Cu-Site Disorder in CuAl2O4 as Studied by XPS Spectroscopy

The results of full study of X-ray photoelectron spectra (XPS) of spin-liquid candidate CuAl2O4 including the measurements of high-energy resolved core level (Cu \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2p$$\end{document}, Al \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$3p$$\end{document}, O \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1s$$\end{document}), Cu LMM Auger and valence band spectra are presented. The comparison of obtained results with spectra of reference samples and specially performed density functional theory calculations has confirmed a finite Cu site-disorder in CuAl2O4, where about 30% of Cu2+ ions occupy the octahedral sites. Obtained valence band spectra can be used in further theoretical studies aimed on the investigation of electronic and magnetic properties of this mysterious ma-terial.


INTRODUCTION
The AB 2 O 4 oxides with spinel structure are one of the most interesting classes of modern ceramic materials due to their physical and chemical properties that determine their technological application. For example, copper aluminate (CuAl 2 O 4 ) is used as a gas sensor [1], a catalyst in the oxidation of benzyl alcohol [2], steam reforming of methanol [3], a photocatalyst for the decomposition of pollutants in an aqueous solution [4], a solar energy absorber [5] solar watersplitting [6], etc. Therefore, for the development of these technologies, so much attention is paid to theoretical and experimental studies of various spinels and, first of all, to the study of fundamental properties that are determined primarily by its crystal structure. In CuAl 2 O 4 spinel the Cu 2+ -ions must be located in the center of A-tetrahedral sites and non-magnetic Al 3+ ions are in the center of octahedral B-sites. In this case, the Cu 2+ d-states split into and e states. The degeneracy of -states can be removed through the following two channels: due to spin-orbit coupling and Jahn-Teller distortion, which lowers the symmetry of the crystal field. As a rule, the results of X-ray and neutron diffraction studies show that the crystal structure of CuAl 2 O 4 at atmospheric pressure is cubic without any signs of tetragonal distortion [4].
However, possible stabilization of the cubic phase (contradicting to the Jahn-Teller theorem) inevitable leads to formation of the spin-orbit entangled state and strong exchange anisotropy, which may result in the spin-liquid ground state [7,8]. This hypothesis was used in particular to explain absence of the long-range magnetic order in CuAl 2 O 4 even at very low temperature [8]. Alternative explanation is based on presence of intrinsic disorder between tetra and octa sites, which prevents onset of antiferromagnetism.
It is no coincidence that in some early works it was assumed that about 30% of Cu 2+ ions occupy octahedral positions [4,5]. In this connection, of particular interest is the use of local spectral methods sensitive to the nearest surrounding of the exciting atoms. In connection with this in recent years, the resonant inelastic X-ray scattering (RIXS) has been used for this purpose [9]. In this work, we applied another method of X-ray photoelectron spectroscopy, which is also an elementand a site-selective probe.

CONDENSED MATTER
color. X-ray powder diffraction (XRPD) data were collected at room temperature on a RIGAKU Mini-Flex 600 diffractometer using Cu K radiation (2 range of 8°-140°, a step width of 0.02°, and a scan rate of 1°/min). XRPD data were analyzed by the Rietveld method using RIETAN-2000 [10].  [11]). X-ray photoelectron spectroscopy (XPS) was used to measure core excitation with help of a PHI XPS 5000 VersaProbe spectrometer (ULVAC-Physical Electronics, USA) with a spherical quartz monochromator and an energy analyzer working in the range of binding energies from 0 to 1500 eV. The X-ray spot size was 200 μm and Al Kα radiation (1486 eV) was used. The energy resolution was eV. The samples were measured at a pressure below 10 -7 Pa. All spectra were calibrated using C 1s binding energies of adventitious carbon (285 eV). Finally, all spectra were processed using PHI MultiPak 9.9.0.8 software.
The calculations of CuAl 2 O 4 and Cu(OH) 2 compounds were carried out using the Vienna ab initio simulation package [12], which was previously used to study such transition metal compounds as, e.g., Ba 4 NbMn 3 O 12 [13] or ZrC l3 [14]. We utilized the projector augmented-wave method with the Perdew-Burke-Ernzerhof type of exchange correlation functional [15] within the general gradient approximation (GGA + U) [16]. Initial crystal structure parameters were taken for CuAl 2 O 4 from [6] ( K). On-site Coulomb repulsion parameter U was taken to be 7 eV, while Hund's intra-atomic exchange is J H = 1 eV. The spin-orbit coupling was taken into account in the calculations. Figure 1 shows the XPS survey spectra of samples under study. The numerical values of the surface composition (in at %) obtained from these spectra are given in Table 1. Note that the ratio of the concentrations of the atoms of the components as a whole follows the chemical formula of the compound and the level of carbon contamination is quite low. CuO arising from multiplet splitting effects due to the interaction between the Cu core hole and the electronic configuration [17]. The Auger Cu LMM spectra (Fig. 2b) also provide the evidence that the main oxidation state of copper in CuAl 2 O 4 is 2+ [18].

RESULTS AND DISCUSSION
The spectra shown in Fig. 3 make it possible to determine the charge state of aluminum and oxygen atoms in the compound under consideration. As follows from the analysis of these spectra, the XPS Al spectrum coincides in energy position with that in Al 2 O 3 , which means that Al atoms are trivalent. XPS O spectrum shows that the contribution of hydroxyl groups [19] is noticeably much smaller than that from the lattice oxygen in CuAl 2 O 4 .
Let us consider the origin of the two-peak structure (Cu 1 and Cu 2 ) of the XPS Cu spectrum of CuAl 2 O 4 shown in Fig. 2a. It is generally believed that CuAl 2 O 4 at atmospheric pressure is in the cubic phase without any signs of tetragonal distortion [8]. This is the spinorbit coupling, which can be responsible for suppression of the Jahn-Teller distortions and absence of corresponding splitting in the Cu-states [20]. However, the local symmetry breaking in this compound induced by Jahn-Teller distortions cannot be completely ruled out, since for this the spin-orbit coupling constant must exceed some critical value , where g is the vibronic coupling parameter and B is the stiffness constant [21].   Fig. 2a), which is much larger than possible splitting due to both the spin-orbit coupling or the crystalfield splitting because of the Jahn-Teller distortions. In fact, this energy difference is comparable with value fixed in splitting of Cu L XAS spectra [9] and the ratio of their intensities suggests that the Cu 1 and Cu 2 peaks correspond to the contributions of tetra-and octasites, respectively, and estimates degree of disorder in 30%. Therefore, we can conclude that the indepen-2 p dent site-selective and element-selective X-ray measurements confirm a finite site-disorder in CuAl 2 O 4 .
The results of the total density of electronic states calculations of CuAl 2 O 4 performed in DFT + U approach and taken into account spin-orbit coupling (Total DOS SOC) and Jahn-Teller distortion (Total DOS ) are presented in Fig. 4. The comparison of the obtained results shows that taking into account the spin-orbit coupling does not lead to significant changes in the distribution of the total density of occupied states. The energy separation of the main peaks of the calculated total density of states is well reproduced by the XPS valence band spectrum also shown in this figure except the presence of an additional feature in the energy range of 8-10 eV. Earlier, a similar feature was recorded in the XPS valence band spectra of other transition metal compounds and was attributed to contribution of (OH)hydroxyl group [22]. To check the origin of this feature in our case, we carried out an additional DFT calculation of the Cu(OH) 2 compound, which actually showed the presence of an additional band in this energy range. This is consistent also with the results of measurements of the XPS O 1s spectra shown in Fig. 3b where the contribution of the hydroxyl group is also recorded.
In conclusion, we studied electronic properties of CuAl 2 O 4 , which is actively investigated now because of possible realization of the spin-liquid state [8], suppression of long-range magnetic ordering and Jahn-Teller distortions due to the spin-orbital entanglement [7,20]. Our results show the presence of a substantial Cu-Al disorder by the XPS measurements. This disorder may affect formation of an antiferromagnetic order and development of static Jahn-Teller distortions.

CONFLICT OF INTEREST
The authors declare that they have no conflicts of interest.

OPEN ACCESS
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