Structural and Magnetic Properties of Substituted Delafossite-Type Oxides CuCr1 － xScxO2 ()
1. Introduction
The family of layered oxides generally referred to as delafossites derive their name from the mineral CuFeO2 [1], with which their crystal structures are isotypic. Denoted by the general chemical formula ABO2, the crystal structures of these materials (Figure 1) are characterized by layers of distorted, edge-sharing octahedra with oxygen coordinating metal cations (B = typically transition or group 13 elements, but also some rare earth species), separated by planar layers of a transition metal (A = typically Cu, Ag, Pd or Pt) which are linearly coordinated along the c-axis by two oxygen sites. The stacking orientation of these two layers results in two basic polymorphs, 2H (space group P 63/mmc) and 3R (space group R-3m), (Figure 1). Characterized by a wide range of possible compositions, the delafossite oxides also exhibit significant richness in properties. For example, depending upon the choice of A, compounds can display metallic (A = Pd, Pt) or semiconducting/insulating (A = Cu, Ag) behavior [2]. Motivated by the relatively high electrical conductivities observed in some delafossite compounds [2-5], in particular observations [3,4] of ptype conductivity and relatively high optical transparency in CuAlO2 thin films, significant efforts in recent years have been focused on the usage of these materials in applications as transparent conductors [5].
Copper-scandium oxide (CSO) is a p-type transparent Cu+-based oxide with a delafossite structure. Holes in CSO are introduced by substituting divalent species for the octahedral Sc-sites and by intercalating with excess oxygen near the Cu+ planes. CSO has the smallest Cu-Cu distance for which excess oxygen intercalation is possible between the layers of Cu [6,7]. In Cu+-based delafossites, the Cu-Cu distances significantly affect the electri-
Figure 1. Delafossite structure (3R) and (2H).
cal conductivity, because holes predominantly pass through the Cu+ planes [7]. Therefore, CSO potentially has the highest p-type electrical conductivity in transparent Cu+- based delafossites. From a crystallographic point of view, CSO has two crystalline phases depending on the stacking periods along the c -axis: rhombohedral (CSO (3R)) and hexagonal (CSO (2H)) [6,7]. The a-axis lattice constants of each phase are 0.3216 nm for CSO (3R) [8] and 0.3215 nm for CSO (2H) [9] and they are very close to that of ZnO (0.3250 nm) [10]. Since ZnO is an n-type transparent conducting oxide with a hexagonal crystalline structure, transparent p-n heterojunctions with excellent interfaces can be fabricated with CSO (0 0 01)/ ZnO (0 0 01) epitaxial films.
CuCrO2 is also reported to exhibit both antiferromagnetic [11] and ferroelectric [12] behavior below its Néel temperature, T = 25 K. The magnetic structure and the mechanism responsible for the ferroelectricity are still under investigation. Recently we report, on the effect of Sc and (Sc + Mg) substitution on the structural and physical properties of delafossite-type CuCrO2 oxide [13]. The present study intends to study in detail the effect of the substitution of Sc3+ for Cr3+ in the widest possible substitution range on the structural, spectroscopic and magnetic properties of CuCr1−xScxO2 0 ≤ x ≤ 0.4.
2. Experimental Details
First, polycrystalline samples of CuCr1−xScxO2 0 ≤ x ≤ 0.4 were prepared by using the standard solid-state reaction. Stoichiometric mixtures (0.5 g) of Cu2O, Cr2O3 Sc2O3 were ground and pressed into pellets, which were set in alumina crucible. The samples were fired several times at 1050˚C for 12 h. The X-ray powder diffraction patterns of the reacted pellets were collected with a PANalytical diffractometer equipped with a CuKα source (Kα1 and Kα2) in the 2q range from 10˚ to 90˚ at room temperature.
Strain and size components were extracted from line widths using the Williamson-Hall (WH) analysis [14]. This method uses the fact that the crystallite size contribution varies as tanθ. The equation used is
Where L is the integral width, λ is the wavelength used, D is the size of the coherent diffraction domain, k is a near-unity constant, and e is the microstrain term. As a result, a plot of (L∙cosq ) as a function of (sinq) yields D from the constant term and e from the slope.
Magnetization dependence on temperature was measured in a Superconducting quantum Interference Device (SQUID) magnetometer while heating from 2.0 to 300 K in 0.1 T.
The Raman spectra were recorded at room temperature with the 514.5 nm line of an Ar+ laser, excitation from a Spectra Physics krypton ion laser. The compounds were studied with a low laser power (102 mW). One scanning of 60 seconds has been used for each spectral range. No damage of the material by the laser has been observed. The beam was focused onto the samples using the macroscopic configuration of the apparatus.
3. Results and Discussions
3.1. Structural Characterizations
The crystal structure of CuCr1−xScxO2 (0 £ x £ 0.4) samples is determined by recording x-ray diffraction patterns shown in Figure 2. According to the standard XRD patterns of CuCrO2 samples they are of single phase with delafossite structure (space group R-3m), without any detectable secondary phase up to 20% Sc content. Above this composition a shoulder like shape appear around 34 in 2q, which increase in intensity with the Sc content. The deconvolution of the observed shoulder points award an overlapping of two peaks (inset of Figure 2), which were assigned to the CuScO2 (3R) and CuScO2 (2H). A mixed-layer form of CuScO2 (intergrown 3R and 2H) generally results when heating mixtures of equimolar amounts of Cu2O and Sc2O3 under various conditions [15]. These mixed-layer products were dominated by a 3R type structure with stacking faults of the 2H type.
In addition, Figure 2 clearly shows that 00l peaks are sharp, but other (h0l) peaks are broadened, shifted and asymmetric due to stacking faults perpendicular to the c axis. In the sample CuCr0.6Sc0.4O2, the peaks broadening are very important in a way that the phase identification is risky. Considering the differences in the radii of Cr3+ (0.615 Å) and Sc3+ (0.75 Å) [16], the possibility of substituting Cr ion by the Sc in the full range 0 < x < 1 is surprising. As a condition for isomorphous miscibility, Vegard’s rule [17] allows deviations in the different radii only up to 15%. In the present case the deviation between Cr3+ and Sc3+ is about 22%. At this percentage, the two secondary phases are appeared. This fact accounts well for a high stabilization potential of the delafossite structure.
Composition dependences of the unit cell volume and lattice constants are given in Table 1 and displayed in Figure 3. For x = 0 (pure CuCrO2) our a and c parameter values are in very good agreement with most of those previously reported for powders as well as for single crystal (e.g. a = 2.9741 (1) Å, c = 17.110 (2) Å from [18]).
The increase of the unit cell volume with increasing x well agrees with the increase of the ionic radius as chromium (r = 0.615 Å) is substituted for scandium (r = 0.75 Å). However, the change is rather anisotropic as it is mainly due to the shrinking of the a parameter whereas
Figure 2. Room temperature XRD patterns of CuCr1−xScxO2 (0 ≤ x ≤ 0.3) samples with a rhombohedral delafossite structure.
Table 1. Cell parameter evolution with Sc content.
Figure 3. lattice constants evolution with the Sc content.
the c parameter remains more or less constant.
As the Cu-O distance does not vary much in delafossite, this tendency reflects a flattening of the MO6 octahedra. A nice geometric account of this evolution is given by J. Tate et al. [19] where the authors suggest that the strong repulsion between M3+ ions across the octahedron shared edges reduces the O-O distance to the contact distance.
Therefore the increase of the size of M cation leads to an increase of the octahedron distortion and in turn of the M-M distance that corresponds to the a parameter. We may recall that in CuMO2, as M changes from Al3+ to La3+ [20,21], the a parameter undergoes a huge increase from 2.8 up to 3.8 Å.
Strain generated by the Sc substitution was determined from the Williamson-Hall relationship. Plots of (L∙cosθ) as a function of (sinθ) are given in Figure 4. They show a remarkable difference in angular dependence of the line width for different families of inter reticular planes: the h0l planes yield an important contribution of microstrains (high slope), while this effect is almost negligible in 00l planes. This shows that this material behaves rather anisotropically, and that strains affects mostly bonding in the basal (ab) planes.
Finally note that the oxygen stoichiometry cannot be reliably obtained by X-ray diffraction data, and the presence of three different mixed-valences precludes a reliable use of chemical redox titration. However, all the samples being prepared in the same conditions (initial oxygen stoichiometry, amount of powder) and their oxygen content is assumed to close to 2 in all cases. This assumption is supported by a previous study of CuCrO2 showing that this compound does not accommodate large oxygen off-stoichiometry [22]. This could be confirmed by neutron diffraction, which was not available to us during this study.
3.2. Raman Spectroscopy
The delafossite structure belongs to point group C3v and space group R-3m. The four atoms in the primitive cell of its rhombohedral R-3m structure give rise to 12 optical phonon modes (Γ = A1g + Eg + 3A2u + 3Eu) in the zone center (k ~ 0): three acoustic and nine optical modes. Among these, the two phonons modes with A1g and Eg symmetry are Raman-active. The former arises due to the Cu-O bond vibration along the c-axis, whereas the doubly degenerate E-modes describe the vibration along the a-axis. Since there is only one mode of each symmetry, the exact eigenvector is determined without any lattice dynamical model required. Pellicer-Porres et al. [23-25] have discussed the phonon dispersion at the zone center for CuGaO2 delafossite. They proposed that the inversion center is lost along the ΓT direction and the symmetry is reduced from D3d to C3v. According to compatibility relations, A1g and A2u on one hand and Eg and Eu modes on the other transform to A1 and E-modes, respectively.
Figure 5 shows the Raman spectra of CuCr1−xScxO2 for different scandium content using a 514.5 nm laser wavelength excitation. The Raman spectrum of CuCrO2