New insights on the structural and optical-vibration properties of noncentrosymmetric lanthanides pyrogermanates

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

“…For these samples, besides the expected shifts, additional peaks corresponding to the tetragonal polymorph appear at x ⩾ 1.80 samples. The results for the Eu 2 Ge 2 O 7 ceramic are in agreement with XRD patterns previously observed in materials produced by the molten-salt method [33]. XRD patterns for samples x > 1.60 are the same as obtained for the mixture of Gd 2 Ge 2 O 7 polymorphs produced at 1200 • C-1220 • C by the solid-state route [28].…”

“…In the sequence, Raman scattering was employed to complete the structural analysis and to understand the optical vibration behavior alongside the entire Eu 2−x Gd x Ge 2 O 7 series. Figure 2 shows the Raman spectroscopic patterns for all ceramic samples synthesized at 1100 • C, for 4 h. A complex band structure was observed as expected according to recent works involving optical-vibration properties of lanthanide pyrogermanates conducted beside a detailed factor-group analysis for triclinic (P1) and tetragonal (P4 1 2 1 2) space groups [29,30,33]. Thus, the focus of the present work was to observe how the changes in the chemical environment can induce structural modifications in our samples.…”

“…Lanthanide oxides were first heated at 900 • C overnight to remove any moisture. The final compositions were obtained by solid-state processing in a conventional furnace under atmosphere conditions at 1100 • C, for 4 h, under heating and cooling rates of 10 • C min −1 (up to 800 • C) and 5 • C min −1 (>800 • C), according previous works [29,33].…”

“…However, lately, they have been investigated mainly due to their interesting magnetic behavior [7][8][9][10]30] and remarkable thermal stability [30][31][32]. Currently, in a set of works for the RE 2 Ge 2 O 7 (RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) complete series of ceramics, Martins et al [28,30,33] were the pioneers in reporting alternative conditions of synthesis via moltensalt process saving energy. Two different crystal phases were obtained, depending on the lanthanide ion, as follows: triclinic P1, for larger lanthanides (La-Eu), and tetragonal P4 1 2 1 2, for Gd-Lu [33].…”

“…Currently, in a set of works for the RE 2 Ge 2 O 7 (RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) complete series of ceramics, Martins et al [28,30,33] were the pioneers in reporting alternative conditions of synthesis via moltensalt process saving energy. Two different crystal phases were obtained, depending on the lanthanide ion, as follows: triclinic P1, for larger lanthanides (La-Eu), and tetragonal P4 1 2 1 2, for Gd-Lu [33]. The same structures were confirmed by the solid-state synthesis at 1100 • C, for 4 h, establishing the limit of triclinic/tetragonal transformation between Eu 2 Ge 2 O 7 and Gd 2 Ge 2 O 7 .…”