Here we report a new versatile approach for the calculation of phonon modes which is applicable to anisotropic, dilute alloys with allowance for a large variety of alloying elements. This approach has significant advantages over previously reported methods, especially for the lattice dynamics of such complex alloys. We use this approach to model the effects of Fe-doping on the vibrational modes in dilute alloys of CuAl 1−x Fe x O 2 (x = 0, 0.01, 0.05, and 0.10) delafossite powders. These samples were structurally characterized with X-ray diffraction (XRD) combined with Rietveld refinement to measure their lattice parameters and Raman and FTIR spectroscopies to measure optical phonon frequencies. To compare experimental results from XRD with calculations for lattice parameters, we use a disordered supercell for x in the range 0−0.1. Both results agree well with Vegard's law. For the phonon calculations, an approach using a disordered supercell is not feasible because it is too computationally expensive. Instead, we developed our weighted dynamical matrix (WDM) approach that uses a straightforward ordered supercell for forceconstant calculations of the CuAlO 2 and CuFeO 2 parent end points, and combines them using a WDM approach leading to an effective medium for vibrational mode calculation in random alloys. Computationally, when Fe is substituted for Al (increasing x), an increase in the bond length is observed leading to a red-shift in the peak positions in all of the phonon modes vs x, in agreement with the experimentally observed trend.

For powder samples of CuAl1- x Fe x O2(x = 0, 0.01, 0.05, and 0.1), measured optical properties are compared with model simulations and phonon spectra are compared with simulations based on weighted dynamical matrix approach.

Delafossites are promising candidates for photocatalysis applications because of their chemical stability and absorption in the solar region of the electromagnetic spectrum. For example, CuAlO2 has good chemical stability but has a large indirect band gap, so that efforts to improve its absorption in the solar region through alloying are investigated. The effect of dilute alloying on the optical absorption of powdered CuAl1–x Fe x O2 (x = 0.0–1.0) is measured and compared to electronic band structures calculations using a generalized gradient approximation with Hubbard parameter and spin. A new absorption feature is observed at 1.8 eV for x = 0.01, which red-shifts to 1.4 eV for x = 0.10. This feature is associated with transitions from the L-point valence band maximum to the Fe-3d state that appears below the conduction band of the spin-down band structure. The feature increases the optical absorption below the band gap of pure CuAlO2, making dilute CuAl1–x Fe x O2 alloys better suited for solar photocatalysis.

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