Two water-stable
zirconium-based metal–organic frameworks
(MOFs) (NU-1000 and UiO-67) have been synthesized in various size
scales (100–2000 nm) for the adsorptive removal of glyphosate
from the aqueous media. Both NU-1000 and UiO-67 possess a three-dimensional
structure; NU-1000 consists of triangular micropores and wide mesoporous
channels (31 Å), whereas UiO-67 has cage-like pores [octahedral
(16 Å) and tetrahedral (14 Å) cages]. NU-1000 comprises
Zr
6
(μ
3
-O)
4
(μ
3
-OH)
4
(H
2
O)
4
(OH)
4
, and
UiO-67 contains Zr
6
O
4
(OH)
4
as secondary
building units. These units act as Lewis acid nodes and can interact
with the Lewis base phosphate group of the glyphosate. The time taken
for reaching equilibrium is found to be reduced considerably as the
size of the MOF decreases. The smaller the particle size, the lesser
is the diffusion barrier for the analyte, which enhances the interaction
between Lewis acidic metal nodes and the Lewis basic center of the
glyphosate molecule. NU-1000 was found to be better compared to UiO-67,
both in terms of efficiency and reusability. This might be due to
the larger pore diameters of the NU-1000. Theoretical calculations
revealed that the interaction energy of glyphosate with the nodes
of NU-1000 is higher (−37.63 KJ mol
–1
) compared
to UiO-67 (−17.37 KJ mol
–1
), which might
be the possible reason for the higher efficiency of NU-1000.
Chemical bonding
in 2D layered materials and van der Waals solids
is central to understanding and harnessing their unique electronic,
magnetic, optical, thermal, and superconducting properties. Here,
we report the discovery of spontaneous, bidirectional, bilayer twisting
(twist angle ∼4.5°) in the metallic kagomé MgCo
6
Ge
6
at
T
= 100(2) K via X-ray
diffraction measurements, enabled by the preparation of single crystals
by the Laser Bridgman method. Despite the appearance of static twisting
on cooling from
T
∼300 to 100 K, no evidence
for a phase transition was found in physical property measurements.
Combined with the presence of an Einstein phonon mode contribution
in the specific heat, this implies that the twisting exists at all
temperatures but is thermally fluctuating at room temperature. Crystal
Orbital Hamilton Population analysis demonstrates that the cooperative
twisting between layers stabilizes the Co-kagomé network when
coupled to strongly bonded and rigid (Ge
2
) dimers that
connect adjacent layers. Further modeling of the displacive disorder
in the crystal structure shows the presence of a second, Mg-deficient,
stacking sequence. This alternative stacking sequence also exhibits
interlayer twisting, but with a different pattern, consistent with
the change in electron count due to the removal of Mg. Magnetization,
resistivity, and low-temperature specific heat measurements are all
consistent with a Pauli paramagnetic, strongly correlated metal. Our
results provide crucial insight into how chemical concepts lead to
interesting electronic structures and behaviors in layered materials.
Combining neutron diffraction with
pair distribution function analysis,
we have uncovered hidden reduced symmetry in the correlated metallic d
1 perovskite, SrVO3. Specifically,
we show that both the local and global structures are better described
using a GdFeO3 distorted (orthorhombic) model as opposed
to the ideal cubic ABO3 perovskite type. Recent reports
of imaginary phonon frequencies in the density functional theory (DFT)-calculated
phonon dispersion for cubic SrVO3 suggest a possible origin
of this observed non-cubicity. Namely, the imaginary frequencies computed
could indicate that the cubic crystal structure is unstable at T =
0 K. However, our DFT calculations provide compelling evidence that
point defects in the form of oxygen vacancies, and not an observable
symmetry breaking associated with calculated imaginary frequencies,
primarily result in the observed non-cubicity of SrVO3.
These experimental and computational results are broadly impactful
because they reach into the thin-film and theoretical communities
who have shown that SrVO3 is a technologically viable transparent
conducting oxide material and have used SrVO3 to develop
theoretical methods, respectively.
Developing the field of quantum information science (QIS) hinges upon designing viable qubits, the smallest unit in quantum computing. One approach to creating qubits is introducing paramagnetic defects into semiconductors or insulators. This class of qubits has seen success in the form of nitrogen-vacancy centers in diamond, divacancy defects in SiC, and P doped into Si. These materials feature paramagnetic defects in a low nuclear spin environment to reduce the impact of nuclear spin on electronic spin coherence. In this work, we report single crystal growth of Ba2CaWO6-δ, and the coherence properties of introduced W 5+ spin centers generated by oxygen vacancies. Ba2CaWO6-δ (δ = 0) is a B-site ordered double perovskite with a temperature-dependent octahedral tilting wherein oxygen vacancies generate W 5+ (d 1 ), S = ½, I = 0, centers. We characterized these defects by measuring the spin-lattice (T1) and spin-spin relaxation (T2) times from T = 5 to 150 K. At T = 5 K, T1 = 310 ms and T2 = 4 μs, establishing the viability of these qubit candidates. With increasing temperature, T2 remains constant up to T = 60 K and then decreases to T2 ~ 1 μs at T = 90 K, and remains roughly constant until T = 150 K, demonstrating the remarkable stability of T2 with increasing temperature. Together, these results demonstrate that systematic defect generation in double perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for QIS.
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