Pressure-dependent structural and
chemical changes of the metal–organic
framework (MOF) compound MIL-47(V) have been investigated up to 3
GPa using different pore-penetrating liquids as pressure transmitting
media (PTM). We find that at 0.3(1) GPa the terephthalic acid (TPA)
template molecules located in the narrow channels of the as-synthesized
MIL-47(V) are selectively replaced by methanol molecules from a methanol–ethanol–water
mixture and form a methanol inclusion complex. Further pressure increase
leads to a gradual narrowing of the channels up to 1.9(1) GPa, where
a second irreversible insertion of methanol molecules leads to more
methanol molecules being inserted into the pores. After pressure release
methanol molecules remain within the pores and can be removed only
after heating to 400 °C. In contrast, when MIL-47(V) is compressed
in water, a reversible replacement of the TPA by H2O molecules
takes place near 1 GPa. The observed structural and chemical changes
observed in MIL-47(V) demonstrate unique high pressure chemistry depending
on the size and type of molecules present in the liquid PTM. This
allows postsynthetic nonthermal pressure-induced removal and insertion
of organic molecules in MOFs forming novel and stable phases at ambient
conditions.
In-situ high-pressure synchrotron X-ray powder diffraction studies up to 21 GPa of CVD-grown silicon 2D-nanosheets establish that the structural phase transitions depend on size and shape. For sizes between 9.3(7) nm and 15.2(8) nm we observe an irreversible phase transition sequence from I (cubic) → II (tetragonal) → V (hexagonal) during pressure increase and during decompression below 8 GPa the emergence of an X-ray amorphous phase. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atomic force microscopy (AFM) images of this X-ray amorphous phase reveal the formation of significant numbers of 1D nanowires with aspect ratios > 10, which are twinned and grow along the <111> direction. We discovered a reduction of dimensionality under pressure from a 2D morphology to a 1D wire in a material with a diamond structure. MD simulations indicate the reduction of thermal conductivity in such nanowires.
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