Mechanistic insights into colossal negative thermal expansion in Ca2RuO4 mott insulator via computational modeling approaches

The enigmatic phenomenon of colossal volumetric and uniaxial negative thermal expansion (NTE) in Ca2RuO4 has long challenged understanding due to its intertwined electronic, structural, and magnetic degrees of freedom. Through ab initio molecular dynamics (AIMD), normal-mode decomposition techniques, and anharmonic Grüneisen analysis, we reveal the microscopic origin of its anisotropic expansion. The uniaxial NTE near the metal-to-insulator transition (MIT) arises from phonons with negative Grüneisen parameters coupled to pronounced elastic anisotropy. Octahedral tilts are central yet non-rigid, involving intrinsic shape changes rather than rigid unit modes. AIMD captures the thermal evolution of RuO6 rotations, tilts, and antipolar Ca displacements, which modify local tetragonal distortions of the RuO6 octahedra and drive the MIT. Our findings provide microscopic insight into NTE in Ca2RuO4 and highlight strategies to engineer phase transitions and thermal responses in A2BO4 oxides via tailored octahedral distortions.