distortion, do not affect the color as they appear in the near-infrared range at 1110 and 1820 nm 2,3. Thanks to the study of electronic and crystal structures under high-pressure condi-tions of this relatively highly compressible material (bulk modulus: K₀=15.0(2) GPa) 4, we are able to establish structural correlations to understand: (i) the electronic properties of Cu¹+ in tetrahedral coordination in the less compressible oxides; and (ii) how a lattice of independent CuCl4^- units under compression evolves towards denser phases. The variation of the crystal structure of Cs2CuCl4 and Cs₂CoCl₄ under pressure has been previously investigated by X-ray diffraction (XRD) in the 0-5 GPa range, where both crystals are in the Pnma crystallograpic phase. However, Cs2CuCl4 under-goes a structural phase transition just above 5 GPa yielding a deep color change from orange to black. The high pressure phase could not be identified by XRD due to amorphization 4. In general, the op-tical properties of Cu²+ chlorides like Cs₂CuCl₄ are strongly dependent on the crystal structure (poly-morphism), particularly, the Cu²+ coordination - symmetry and crystal-field strength- and the way Cu²+ ions are coupled to each other, i.e. either as isolated units or as interconnected Cu-Cu links through Cl^- ligand sharing 5,6. Therefore, the knowledge of how these links and crystal-field ef-fects express in the optical spectra are essential to extract structural information from the electronic spectra at high-pressure conditions. An important goal is to establish correlations between structure and electronic properties 3. In this work, we in-vestigate the relationship between dihedral Cl-Cu-Cl angle of the JT-distorted flattened tetrahedra and the Cu²+ d-orbital splitting experimentally ob-served by optical absorption and its pressure depen-dence. These correlations will be used to analyze how the band gap energy and d-d bands vary with pressure in the Cs₂CuCl₄ Pnma phase, and how they change after the structural phase transition above 5 GPa.

II. Experimental

Single crystals of Cs2CuCl4 were grown by slow evaporation at 30^o C from acidic (HCl) solution containing a 2:1 stoichiometric ratio of the CsCl and CuCl₂.H₂O . The Pnma space group was checked by XRD on powder samples using a Bruker D8 Advance diffractometer. The measured cell parameters at ambient conditions were: a = 9.770 A, b = 7.617 Á, c = 12.413 A.

A Boehler-Almax Plate diamond anvil cell (DAC) was used for the high-pressure studies. 200 pm thickness Inconel gaskets were pre-indented and suitable 200 pm diameter holes were perforated with a BETSA motorized electrical discharge machine. Given that Cs₂CuCl₄ is soluble in common pressure transmitting media like methanol-ethanol-water (16:4:1), spectroscopic paraffin oil (Merck) was used as alternative pressure transmitting media. It must be noted, however, that according to the ruby line broadening non-hydrostatic effects were significant in the explored range, as previously reported 6.

The microcrystals used for optical absorption in the high-pressure experiments were extracted by cleavage from a Cs2CuCl4 single crystal. The crystal quality was checked by means of a polar-izing microscope. The d-d spectra were obtained using powdered Cs₂CuCl₄ filling the gasket hole of the DAC for obtaining suitable optical and in-frared absorption spectra due to the high oscilla-tor strength of these transitions. The experimental set-up for optical absorption measurements with a DAC has been described elsewhere 8-11. The spectra were obtained by means of an Ocean Optics USB 2000 and a NIRQUEST 512 monochromators equipped with Si- and InGaAs-CCD detectors for the VIS and NIR, respectively. A Thermo Nicolet Continupm FTIR provided with a reflective-optic microscope was used in the IR range. Pressure was calibrated from the ruby R-line luminescence shift.

III. Results and discussion

i. Electronic structure, optical absorption spectra and piezochromism of Cs₂CuCl₄

The optical absorption spectrum of Cs₂CuCl₄ at ambient pressure in the Pmna phase consists of two intense bands in the near infrared associated with d-d electronic transitions within the CuCl4^-(D2d) and a ligand-to-metal CT absorption in the visible, which is responsible for the band-gap and the concomitant yellow-orange color of this crystal (Fig. 1). d-d peaks can be assigned to tetrahedral crystal-field transitions of CuCl4^- using both T_d and D_2d irreps notation) 12. Within D_2d, the two main absorption peaks correspond to spin-allowed d-d electric-dipole transitions from the ²B₂ ground state to the ²E and ²A₄ excited states and are lo-cated at 0.55 and 1.3 eV, respectively. It must be noted that the first transition ²B₂ ^²E is associated with the splitting of the parent-tetrahedral ²T₂ state into ²B₂+²E due to the JT distortion of D2d symmetry with ²B2 being the electronic ground state corresponding to a flattened tetra-hedron (inset of Fig. 1). Thus, the presence of this transition in the optical spectra constitutes the fingerprint of a JT distortion; in T_d the corresponding transition energy, i.e. ²T₂ splitting, would be zero besides splitting contributions caused by the spin-orbit interaction. As it is shown in Fig. 1, the splitting of ²B₂ ^²A₁ +²B₁ transitions (a single ²T₂ ^²E transition in T_d) is not observed spectroscopically as they appear as a single band in the absorption spectrum due to symmetry selection rules. Actually, in D_2d, there are only two allowed electric-dipole transitions from the ²B₂ groundstate: ²B₂ ^²E (x,y-polarized) and ²B₁ ^²A₁ (z-polarized) 7, in agreement with experimental observations.

Figure 1: Optical absorption spectrum of CS2CUCI4 at ambient conditions. The blue line represents the fit of the experimental points to the sum of two Gaussian profiles. The crystal-field bands correspond to crystal-field d-d transitions: ²B₂ ^ ²E (0.55 eV) and ²B₂^ ²Ái (1.30 eV) in D_2d symmetry. The high-energy absorption threshold corresponds to the Cl^-^Cu²⁺ CT band gap, which is E_g = 2.52 eV.

 

Figure 2 shows the peak energy variations of d-d transitions as a function of pressure in both Pnma and high-pressure phases of Cs₂CuCl4. Their transition energies and corresponding pressure rates are given in Fig. 2. Interestingly, the first JT-related band associated to the ²B₂ ^²E transition shows a large redshift with pressure at a rate of -73 meV/GPa, while the second one, associated to ²B₂ ^²A₁, shifts slightly towards higher energies (+7.5 meV/GPa). It must be noted that the transition energy variation of both bands E(P) un-dergoes a change of slope at the structural phase transition at 4.9 GPa, thus being an adequate probe to explore phase transition phenomena.

The CT direct band gap is also very sensitive to pressure. Unlike d-d bands, pressure-induced CT redshift is responsible for the strong piezochromism of Cs₂CuCl₄, the color of which changes with pres-sure from yellow-orange to black, particularly at the structural transition to the high-pressure phase (Fig. 3). Cs₂CuCl₄ is a CT semiconductor with a direct gap of 2.52 eV at ambient conditions which redshifts with pressure at a rate of -20 meV/GPa. This means that significant color changes are ex-pected at pressures well above 5 GPa as shown in Fig. 3. The direct band gap, E_g, was determined from the tail of the absorption threshold by plot-ting (hv x a)² against hv, with a being the absorption coefficient, once the absorption background was subtracted. E_g, was obtained by the intercep-tion of this plot with a= 0. As Fig. 3 shows, E_g (P) experiences an abrupt jump of about 0.3 eV at the phase transition at 4.9 GPa. Above this pressure, we observe a band structure with at least two no-ticeable absorption peaks at 0.43 and 1.43 eV, the pressure dependence of which is shown in Fig. 2. The Pnma phase is recovered in down-stroke below about 3 GPa, thus having a hysteresis of 2 GPa at room temperature. It must be also noted that the difference between the transition pressure measured in single crystal (5.4 GPa) and powder (4.9 GPa) of Cs₂ CuCl₄ must be associated to a lack of hydro-staticity in powdered samples, which reduces the transition pressure. However, the phase transition can be established at 4.9 GPa in upstroke, which corresponds to initial observation of traces of the high-pressure phase within the pressure range of phase coexistence.

ii. Angular overlap model for CuCl4^-

The unusual pressure shifts of the two d-d bands (Fig.2) can be explained semi-quantitatively within the ligand-field theory through the Angular Overlap Model (AOM) 13,15-17. The initial flattened-tetrahedron symmetry (D_2d) of CuCl;)^-, which splits the parent tetrahedral t₂ and e orbitals into b₂ + e and a₁ + b₁, respectively, will change upon Cs₂CuCl₄ compression. The corresponding split-ting will change depending on how the relative variations of the T_d crystal-field strength and the JT-related dihedral Cl-Cu-Cl angle evolve with pressure. According to crystal-field theory and experimental observations 1,3, the crystal-field strength usually increases by decreasing the Cu-Cl bond distance, R, whereas the dihedral angle tends to decrease with pressure, approaching the T_d angle (109.47^o) under high compression. XRD results show that R and ycí-Cu-cí change from R = 2.230 A and ycí3-Cu-cí3 = 127.4^o 17 at ambient pressure to R = 2.199 A and y_cl3_-Cu-Cl3 = 122.3^o at 3.9 GPa 4, in agreement with ex-pectations for a JT system. In order to apply the AOM to determine the d-d transition ener-gies of CuCl4^- as a function of structural param-eters, instead of 7_Ci_-Cu-Ci we will use the angle P = 1/2(y_C;_-Cu-C; - 109.47^o), which represents the deviation of the Cl-Cu-Cl angle from its T_d value. Then, P = 0 in T_d symmetry and P = 35.3^o in a square-planar D_4h symmetry. For CuClp in Cs₂CuCl₄, P = 8.5±0.5^o at ambient conditions and P = 6.4±0.5^o at 3.9 GPa. We will use the AOM to simulate the transition energies as a function of R and P for explaining why the first band largely shifts to lower energy whereas the second one, more sensitive to the crystalfield strength, shifts slightly to higher energies with pressure.

 

Within the AOM, the expressions to calculate the electronic energies in a MX4^- system are given as a function of the AOM parameters e_a,e_n, e_sd and e_pd and the X-M-X bond angle y as shown in Eq. (1) 16.

 

 

These expressions are suitable to account for the transition energies in different MX^^- systems hav-ing different dihedral angles 17. This has been es-pecially useful for explaining the variation of transition energies obtained from absorption spectra as a function of the dihedral angle in series of Cu²+ chlo-rides, providing dihedral angles for CuCl;)^- ranging between 127^o and 180^o or, equivalently, from 3 = 8.5^o to 3 = 35.3^o 13,15-17. The spectroscopic series of CuCl4 can be explained using the following AOM parameters: e_a =0.635 eV, e_n = 0.113 eV, e_sd = 0.114 eV and e_pd = -0.0025 eV 13,16,17. Figure 4 shows the energy of the d-d transitions of CuCl4^- as a function of 3, where, additionally, we have included the spin-orbit interaction -AL.S using A = 0.103 eV 17. The D_2d states appear additionally split as ²B₂(r₇); ²E(r6+r₇); ²B₁(r₇); and ²A₁(r₆) following double group irrep notation (see Fig. 4a) 7.

The pressure-induced energy shifts in Cs₂CuCl₄ have been simulated by scaling the AOM parameters to structural data at 3.9 GPa on the assump-tion of a power law for the volume as (VO/V)^5/3 using the equation of state of Cs₂CuCl₄ in the Pnma phase 4. So, we obtained the following AOM parameters at 3.9 GPa: e_a = 0.78 eV, e_n = 0.139 eV, e_sd = 0.172 eV and e_pd = -0.0025 eV. Although this may be a rough approximation for describ-ing the variation of AOM parameters with pres-sure/volume, the result of these simulations allows us to explain the band shifts with pressure (Fig.

4). Pressure-induced R (or V) reduction increases the energy separation of the parent T_d orbitals, e and t₂, by 10Dq, while reduction of 0 decreases the t₂ and e orbital splittings in D_2d. As illustrated in Fig. 4, both effects induce band shifts in the two d-d bands similar to those observed experimentally. Therefore, these structural correlations, which are based on the energy shifts of the crystal-field bands in Cs₂CuCl4, indicate that the main pressure effect on the JT-flattened CuCl^^- is reducing the Cl-Cu-Cl bond angle from 8.5^o at ambient pressure to 6.4^o at 3.9 GPa, consistently with structural data 4. Therefore, these results support the adequacy of the d-d spectra to explore structural changes induced by pressure in transition-metal chlorides in-volving JT ions like Cu²+.

IV. Conclusions

Electronic absorption spectra allow us to eluci-date that the pressure dependence of the electronic structure of Cs₂CuCl₄ can be explained to a great extent on the basis of T_d CuCl4^-, the volume of which is roughly eight times more incompressible than Cs₂CuCl₄ bulk. The piezochromic phase tran-sition at 4.9 GPa is mainly associated with the CT redshifts, particularly in the high-pressure phase well above 4.9 GPa. The new high-pressure phase, although it has not been identified yet, probably involves a change of coordination from CuCl4^- flat-tened tetrahedra to a structure consisting of ligand sharing CuCl))^- octahedra as suggested by its d-d transition energies. Correlations between crystal-field bands and structure of CuCl4^- through the AOM allow us to infer structural changes under-gone by CuCl4^- induced by pressure on the basis of the crystal-field energy shifts. The measured shifts are consistent with a reduction of both the bond distance and the Cl-Cu-Cl angle, i.e. reduction of the JT distortion, with pressure in agreement with previous XRD data.

 

Acknowledgements - Financial support from the Spanish Ministerio de Economía, Industria y Competitividad (Project Ref. MAT2015-69508-P) and MALTA-CONSOLIDER (Ref. MAT2015-71010REDC). EJ also thanks the Spanish Ministerio de Ciencia, Innovación y Universidades for a FPI research grant (Ref. No. BES-2016-077449).

 

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