Showing papers on "Tetragonal crystal system published in 2018"
TL;DR: A general and practical method is demonstrated to obtain super-tetragonality and giant polarization using interphase strain to create an out-of-plane–to–in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO3), compared to 1.065 in bulk.
Abstract: Strain engineering has emerged as a powerful tool to enhance the performance of known functional materials. Here we demonstrate a general and practical method to obtain super-tetragonality and giant polarization using interphase strain. We use this method to create an out-of-plane-to-in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO3), compared to 1.065 in bulk. These thin films with super-tetragonal structure possess a giant remanent polarization, 236.3 microcoulombs per square centimeter, which is almost twice the value of known ferroelectrics. The super-tetragonal phase is stable up to 725°C, compared to the bulk transition temperature of 490°C. The interphase-strain approach could enhance the physical properties of other functional materials.
167 citations
TL;DR: In this article, tetragonal zirconia (t-ZrO2) nanoparticles were successfully prepared by a hydrothermal method and characterized by various measurement techniques.
143 citations
TL;DR: It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.
Abstract: Hf1-xZrxO2 (x ∼ 0.5-0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca21) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.
138 citations
TL;DR: The temperature-dependent structure evolution of the hybrid halide perovskite compounds, FASnI3 and FAPbBr3, has been monitored using high-resolution synchrotron X-ray powder diffraction to suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the in organic network and dynamic lone pair stereochemical activity on the B-site.
Abstract: The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI3, FA+ = CH[NH2]2+) and formamidinium lead bromide (FAPbBr3), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic Pm3m (No. 221) to tetragonal P4/mbm (No. 127) for both materials upon cooling; this occurs for FAPbBr3 between 275 and 250 K, and for FASnI3 between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic Pnma (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI3 and FAPbBr3 are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K–1 for FASnI3 at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the...
120 citations
TL;DR: The findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.
Abstract: Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline ( C H 3 N H 3 ) P b I 3 with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal ( C H 3 N H 3 ) P b I 3 . At room temperature, the effect peaks for excitation photon energies Δ E = 110 meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic–tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both Δ E and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.
109 citations
TL;DR: In this paper, an equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying, and the analysis of in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation.
108 citations
TL;DR: The dynamics of the methylammonium cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy, and 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase.
Abstract: In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C–N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition tem...
101 citations
TL;DR: HEAs have potential as hydrogen storage materials because of favorable absorption in all interstitial sites within the structure, and the large lattice strain in the HEA seems favorable for absorption in both octahedral and tetrahedral sites.
Abstract: A high-entropy alloy (HEA) of HfNbTiVZr was synthesized using an arc furnace followed by ball milling The hydrogen absorption mechanism was studied by in situ X-ray diffraction at different temperatures and by in situ and ex situ neutron diffraction experiments The body centered cubic (BCC) metal phase undergoes a phase transformation to a body centered tetragonal (BCT) hydride phase with hydrogen occupying both tetrahedral and octahedral interstitial sites in the structure Hydrogen cycling of the alloy at 500 °C is stable The large lattice strain in the HEA seems favorable for absorption in both octahedral and tetrahedral sites HEAs therefore have potential as hydrogen storage materials because of favorable absorption in all interstitial sites within the structure
100 citations
TL;DR: High-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry).
Abstract: All inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) belong to the novel class of confined metal–halide perovskites which are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to outstanding properties. A deep knowledge of the band-edge excitonic properties of these materials is thus crucial to further optimize their performances. Here, high-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry). Intelligible equations are found that show how the fundamental parameters (spin–orbit coupling, ΔSO, crystal field term, T, and electron–hole exchange energy, J) rule the energy spacings in doublets and triplets. From experimental data, fine estimations of each parameter are obtained. The analysis of the absorption spectra of an ensemble of NCs with a “quasi-bulk” behavior leads to ΔSO = 1.20 ± 0.06 eV and T = −0.34 ± 0.05 eV in CsPbBr3. The study of individual luminescence responses of NCs having sizes comparable to the exciton Bohr diameter, 7 nm, allows us to estimate the value of J to be around ≈3 meV in both tetragonal and orthorhombic phases. This value is already enhanced by confinement.
96 citations
TL;DR: In this paper, the low-temperature phases of formamidinium lead iodide (FAPbI3) have been investigated using rapid neutron powder diffraction and the orientation of disordered FA cation over this temperature range determined.
Abstract: A greater understanding of the structure–property relationships of hybrid perovskites for solar cells is crucial for enhancing their performance. The low-temperature phases of formamidinium lead iodide (FAPbI3) have been investigated using rapid neutron powder diffraction. On cooling, the metastable α-polymorph descends in symmetry from the cubic unit cell phase present at room temperature through two successive phase transitions. Between 285 and 140 K a tetragonal phase, adopting the space group P4/mbm, is confirmed and the orientation of the disordered FA cation over this temperature range determined. The cation dynamics have also been investigated, over the same temperature range, at the atomic scale by using ab initio molecular dynamics simulations, which indicate contrasting FA motion in the cubic and tetragonal structures. Below 140 K the neutron powder diffraction data display weak Bragg scattering intensities not immediately indexable to a related unit cell. Data collected at 100 K from N-deuterat...
95 citations
TL;DR: In this paper, the crystal structure, elastic properties, Vickers hardness, elastic anisotropy, electronic and thermodynamic properties of transition metal silicides are investigated by using the first-principles calculations.
TL;DR: Interestingly, black zirconia exemplified relatively limited activity albeit presence of oxygen vacancies, which was attributed to the presence of tetragonal phase and possibly, the insufficient creation of new energy states near valence and conduction band towards Fermi energy level.
Abstract: Nanometric powder particles of white zirconia were synthesized through precursor route by the pyrolysis of zirconium (IV) butoxide at varied temperatures in air ranging from 900–1400 °C and were predominantly monoclinic in nature. To control the defect chemistry, the precursor was also pyrolyzed in a reduced atmosphere at 900 °C, eventually resulting in black zirconia. The stabilization of tetragonal phase and observed color change from white to black in samples pyrolyzed under reduced atmosphere was attributed to the creation of oxygen vacancies and disorder. The black and white zirconia produced delineated the influence of crystal structure and oxygen vacancies on the photocatalytic performance. Furthermore, zirconia synthesized at lower temperatures (600 and 800 °C) in air confirmed the detrimental role of tetragonal phase on the degradation behavior of methylene blue dye. High photocatalytic degradation rate for white zirconia was attributed to the presence of increased density of nano-sized pores and low recombination rate of electron-hole pairs as confirmed by PL measurements. Interestingly, black zirconia exemplified relatively limited activity albeit presence of oxygen vacancies. This negative effect was attributed to the presence of tetragonal phase and possibly, the insufficient creation of new energy states near valence and conduction band towards Fermi energy level.
TL;DR: This work demonstrates that even in ionic conductors differences can be observed between the average and local crystal structures, and it reasserts that the high ionic Conductivity in Na3PS4 is not related to the crystal structure but rather differences in the defect concentration.
Abstract: The sodium superionic conductor Na3PS4 is known to crystallize in one of two different structural polymorphs at room temperature (i.e., cubic or tetragonal, depending on the synthetic conditions). Experimentally, the cubic structure is known to exhibit a higher ionic conductivity than the tetragonal structure, despite theoretical investigations suggesting that there should be no difference at all. Employing a combination of Rietveld and pair distribution function (PDF) analyses, as well as electrochemical impedance spectroscopy, we investigate the open question of how the crystal structure influences the ionic transport in Na3PS4. Despite the average structures of Na3PS4 prepared via ball-milling and high-temperature routes being cubic and tetragonal, respectively, the structural analysis by PDF indicates that both compounds are best described by the structural motifs of the tetragonal polymorph on the local scale. Ultimately, the high ionic conductivity of Na3PS4 prepared by the ball-milling approach is ...
TL;DR: In this article, multicomponent carbide thin films of (CrNbTaTiW)C (30-40% C) with different metal contents were deposited at different temperatures using non-reactive DC magnetron sputtering.
TL;DR: In this article, the authors carried out first-principles calculations of bandgaps of 260 all-inorganic halide perovskites belonging to the class ABX3, with Li, Na, K, Rb, Cs, B = Pb, Sn, and Ge, and X = F, Cl, Br, I.
Abstract: In order to address an all-inorganic halide lead-free perovskite for potential photovoltaic applications, we carried out first-principles calculations of bandgaps of 260 all-inorganic halide perovskites belonging to the class ABX3, with A = Li, Na, K, Rb, Cs, B = Pb, Sn, and Ge, and X = F, Cl, Br, I. Three most common crystal symmetries were chosen, including cubic, tetragonal, and two orthorhombic phases. The bandgap exhibited increase with the decreasing of the anions radius (I, Br, Cl, F) and lowering the symmetry of the structures. With consideration of multiple factors forming perovskites, we reported three all-inorganic lead-free halides perovskites including cubic-KSnCl3, cubic-RbSnCl3, and trigonal-NaGeBr3 as candidates with desirable bandgap (1.24–1.44 eV) for photovoltaic applications.
TL;DR: In this article, pure and dysprosium (Dy) doped SnO2 films have been fabricated through sol-gel spin coating technique and X-ray diffraction study revealed that deposited films exhibit tetragonal crystal structure with preferentially grown along (2' 0'0'0) plane.
Abstract: In the present work, pure and dysprosium (Dy) doped SnO2 films have been fabricated through sol-gel spin coating technique. Strong influence of Dy doping is observed on structural, morphological, vibrational, linear and nonlinear optical properties of SnO2 films. X-ray diffraction study revealed that deposited films exhibit tetragonal crystal structure with preferentially grown along (2 0 0) plane. With increase of doping concentration in SnO2, the crystallite size decreases while dislocation density and lattice distortion ratio increases. The characteristics Raman peaks of doped SnO2 thin films broaden, shifted and intensity decreases as compared to pure film which confirm the bonding between Dy and SnO2. Optical study shows that the prepared thin films are highly transparent and absorption increases with doping concentrations owing to increase of defects states. It is also observed that the optical band gap first increases and then lessens with rise of Dy-doping concentration which attributed to the Burstein-Moss (BM) effect. Additionally, dielectric constant and refractive index first decreasing with small doping concentration (1–3%) due to increase of carrier concentration, and then increases for higher doping (5–7%) due to increase of defect in SnO2 lattice. The values χ 3 and β obtained by Z-scan measurement are observed in range of 0.31 × 10−7 to 1.28 × 10−7 and 1.27 to 5.32 × 10−4 cm W−1, respectively. The limiting threshold of pure and Dy doped SnO2 nanostructured films were calculated to be in the range of 5.37–11.18 kJ/cm2.
TL;DR: Zirconia supports were synthesized via a precipitation method, and the effects of crystalline structure and textural properties of ZrO2 on the methane combustion reaction were investigated using the Pd/ZrO 2 catalysts.
Abstract: Zirconia supports were synthesized via a precipitation method, and the effects of crystalline structure and textural properties of ZrO2 on the methane combustion reaction were investigated using the Pd/ZrO2 catalysts. Upon increasing the digestion temperature, the crystalline structure of ZrO2 was transformed from the mixed phase (monoclinic and tetragonal) to the pure tetragonal phase, and these tetragonal ZrO2 supported Pd catalysts exhibited good hydrothermal stabilities. In addition, the BET surface areas of the tetragonal ZrO2 supports could also be tuned by varying the digestion time and calcination temperature, and the catalysts supported on ZrO2 having lower BET surface areas exhibited enhanced catalytic activities. Therefore, the Pd/ZrO2(85) catalyst, which had a tetragonal structure in addition to the smallest BET surface area, gave the optimal catalytic performance. This is likely due to larger PdO particles being formed on this ZrO2(85) support, thereby resulting in a highly reducible PdO species. Finally, variations in reducibility between the monoclinic and the tetragonal supported catalysts were clearly calculated by the density functional theory.
TL;DR: In this paper, the fundamental properties of MAPbBr3 single crystal were investigated by applying temperature-dependent x-ray diffraction and photoluminescence (PL) measurements from 10 K to 270 K.
Abstract: We investigated the fundamental properties of MAPbBr3 single crystal by applying temperature-dependent x-ray diffraction and photoluminescence (PL) measurements from 10 K to 270 K. The structural and spectral analyses illustrate the phase transitions of MAPbBr3 single crystal from cubic phase (-MAPbBr3) to tetragonal phases (-MAPbBr3 and -MAPbBr3), and then to orthorhombic phase (-MAPbBr3) at ~230 K, ~160 K and ~150 K, respectively. The Time-resolved PL results show that both free excitons (FE) and bound excitons (BE) have contributions to the PL emissions. The PL at high temperature mainly originates from the BE transitions, while the PL at low temperature (-MAPbBr3 phase) comes from both BE and FE transitions, and the FE related PL becomes more and more dominant at lower temperatures. The first-principle calculation at zero temperature reveals that the BE-related PL was caused by defects from the anti-site substitution of MA and Br ions.
TL;DR: Though 1 and 2 are not isomorphous, their crystal structures display the same structural organization; namely, the BiI6 octahedra are linked by I2 units to form disordered layers in 1 and perfectly ordered chains in 2, despite the strong quenching propensity of I2 moieties.
Abstract: We report the synthesis, crystal structures, and optical properties of two new compounds, K18Bi8I42(I2)0.5·14H2O (1) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2), as well as the electronic structure of the latter. They crystallize in tetragonal space group P4/mmm with the unit cell parameters a = 12.974(1) and c = 20.821(3) A for 1 and a = 13.061(3) and c = 15.162(7) A for 2. Though 1 and 2 are not isomorphous, their crystal structures display the same structural organization; namely, the BiI6 octahedra are linked by I2 units to form disordered layers in 1 and perfectly ordered chains in 2. The I–I bond distances in the thus formed I–I–I–I linear links are not uniform; the central bond is only slightly longer than in a standalone I2 molecule, whereas the peripheral bonds are significantly shorter than longer bonds typical for various polyiodides, which is confirmed by Raman spectroscopy. The analysis of the electronic structure shows that the atoms forming the I–I–I–I subunits transfer electron density from their o...
TL;DR: XRD studies reveals that sintered powder which exhibits tetragonal crystal structure and both crystallinity as well as crystal size increase with increase in temperature, while morphological studies reveal randomly arranged grains with compact nature grain size increases with sintering temperature.
Abstract: Nanoparticles of tin oxide (SnO2) powders were prepared by co-precipitation method at 500 °C, 700 °C and 900 °C temperature. The sintered SnO2 nanoparticles, structural, optical, magnetic, morphological properties and microbial activity have been studied. XRD studies reveals that sintered powder which exhibits tetragonal crystal structure and both crystallinity as well as crystal size increase with increase in temperature. The morphological studies reveal randomly arranged grains with compact nature grain size increases with sintering temperature. The compositional analyses of SnO2 nanoparticles have been studied using X-ray photoelectron spectroscopy analysis. The optical band gap values of SnO2 nanoparticles were calculated to be about 4.3 eV in the temperature 500 °C, comparing with that of the bulk SnO2 3.78 eV, by optical absorption measurement. Room temperature M-H curve for pure SnO2 nanoparticles exhibits ferromagnetic behaviour. The tin oxide nanoparticles are acted as potential candidate material for bacterial and fungal activity.
TL;DR: In this article, a complete phonon assignment of the oxide garnet Y3Al5O12 (YAG) was provided and the general spectral trends upon variation of the Ce3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y3-xCexAl 5O12.
Abstract: The oxide garnet Y3Al5O12 (YAG), when a few percent of the activator ions Ce3+ substitutes for Y3+, is a luminescent material widely used in phosphor-converted white lighting However, fundamental questions surrounding the defect chemistry and luminescent performance of this material remain, especially in regard to the nature and role of vibrational dynamics Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y3–xCexAl5O12 Increasing the Ce3+ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO8 moieties Decreasing the Ce3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon
TL;DR: In this article, the XRD Rietveld refinement was performed to explore crystal structures, phase fractions and atomic parameters of Fe2O3-added (0.995-x)K0.48Na0.5Na 0.5ZrO3 0.005BiScO3 ceramics.
Abstract: Although phase boundary engineering has made notable progress in improving the electrical properties of (K,Na)NbO3-based piezoceramics, lattice distortion and spontaneous polarization of multiphase coexisting systems are a few of the remaining concerns. Here, new research employing XRD Rietveld refinement was performed to explore crystal structures, phase fractions and atomic parameters of Fe2O3-added (0.995-x)K0.48Na0.52NbO3-xBi0.5Na0.5ZrO3-0.005BiScO3 ceramics. The distortion of the oxygen octahedron and the spontaneous polarization were presented. Central cation displacement provides a much larger contribution to polarization and the electric dipole moment in orthorhombic phase is much larger than that in tetragonal phase. Benefiting from tetragonal-orthorhombic phase coexistence and lattice distortion, optimized ferroelectric and piezoelectric properties (d33 ∼ 381 pC/N, Pr ∼ 20.47 μC/cm2) were obtained. The ceramic still holds a large d33 (313 pC/N) after up to 300 ℃ of thermal annealing. A series of material constants was also calculated and compared to lead-based ones.
TL;DR: In this article, single crystals of the methylammonium (MA) lead halides MAPbI3, MAPbBr3, and MAPbCl3 have been investigated using infrared spectroscopy with the aim of analyzing structural and dynamical aspects of processes that enable the ordering of the MA molecule in the orthorhombic crystal structure of these hybrid perovskites.
Abstract: Single crystals of the methylammonium (MA) lead halides MAPbI3, MAPbBr3, and MAPbCl3 have been investigated using infrared spectroscopy with the aim of analyzing structural and dynamical aspects of processes that enable the ordering of the MA molecule in the orthorhombic crystal structure of these hybrid perovskites Our temperature-dependent studies were focused on the analysis of the CH/NH rocking, C–N stretching, and CH/NH bending modes of the MA molecule in the 800–1750 cm–1 frequency range They deliver a direct comparison of the behaviors of the three halides on crossing the orthorhombic–tetragonal phase transition in MA lead halide single crystals Drastic changes of all vibrational modes close to the phase transition were clearly observed Additional spectral features that were not discussed previously are pointed out The transformation of the two-dimensional orthorhombic hydrogen bond layers into a more three-dimensional arrangement in the tetragonal phase seems to be an important feature provid
TL;DR: In this article, alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA+), HO(CH2)2NH3+, were introduced into formamidinium (FA+) tin-based perovskites (HEAxFA1-xSnI3; x = 0-1) to absorb light in carbon-based mesoscopic solar cells.
Abstract: Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA+), HO(CH2)2NH3+, were introduced into formamidinium (FA+) tin-based perovskites (HEAxFA1–xSnI3; x = 0–1) to absorb light in carbon-based mesoscopic solar cells. We found that HEA+ cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2–0.4) with greater symmetry. When x was increased to 0.6–1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEAxFA1–xSn0.67I2.33, x ≥ 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethylenediammonium diiodide (EDAI2) as an additive. After optimization of device performance with the SE + 3% EDAI2 approach, the HEA0.4FA0.6SnI3 (HEAI = 40%) device gave the best photovoltaic performance with JSC = 18.52 mA cm–2, VOC = 371 mV, FF = 0.562, and overall effici...
TL;DR: Electronic structure calculations suggest that asymmetric hopping p orbital electrons of Br(3) ions are responsible for the enhancement in electric polarization in tetragonal CsPbBr3 crystals, and this can broaden the application field from optoelectronics to ferroelectrics.
Abstract: Ab initio simulations combined with the Berry phase method are employed to investigate ferroelectric polarization of tetragonal CsPbBr3 crystals by applying hydrostatic pressure varying from 0 to 19 GPa; we find that the object research belongs to the P4mm space group. The calculated results show that the materials undergo a paraelectric-ferroelectric phase transition when the pressure increases to a critical value 15 GPa. The polarization is strongly enhanced and attains a high value of about 23 μC cm-2, owing to the increase in the ionic and electric contributions to the polarization under compressive strain. We present a detailed theoretical investigation to analyze the origin of polarization. The ionic polarization is mainly ascribed to the central displacements of Pb2+ cations and Br- anions induced by a highly distorted octahedral PbBr6- framework. Electronic structure calculations suggest that asymmetric hopping p orbital electrons of Br(3) ions are responsible for the enhancement in electric polarization. These discoveries suggest that tetragonal CsPbBr3 has significant potential in future ferroelectric applications, and this can broaden the application field from optoelectronics to ferroelectrics.
14 Mar 2018
TL;DR: In this paper, the spontaneous polarization of CH3NH3PbI3 was studied using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films.
Abstract: We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH3NH3PbI3) that is currently being investigated for use in thin film solar cells and light-emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-to-cubic phase transition temperature. Importantly, we observe these domains to exhibit a piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias but relaxing back on time scales of several minutes. Our results suggest the existence ...
TL;DR: The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.
Abstract: Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron-doped giant tetragonal perovskite compound Pb1-x Bix VO3 (x=0.2 and 0.3). A polar tetragonal (P4mm) to non-polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb0.76 La0.04 Bi0.20 VO3 exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb0.76 La0.04 Bi0.20 VO3 polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.
TL;DR: In this article, a nanoparticle-composed layered ZnMn2O4 is synthesized using a co-precipitation method, which shows an initial lithiation capacity of ∼1400 mA h g−1 and a high reversible capacity at a specific current of 0.5 A g− 1.
Abstract: Tetragonal spinel ZnMn2O4 provides extremely high capacity as an anode for Li-ion batteries owing to a conversion-type mechanism. In this work, nanoparticle-composed layered ZnMn2O4 is synthesized using a co-precipitation method. Calcination parameters are optimized through thermal gravimetric analysis and in situ high temperature synchrotron X-ray powder diffraction. The ZnMn2O4 shows an initial lithiation capacity of ∼1400 mA h g−1 and a high reversible capacity of ∼900 mA h g−1 at a specific current of 0.5 A g−1. In situ synchrotron X-ray powder diffraction reveals phase evolution during the 1st cycle. An intermediate phase, tetragonal spinel LiZnMn2O4, is formed and coexists with the original ZnMn2O4 during the 1st lithiation. Electrochemical impedance spectroscopy applied at varying potentials during the 1st cycle provides evidence of the high Li+ diffusion coefficient and low resistance of the electrode in the lithiated state, which enables a high rate performance with 810 mA h g−1 at 1 A g−1 and 580 mA h g−1 at 2 A g−1. X-ray photoelectron spectroscopy reveals that the solid-electrolyte interphase is mainly composed of LiOH and Li2CO3, which can contribute additional capacity. In addition, an Mn(II)/Mn(III) redox reaction appearing during the 60th to 100th cycles is reported for the first time and could be another reason for the capacity increase upon cycling (the maximum capacity is ∼1250 mA h g−1 at the 90th cycle). This redox reaction is facilitated by the increase in the electronic conductivity upon cycling. Based on these investigations, fundamental insights into the energy storage mechanism of ZnMn2O4 conversion anodes in Li-ion batteries are clarified. This work can shed light on an understanding of other conversion-type electrode materials.
TL;DR: In this article, the effect of Ca substitution on the structure, electrical and dielectric properties of BTC100x ceramics was systematically investigated, and it was found that pure BaTiO3 [BT] and pure BC3T [BC3T] had the similar structural and energy properties, whereas BTC3 ceramic showed much difference，XRD patterns, Raman spectrum, impedance spectra and die-lectric-temperature spectra provided strong evidence of Ca2+ substitution at Ti site in BT lattice.
TL;DR: In this paper, the growth and characterization of atomically thick NbS2, TaS2 and FeS films on a 6H-SiC(0001) substrate terminated with monolayer or bilayer epitaxial graphene was reported.
Abstract: We report the growth and characterization of atomically thick NbS2, TaS2, and FeS films on a 6H-SiC(0001) substrate terminated with monolayer or bilayer epitaxial graphene. The crystal and electronic structures are studied by scanning tunneling microscopy and reflection high-energy electron diffraction. The NbS2 monolayer is solely in the 2H structure, while the TaS2 monolayer contains both 1T and 2H structures. Charge-density waves are observed in all phases. For the FeS films, the tetragonal structure coexists with the hexagonal one and no superconductivity is observed.