Fig. 2.

Size-dependent mechanism underpinning the phase transition in MIL-53(Al). Visualization of the lp-to-cp transition in MIL-53(Al) during a constant-temperature constant-pressure simulation at 300 K and 40 MPa, using a a nanocell (1 × 2 × 1 unit cell), which is indicated with a red rectangle and periodically repeated to match the size of the mesocell, and b a mesocell (8 × 2 × 8 unit cell). The atomic visualizations are supplemented by schematic representations that are color coded based on the volume of the different subcells (structural classification). Furthermore, the mechanically stable closed-pore (cp) and large-pore (lp) phases and the intermediate unstable region are indicated by circles, squares, and crosses, respectively (mechanical classification). Due to periodic boundary conditions, spatial disorder is only tolerated within the simulation cell