Mechanisms of Negative Thermal Expansion in 2D Materials (IMAGE)

Tsinghua University Press

Mechanisms of Negative Thermal Expansion in 2D Materials

Caption

This schematic illustrates the primary mechanisms causing negative thermal expansion in 2D materials. 1) Phonon Modes: Heating excites out-of-plane rippling vibrations (flexural ZA phonons), which pull the atoms closer together in-plane, causing net shrinkage. 2) Rigid Unit Modes (RUMs): In framework structures, the collective rotation of rigid polygonal units (e.g., squares or carbon rings) reduces the overall lattice dimensions. 3) Spin-Lattice Coupling: In magnetic 2D materials, the establishment of magnetic order upon cooling can pull atoms closer together, leading to contraction when heated through this transition point.

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Nano Research, Tsinghua University Press

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