The crystal structure of CuCr_1−xSc_xO₂ (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 CuCr_0.6Sc_0.4O₂, 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

Table 1. Cell parameter evolution with 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.

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 = A_1g + E_g + 3A_2u + 3E_u) in the zone center (k ~ 0): three acoustic and nine optical modes. Among these, the two phonons modes with A_1g and E_g 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 Γ_T direction and the symmetry is reduced from D3d to C3v. According to compatibility relations, A1g and A2u on one hand and E_g and E_u modes on the other transform to A₁ and E-modes, respectively.

Figure 5 shows the Raman spectra of CuCr_1−xSc_xO₂ for different scandium content using a 514.5 nm laser wavelength excitation. The Raman spectrum of CuCrO₂