PBEsol-DFT Insights into Anisotropic Compression of Diopside: Chain Kinking versus Bond Rupture

Abstract Diopside (CaMgSi2O6), a key constituent of planetary mantles and high-pressure ceramics, exhibits unique anisotropic compression mechanisms under extreme conditions. This study resolves longstanding discrepancies in diopside’s compressibility by establishing a novel hierarchy of bond-specific rigidity and deformation pathways. Using PBEsol-DFT, we demonstrate that strain accommodation occurs via silicate chain kinking rather than bond rupture - a paradigm shift in understanding silicate mineral resilience. Our results reveal a compressibility hierarchy (b-axis > c-axis > a-axis), driven by CaO₈ polyhedral collapse and chain kinking (ΔR/R₀ ≈ 8.4% for Ca–O bonds), with PBEsol-DFT accurately predicting equilibrium lattice parameters (a = 9.7832 Å, b = 8.8950 Å, c = 5.2608 Å) and bulk modulus (B₀ = 114 GPa vs. experimental 111.3 GPa). Bond-specific moduli highlight Si–O bond rigidity (BSi-O = 1048.3 GPa) versus Ca/Mg–O flexibility, with strain redistributed via tetrahedral tilting (decrease in the O3–O3–O3 angle at a rate of -0.56°/GPa). The elastic constants and moduli were calculated, which in the normal state have values for the bulk modulus of 115. GPa and the shear modulus of 69.8 GPa, and under pressure they increase at rates of 4.20 and 1.85, respectively. These findings bridge crystallochemical principles with extreme-condition materials physics, offering design rules for high-pressure ceramics, planetary interior models, and materials for aerospace applications.