Effect of Pressure and Temperature on the Local Structure and Lattice Dynamics of Copper(II) Oxide

Available online at www.sciencedirect.com

ScienceDirect Physics Procedia 85 (2016) 27 – 35

EMRS Symposium: In situ studies of functional nano materials at large scale facilities: From model systems to applications, EMRS Spring Meeting

Effect of pressure and temperature on the local structure and lattice dynamics of copper(II) oxide A. Kuzmina,∗, A. Anspoksa , A. Kalinkob , A. Rumjancevsa , J. Timoshenkoa , L. Natafc , F. Baudeletc , T. Irifuned b Universit¨ at

a Institute of Solid State Physics, University of Latvia, Kengaraga street 8, Riga LV-1063, Latvia Paderborn, Naturwissenschaftliche Fakult¨at, Department Chemie, Warburger Straße 100, Paderborn 33098, Germany c Synchrotron SOLEIL, l’Orme des Merisiers, Saint-Aubin 91190, France d Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan

Abstract Microcrystalline and nanocrystalline (6 nm) CuO were studied in situ by the Cu K-edge X-ray absorption spectroscopy as a function of pressure (0-20 GPa) and temperature (10-300 K). Pressure dependence of X-ray absorption near edge structure (XANES) was interpreted within the full-multiple-scattering formalism based on the relaxed atomic structure determined by ab initio linear combination of atomic orbital (LCAO) calculations. Temperature dependence of the mean-square relative displacement (MSRD) for the four shortest Cu–O distances was obtained from the analysis of extended X-ray absorption fine structure (EXAFS) and described by the correlated Einstein model with the characteristic temperature θE =589 K. It was found that the thermal motion of copper and four oxygen atoms forming square-planar coordination is strongly correlated. © Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license c 2016 by Elsevier  2016The TheAuthors. Authors. Published (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EMRS Spring Meeting 2016. Peer-review under responsibility of the organizing committee of the EMRS Spring Meeting 2016

Keywords: CuO; XANES; EXAFS; high pressure; low temperature

1. Introduction Copper(II) oxide (CuO) is a hot topic of fundamental and applied research for many years. In bulk form it is a p-type semiconductor with a narrow band gap of 1.4-1.7 eV (Heinemann et al. (2013)), whose exact size and the character (direct or indirect) are still a subject of experimental and theoretical studies (Meyer et al. (2012)). CuO finds broad range of applications in nanostructured form, including in photocatalysis, photodetectors, spintronic devices, sensors, supercapacitors, nanoenergetic materials, field emission displays and as electrode material in lithium-ion batteries (Yang et al. (2003); Habisreutinger et al. (2013); Zhang et al. (2014)). Electron-phonon and spin-lattice interactions play an important role in bulk CuO, which has monoclinic structure (space group C2/c (No. 15), Z=4, 4 Cu in 4(c) and 4O in 4(e)) built up of CuO6 octahedra (Fig. 1) strongly distorted ∗

Corresponding author. Tel.: +00371-67251691 ; fax: +00371-67132778. E-mail address: a.kuzmin@cfi.lu.lv

1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EMRS Spring Meeting 2016 doi:10.1016/j.phpro.2016.11.077

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A. Kuzmin et al. / Physics Procedia 85 (2016) 27 – 35

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b a Fig. 1. Left panel: crystalline structure of monoclinic CuO (space group C2/c (No. 15)) with the indicated unit cell (Åsbrink and Norrby (1970)). Right panel: four-fold planar coordination of copper by oxygen atoms; the Cu–O bond lengths are given.

by the first-order Jahn-Teller effect (Åsbrink and Wa´skowska (1991)). As a result, the two axial Cu–O bonds are significantly longer (∼2.79 Å) than the four others (∼1.95 Å), so that the coordination of copper atoms can be described as a square-planar one. Strong spin-lattice coupling in CuO, leading to low temperature (below the N´eel temperature T N =230 K) anomalies in the lattice constants at the antiferromagnetic phase transitions, was evidenced by high resolution X-ray diffraction measurements (Yamada et al. (2004)). The magnetostriction effect was also proposed recently as an explanation of the giant negative thermal expansion observed in small CuO nanocrystals of ∼5 nm below the magnetic ordering temperature (∼200 K) (Zheng et al. (2008)). Investigations of the interplay between the bonding and magnetism in CuO are also important for understanding of room temperature ferromagnetism observed in nanoscaled CuO (Gao et al. (2010); Qin et al. (2010)), multiferroic properties of CuO (Kimura et al. (2008)) and due to the connection with the physics of high-Tc superconducting materials, whose basic units are Cu–O chains or layers (Filippetti and Fiorentini (2005)). CuO is also well suited to study the charge ordering phenomenon (Zheng et al. (2000)) and compressibility of copper–oxygen bonds (Malinowski et al. (1990); Ehrenberg et al. (1999)). Among different experimental techniques, X-ray absorption spectroscopy using synchrotron radiation is an ideal tool for in situ studies of the local atomic and electronic structure in both bulk and nanosized materials (Bordiga et al. (2013)). In this study we report on the Cu K-edge X-ray absorption spectroscopy of microcrystalline and nanocrystalline CuO as a function of pressure (0-20 GPa) and temperature (10-300 K). The obtained experimental results on the local atomic structure and lattice dynamics in CuO are discussed and interpreted based on the results of ab initio calculations. 2. Experimental Polycrystalline CuO was commercial powder (Adrich, 99+% purity). CuO nanoparticles were prepared by a decomposition in air at 130◦ C of Cu(OH)2 precipitate, produced during reaction of aqueous solutions of copper nitrate and sodium hydroxide (Tran and Nguyen (2014)). The size of CuO nanocrystallites (6 nm) was estimated by the Scherrer method. Room-temperature pressure-dependent (P=0-20 GPa) Cu K-edge X-ray absorption spectroscopy study of microcrystalline and nanocrystalline (6 nm) CuO samples was performed using the dispersive setup of the ODE beamline at SOLEIL synchrotron (Baudelet et al. (2011)) with a membrane-type nano-polycrystaline diamond anvil cell (NDAC) (Tetsuo et al. (2003); Ishimatsu et al. (2012)). The pressure in the cell was controlled using ruby luminescence. The SOLEIL synchrotron operated in the top-up mode with the energy E=2.75 GeV and current I=450 mA. The X-ray synchrotron radiation, produced by bending magnet, was dispersed and focused by a cooled single-crystal Si(111) monochromator bent in four points. Two mirrors installed before and after the monochromator were used for a harmonic rejection. The X-ray absorption spectrum was detected by a Princeton Instruments PIXIS-400 CCD camera coupled with a scintillator.

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A. Kuzmin et al. / Physics Procedia 85 (2016) 27 – 35 Table 1. Comparison of the lattice parameters (a, b, c, β), the fractional coordinate of oxygen atom (y(O)) and the unit cell volume (V0 ), calculated using the LCAO DFT/HF method and determined by high-resolution powder X-ray diffraction (HRPXRD) measurements at T =100 K (Yamada et al. (2004)). HRPXRD P=0 GPa a (Å) b (Å) c (Å) β (◦ ) y(O) V0 (Å3 ) R(Cu-O1 ) (Å) R(Cu-O2 ) (Å) R(Cu-O3 ) (Å)

4.68457 3.42219 5.12887 99.7037 0.4195 81.047 1.946 1.964 2.785

4.818 3.290 5.159 99.81 0.4098 80.59 1.959 1.980 2.701

LCAO calculations P=10 GPa 4.806 3.141 5.087 101.92 0.3820 75.15 1.941 1.966 2.524

P=20 GPa 4.781 3.053 5.030 103.11 0.3653 71.50 1.926 1.953 2.420

Temperature dependent (T =10-300 K) Cu K-edge X-ray absorption spectroscopy of microcrystalline CuO was performed at normal pressure in transmission mode at the DORIS III synchrotron (HASYLAB/DESY, Hamburg) at the C bending-magnet beamline (Rickers et al. (2007)). The storage ring DORIS III operated at E = 4.44 GeV and Imax = 140 mA. The higher order harmonics were effectively eliminated by detuning of the monochromator Si(111) crystals to 60% of the rocking curve maximum, using the beam-stabilization feedback control. The X-ray beam intensity before and after the sample was measured by ionization chambers filled with argon and krypton gases. The Oxford Instruments liquid helium flow cryostat was used to maintain the required sample temperature. The powder samples, deposited on Millipore filter and fixed by Scotch tape, were used in the experiments.

3. Ab initio quantum-mechanical calculations The atomic structure of CuO under high pressure was calculated using the ab initio linear combination of atomic orbital (LCAO) method based on the hybrid exchange-correlation density functional (DFT)/Hartree-Fock (HF) scheme. The calculations were performed using the CRYSTAL14 code (Dovesi et al. (2014)). We used the same all-electron basis sets for copper (Ruiz et al. (2003)) and oxygen (Piskunov et al. (2004)) atoms as previously for CuWO4 (Kuzmin et al. (2013)). The evaluation of the Coulomb and exchange integrals was performed with the tolerance factors (Dovesi et al. (2014)) of 10−8 , 10−8 , 10−8 , 10−8 , 10−16 and the convergence criterion for the energy was set to 10−10 a.u. The Monkhorst-Pack scheme (Monkhorst and Pack (1976)) for an 8×8×8 k-point mesh in the Brillouin zone was applied. The SCF calculations were performed for hybrid DFT/HF functional (Adamo and Barone (1999)) with the Hartree-Fock admixture of 13% in the exchange part of DFT functional (Kuzmin et al. (2013)). The results of our LCAO DFT/HF calculations do not account for temperature effects and, therefore, should be compared with the experimental data at low temperature to minimise thermal effects. Since CuO is an antiferromagnetic semiconductor below the N´eel temperature T N of about 230 K (Yang et al. (1988)), the spin-polarised calculations were performed upon applying the external hydrostatic pressure between 0 and 20 GPa and optimizing the lattice parameters (a, b, c, β) and the Wyckoff position y(O). The obtained structural parameters, including three nearest interatomic distances Cu–O, are compared with the experimental high-resolution powder X-ray diffraction data at 100 K (Yamada et al. (2004)) in Table 1. Rather good agreement between calculated at P=0 GPa and experimental values of the lattice parameters (a, b, c and β) is observed: the average error is about 2%, being smaller than in the previous works reviewed recently by Gattinoni and Michaelides (2015). An increase of pressure up to 20 GPa produces the strongest effect on the b-axis and the Wyckoff position y(O) in agreement with the results of high-pressure neutron powder diffraction study of bulk CuO (Malinowski et al. (1990); Ehrenberg et al. (1999)). As a result of applied pressure, the unit cell volume V0 is reduced by ∼11% and the axial

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R(Cu–O3 ) bonds in CuO6 octahedra are shortened by ∼10%. The remaining four Cu–O bonds (2×R(Cu–O1 ) and 2×R(Cu–O2 )) are less affected and decrease by less than 2%.

4. XANES and EXAFS analysis Normalized experimental Cu K-edge X-ray absorption near edge structure (XANES) spectra of microcrystalline and nanocrystalline CuO are shown in Fig. 2 for three selected pressures. The variation of XANES for both samples upon increasing pressure is not dramatic and is related to a decrease of the unit cell volume, predicted by our LCAO DFT/HF calculations (Table 1). To understand the origin of the main features in the XANES of CuO, the theoretical Cu K-edge XANES spectra were calculated as a function of the cluster radius R from 2.8 Å to 8.0 Å around the absorbing Cu atom (Fig. 3(a)). The smallest cluster with the size of 2.8 Å was composed of six oxygen atoms forming the first coordination shell of copper plus the absorber Cu atom, whereas the largest cluster with the size of 8.0 Å included 111 copper and 106 oxygen atoms. The gradual increase of the cluster size allows one to estimate the contribution of the outer coordination shells into the total XANES and to control the convergence of the simulations. The calculations were performed by the ab initio real-space FDMNES code (Joly (2001); Bunau and Joly (2009)) using the full-multiple-scattering (FMS) formalism and muffin-tin (10% overlap) self-consistent potential. The dipole and quadrupole transitions were taken into account, and the real Hedin-Lundqvist exchange-correlation potential was employed. The calculated XANES spectra were broadened to account for the core hole level width (1.8 eV at the Cu K-edge) and other multielectronic phenomena. Note that all main peaks in the experimental XANES are reproduced considering the cluster of 4.6 Å radius with 36 atoms around the absorber. Our results are in agreement with previous ˇ (1992); Sipr ˇ and Sim˚ ˇ unek (2001); Bocharov et al. (2001)). XANES studies of bulk CuO (Norman et al. (1985); Sipr Based on the results of the LCAO DFT/HF simulations, the FMS calculations of the Cu K-edge XANES spectra of bulk CuO were conducted at selected pressures for the cluster radius R=8.0 Å (Fig. 3(b)). Note that the shape of the calculated spectra resembles closely that of the experimental XANES suggesting that the atomic structure compression accompanied by some relaxation is well reproduced. The experimental Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra χ(k)k2 (k is the wavenumber of the photoelectron) of microcrystalline CuO (Fig. 4) were analyzed using the EDA software package (Kuzmin (1995)) following  conventional procedure (Rehr and Albers (2000); Aksenov et al. (2006)). The wavenumber is defined as k = (2m/2 )(E − E0 ) where m is the electron mass,  is the Planck constant, and E is the photoelectron energy. The value of the parameter E0 was found for the experimental data so to match the energy scale of the theoretical Cu K-edge EXAFS calculated by the ab initio MS real-space FEFF9 code (Rehr et al. (2010)): it was fixed during the fits.

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Fig. 3. Left panel: full-multiple scattering (FMS) calculations of the Cu K-edge XANES for bulk CuO (P=0 GPa) as a function of the cluster radius R. The spectra are vertically shifted for clarity. Right panel: FMS calculations of the Cu K-edge XANES for bulk CuO structure obtained from LCAO DFT/HF simulations for several pressures. The energy scale is relative to the theoretical Fermi level E F . The cluster radius R=8.0 Å. 







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Fig. 4. Temperature dependence of the experimental Cu K-edge EXAFS χ(k)k2 (left panel) and their Fourier transforms (FTs) (right panel) for bulk CuO.

The Fourier transforms (FTs) of the EXAFS spectra were calculated using the 10%-Gaussian window function and were not corrected for the backscattering phase shift of atoms, therefore the positions of all peaks are displaced to smaller distances relative to their crystallographic values. An increase of temperature leads to the EXAFS oscillation damping at high k-values and a reduction of all peaks in their FTs. The effect is relatively weak for the first peak in the FTs at 1.5 Å due to the nearest four oxygen atoms located at ∼1.95 Å from the absorbing Cu atom but is much stronger at longer distances. This fact indicates on the importance of the correlation effects for the four shortest Cu–O bonds. Note that the remaining two axial oxygen atoms of the CuO6 octahedra are located at ∼2.79 Å and contribute to the second peak in FTs at 2.4 Å. First, we estimated the influence of the multiple-scattering (MS) effects on the EXAFS of CuO using the FEFF9 code (Rehr et al. (2010)). The self-consistent calculations were performed for a cluster of 8 Å radius with the CuO structure (Yamada et al. (2004)) and centered at the absorbing Cu atom. The inelastic losses were taken into account using the complex exchange-correlation Hedin-Lundqvist potential. The default values of muffin-tin radii (Rmt (Cu)=1.32 Å and Rmt (O)=1.10 Å) were used in the construction of cluster potential. Calculated single-scattering

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Fig. 5. Calculated single-scattering, double-scattering, triple-scattering and higher order (>3) multiple-scattering contributions to the Cu K-edge EXAFS χ(k)k2 and their Fourier transforms (FTs) for bulk CuO. The range of the nearest four oxygen atoms (Cu–O4 ) is also indicated in the right panel.

(SS), double-scattering (DS), triple-scattering (TS) and higher order (>3) MS EXAFS contributions for a cluster of 8 Å radius around the absorbing Cu atom in CuO are shown in Fig. 5. One can conclude that only SS, DS and TS contributions from the atoms located up to ∼6 Å are significant, and the first peak in FT at 1.5 Å contains only the SS contribution. Therefore, the contribution of the nearest four oxygen atoms (peak at 1.5 Å in Fig. 4 (right panel)) was isolated by Fourier filtering procedure. Next, assuming closeness of the four interatomic Cu–O distances, it was analysed at each temperature using the single-scattering EXAFS equation (Beni and Platzman (1976); Kuzmin and Chaboy (2014)) χl2 (k) = S 02

N l f (π, k, R) exp(−2σ2 k2 ) sin(2kR + φl (π, k, R) + 2δlc (k)), kR2

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where S 02 is a many-body reduction factor; N is the coordination number of copper atoms; R is the radius of the first coordination shell; σ2 is the mean-square relative displacement (MSRD) for the Cu–O bond; f l (π, k, R) and φl (π, k, R) are the photoelectron backscattering amplitude and phase shift functions due to the oxygen atoms of the first coordination shell of copper; 2δlc (k) is the final-state phase shift at the absorbing atom; l is the angular momentum of the photoelectron. The coordination number was fixed at N=4, and S 02 value was equal to 0.6. The backscattering functions f l (π, k, R) and φl (π, k, R) were calculated by the FEFF9 code (Rehr et al. (2010)). The results of the best fits with only two free parameters (R and σ2 ) are shown in Fig. 6 (left panel): they indicate that the model (1) gives good agreement with the experimental EXAFS data at all studied temperatures. Note that an extension of the analysis beyond the first coordination shell is not trivial and will be performed in the future using reverse Monte Carlo technique (Timoshenko et al. (2014a,b)). The temperature dependence of the interatomic distance R(Cu–O) was negligible, whereas that of the MSRD σ2 is shown in Fig. 6 (right panel) and can be approximated by the correlated Einstein model (Sevillano et al. (1979)) with the characteristic temperature θE =589 K. The sum of the mean-square displacements (MSDs) for Cu and O atoms, obtained from powder X-ray diffraction measurements (Yamada et al. (2004)), is also shown in Fig. 6 (right panel) for comparison and can be described by the Einstein model with the characteristic temperature θE =305 K. Note that the MSRD and MSD values for the Cu–O bonds are related as (Booth et al. (1995))   MSRDCu−O = MSDCu + MSDO − 2φ MSDCu MSDO , (2) where φ is a dimensionless correlation parameter, which is equal to +1 for perfectly in-phase atom motion, to zero for completely independent motion, and to −1 for perfectly antiphase motion. From data reported in Fig. 6 (right panel) one can find φ ≈ 0.81 in the temperature range from 100 to 300 K. This means that the thermal motion of copper and four oxygen atoms forming square-planar coordination is strongly correlated. The large value of θE = 589 K, due to small values of the MSRD factor and its weak temperature dependence, suggests relatively strong Cu–O bonding.

33

A. Kuzmin et al. / Physics Procedia 85 (2016) 27 – 35

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Fig. 6. Left panel: temperature dependence of the Cu K-edge EXAFS χ(k)k2 (open circles – experiment, solid lines – best fit) for the nearest Cu–O atom pairs in bulk CuO. Right panel: temperature dependence of the mean-square relative displacements (MSRD) for the four nearest Cu–O atom pairs (solid circles) and of a sum of the mean-square displacements (MSD) for Cu and O atoms (open squares) from high-resolution powder X-ray diffraction measurements (Yamada et al. (2004)). Solid and dashed lines are the Einstein models (Sevillano et al. (1979)) with the characteristic temperatures θE = 589 K and 305 K, respectively.

5. Conclusions In situ Cu K-edge X-ray absorption spectroscopy was used to probe the local atomic structure and lattice dynamics in copper(II) oxide as a function of pressure (0-20 GPa) and temperature (10-300 K). The atomic structure compression in microcrystalline CuO upon increasing pressure was determined from ab initio linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange-correlation density functional (DFT)/Hartree-Fock (HF) scheme. This structural information was employed in the full-multiple-scattering calculations of the Cu K-edge XANES spectra, which reproduce well experimentally observed behaviour, thus supporting the accuracy of our theoretical calculations. The main effect of pressure is related to a reduction of the unit cell volume and a change of the oxygen atom Wyckoff position y(O). The axial distortion of the CuO6 octahedra is reduced under pressure mainly due to shortening of two longest Cu–O bonds. Similar effects are expected to occur in nanocrystalline (6 nm) CuO due to closeness of the experimentally observed pressure dependencies of the Cu K-edge XANES spectra in two samples. The analysis of the Cu K-edge EXAFS temperature dependence for microcrystalline CuO was performed based on the detailed evaluation of the multiple-scattering effects. It was shown that the contribution from the first peak in the Fourier transform of the EXAFS can be accurately simulated within the single-scattering approach. Its analysis provides the evidence of strong bonding and significant correlation of atomic thermal motion between copper and four nearest oxygen atoms forming square-planar coordination. These results correlate well with above mentioned findings on the compression of the CuO6 octahedra under pressure. The obtained structural information on the behaviour of the local environment in microcrystalline CuO provides the basis towards understanding of the structure and properties of copper oxide nanoparticles. This question will be addressed in the forthcoming publications. Acknowledgements The present research was supported by the Latvian National Research Program IMIS2. References Adamo, C., Barone, V., 1999. Toward reliable density functional methods without adjustable parameters: the PBE0 model. J. Chem. Phys. 110, 6158–6170.

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