We report the observation of local structural dipoles that emerge from an undistorted ground state on warming, in contrast to conventional structural phase transitions in which distortions emerge on ...cooling. Using experimental and theoretical probes of the local structure, we demonstrate this behavior in binary lead chalcogenides, which were believed to adopt the ideal, undistorted rock-salt structure at all temperatures. The behavior is consistent with a simple thermodynamic model in which the emerging dipoles are stabilized in the disordered state at high temperature due to the extra configurational entropy despite the fact that the undistorted structure has lower internal energy. Our findings shed light on the anomalous electronic and thermoelectric properties of the lead chalcogenides. Similar searches may show that the phenomenon is more widespread.
We have synthesized a new layered BiS2-based compound, SrFBiS2. This compound has a similar structure to LaOBiS2. It is built up by stacking up SrF layers and NaCl-type BiS2 layers alternatively ...along the c axis. Electric transport measurement indicates that SrFBiS2 is a semiconductor. Thermal transport measurement shows that SrFBiS2 has a small thermal conductivity and large Seebeck coefficient. First principle calculations are in agreement with experimental results and show that SrFBiS2 is very similar to LaOBiS2, which becomes a superconductor with F doping. Therefore, SrFBiS2 may be a parent compound of new superconductors.
The atomic pair distribution function (PDF) analysis of X-ray powder diffraction data has been used to study the structure of small and ultra-small CdSe nanoparticles. A method is described that uses ...a wurtzite and zinc-blende mixed phase model to account for stacking faults in CdSe particles. The mixed-phase model successfully describes the structure of nanoparticles larger than 2 nm yielding a stacking fault density of about 30%. However, for ultrasmall nanoparticles smaller than 2 nm, the models cannot fit the experimental PDF showing that the structure is significantly modified from that of larger particles and the bulk. The observation of a significant change in the average structure at ultra-small size is likely to explain the unusual properties of the ultrasmall particles such as their white light emitting ability.
For ultrasmall CdSe nanoparticles smaller than 2 nm, the average structure is significantly modified from that of larger particles and the bulk.
The improper ferroelectricity inYMnO3and other related multiferroic hexagonal manganites is known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical ...phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here, we reveal the evolution of the local structure with temperature inYMnO3using neutron total scattering techniques, and we interpret them with the help of first-principles calculations and with a first-principles-based effective Hamiltonian. The results show that, at room temperature, the local and average structures are consistent with the established ferroelectricP63cmsymmetry. On heating, both local and average structural analyses show striking anomalies from about 800 K up to the Curie temperature and signatures of a locally more preserved structure than on average, consistent with increasing fluctuations of the order-parameter angle. These fluctuations result in an unusual local symmetry lowering into a continuum of structures on heating. This local symmetry breaking persists into the high-symmetry nonpolar phase, constituting an unconventional type of order-disorder transition, and we pinpoint it as the reason for the anomalous behavior near the phase transition. The hidden disorder revealed inYMnO3by total scattering is expected to find analogies in other materials with structural frustration or characteristic energy barriers of different magnitudes.
Two-dimensional iron chalcogenide intercalates display a remarkable correlation of the interlayer spacing with enhancement of the superconducting critical temperature (Tc). In this work, synchrotron ...X-ray absorption (XAS; at the Fe and Se K-edges) and emission (XES; at the Fe Κβ) spectroscopies allow one to discuss how the important rise of Tc (~44 K) in the molecule-intercalated Lix(C5H5N)yFe2-zSe2 relates to the electronic and local structural changes felt by the inorganic host upon doping (x). XES shows that widely separated layers of edge-sharing FeSe4 tetrahedra carry low-spin moieties, with a local Fe magnetic moment slightly reduced compared to the parent β-Fe2-zSe2. Pre-edge XAS expresses the progressively reduced mixing of metal 3d-4p states upon lithiation. Doping-mediated local lattice modifications, probed by conventional Tc optimization measures (cf. the anion height and FeSe4 tetrahedra regularity), become less relevant when layers are spaced far away. On the basis of extended X-ray absorption fine structure, such distortions are compensated by a softer Fe network that relates to Fe-site vacancies, alleviating electron-lattice correlations and superconductivity. Density functional theory (DFT) guided modification of the isolated Fe2-zSe2 (z, vacant sites) planes, resembling the host layers, identify that Fe-site deficiency occurs at low energy cost, giving rise to stretched Fe sheets, in accordance with experiments. Here, the robust high-Tc in Lix(C5H5N)yFe2-zSe2, arises from the interplay of electron-donating spacers and the iron selenide layer's tolerance to defect chemistry, a tool to favorably tune its Fermi surface properties.
Typically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid ...compound AgGaTe2 is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd3 orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic–optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m−1 K−1 at 850 K in AgGaTe2. The findings explain the underlying reason for the unexpectedly low thermal conductivities of silver‐based compounds compared to copper‐based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.
Weak sd3 orbital hybridization of tetrahedral Ag atoms leads to the local symmetry breaking in Ag‐based diamondoid compounds. This emphanitic local geometric distortion of Ag causes a global distortion of the crystal structure and results in the ultralow thermal conductivity. Based on this, the heat conductivity of the diamondoid structure can be directly evaluated by the crystallographic distortion parameter.
The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total ...scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy their vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation still creates a spinel-like shell, but with a higher volume fraction of the rock salt Co-containing phase in the core. The higher divalent metal stoichiometry in the rock salt phase, with increasing Co content, results in a population of unoccupied tetrahedral metal sites in the spinel part, likely through oxidative shell creation, involving an ordered defect-clustering mechanism, directly correlated to core stabilization. To shed light on the effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo simulations suggest that the designed NCs, with desirable, enhanced magnetic properties (cf. exchange bias and coercivity), are developed with magnetocrystalline anisotropy which increases due to a relatively low content of Co ions in the lattice. The growth of optimally performing candidates combines also a strongly exchange-coupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, specific absorption rate (SAR) calculations demonstrate that the rock salt core sufficiently protected from oxidation and the heterostructure preserved over time, play a key role in magnetically mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications.
When synthesized under certain conditions, barium titanate (BaTiO3, BTO) nanoparticles are found to have the non‐thermodynamic cubic structure at room temperature. These particles also have a ...several‐fold enhanced dielectric constant, sometimes exceeding 6000, and are widely used in thin‐layer capacitors. A hydrothermal approach is used to synthesize BTO nanocrystals, which are characterized by a range of methods, including X‐ray Rietveld refinement and the Williamson–Hall approach, revealing the presence of significant inhomogeneous strain associated with the cubic phase. However, X‐ray pair distribution function measurements clearly show the local structure is lower symmetry than cubic. This apparent inconsistency is resolved by examining 3D Bragg coherent diffraction images of selected nanocrystals, which show the existence of ≈50 nm‐sized domains, which are interpreted as tetragonal twins, and yet cause the average crystalline structure to appear cubic. The ability of these twin boundaries to migrate under the influence of electric fields explains the dielectric anomaly for the nanocrystalline phase.
A hydrothermal synthesis is used to prepare metastable cubic‐phase barium titanate nanoparticles, similar to those used in multilayer capacitors for their enhanced dielectric constant. A strain analysis reveals the presence of large inhomogeneous strain, while coherent X‐ray imaging reveals the existence of domains, interpreted as twins. The twin boundaries can move under an electric field, to explain the dielectric response.
A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron–selenide host by ...organic molecular donors, aiming at the formation of the expanded-lattice Lix(C5H5N)yFe2–zSe2 hybrid superconductor. The intercalates are found to crystallize in the tetragonal ThCr2Si2-type structure at the average level, however, with an enhanced interlayer iron–selenide spacing (d = 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution function (PDF) analysis at variable times suggests distorted FeSe4 tetrahedral local environments that appear swollen with respect to those in the parent β-FeSe. Simultaneously acquired in situ synchrotron X-ray powder diffraction data disclose that secondary phases (α-Fe and Li2Se) grow significantly when a higher lithium concentration is used in the solvothermal reaction or when the solution is aged. These observations are in line with the strongly reducing character of the intercalation medium’s solvated electrons that mediate the defect chemistry of the expanded-lattice superconductor. In the latter, intralayer correlated local distortions indicate electron-donating aspects that reflect in somewhat enlarged Fe–Se bonds. Additionally, they also reveal a degree of relief of chemical pressure associated with a large distance between Fe and Se sheets (“taller” anion height) and a stretched Fe–Fe square planar topology. The elongation of the latter, derived from the in situ PDF study, speaks for a plausible increase in the Fe-site vacancy concentration. The evolution of the local structural parameters suggests an optimum reaction window where kinetically stabilized phases resemble the distortions of the edge-sharing Fe–Se tetrahedra, required for a high-Tc in expanded-lattice iron-chalcogenides.
A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by ...organic molecular donors, aiming at the formation of the expanded-lattice Li
(C
H
N)
Fe
Se
hybrid superconductor. The intercalates are found to crystallize in the tetragonal ThCr
Si
-type structure at the average level, however, with an enhanced interlayer iron-selenide spacing (
= 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution function (PDF) analysis at variable times suggests distorted FeSe
tetrahedral local environments that appear swollen with respect to those in the parent β-FeSe. Simultaneously acquired in situ synchrotron X-ray powder diffraction data disclose that secondary phases (α-Fe and Li
Se) grow significantly when a higher lithium concentration is used in the solvothermal reaction or when the solution is aged. These observations are in line with the strongly reducing character of the intercalation medium's solvated electrons that mediate the defect chemistry of the expanded-lattice superconductor. In the latter, intralayer correlated local distortions indicate electron-donating aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a degree of relief of chemical pressure associated with a large distance between Fe and Se sheets ("taller" anion height) and a stretched Fe-Fe square planar topology. The elongation of the latter, derived from the in situ PDF study, speaks for a plausible increase in the Fe-site vacancy concentration. The evolution of the local structural parameters suggests an optimum reaction window where kinetically stabilized phases resemble the distortions of the edge-sharing Fe-Se tetrahedra, required for a high-
in expanded-lattice iron-chalcogenides.
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