Structural, vibrational and optical properties of Sc2-xLaxGe2O7 ceramics

Sc2-xLaxGe2O7 (0.00 <= x <= 2.00) ceramics were synthesized by the solid-state method and their structural, vibrational, and optical properties were investigated by XRD, Raman scattering, infrared and diffuse reflectance spectroscopies. Phase-pure Sc2Ge2O7 is monoclinic, belonging to the C2/m (#12) space group, while La2Ge2O7 belongs to the triclinic (P1) over bar (#2) space group, which was confirmed by second harmonic generation measurements. It was verified that the solid solubility was attained at two distinct compositional ranges. For 0.00 < x <= 0.04, the monoclinic structure incorporates La3+ ions, while in the range of 1.80 <= x < 2.00, Sc3+ replaces La3+ into the triclinic space group. The results from vibrational spectroscopic data were essential to observe the crystal phase evolution as the optical phonon modes change, tracking gradually the structural modifications observed through the analysis of 22 different chemical compositions. In addition, the resulting compositional effects on the band gap energies of the pyrogermanate series showed values that varied between 5.75 (pure La2Ge2O7) and 6.00 eV (pure Sc2Ge2O7).