Mapping anisotropic compression and interatomic interactions in diopside (CaMgSi2O6) through a Hirshfeld-volume-driven equation of state

Abstract

We introduce a Hirshfeld-volume-driven equation of state (EoS) to resolve atomistic compression mechanisms in diopside (CaMgSi₂O₆) under high pressure (0–10.16 GPa). Our approach integrates topological electron density partitioning via Hirshfeld surface analysis with third order Birch–Murnaghan EoS, achieving <2% error in Hirshfeld volume (V_H) predictions versus experimental benchmarks. Critically, this method visualizes and quantifies how interatomic contacts and crystal packing evolve under compression. Hirshfeld analysis reveals a stark differential atomic compressibility: Mg atoms dominate strain absorption (ΔV_atom/V_atom = −16.2% at 10.16 GPa), followed by Ca (−12.7%), Si (−8.54%), and O (−4.60%). This hierarchy arises from the flexible coordination environments of Mg/Ca–O polyhedra (bulk modulus B₀ ≈ 85 GPa) accommodating compression via bond shortening, while the rigid SiO₄ tetrahedra (B₀ > 150 GPa) preserve the supramolecular architecture. Calibrated Hirshfeld volume-EoS parameters (V_H = 438.72 ų, B₀ = 119.0 GPa, = 3.44) align with experiments (ΔV_H < 0.03%), providing a profound link between microscopic interactions and macroscopic properties. This work establishes the Hirshfeld-driven EoS as a transformative tool for decoding structure–property relationships in molecular crystals and designing pressure-resilient functional materials.