A Combined Theoretical and Experimental Raman Scattering Study of BaZrS₃-BaHfS₃ Solid Solutions

Ba(Zr,Hf)S₃ solid solutions are proposed for photovoltaic applications and a fast non-destructive measurement of the composition of these solutions and the identification of any possible secondary phases is a prerequisite for understanding their opto-electronic properties. Previously multi-wavelength Raman spectroscopy has been used for such purposes in other chalcogenide solution series. Here we calculate the non-resonant one-phonon Raman spectra of pure BaHfS₃ and BaZrS₃, which show only subtle differences between them, since the most prominent modes are dominated by the sulfur atoms and the change in mass of going from Hf to Zr is balanced by a near equal and opposite change in the bonding. To test this experimentally, a solution series of BaZr_1-xHf_xS₃ (0≤x≤1) powders was synthesised and free of a secondary phase, HfS₃ identified by 633 nm excitation Raman spectroscopy. The veracity of the synthesis method was confirmed by comparing X-ray diffractograms and optical absorption spectra of the BaZr_0.5Hf_0.5S₃ alloy to a 50 : 50 mixture of the pure ternary compounds. Experimental non-resonant Raman measurements on the alloy powders confirm that only slight variations in the spectra are visible, making an alloy composition determination difficult. However, exciting the alloys resonantly, leads to the appearance of new two-phonon modes in the Raman spectrum, which change significantly across the alloy series. We consequently suggest that a rapid alloy composition measurement can be made unambiguously by measuring the ratio of the intensities of the 825 cm^-1 and 625 cm^-1 Raman features.