X-ray detectors are demonstrated using composite films of lead-free Cs
2
AgBiBr
6
halide double perovskite embedded in a polymer matrix as the X-ray photoconductors. Polymers with hydroxyl functional ...groups greatly improve the uniformity of the composite films, and large area dense films are obtained using a simple drop-casting process. X-ray detectors based on the composite films exhibit a sensitivity of 40 μC Gy
air
−1
cm
−2
, comparable to the reported sensitivity using Cs
2
AgBiBr
6
single crystals. Such detectors can also tolerate a 5% tensile/compressive strain in the composite films without performance degradation. Pixelated X-ray detectors fabricated on the same composite film can realize X-ray imaging and resolve a proof-of-concept geometric pattern.
Organic–inorganic hybrid metal halide perovskites have emerged as a highly promising class of light emitters, which can be used as phosphors for optically pumped white light-emitting diodes (WLEDs). ...By controlling the structural dimensionality, metal halide perovskites can exhibit tunable narrow and broadband emissions from the free-exciton and self-trapped excited states, respectively. Here, we report a highly efficient broadband yellow light emitter based on zero-dimensional tin mixed-halide perovskite (C4N2H14Br)4SnBr x I6–x (x = 3). This rare-earth-free ionically bonded crystalline material possesses a perfect host-dopant structure, in which the light-emitting metal halide species (SnBr x I6–x 4–, x = 3) are completely isolated from each other and embedded in the wide band gap organic matrix composed of C4N2H14Br–. The strongly Stokes-shifted broadband yellow emission that peaked at 582 nm from this phosphor, which is a result of excited state structural reorganization, has an extremely large full width at half-maximum of 126 nm and a high photoluminescence quantum efficiency of ∼85% at room temperature. UV-pumped WLEDs fabricated using this yellow emitter together with a commercial europium-doped barium magnesium aluminate blue phosphor (BaMgAl10O17:Eu2+) can exhibit high color rendering indexes of up to 85.
Abstract
The single-ion anisotropy and magnetic interactions in spin-ice systems give rise to unusual non-collinear spin textures, such as Pauling states and magnetic monopoles. The effective spin ...correlation strength (
J
e
f
f
) determines the relative energies of the different spin-ice states. With this work, we display the capability of capacitive torque magnetometry in characterizing the magneto-chemical potential associated with monopole formation. We build a magnetic phase diagram of Ho
2
Ti
2
O
7
, and show that the magneto-chemical potential depends on the spin sublattice (
α
or
β
), i.e., the Pauling state, involved in the transition. Monte Carlo simulations using the dipolar-spin-ice Hamiltonian support our findings of a sublattice-dependent magneto-chemical potential, but the model underestimates the
J
e
f
f
for the
β
-sublattice. Additional simulations, including next-nearest neighbor interactions (
J
2
), show that long-range exchange terms in the Hamiltonian are needed to describe the measurements. This demonstrates that torque magnetometry provides a sensitive test for
J
e
f
f
and the spin-spin interactions that contribute to it.
At elevated temperatures SnSe is reported to undergo a structural transition from the low symmetry orthorhombic GeS-type to a higher symmetry orthorhombic TlI-type. Although increasing symmetry ...should likewise increase lattice thermal conductivity, many experiments on single crystals and polycrystalline materials indicate that this is not the case. Here we present temperature dependent analysis of time-of-flight (TOF) neutron total scattering data in combination with theoretical modeling to probe the local to long-range evolution of the structure. We report that while SnSe is well characterized on average within the high symmetry space group above the transition, over length scales of a few unit cells SnSe remains better characterized in the low symmetry GeS-type space group. Our finding from robust modeling provides further insight into the curious case of a dynamic order-disorder phase transition in SnSe, a model consistent with the soft-phonon picture of the high thermoelectric power above the phase transition.
The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accommodating the predicted chiral charge of the bulk Weyl points, has been difficult in ...these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and first-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projected Weyl nodes, and show how they facilitate a topological classification of the surface band structure. Our findings establish a critical role of the orbital degrees of freedom in mediating the surface-bulk connectivity in Weyl semimetals.
The nitride-hydride Ba3CrN3H was obtained in single crystalline form using flux growth techniques based on alkaline earth metals. Ba3CrN3H crystallizes in the hexagonal space group P63/m (Nr 176), ...with the lattice parameters a = 8.0270(2) Å, c = 5.6240(1) Å, and Z = 2. The structure comprises CrN35– trigonal planar units and HBa611+ octahedral units. The presence of anionic hydrogen in the structure has been verified by 1H NMR experiments. DFT calculations show that the addition of hydrogen increases the stability of the phase versus Ba3CrN3. The two d-electrons of Cr4+ are located in the nonbonding d z 2 orbital, rendering Ba3CrN3H nonmagnetic and insulating.
Single crystals of a new family of layered lanthanide oxychlorides, Ba3Ln2O5Cl2 (Ln = Gd–Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/mmm (No. ...139; Z = 2) with lattice parameters a = 4.3384(1)–4.4541(1) Å and c = 24.5108(7)–24.8448(9) Å. Ba3Ln2O5Cl2 phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba2Ln2O5 formed by corner-connected LnO5 tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl2. A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. The synthesis of this family suggests a route to other potential multianion phases.
The nitride-hydride Ba
CrN
H was obtained in single crystalline form using flux growth techniques based on alkaline earth metals. Ba
CrN
H crystallizes in the hexagonal space group P6
/ m (Nr 176), ...with the lattice parameters a = 8.0270(2) Å, c = 5.6240(1) Å, and Z = 2. The structure comprises CrN
trigonal planar units and HBa
octahedral units. The presence of anionic hydrogen in the structure has been verified by
H NMR experiments. DFT calculations show that the addition of hydrogen increases the stability of the phase versus Ba
CrN
. The two d-electrons of Cr
are located in the nonbonding d
orbital, rendering Ba
CrN
H nonmagnetic and insulating.
A series of f-block chromates, CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the AmIII derivatives and their lanthanide analogs. While all compounds ...form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the LnIII compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm(CrO4)2, α-CsEu(CrO4)2, and α-CsAm(CrO4)2 were studied using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods. Notably, the covalent contributions to bonding by the f orbitals were found to be more than twice as large in the AmIII chromate than in the SmIII and EuIII compounds, and even larger in magnitude than the Am-5f spin–orbit splitting in this system. Our analysis indicates also that the Am–O covalency in α-CsAm(CrO4)2 is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.
Single crystals of a new family of layered lanthanide oxychlorides, Ba
Ln
O
Cl
(Ln = Gd-Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/mmm (No. ...139; Z = 2) with lattice parameters a = 4.3384(1)-4.4541(1) Å and c = 24.5108(7)-24.8448(9) Å. Ba
Ln
O
Cl
phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba
Ln
O
formed by corner-connected LnO
tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl
. A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. The synthesis of this family suggests a route to other potential multianion phases.
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