Raman spectroscopic signatures of Er₂Ti₂O₇ pyrochlore and ErPO₄ xenotime: structural mode assignments and comparative analysis

Er₂Ti₂O₇ and ErPO₄ are rare-earth compounds whose distinct crystal frameworks—pyrochlore (Fd-3m) and xenotime (I4₁/amd), respectively—give rise to contrasting vibrational and functional properties. We present a comparative Raman spectroscopic study of these materials, combining high-resolution measurements with symmetry-guided mode assignments. Both phases were synthesized via a controlled sol–gel process and examined under identical experimental conditions. For Er₂Ti₂O₇, the spectrum is marked by three main features in the low- to mid-frequency range, linked to Er–O lattice translations, O–Ti–O bending, and symmetric Ti–O stretching in TiO₆ octahedra. In contrast, ErPO₄ is dominated by internal PO₄³⁻ vibrations, with intense ν₁ and ν₃ stretching modes above 1000 cm⁻¹. The large frequency separation between TiO₆- and PO₄³⁻-related modes enables unambiguous phase identification, even in mixed systems. The narrow linewidths in both compounds point to long phonon lifetimes and high crystalline quality. These results establish a concise, reliable set of Raman signatures for these Er-based phases, supporting precise phase discrimination and the evaluation of structural order, with relevance for optical and functional materials design.