New insight on the use of diffuse reflectance spectroscopy for the optical characterization of Ln 2Ge2O7 (Ln = lanthanides) pyrogermanates

Highlights: • The optical properties of triclinic/tetragonal Ln₂Ge₂O₇ pyrocompounds were reported. • Diffuse reflectance spectroscopy (DRS) was employed to investigate their optical features. • Charge transfer band and energy gap were evaluated throughout the systems. • Intraconfigurational f-f transitions were fully identified in the range 200–800 nm. • DRS appeared as a promising, powerful tool for the full optical characterization of materials. In this paper, the optical properties of Ln₂Ge₂O₇ ceramics (Ln = lanthanides) were evaluated by diffuse reflectance spectroscopy. Charge transfer band, energy gap, and intraconfigurational f-f transitions were investigated, identified, and discussed as a function of the structural features of the pyrocompounds. A medium band gap of 5.8 eV indicates that the space group has a small influence on this parameter (the triclinic pyrogermanates have slightly lower band gaps than the tetragonal ones), while the f-f electronic transitions could be entirely observed in all samples. In this respect, diffuse reflectance spectroscopy emerged as a powerful tool for the analysis of the intraconfigurational features in lanthanide-containing materials in association with photoluminescence data. The optical features reported in the present work represent an important evolution aiming at the deep knowledge of lanthanide pyrogermanates, which can assist in the development of devices with properties that meet the most advanced technological demands.