5 research outputs found
Two-Step Pressure-Induced Superhydration in Small Pore Natrolite with Divalent Extra-Framework Cations
In
situ high pressure X-ray powder diffraction studies of natrolite
(NAT) containing the divalent extra-framework cations (EFC) Sr<sup>2+</sup>, Ca<sup>2+</sup>, Pb<sup>2+</sup>, and Cd<sup>2+</sup> reveal
that they can be superhydrated in the presence of water. In the case
of Ca-NAT, Sr-NAT, and Pb-NAT pressure-induced hydration (PIH) inserts
40 H<sub>2</sub>O/unit cell into the zeolite compared to 32 in superhydrated
natrolites containing monovalent EFC. Cd-NAT is superhydrated in one
step to a zeolite containing 32 H<sub>2</sub>O/unit cell. PIH of Ca-NAT
and Sr-NAT occurs in two steps. During PIH of Pb-NAT three distinct
steps have been observed. The excess H<sub>2</sub>O in natrolites
with divalent EFC are accommodated on sites no longer required for
charge compensation. Two distinct families with ordered and disordered
EFC–water topologies have been found. Our work established
the importance of both size and charge of the EFC in PIH
Role of Cation–Water Disorder during Cation Exchange in Small-Pore Zeolite Sodium Natrolite
By
combining X-ray diffraction with oxygen K-edge absorption spectroscopy
we track changes occurring during the K<sup>+</sup>–Na<sup>+</sup> cation exchange of Na-natrolite (Na-NAT) as tightly bonded
Na<sup>+</sup> cations and H<sub>2</sub>O molecules convert into a
disordered K<sup>+</sup>–H<sub>2</sub>O substructure and the
unit cell expands by ca. 10% after 50% cation exchange. The coordination
of the confined H<sub>2</sub>O and nonframework cations change from
a tetrahedral configuration, similar in ice <i>I</i><sub><i>h</i></sub>, with Na<sup>+</sup> near the middle of
the channels in Na-NAT to two-bonded configuration, similar in bulk
water, and K<sup>+</sup> located near the walls of the framework in
K-NAT. This is related to the enhanced ion-exchange properties of
K-NAT, which, in marked contrast to Na-NAT, permits the exchange of
K<sup>+</sup> by a variety of uni-, di-, and trivalent cations
Pressure-Induced Metathesis Reaction To Sequester Cs
We
report here a pressure-driven metathesis reaction where Ag-exchanged
natrolite (Ag<sub>16</sub>Al<sub>16</sub>Si<sub>24</sub>O<sub>80</sub>·16H<sub>2</sub>O, Ag-NAT) is pressurized in an aqueous CsI
solution, resulting in the exchange of Ag<sup>+</sup> by Cs<sup>+</sup> in the natrolite framework forming Cs<sub>16</sub>Al<sub>16</sub>Si<sub>24</sub>O<sub>80</sub>·16H<sub>2</sub>O (Cs-NAT-I) and,
above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the
initial cation exchange, the precipitation of AgI occurs. Additional
pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to
a pollucite-related, highly dense, and water-free triclinic phase
with nominal composition CsAlSi<sub>2</sub>O<sub>6</sub>. At ambient
temperature after pressure release, the Cs remains sequestered in
a now monoclinic pollucite phase at close to 40 wt % and a favorably
low Cs leaching rate under back-exchange conditions. This process
thus efficiently combines the pressure-driven separation of Cs and
I at ambient temperature with the subsequent sequestration of Cs under
moderate pressures and temperatures in its preferred waste form suitable
for long-term storage at ambient conditions. The zeolite pollucite
CsAlSi<sub>2</sub>O<sub>6</sub>·H<sub>2</sub>O has been identified
as a potential host material for nuclear waste remediation of anthropogenic <sup>137</sup>Cs due to its chemical and thermal stability, low leaching
rate, and the large amount of Cs it can contain. The new water-free
pollucite phase we characterize during our process will not display
radiolysis of water during longterm storage while maintaining the
Cs content and low leaching rate
Pressure-Dependent Structural and Chemical Changes in a Metal–Organic Framework with One-Dimensional Pore Structure
Pressure-dependent structural and
chemical changes of the metal–organic
framework (MOF) compound MIL-47Â(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-47Â(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-47Â(V) is compressed
in water, a reversible replacement of the TPA by H<sub>2</sub>O molecules
takes place near 1 GPa. The observed structural and chemical changes
observed in MIL-47Â(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
Atomically Engineered Metal–Insulator Transition at the TiO<sub>2</sub>/LaAlO<sub>3</sub> Heterointerface
We demonstrate that the atomic boundary
conditions of
simple binary
oxides can be used to impart dramatic changes of state. By changing
the substrate surface termination of LaAlO<sub>3</sub> (001) from
AlO<sub>2</sub> to LaO, the room-temperature sheet conductance of
anatase TiO<sub>2</sub> films are increased by over 3 orders of magnitude,
transforming the intrinsic insulating state to a high mobility metallic
state, while maintaining excellent optical transparency