Freezing of water in hydrophilic nanopores (D=1.2 nm) is probed at the microscopic scale using x-ray diffraction, Raman spectroscopy, and molecular simulation. A freezing scenario, which has not been observed previously, is reported; while the pore surface induces orientational order of water in contact with it, water does not crystallize at temperatures as low as 173 K. Crystallization at the surface is suppressed as the number of hydrogen bonds formed is insufficient (even when including hydrogen bonds with the surface), while crystallization in the pore center is hindered as the curvature prevents the formation of a network of tetrahedrally coordinated molecules. This sheds light on the concept of an ubiquitous unfreezable water layer by showing that the latter has a rigid (i.e., glassy) liquidlike structure, but can exhibit orientational order.
Carbon nanothreads are likely the most attracting new materials produced under high pressure conditions. Their synthesis is achieved by compressing crystals of different small aromatic molecules also exploiting the applied...
The siliceous zeolite
TON with a 1-D pore system was studied at
high pressure by X-ray diffraction, infrared spectroscopy, and DFT
calculations. The behavior of this material was investigated using
nonpenetrating pressure-transmitting media. Under these conditions,
a phase transition from the Cmc21 to a Pbn21 structure occurs at close to 0.6 GPa with
doubling of the primitive unit cell based on Rietveld refinements.
The pores begin to collapse with a strong increase in their ellipticity.
Upon decreasing the pressure below this value the initial structure
was not recovered. DFT calculations indicate that the initial empty
pore Cmc21 phase is dynamically unstable.
Irreversible, progressive pressure-induced amorphization occurs upon
further increases in pressure up to 21 GPa. These changes are confirmed
in the mid- and far-infrared spectra by peak splitting at the Cmc21 to Pbn21 phase
transition and strong peak broadening at high pressure due to amorphization.
Microporous AlPO 4 -54•xH 2 O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H 2 O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H 2 O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO 4 -54•xH 2 O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IV Al, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.
Synthesis of carbon nanothreads from pyridine under variable high-pressure and high-temperature conditions discloses the role of H-bonding in the kinetic control of the reaction.
Microporous AlPO4-54,
which exhibits the largest pores
among zeolites and aluminophosphates with a diameter of 12.7 Å,
was investigated at high pressure by X-ray powder diffraction (XRD),
mid- and far-infrared (IR) spectroscopy in diamond anvil cells. The
material undergoes a phase transition beginning around 0.8 GPa. The
amount of AlPO4-8 gradually increases with pressure and
the phase transition is complete between 2 and 3 GPa. The closure
of the (POAl) angle destabilizes the structure of
AlPO4-54, which drives the transition to AlPO4-8. The pressure-induced phase transformation of AlPO4-54 to AlPO4-8 is associated with a symmetry reduction
from hexagonal to orthorhombic and with a change in the unidirectional
ring channel parallel to the c-axes from 18 to 14
AlO4 and PO4 tetrahedra. An abrupt decrease
along the b direction is linked to the formation of 4
new rings of 6 tetrahedra with significant structural reorganization.
The transition is followed by irreversible amorphization beginning
around 3.5 GPa due to the collapse of the pores. The amorphization
of AlPO4-8 was detected from the disappearance of the XRD
lines, abrupt shifts, and strong broadening of the mid-IR modes, and
by changes in the pressure dependence of the mid- and far-IR modes,
indicating a lower compressibility for the more dense amorphous form.
Far-IR spectra indicate that the new amorphous form retains the local
structure of AlPO4-8.
Saturated
carbon nanothreads are one of the most attractive new
materials produced under high pressure in the last years. Nanothreads
can be considered as a monodimensional diamond; in fact, they preserve
some of the mechanical properties of the diamond itself, like stiffness,
but their intrinsic flexibility makes them excellent nanowires. Since
their discovery, many advancements have been made, and nowadays, they
can be obtained from the compression of several aromatic molecular
crystals. However, it is often not clear why certain starting crystals
give high-quality nanothreads while others do not or which are the
best conditions for the synthesis in terms of pressure, temperature,
compression rate, and reaction time. In other words, the mechanisms
that allow their formation with respect to other byproducts are often
unclear. This is an important piece of information that can be used
for the design of a synthetic strategy for the production of functional
materials with targeted characteristics, like conductivity and electro-optical
properties, while preserving the mechanical ones. Here, we report
an X-ray diffraction study in which we followed the transformation
induced by the pressure of trans-azobenzene using polycrystalline
samples compressed with and without a pressure-transmitting medium.
With this approach, we were able to highlight the structural relations
along the reactive path leading to double-core saturated carbon nanothreads.
The features that we discovered could be common to all pseudo-stilbene
crystals, a class of compounds isostructural to azobenzene and characterized
by two phenyl rings connected by a variety of different linkers, thus
representing excellent starting materials for the synthesis of functional
nanothreads.
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