Raman scattering measurements of phonon anharmonicity in the delafossite CuGa_1-xCr_xO₂ (0 ≤ x ≤ 1) films

Raman spectra of the delafossite CuGa_1-xCr_xO₂ (0 ≤ x ≤ 1) solid solution films have been measured in the temperature range from 80 to 823 K. Raman phonon modes are observed at 224 (A_g), 328 (A_2u), 396 (E_g), and 757 (A_1g) cm⁻¹ for the CuGaO₂ film, and phonon modes of the CuCrO₂ film are around 211 (A_g), 324 (A_2u), 458 (E_g), and 712 (A_1g) cm⁻¹, respectively. The A_1g phonon mode has a distinct redshift with increasing Cr composition, which indicates that the interatomic potential becomes weaker between Cu and O atoms. Except Raman modes at the Brillouin zone center, the additional phonon modes in the spectral range of 500−670 cm⁻¹ were observed due to Raman selection rules being relaxed by the crystal defects. With increasing temperature, the A_1g phonon mode undergoes a larger phonon softening but a smaller line broadening relative to that of the E_g phonon mode, which indicated the anisotropic behavior for the delafossite CuGa_1-xCr_xO₂ films. The nonlinear temperature dependence of the phonon frequency and linewidth is an indication of nonharmonic lattice dynamics. The Raman shift and the linewidth of the E_g and A_1g optical modes as a function of temperature were studied as an experimental evidence of the combined effect of lattice expansion and anharmonic phonon–phonon interaction. And the three-phonon anharmonic coupling plays the dominant role in the anharmonicity behavior. The phonon anharmonicity behavior of the CuGaO₂ and CuCrO₂ films is slightly different. The larger Γ₀ is intrinsic for the A_1g mode of the CuGa_0.8Cr_0.2O₂ film due to the broadening by the crystalline defects, which perturb the translation symmetry of the harmonic crystal, making the momentum conservation rule for phonon scattering partially relaxed.