Papers in Physics, vol. 11, art. 110004 (2019) Received: 19 November 2018, Accepted: 10 May 2019 Edited by: A. Goñi, A. Cantarero, J. S. Reparaz Licence: Creative Commons Attribution 4.0 DOI: http://dx.doi.org/10.4279/PIP.110004 www.papersinphysics.org ISSN 1852-4249 Structural correlations in Cs2CuCl4: Pressure dependence of electronic structures E. Jara,1 J. A. Barreda-Argüeso,1 J. González,1 R. Valiente,2 F. Rodŕıguez1∗ We have investigated the crystal structure of Cs2CuCl4 in the 0-20 GPa range as a func- tion 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 CuCl2−4 units dis- torted 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, RCu−Cl. Interestingly, the opposite shift of the charge-transfer band-gap and the Cu2+ 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 associ- ated 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. I. Introduction Cs2CuCl4 (orthorhombic Pnma at ambient pres- sure) is a wide-band-gap Charge-Transfer (CT) semiconductor (E g = 2.52 eV), which exhibits a puzzling optical behaviour under pressure, associ- ated with the Cu2+ absorption and its structural changes [1]. Both Cl−→Cu2+ CT and d-d absorp- tion 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 ∗E-mail: fernando.rodriguez@unican.es 1 MALTA TEAM, DCITIMAC, Facultad de Ciencias, Uni- versidad de Cantabria, 39005 Santander, Spain. 2 Nanomaterials Group-IDIVAL, Dpto. F́ısica Aplicada, Universidad de Cantabria, 39005 Santander, Spain. CuCl2−4 tetrahedra in the Pnma phase show a flat- tened (D 2d) distortion by the Jahn-Teller (JT) ef- fect, which is responsible for the low-lying CT band gap, and thus its yellow–orange color, in compari- son to other transition-metal ion (M ) isomorphous compounds Cs2MCl4 (M = Co, Zn) [3]. Unlike Cs2CoCl4, the d-d bands of Cu 2+ (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 Cu2+ in tetrahe- dral coordination in the less compressible oxides; and (ii) how a lattice of independent CuCl2−4 units under compression evolves towards denser phases. The variation of the crystal structure of Cs2CuCl4 and Cs2CoCl4 under pressure has been previously 110004-1 Papers in Physics, vol. 11, art. 110004 (2019) / E. Jara et al. 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 30oC 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 Å, b = 7.617 Å, c = 12.413 Å. A Boehler-Almax Plate diamond anvil cell (DAC) was used for the high-pressure studies. 200 µm thickness Inconel gaskets were pre-indented and suitable 200 µm diameter holes were perforated with a BETSA motorized electrical discharge ma- chine. 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 me- dia. 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 experimen- tal 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 Continuµm 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 CuCl2−4 (D 2d) 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 CuCl2−4 using both Td and D2d 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 2A1 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 asso- ciated 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 110004-2 Papers in Physics, vol. 11, art. 110004 (2019) / E. Jara et al. 0 200 400 600 800 0.4 0.8 1.2 1.6 2.0 2.4 2.8 α (c m -1 ) E g =2.52 eV Charge-transfer band gap Crystal field 2B 2 2E 2B 2 2A 1 +2B 1 E (Ev) 2E 2T2 2B1 2A1 2E 2B2 E2 E1 Figure 1: Optical absorption spectrum of Cs2CuCl4 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→ 2A1 (1.30 eV) in D2d symmetry. The high-energy ab- sorption threshold corresponds to the Cl−→Cu2+ CT band gap, which is Eg = 2.52 eV. the fingerprint of a JT distortion; in Td the cor- responding 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 transi- tions (a single 2T2 →2E transition in Td) is not observed spectroscopically as they appear as a sin- gle band in the absorption spectrum due to sym- metry selection rules. Actually, in D2d, there are only two allowed electric-dipole transitions from the 2B2 ground state: 2B2 →2E (x,y-polarized) and 2B1 →2A1 (z-polarized) [7], in agreement with ex- perimental observations. 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 tran- sition 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 ener- gies (+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 (hν×α)2 against hν, with α being the absorp- tion coefficient, once the absorption background was subtracted. E g, was obtained by the intercep- tion of this plot with α= 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 Cs2CuCl4 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 CuCl2−4 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 Over- lap Model (AOM) [13,15–17]. The initial flattened- tetrahedron symmetry (D2d) of CuCl 2− 4 , 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 vari- ations of the T d crystal-field strength and the JT- related dihedral Cl–Cu–Cl angle evolve with pres- sure. According to crystal-field theory and experi- mental observations [1, 3], the crystal-field strength 110004-3 Papers in Physics, vol. 11, art. 110004 (2019) / E. Jara et al. (a) (b) 0 0.5 1 1.5 2 2.5 E (eV) O pt ic al D en si ty (a rb . u n. ) 0.0 GPa 3.5 GPa 4.9 GPa 6.3 GPa 7.4 GPa 0 0.5 1 1.5 2 0 1 2 3 4 5 6 7 8 E (e V ) P(GPa) Pnma HP phasePT 1.25 +0.0075 P 0.66 - 0.073 P 0.81-0.068 P 0.51+0.178 P Figure 2: (a) Variation of the absorption spectrum of Cs2CuCl4 with pressure in the Pnma and high-pressure phase (P > 4.9 GPa). (b) Variation of the peak energy of the main absorption bands with pressure. Pressure coefficients dereived by fitting in the low-pressure Pnma phase are included. 1.8 2.0 2.2 2.4 2.6 2.8 0 5 10 15 E (e V ) P(GPa) Pnma HP phaseP T Direct Gap 2.53-0.02 P 2.29-0.03 P 0.1 GPa 2.1 GPa 5.4 GPa 7.9 GPa 18.2 GPa11.5 GPa (b)(a) Figure 3: (a) Variation of the charge-transfer band gap with pressure in both Pnma phase and high-pressure phase. The direct gap shifts toward lower energies with pressure. (b) Images of a single crystal of Cs2CuCl4 in a DAC at different pressures. Note how the crystal varies from yellow to dark red and eventually to black upon increasing pressure. The piezochromism is associ- ated with the redshifted charge-transfer band gap with pressure. Single crystal dimensions (ambient pressure): 80 × 80 × 22 µm3. usually increases by decreasing the Cu–Cl bond distance, R, whereas the dihedral angle tends to decrease with pressure, approaching the Td angle (109.47o) under high compression. XRD results show that R and γCl−Cu−Cl change from R = 2.230 Å and γCl3−Cu−Cl3 = 127.4 o [17] at am- bient pressure to R = 2.199 Å and γCl3−Cu−Cl3 = 122.3o 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 CuCl2−4 as a function of structural param- eters, instead of γCl−Cu−Cl we will use the angle β = 1/2(γCl−Cu−Cl − 109.47o), which represents the deviation of the Cl–Cu–Cl angle from its Td value. Then, β = 0 in Td symmetry and β = 35.3 o in a square-planar D4h symmetry. For CuCl 2− 4 in Cs2CuCl4, β = 8.5±0.5o at ambient conditions and β = 6.4±0.5o at 3.9 GPa. We will use the AOM to simulate the transition energies as a function of R and β for explaining why the first band largely shifts to lower energy whereas the second one, more sensitive to the crystal-field strength, shifts slightly to higher energies with pressure. Within the AOM, the expressions to calculate the electronic energies in a MX2−4 system are given as a function of the AOM parameters eσ,eπ,esd and epd and the X–M–X bond angle γ as shown in Eq. (1) [16]. 110004-4 Papers in Physics, vol. 11, art. 110004 (2019) / E. Jara et al. D2d D2dTd Td β = 8.5º β = 0º β = 0º β = 6.4º Pressure β d-levels 2T2 2E 10Dq10Dq0 0.0 0.5 1.0 1.5 2.0 2.5 0 5 10 15 20 25 30 35 40 Pe ak E ne rg y, E (e V ) β (º) CuCl4 2- Experimental Cs2CuCl4 P=0 GPa P=0.0 GPa P=3.9 GPa2B2 2A1 2E 2B1 2A1 2B1 2E Experimental Cs2CuCl4 P=3.9 GPa 2E 2T2 2E 2B2 2B1 2A1 2E 2B2 2B1 2A1 (a) (b) Figure 4: (a) Calculated crystal-field energies for CuCl2−4 as a function of the distortion angle (β) using AOM. β = 0◦ corresponds to Td (regular tetrahedron), and β = 32.5 ◦ to D4h CuCl 2− 4 (square-planar). Solid and dashed lines correspond to calculations at ambient pressure and 3.9 GPa, respectively. The spin-orbit coupling have been included in the calculations with λ= -829 cm−1 (see text for details). Filled color symbols correspond to experimental data from the compound series providing different Cl–Cu–Cl bond angles for CuCl2−4 [15, 17]. Empty circles correspond to present experimental data for Cs2CuCl4 at ambient pressure (orange) and 3.9 GPa (dark red). Note that the trends of the variation is in agreement with structural data obtained by XRD [4]. (b) Schematic diagram of the Cu2+ d-orbital splitting in D2d and Td symmetry for four different configurations corresponding to ambient pressure (left) and high-pressure conditions (3.9 GPa, right). In Td only R is changed whereas both R and β are modified in D2d. ∆E(2B2 →2 E) = 3[sin4(γ/2) − 1/2 sin2(γ)]eσ + [sin2(γ) − 2 cos2(γ) − 2 cos2(γ/2)]eπ, ∆E(2B2 →2 B1) = 3 sin4(γ/2)eσ + [sin2(γ) − 4 sin2(γ/2)]eπ − 13.3 sin4(γ/2) cos2(γ/2)epd, (1) ∆E(2B2 →2 A1) = 3 sin4(γ/2)eσ − 4[cos2(γ/2) − 1/2 sin2(γ/2)]2eσ − 2 sin2(γ)eπ + 16[cos2(γ/2) − 1/2 sin2(γ/2)]2esd − 13.3 sin4(γ/2) cos2(γ/2)epd. These expressions are suitable to account for the transition energies in different MX2−4 systems hav- ing different dihedral angles [17]. This has been es- pecially useful for explaining the variation of transi- tion energies obtained from absorption spectra as a function of the dihedral angle in series of Cu2+ chlo- rides, providing dihedral angles for CuCl2−4 ranging between 127o and 180o or, equivalently, from β = 8.5o to β = 35.3o [13, 15–17]. The spectroscopic se- ries of CuCl2−4 can be explained using the following AOM parameters: eσ =0.635 eV, eπ = 0.113 eV, esd = 0.114 eV and epd = −0.0025 eV [13, 16, 17]. Figure 4 shows the energy of the d-d transitions of CuCl2−4 as a function of β, where, additionally, we have included the spin-orbit interaction -λ~L.~S using λ = 0.103 eV [17]. The D2d states appear additionally split as 2B2(Γ7); 2E(Γ6+Γ7); 2B1(Γ7); and 2A1(Γ6) following double group irrep notation (see Fig. 4a) [7]. The pressure-induced energy shifts in Cs2CuCl4 have been simulated by scaling the AOM param- eters to structural data at 3.9 GPa on the assump- tion of a power law for the volume as (V0/V ) 5/3 us- ing the equation of state of Cs2CuCl4 in the Pnma phase [4]. So, we obtained the following AOM pa- rameters at 3.9 GPa: eσ = 0.78 eV, eπ = 0.139 eV, esd = 0.172 eV and epd = −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. 110004-5 Papers in Physics, vol. 11, art. 110004 (2019) / E. Jara et al. 4). Pressure-induced R (or V ) reduction increases the energy separation of the parent Td orbitals, e and t2, by 10Dq, while reduction of β decreases the t2 and e orbital splittings in D2d. 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 CuCl2−4 is reducing the Cl–Cu– Cl bond angle from 8.5o at ambient pressure to 6.4o at 3.9 GPa, consistently with structural data [4]. Therefore, these results support the adequacy of the d-d spectra to explore structural changes in- duced 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 Td CuCl 2− 4 , 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 CuCl2−4 flat- tened tetrahedra to a structure consisting of ligand sharing CuCl4−6 octahedra as suggested by its d-d transition energies. 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