The structural origin of the germanate anomaly in glasses, which involves complex Ge–O coordination environments, is frequently studied using crystalline analogs. This study aims to provide reliable spectroscopic fingerprints by performing a detailed structural and thermal analysis of crystalline A₄Ge₉O₂₀ model systems with A = Li, Na, K. The compounds were synthesized via melt crystallization and characterized using powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and Raman spectroscopy techniques. The results demonstrate clear cation-dependent crystallization pathways. The Li-containing system predominantly forms Li₂Ge₇O₁₅ in mixture with Li₄Ge₉O₂₀, indicating a preference for thermodynamically stable phases. The Na-system successfully yields the target Na₄Ge₉O₂₀ compound. In contrast, the K-system primarily produces the likely metastable K₂Ge₄O₉ phase with a significant amorphous fraction, highlighting the role of kinetic limitations. This comparative study demonstrates that the size of the alkali cation is a critical factor for controlling phase formation under identical stoichiometric and thermal conditions.

Ab initio calculations were performed to estimate the energy preference of Ca2+ substitution by La3+ with compensating incorporation of an additional F- ion at the (111) twin boundary of fluorite (CaF2). Model structures of polytypes and polysomes, which are periodic stacking faults within the cubic fluorite matrix, were constructed and optimized using density functional theory (DFT). Our results indicate that the twin boundary has a strong tendency to trap lanthanum cations. These findings suggest that the twin boundaries in CaF2 may act as active sites for the accumulation of rare-earth elements, providing potential routes for modifying the functional properties of the materials. Conversely, addition of lanthanoids may promote growth twinning of fluorite.