High-Pressure Structural Evolution of Na ₂ ZrSi ₂ O ₇ and Na ₂ ZrSi ₂ O ₇ ·H ₂ O: Topology-Driven Compression Behaviors, Phase Stability, and Electronic Transitions

Silicate frameworks exhibit diverse structural responses under extreme conditions, which are strongly influenced by hydration. Here, we present a comparative high-pressure synchrotron X-ray diffraction study of Na₂ZrSi₂O₇ and its hydrated analogue Na₂ZrSi₂O₇·H₂O up to 30 GPa, combined with electronic structure calculations. At ambient conditions, both phases share the same primary building units (PBUs: [ZrO₆] and [SiO₄]) but differ in secondary building units (SBUs, M₂T₄ vs M₂T₆). Under compression, Na₂ZrSi₂O₇ undergoes a phase transition near 15 GPa, while the hydrated phase remains stable throughout the pressure range. The anhydrous compound exhibits a higher bulk modulus (B₀ = 77.1 GPa) and less anisotropic compression compared with those of the hydrated phase (B₀ = 66.3 GPa). Distinct deformation mechanisms are observed: the anhydrous framework accommodates pressure through [ZrO₆] octahedral distortion, whereas the hydrated framework compresses via [Si₂O₇] group tilting. Electronic structure calculations indicate band gap widening with pressure in both phases; notably, Na₂ZrSi₂O₇ shows a direct-to-indirect band gap transition, whereas the hydrated phase retains a direct gap. These results reveal how hydration-driven topological modifications at the SBU scale dictate the pressure-induced structural evolution, phase stability, and electronic properties of zirconosilicate frameworks.