La ³⁺ -Driven Structure Transformations, Site-Related Luminescence, and Crystal Growth of G-(Gd,La) ₂ Si ₂ O ₇ :Ce ³⁺ toward Improved High-Temperature Scintillation

(Gd,La)2Si2O7:Ce3+ crystals show promise as high-temperature scintillators for ultradeep well logging because of the outstanding performance at >200 °C; however, the crystal structures and site-related luminescence are still debated. In this work, we revisit the structures by X-ray single-crystal and powder diffraction and disclose structural transformations among three modifications: E(δ) (Pnma) → F (P1¯) → G (P21/n) driven by the incremental La3+ content. The E(δ) ↔ F transformation is reconstructive, whereas the F-type features a translationengleiche subgroup of the G-type, with critical La3+ contents of ∼0.1 and 0.34. Synchrotron-radiation VUV–UV and X-ray excited luminescence reveal that the structural variations significantly modulate the band gap of Gd2–xLaxSi2O7 and the Ce3+ 4f–5d transitions associated with multiple crystallographic sites. The thermal stability of Ce3+ radioluminescence, governed by La3+ content, is understood through vacuum referred binding energy schemes to involve La3+ modulation of the delocalization channel for Ce3+ 5d1 electrons to the conduction band. Furthermore, we develop a G-Gd1.65La0.35Si2O7:Ce3+ crystal by the Czochralski method with the minimum La3+ content in the G-type crystals. This specific composition exhibits enhanced thermal stability and light yield, thereby bearing out that the high-temperature scintillation property of the well-established G-type could be fine-tuned while comprehending the structure–luminescence relationship.