Reddish-orange emitting Eu2−x Gd x Ge2O7 pyrogermanate ceramic phosphors: synthesis and role of Eu3+/Gd3+ substitution on the optical features

Abstract In this work, highly luminescent pyrogermanates were successfully synthesized through the solid-state reaction route, and the role of gradual Eu 3+ substitution for Gd 3+ on the structure and photoluminescent properties of Eu 2− x Gd x Ge 2 O 7 pyrogermanates was investigated. Pure Eu 2 Ge 2 O 7 ceramic is triclinic, belonging to the P1 (#1) space group, and by increasing the x values, the crystal structure changes with the partial substitution of Eu 3+ ions by the Gd 3+ ions into the chemical lattice. The other phase-pure ceramic, Gd 2 Ge 2 O 7 , belongs to the tetragonal P4 1 2 1 2 (#92) space group. Complete solid solubility was attained for x ⩽ 1.6, where Gd 3+ replaced Eu 3+ in the triclinic structure. It was verified that the band gap energies are dependent on the crystalline structure, increasing as Gd replaced Eu in the Eu 2− x Gd x Ge 2 O 7 . The lowest band gap value (5.13 eV) was observed for the triclinic Eu 2 Ge 2 O 7 , and the highest one (5.88 eV) for the tetragonal Gd 2 Ge 2 O 7 . Highly intense reddish-orange emission (quantum efficiency up to 91.9%), through excitation at charge transfer band and Ln 3+ f – f transitions were evaluated as the Gd substitution rises. Substitution-sensitive phase change at the nanoscale was monitored by Eu 3+ emission, validating the presence of Eu 3+ in the triclinic and/or tetragonal phase depending on the chemical composition (or the Gd 3+ /Eu 3+ ratio). The chromaticity diagram figured out a reddish-orange emission, making them promising materials for high-entropy and photonic devices as solid-state lighting using excitation by near UV light-emitting devices.