Fig. 3: Intrusion/extrusion experiment in ZIF-67.

A Synchrotron data allow to track the changes in the lattice parameter a as the hydrostatic pressure in the water/ZIF-67 system is increased from low values to above the intrusion pressure (black) and back to ambient values, triggering extrusion (red). The system display the two NC (negative compressibility) transitions, in agreement with the model shown in Fig. 2. Data were collected at beamline 17-BM at the Advanced Photon Source, Argonne National Laboratory. Source data are provided as a Source Data file. B Molecular structure (left) and topological simplification scheme (right) of ZIF-67 (Zeolitic imidazolate framework); violet denotes cobalt ions and the related tetrahedra connected by imidazolate linkers; the yellow sphere highlights the cavity. C Conceptual scheme of the behaviour of a single cage of ZIF-67 during the initial compression (ii-iii) and following water intrusion (iv=v). Blue background stands for water; yellow highlights the empty cavity. D Experimental setup and TEM image of the ZIF-67 nanoparticles. The sample is loaded into a single crystal sapphire capillary with a K type thermocouple inside the sample. The sample is composed of nanoparticles having an average size of 509 ± 13 nm (standard deviation 124 nm). TEM measurements were performed on a Tecnai G2 F20 Super Twin under 200 kV acceleration voltages; sample powder was dispersed in ethanol, sonicated using a water bath and placed in a holey carbon grid.