Vibrational Spectra of the “One-Mode” (Y1−xLax)2O3 Solid Solution Ceramics

Y2O3 is widely used in IR windows and optoelectronics, but its vibrational and spectral properties under La3+ substitutions remain unclear. This work investigates xLa2O3-(1−x)Y2O3 with x = 0–0.2 via XRD, SEM, Raman, and IR spectroscopy to address the lack of comprehensive data on structure–property correlations. The solid solution (Y1−xLax)2O3 with 0 ≤ x < 0.15 was determined. The cell parameter a increases from 10.6113(5) Å to 10.7116(1) Å as x increases from 0 to 0.15 according to the Rietveld refinements. Both the Raman spectra and infrared (IR) reflection spectra show that (Y1−xLax)2O3 is a “one-mode” system, in which eight of the 22 theoretical first-order Raman modes and 12 of 16 theoretical IR modes for the (Y1−xLax)2O3 are recognized. A local vibrational mode at approximately 680 cm−1 is observed in the Raman spectra as x > 0. The Raman modes and IR modes of (Y1−xLax)2O3 show red shift and obvious peak broadening as x increases, which is caused by the expansion of unit cell and local distortions of the crystal structure. As Y3+ is substituted by La3+, the extinction coefficient κ decreases significantly, which can lead to a lower IR absorption and should be beneficial for the IR window applications.