On the Crystal Chemistry of RE₂Si₂O₇: Revisited Structures, Group–Subgroup Relationship, and Insights of Ce³⁺-Activated Radioluminescence

Polymorphic rare-earth disilicates RE2Si2O7 (RE = La–Lu, Y, and Sc) are attractive materials as thermal barrier coatings and scintillators; however, the orthorhombic E(δ)-type (RE = Eu–Ho and Y) and the triclinic F-type (RE = Sm and Eu) remain structurally controversial hitherto. In this work, we revisit the crystal structures of E(δ)-RE2Si2O7 (Pnma), F-RE2Si2O7 (P1̅), and the monoclinic G-La2Si2O7 (P21/n) by X-ray single-crystal/powder diffraction. The second-harmonic generation (SHG) and/or the local-structure-sensitive VUV–UV–vis luminescence of Ce3+/Eu3+ validate the structure determination. E(δ)-RE2Si2O7 (RE = Eu–Ho) crystallizes in the centrosymmetric Pnma instead of the previous noncentrosymmetric Pna21 (a subgroup of Pnma). Increasing the RE3+ radius results in structural transformations of the reconstructive-type E(δ) ↔ F and translationengleiche subgroup–group relationship F ↔ G (RE = La–Nd). More importantly, we perform a complete survey of 17 structure-types over 63 compounds of RE2Si2O7 and their related transformations depending on the radius of RE3+. On the basis of these studies, one could (i) predict the potential polymorphs of RE2Si2O7, including the high-pressure phase and the RE2Si2O7 solid solutions; (ii) correlate the coefficient of thermal expansion (CTE) with the crystal structure, and (iii) understand the unusual radioluminescence of (Gd,La)2Si2O7:Ce3+ high-temperature scintillators.