Soft-phonon mode observation in Li ₂Ge ₄O ₉ phase above room temperature

Li ₂Ge ₄O ₉ is known to be a ferroelectric phase with Curie temperature at T _c ∼ -93 C. In this study, we found both the lowest-frequency phonon and heat-capacity anomalies around 100 C in the Li ₂Ge ₄O ₉ phase, demonstrating their presence in the structural phase-transition. The phase transition is probably triggered by regularization of distorted GeO ₆ octahedra in the paraelectric phase, resulting in another phase consisting of the regular octahedral GeO ₆ unit, which was supported by in situ measurement of emission of ²E → ⁴A ₂ transition in the Mn ⁴-doped phase. It is deduced that the phase transition is more dynamic than ferroelectric phase-transition originating in ordering of the Li ions.