Raman scattering measurements of phonon anharmonicity in the delafossite CuGa1‐xCrxO2 (0 ≤ x ≤ 1) films

Abstract

AbstractRaman spectra of the delafossite CuGa1‐xCrxO2 (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 (Ag), 328 (A2u), 396 (Eg), and 757 (A1g) cm−1 for the CuGaO2 film, and phonon modes of the CuCrO2 film are around 211 (Ag), 324 (A2u), 458 (Eg), and 712 (A1g) cm−1, respectively. The A1g 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 A1g phonon mode undergoes a larger phonon softening but a smaller line broadening relative to that of the Eg phonon mode, which indicated the anisotropic behavior for the delafossite CuGa1‐xCrxO2 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 Eg and A1g 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 A1g mode of the CuGa0.8Cr0.2O2 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.