Piezoelectric Actuation Mechanism Involving Extrinsic Nanodomain Dynamics in Lead-Free Piezoelectrics.

Piezoelectric materials play a key role in applications, while there are physically open questions. The physical origin of piezoelectricity is understood as the sum of contributions from intrinsic effects on lattice dynamics and those from extrinsic effects on ferroic-domain dynamics, but there is an incomplete understanding that all but intrinsic effects are classified as extrinsic effects. Therefore, the accurate classification of extrinsic effects is important for understanding the physical origin of piezoelectricity. In this work, high-energy synchrotron radiation X-ray diffraction is utilized to measure the response of BiFeO3 -BaTiO3 piezoelectrics and the intrinsic/extrinsic contribution to electric fields. It is found from crystal structure and intrinsic/extrinsic contribution, using the analysis involving structure refinement with various structural model and micromechanics-based calculations, that Bi3+ -ion disordering is important for realization of piezoelectricity and nanodomains. Here, an extrinsic effect on the rearrangement of nanodomains is suggested. The nanodomains, which are formed by the locally distorted structure around the A-site by Bi-ion disordering, can significantly deform the material in the BiFeO3 -BaTiO3 system, which contributes to the piezoelectric actuation mechanism apart from the extrinsic effect on ferroic-domain dynamics. Bi-ion disordering plays an important role in realizing piezoelectricity and nanodomains and can provide essential material design clues to develop next-generation Bi-based lead-free piezoelectric ceramics.

Observing and Modeling the Sequential Pairwise Reactions that Drive Solid-State Ceramic Synthesis.

Solid-state synthesis from powder precursors is the primary processing route to advanced multicomponent ceramic materials. Designing reaction conditions and precursors for ceramic synthesis can be a laborious, trial-and-error process, as heterogeneous mixtures of precursors often evolve through a complicated series of reaction intermediates. Here, ab initio thermodynamics is used to model which pair of precursors has the most reactive interface, enabling the understanding and anticipation of which non-equilibrium intermediates form in the early stages of a solid-state reaction. In situ X-ray diffraction and in situ electron microscopy are then used to observe how these initial intermediates influence phase evolution in the synthesis of the classic high-temperature superconductor YBa2 Cu3 O6+ x   (YBCO). The model developed herein rationalizes how the replacement of the traditional BaCO3 precursor with BaO2 redirects phase evolution through a low-temperature eutectic melt, facilitating the formation of YBCO in 30 min instead of 12+ h. Precursor selection plays an important role in tuning the thermodynamics of interfacial reactions and emerges as an important design parameter in planning kinetically favorable synthesis pathways to complex ceramic materials.

Formation Mechanism of ß-Li3PS4 through Decomposition of Complexes.

ß-Li3PS4 is a solid electrolyte with high Li+ conductivity, applicable to sulfide-based all-solid-state batteries. While a ß-Li3PS4-synthesized by solid-state reaction forms only in a narrow 300-450 °C temperature range upon heating, ß-Li3PS4 is readily available by liquid-phase synthesis through low-temperature thermal decomposition of complexes composed of PS43- and various organic solvents. However, the conversion mechanism of ß-Li3PS4 from these complexes is not yet understood. Herein, we proposed the synthesis mechanism of ß-Li3PS4 from Li3PS4·acetonitrile (Li3PS4·ACN) and Li3PS4·1,2-dimethoxyethane (Li3PS4·DME), whose structural similarity with ß-Li3PS4 would reduce the nucleation barrier for the formation of ß-Li3PS4. Synchrotron X-ray diffraction clarified that both complexes possess similar layered structures consisting of alternating Li2PS4- and Li+-ACN/DME layers. ACN/DME was removed from these complexes upon heating, and rotation of the PS4 tetrahedra induced a uniaxial compression to form the ß-Li3PS4 framework.

Size effect of the guest cation on the AlO4 framework in aluminate sodalite-type oxides M8[Al12O24](SO4)2 (M = Sr2+ and Ca2+) in the I43m phase.

Sr8[Al12O24](SO4)2 (SAS) and Ca8[Al12O24](SO4)2 (CAS) are members of the aluminate sodalite-type oxides with the general chemical formula M8[Al12O24](XO4)2 (M2+ is the guest cation and XO42- is the guest anion). To discuss the role of the guest cations (M2+ = Sr2+ and Ca2+) on the rotation of AlO4 in the oxygen tetrahedral framework in the I43m phase, the crystal structure parameters and the probability density function of the guest ions in SAS and CAS have been investigated via synchrotron radiation X-ray powder diffraction by considering Gram-Charlier expansions. The interatomic distances between the M2+ and O2- ions evaluated from the maximum positions in the probability density distribution are almost equal to the sum of the ideal ionic radii of the M2+ and O2- ions. This result suggests that the geometry of the AlO4 tetrahedral framework and the fluctuation of the guest ions are mainly caused by steric effects between the M2+ and O2- ions.

Structural Transition with a Sharp Change in the Electrical Resistivity and Spin-Orbit Mott Insulating State in a Rhenium Oxide, Sr3Re2O9.

We report the successful synthesis, crystal structure, and electrical properties of Sr3Re2O9, which contains Re6+ with the 5d1 configuration. This compound is isostructural with Ba3Re2O9 and shows a first-order structural phase transition at ∼370 K. The low-temperature (LT) phase crystallizes in a hettotype structure of Ba3Re2O9, which is different from that of the LT phase of Sr3W2O9, suggesting that the electronic state of Re6+ plays an important role in determining the crystal structure of the LT phase. The structural transition is accompanied by a sharp change in the electrical resistivity. This is likely a metal-insulator transition, as suggested by the electronic band calculation and magnetic susceptibility. In the LT phase, the ReO6 octahedra are rotated in a pseudo-a0a0a+ manner in Glazer notation, which corresponds to C-type orbital ordering. Paramagnetic dipole moments were confirmed to exist in the LT phase by muon spin rotation and relaxation measurements. However, the dipole moments shrink greatly because of the strong spin-orbit coupling in the Re ions. Thus, the electronic state of the LT phase corresponds to a Mott insulating state with strong spin-orbit interactions at the Re sites.

Evolution of two bulk-superconducting phases in Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, Sm) by external hydrostatic pressure effect.

Polycrystalline samples of Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transition temperatures (Tc) within the range of 2.1-2.7 K under ambient pressure, the estimated superconducting volume fraction was small, which indicates non-bulk nature of superconductivity in those samples under ambient pressure. A dramatic increase in shielding fraction, which indicates the emergence of the bulk superconductivity was achieved by applying external hydrostatic pressures. We found that two phases, low-P phases with Tc = 2.5-2.8 K and high-P phases with Tc = 10.0-10.8 K, were induced by the pressure effect for samples with RE = La, Ce, Pr, and Nd. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions, which was explained by the correlation between external and chemical pressure effects. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for understanding the effects of both external and chemical pressures to the emergence of bulk superconductivity and pairing mechanisms in BiCh2-based superconductors.

Crystal Structure and Thermoelectric Transport Properties of As-Doped Layered Pnictogen Oxyselenides NdO0.8F0.2Sb1-xAsxSe2.

We report the synthesis and thermoelectric transport properties of As-doped layered pnictogen oxyselenides NdO0.8F0.2Sb1-xAsxSe2 (x ≤ 0.6), which are predicted to show high-performance thermoelectric properties based on first-principles calculation. The crystal structure of these compounds belongs to the tetragonal P4/nmm space group (No. 129) at room temperature. The lattice parameter c decreases with increasing x, while a remains almost unchanged among the samples. Despite isovalent substitution of As for Sb, electrical resistivity significantly rises with increasing x. Very low thermal conductivity of less than 0.8 Wm-1K-1 is observed at temperatures between 300 and 673 K for all the examined samples. For As-doped samples, the thermal conductivity further decreases above 600 K. Temperature-dependent synchrotron X-ray diffraction indicates that an anomaly also occurs in the c-axis length at around 600 K, which may relate to the thermal transport properties.

Antithermal Quenching of Luminescence in Zero-Dimensional Hybrid Metal Halide Solids.

Zero-dimensional (0D) hybrid metal halides have emerged as a new generation of luminescent phosphors owing to their high radiative recombination rates, which, akin to their three-dimensional cousins, commonly demonstrate thermal quenching of luminescence. Here, we report on the finding of antithermal quenching of luminescence in 0D hybrid metal halides. Using (C9NH20)2SnBr4 single crystals as an example system, we show that 0D metal halides can demonstrate antithermal quenching of luminescence. A combination of experimental characterizations and first-principles calculations suggests that antithermal quenching of luminescence is associated with trap states introduced by structural defects in (C9NH20)2SnBr4. Importantly, we find that antithermal quenching of luminescence is not only limited to (C9NH20)2SnBr4 but also exists in other 0D metal halides. Our work highlights the important role of defects in impacting photophysical properties of hybrid metal halides and may stimulate new efforts to explore metal halides exhibiting antithermal quenching of luminescence at higher temperatures.

Flux Growth and Superconducting Properties of (Ce,Pr)OBiS2 Single Crystals.

Ce1-x Pr x OBiS2 (0. 1 ≤ x ≤ 0.9) single crystals were grown using a CsCl flux method. Their structural and physical properties were examined by X-ray diffraction, X-ray absorption, transmission electron microscopy, and electrical resistivity. All of the Ce1-x Pr x OBiS2 single crystals with 0.1 ≤ x ≤ 0.9 exhibited tetragonal phase. With increasing Pr content, the a-axis and c-axis lattice parameters decreased and increased, respectively. Transmission electron microscope analysis of Ce0.1Pr0.9OBiS2 (x = 0.9) single crystal showed no stacking faults. Atomic-resolution energy dispersive X-ray spectrometry mapping revealed that Bi, Ce/Pr, O, and S occupied different crystallographic sites, while Ce and Pr randomly occupied the same sites. X-ray absorption spectra showed that an increase of the Pr ratio increased the ratio of Ce4+/Ce3+. All of the Ce1-x Pr x OBiS2 crystals showed superconducting transition, with a maximum transition temperature of ~4 K at x = 0.9.

Antiferroelectric to Antiferroelectric-Relaxor Phase Transition in Calcium Strontium Sulfoaluminate.

Structural phase transitions of calcium strontium sulfoaluminate series, (Ca1-xSrx)8[AlO2]12(SO4)2 ((CS)AS-x) with x = 0.80-1.00, are systematically investigated by powder X-ray diffraction, dielectric measurements, and pyroelectric measurements, to clarify a phase diagram of (CS)AS-x (x = 0.80-1.00). A pure strontium sulfoaluminate, (CS)AS-1.00, is found to undergo three phase transitions, which take place successively on cooling from a prototypical cubic phase with the symmetry of Im3̅m. Though the room-temperature phase of (CS)AS-1.00 was previously reported to be of polar Pcc2, the pyroelectric measurements clarified a nonpolar character of the crystal symmetry. The dielectric measurements suggest a possibility of an antiferroelectric ground state of (CS)AS-x in the Sr-rich compositions. As x decreases, the ground state changes to a short-range-ordered state, implying a unique phase transition from the antiferroelectric state to the antiferroelectric-relaxor state. The present study provides an intriguing playground for designing new ferro/antiferroelectric materials.

Defective [Bi2 O2 ]2+ Layers Exhibiting Ultrabroad Near-Infrared Luminescence.

Aurivillius phases have been routinely known as excellent ferroelectrics and have rarely been deemed as materials that luminesce in the near-infrared (NIR) region. Herein, it is shown that the Aurivillius phases can demonstrate broadband NIR luminescence that covers telecommunication and biological optical windows. Experimental characterization of the model system Bi2.14 Sr0.75 Ta2 O9-x , combined with theoretical calculations, help to establish that the NIR luminescence originates from defective [Bi2 O2 ]2+ layers. Importantly, the generality of this finding is validated based on observations of a rich bank of NIR luminescence characteristics in other Aurivillius phases. This work highlights that incorporating defects into infinitely repeating [Bi2 O2 ]2+ layers can be used as a powerful tool to space-selectively impart unusual luminescence emitters to Aurivillius-phase ferroelectrics, which not only offers an optical probe for the examination of defect states in ferroelectrics, but also provides possibilities for coupling of the ferroelectric property with NIR luminescence.

Hydrothermal Synthesis and Crystal Structure of a (Ba0.54K0.46)4Bi4O12 Double-Perovskite Superconductor with Onset of the Transition Tc ∼ 30 K.

A new superconducting double perovskite was successfully synthesized by a low-temperature hydrothermal reaction at 240 °C. The crystal structure refinement of this double perovskite was done by single-crystal X-ray diffraction, and it had a cubic unit cell of a = 8.5207(2) Å with space group Im3̅m (No. 229). This superconducting double-perovskite chemical composition was estimated by electron probe microanalysis and was similar to the refined data. The superconducting transition temperature of the double perovskite was ∼30 K; the electrical resistivity began to fall at ∼25 K, and zero resistivity occurred below 7 K. Moreover, temperature-dependent resistivity under various magnetic fields and isothermal magnetization measurements ensured the nature of a type II superconductor for the sample. Finally, the metallic nature of the material was investigated by a first-principles study.

An electronic structure governed by the displacement of the indium site in In-S6 octahedra: LnOInS2 (Ln = La, Ce, and Pr).

An extremely large displacement of the indium site in In-S6 octahedra in LnOInS2 (Ln = La, Ce, and Pr) was found in synchrotron X-ray diffraction. LaOInS2 with off-center indium in In-S6 octahedra exhibited a wider optical band gap than CeOInS2 and PrOInS2 with on-center indium. Therefore, the electronic structure of LnOInS2 is governed by the indium site with an extremely large displacement. All LnOInS2 produced H2 gas under visible light irradiation in the presence of sacrificial electron donors.

Doping-Induced Polymorph and Carrier Polarity Changes in Thermoelectric Ag(Bi,Sb)Se2 Solid Solution.

Silver bismuth diselenide (AgBiSe2) is an n-type thermoelectric material that exhibits a complex structural phase transition from the hexagonal to cubic phase, while silver antimony diselenide (AgSbSe2) is a p-type thermoelectric material that crystallizes in the cubic phase at all temperatures. Here, we investigate the crystal structure and thermoelectric properties of Ag(Bi,Sb)Se2 solid solution, employing AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 as representative samples. The carrier polarity of AgBi0.9Sb0.1Se2 is converted from the n-type to p-type by Pb doping, accompanied by a polymorphic change to the cubic phase. It is difficult to obtain highly conductive p-type hexagonal AgBiSe2-based materials, although first-principles calculations predict high-performance thermoelectric properties for these systems. We also demonstrate that cubic AgBi0.7Sb0.3Se2 undergoes a polymorphic change to the hexagonal phase upon Nb doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se2 solid solution.

Hydrothermal Synthesis of Pyrochlore-Type Pentavalent Bismuthates Ca2Bi2O7 and Sr2Bi2O7.

The pyrochlore-type Ca2Bi2O7 and Sr2Bi2O7 have been synthesized from a low-temperature hydrothermal route using NaBiO3·nH2O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca2Bi2O7 and Sr2Bi2O7, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.

X-ray-activated long persistent phosphors featuring strong UVC afterglow emissions.

Phosphors emitting visible and near-infrared persistent luminescence have been explored extensively owing to their unusual properties and commercial interest in their applications such as glow-in-the-dark paints, optical information storage, and in vivo bioimaging. However, no persistent phosphor that features emissions in the ultraviolet C range (200-280 nm) has been known to exist so far. Here, we demonstrate a strategy for creating a new generation of persistent phosphor that exhibits strong ultraviolet C emission with an initial power density over 10 milliwatts per square meter and an afterglow of more than 2 h. Experimental characterizations coupled with first-principles calculations have revealed that structural defects associated with oxygen introduction-induced anion vacancies in fluoride elpasolite can function as electron traps, which capture and store a large number of electrons triggered by X-ray irradiation. Notably, we show that the ultraviolet C afterglow intensity of the yielded phosphor is sufficiently strong for sterilization. Our discovery of this ultraviolet C afterglow opens up new avenues for research on persistent phosphors, and it offers new perspectives on their applications in terms of sterilization, disinfection, drug release, cancer treatment, anti-counterfeiting, and beyond.

Na1-xSn2P2 as a new member of van der Waals-type layered tin pnictide superconductors.

Superconductors with a van der Waals (vdW) structure have attracted a considerable interest because of the possibility for truly two-dimensional (2D) superconducting systems. We recently reported NaSn2As2 as a novel vdW-type superconductor with transition temperature (Tc) of 1.3 K. Herein, we present the crystal structure and superconductivity of new material Na1-xSn2P2 with Tc = 2.0 K. Its crystal structure consists of two layers of a buckled honeycomb network of SnP, bound by the vdW forces and separated by Na ions, as similar to that of NaSn2As2. Amount of Na deficiency (x) was estimated to be 0.074(18) using synchrotron X-ray diffraction. Bulk nature of superconductivity was confirmed by the measurements of electrical resistivity, magnetic susceptibility, and specific heat. First-principles calculation using density functional theory shows that Na1-xSn2P2 and NaSn2As2 have comparable electronic structure, suggesting higher Tc of Na1-xSn2P2 resulted from increased density of states at the Fermi level due to Na deficiency. Because there are various structural analogues with tin-pnictide (SnPn) conducting layers, our results indicate that SnPn-based layered compounds can be categorized into a novel family of vdW-type superconductors, providing a new platform for studies on physics and chemistry of low-dimensional superconductors.

Crystal Structure, Thermal Behavior, and Photocatalytic Activity of NaBiO3· nH2O.

The crystal structure of NaBiO3· nH2O was refined using synchrotron powder X-ray diffraction and was assigned to a trigonal unit cell (space group P3̅) consisting of layered structures formed by edge-sharing BiO6 octahedra and consisting of an interlayer composed of water molecules sandwiched between two layers of sodium atoms, perpendicular to the c axis. An intermediate phase was observed during the dehydration of the hydrated compound. Density of state calculations showed hybridization of the Bi 6s and O 2p orbitals at the bottom of the conduction bands for both the hydrated and the dehydrated phases, which narrows the band gap and promotes their photocatalytic activity in the visible region.

Cs4PbBr6/CsPbBr3 Perovskite Composites with Near-Unity Luminescence Quantum Yield: Large-Scale Synthesis, Luminescence and Formation Mechanism, and White Light-Emitting Diode Application.

All-inorganic perovskites have emerged as a new class of phosphor materials owing to their outstanding optical properties. Zero-dimensional inorganic perovskites, in particular the Cs4PbBr6-related systems, are inspiring intensive research owing to the high photoluminescence quantum yield (PLQY) and good stability. However, synthesizing such perovskites with high PLQYs through an environment-friendly, cost-effective, scalable, and high-yield approach remains challenging, and their luminescence mechanisms has been elusive. Here, we report a simple, scalable, room-temperature self-assembly strategy for the synthesis of Cs4PbBr6/CsPbBr3 perovskite composites with near-unity PLQY (95%), high product yield (71%), and good stability using low-cost, low-toxicity chemicals as precursors. A broad range of experimental and theoretical characterizations suggest that the high-efficiency PL originates from CsPbBr3 nanocrystals well passivated by the zero-dimensional Cs4PbBr6 matrix that forms based on a dissolution-crystallization process. These findings underscore the importance in accurately identifying the phase purity of zero-dimensional perovskites by synchrotron X-ray technique to gain deep insights into the structure-property relationship. Additionally, we demonstrate that green-emitting Cs4PbBr6/CsPbBr3, combined with red-emitting K2SiF6:Mn4+, can be used for the construction of WLEDs. Our work may pave the way for the use of such composite perovskites as highly luminescent emitters in various applications such as lighting, displays, and other optoelectronic and photonic devices.

Transformation of Perovskite BaBiO3 into Layered BaBiO2.5 Crystals Featuring Unusual Chemical Bonding and Luminescence.

Engineering oxygen coordination environments of cations in oxides has received intense interest thanks to the opportunities for the discovery of novel oxides with unusual properties. Herein, the synthesis of stoichiometric layered BaBiO2.5 by a nontopotactic phase transformation of perovskite BaBiO3 is presented. By analyzing the synchrotron X-ray diffraction data by the maximum-entropy method/Rietveld technique, it was found that Bi is involved in an unusual chemical bonding situation with four oxygen atoms featuring one ionic bond and three covalent bonds, which results in an asymmetric coordination geometry. Photophysical characterization revealed that this peculiar structure shows near-infrared luminescence differing from that of conventional Bi-containing compounds. Experimental and theoretical results led to the proposal of an excitonic nature of the luminescence. This work highlights that synthesizing materials with uncommon Bi-O bonding and Bi coordination geometry provides a pathway to the discovery of systems with new functionalities. This could inspire interest in the exploration of a range of materials containing heavier p-block elements with prospects for finding systems with unusual properties.