Distortion Of Octahedron Research Articles

Lead-free hybrid perovskite N(CH3)4SnI3 with robust ferroelectricity induced by large and non-polar N(CH3)4+ molecular cation

The ferroelectricity in the hybrid perovskite CH3NH3PbI3 is under debate because it results from the polar molecular cation CH3NH3+ while the molecular orientation was reported to be random. Here we predict that a Pb-free hybrid perovskite N(CH3)4SnI3 with non-polar molecular cation N(CH3)4+ has strong ferroelectricity with a spontaneous polarization of 16.13 μC cm−2. The large polarization results from the distortion of SnI6 octahedron induced by the large N(CH3)4+ and is independent of the molecular orientation, so the ferroelectricity is robust. The ferroelectric R3m perovskite structure of N(CH3)4SnI3 can be synthesized as the ground state under a hydrostatic pressure over 3 GPa and remains stable under ambient pressure. Given the strong ferroelectricity, good stability and high visible-light absorption, N(CH3)4SnI3 may be an ideal light-absorber semiconductor for high-efficiency solar cells because its ferroelectric polarization can facilitate electron-hole separation and produce large bulk photovoltaic effect, making the design of homogeneous bulk photovoltaic devices possible.

Crystal structures and electrical properties of Sr/Fe‐modified KNbO3 ferroelectric semiconductors with narrow bandgap

AbstractIn the process of exploring ferroelectric semiconductors, a new system of (1−x) KNbO3–xSrFeO3−δ (x = 0.00‐0.20) was successfully synthesized via solid‐state reaction. The crystal structures, ferroelectric, dielectric, optical, and electrical properties were systematically characterized. The orthorhombic phase with Amm2 space group is detected in all the ceramics. In addition, the orthorhombic and tetragonal phases coexist in 0.80KNbO3‐0.20SrFeO3‐δ ceramic. The decrease in oxygen octahedron distortion induces a weak ferroelectric polarization. The existence of long‐range ferroelectric polarization order in all the ceramics is verified and the bandgap of the ceramics can be tuned to ~2.18 eV. The improved short‐circuit photocurrent density (Jsc) and open‐circuit voltage (Voc) of the poled 0.95KNbO3‐0.05SrFeO3−δ ceramic at 30 kV/cm are ~6.90 nA/cm2 and 0.04 V, respectively. The activation energies for electrical conductivity of the grains and grain boundaries from 0.90KN–0.10SF ceramic are 0.67 and 0.77 eV, respectively, which indicate the doubly ionized oxygen vacancies. This work provides a new way to tune the optical bandgap/ferroelectric properties of KNbO3‐based ceramics for potential application in ferroelectric photovoltaic and energy fields.

First-Principles Calculation of Defect Properties on Copper Doped KTaO3 Crystal

A first-principles calculation is applied to perform a study of the Cu: KTaO3 system. The defect formation energy, the doping induced lattice distortion and the electronic structure properties have been calculated. The defect formation energy found that Cu is prefer to substitute K (CuK) at corresponding defect charge state +3(), +1() and 0 () with Fermi energy increase; The doping of Cu leads to an obvious distortion of the oxygen octahedron, along with the change of ionic bond characteristics; Further research on electronic structure shows that the doped Cu atom mainly occupies the top of the valence band, resulting in the decrease of the band gap.

Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides.

High-entropy oxides (HEOs) have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge toward realizing their vast potential. Here we report the first examples of pyrochlore HEOs with five M-site cations, for Nd2M2O7, in which the local structure has been investigated by neutron diffraction and pair distribution function (PDF) analysis. The average structure of the pyrochlores is found to be orthorhombic Imma, in agreement with radius-ratio rules governing the structural archetype. The computed PDFs from density functional theory relaxed special quasirandom structure models are compared with real space PDFs in this work to evaluate M-site order/disorder. Reverse Monte Carlo combined with ab initio molecular dynamics and Metropolis Monte Carlo simulations demonstrates that Nd2(Ta0.2Sc0.2Sn0.2Hf0.2Zr0.2)2O7 is synthesized with its M-site local to nanoscale order highly randomized/disordered, while Nd2(Ti0.2Nb0.2Sn0.2Hf0.2Zr0.2)2O7+x exhibits a strong distortion of the TiO6 octahedron and small degree of Ti chemical short-range order (SRO) on the subnanometer scale. Calculations suggest that this may be intrinsic, energetically favored SRO rather than due to sample processing. These results offer an important demonstration that the engineered variation of participating ions in HEOs, even among those with very similar radii, provides richly diverse opportunities to control local order/disorder motifs-and therefore materials properties for future designs. This work also hints at the exquisite level of detail that may be needed in computational and experimental data analysis to guide structure-property tuning in the emerging HEO materials class.

Correlations between crystal structure and microwave dielectric properties of Mn2+-substituted ZnTiNb2O8 ceramics

Zn1−xMnxTiNb2O8 (0.05 ≤ x ≤ 0.30) microwave dielectric ceramics were synthesized by the solid-state reaction method. The impacts of partial substitution of Mn2+ for Zn2+ on the crystal phase, crystal structure, grain morphology, and microwave dielectric properties were investigated in detail. Minor ixiolite phase of MnTiNb2O8 was detected via the X-ray diffraction at higher x levels. Structural characteristics, such as porosity, packing fraction, and [Zn/Ti/NbO6] octahedron distortion, were discussed to reveal the correlations between the microwave dielectric characteristics and the crystal structure. It was suggested that the permittivity was related to the porosity and the minor phase, while the quality factor was mainly determined by the packing fraction and relative density. The temperature coefficient of the resonant frequency was positively correlated with the distortion of [Zn/Ti/NbO6] octahedron. Among all Zn1−xMnxTiNb2O8 ceramics, the sample of x = 0.05 sintered at 1100 °C exhibited good microwave dielectric properties: er = 35.91, Q × f = 63,672 GHz (at 6.52 GHz), and τf = − 68.68 ppm/°C, which provided an effective performance modulating approach for microwave dielectric ceramic design.

Rashba band splitting in two-dimensional Ruddlesden–Popper halide perovskites

Due to the presence of heavy elements and the dynamic nature of hybrid halide perovskites, the strong spin–orbit coupling effect can give rise to Rashba band splitting in these materials. Despite many reports on the Rashba effect in 3D perovskites like CH3NH3PbI3, little is known about its presence in two-dimensional Ruddlesden–Popper (2DRP) perovskites. In this work, we use first-principle calculations to investigate the magnitude and origin of the Rashba effect in three families of 2DRP perovskites. We demonstrate the correlation between the splitting magnitude and the octahedron distortions. Moreover, different numbers of inorganic layers, spacer cations, and A-site cations have a great influence on the Rashba splitting through different mechanisms. While structures with C6H5C2H4NH3 (PEA) have a significant Rashba splitting only in the monolayer condition, C4H9NH3 (BA) induces large distortion by tilting the CH3NH3 (MA) cations around all octahedrons, giving rise to a larger Rashba splitting with an increasing number of inorganic layers. Our work elucidates the magnitude and origin of the Rashba splitting in 2DRP perovskites and provides guidelines for the manipulation of the Rashba splitting in these materials.

Effective Ca2+-doping in Sr1-xCaxFeO3-δ oxygen carriers for chemical looping air separation: A theoretical and experimental investigation

Perovskite iron oxides are promising oxygen carrying materials due to their effectiveness and the low cost of iron. The effect of Ca2+ doping on oxygen ion diffusion in Sr1-xCaxFeO3-δ (x = 0, 0.125, 0.25, 0.375, 0.5) is investigated by combining density functional theory (DFT) calculations and experimental measurements. The oxygen ion diffusion is determined by two key factors of oxygen vacancy formation and migration. The DFT results show that the oxygen vacancy formation energies greatly decrease as Ca2+ content reaches x = 0.125, then gradually decrease with Ca2+ contents up to 0.375, and finally increase as the Ca content reaches x = 0.5. A combination of experimental thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results corroborate this trend for Ca2+ contents between 0 and 0.4. The Fe-O bonding dominates the effect of Ca2+ doping on the oxygen vacancy formation. Shortened Fe-O bonds cause the decrease in the formation energy at lower Ca2+ contents, while the lengthened bonds by FeO6 octahedron distortion cause the increase in the formation energy at higher Ca2+ contents. Kinetically, the oxygen migration barrier is lowered upon Ca2+ doping through the increasing lattice spacing for oxygen diffusion. Therefore, an appropriate Ca2+ doping of x = 0.125–0.375 promotes the oxygen ion diffusion in Sr1-xCaxFeO3-δ. Our findings provide the effective Ca2+ doping value for Sr1-xCaxFeO3-δ and a material design clue for the isovalent A-site doping system of oxygen carriers.

Structure and infrared reflectivity spectra of novel Mg3Ga2GeO8 microwave dielectric ceramic with high Q

Mg3Ga2GeO8 ceramics were synthesised by means of the high-temperature solid-state reaction method. The Mg3Ga2GeO8 ceramics of the orthorhombic structure type comprised an Imma space group composed of three different octahedrons and one tetrahedron. The dense ceramic was obtained at 1340 °C and presented excellent microwave dielectric performances, namely, low εr = 9.41, high Q × f = 133, 113 GHz, and τf = −63.54 ppm/°C. The intrinsic microwave dielectric performances were analysed on the basis of the packing fraction, octahedron distortion, and infrared reflectivity spectra of the Mg3Ga2GeO8 ceramics. These merits render the Mg3Ga2GeO8 ceramics an appropriate candidate for millimetre-wave frequencies.

First principles study on the effect of Bi3+ and Y3+ doping on electronic structure, thermal and optical properties of KSr2Nb5O15

The electronic structure, thermal and optical properties of KSr2Nb5O15 with Bi3+ doping (KSN-Bi) and Y3+ doping (KSN-Y) are studied using first principles. When Bi3+ and Y3+ doping in the KSN, the band gap becomes smaller and satisfies to KSN > KSN-Bi > KSN-Y, the original tetragonal structure changes to be orthorhombic due to the great distortion of Nb-O octahedron affected by Bi3+ and Y3+. The specific heat capacity under constant volume Cv, specific heat capacity under constant pressure Cp, entropy Svib, thermal expansion coefficient α(T) and volume change decreased after doping. The calculated Gibbs results suggest that KSN-Y is more stable than KSN-Bi. Furthermore, the dielectric constant is increased and the imaginary part of dielectric function decreased, while the refraction property is increased and reflectivity is decreased after Bi3+ and Y3+ doping. Both the KSN-Bi and KSN-Y show ultraviolet absorption in 100–400 nm, refractivity in 0–5 eV and reflectivity 5–8 eV. KSN-Bi and KSN-Y show optical anisotropy properties and KSN-Bi has better optical properties than that of KSN-Y.

Revealing the A-Site Effect of Lead-Free A3 Sb2 Br9 Perovskite in Photocatalytic C(sp3 )-H Bond Activation.

The lead-free halide perovskite A3 Sb2 Br9 is utilized as a photocatalyst for the first time for C(sp3 )-H bond activation. A3 Sb2 Br9 nanoparticles (A3 Sb2 Br9 NPs) with different ratios of Cs and CH3 NH3 (MA) show different photocatalytic activities for toluene oxidation and the photocatalytic performance is enhanced when increasing the amount of Cs. The octahedron distortion caused by A-site cations can change the electronic properties of X-site ions and further affect the electron transfer from toluene molecules to Br sites. After the regulation of A-site cations, the photocatalytic activity is higher with A3 Sb2 Br9 NPs than that with classic photocatalysts (TiO2 , WO3 , and CdS). The main active species involved in photocatalytic oxidation of toluene are photogenerated holes (h+ ) and superoxide anions (. O2 - ). The octahedron distortion by A-site cations affecting photocatalytic activity remains unique and is also a step forward for understanding more about halide-perovskite-based photocatalysis. The relationship between octahedron distortion and photocatalysis can also guide the design of new photocatalytic systems involving other halide perovskites.

Revealing the A‐Site Effect of Lead‐Free A3Sb2Br9 Perovskite in Photocatalytic C(sp3)−H Bond Activation

AbstractThe lead‐free halide perovskite A3Sb2Br9 is utilized as a photocatalyst for the first time for C(sp3)−H bond activation. A3Sb2Br9 nanoparticles (A3Sb2Br9 NPs) with different ratios of Cs and CH3NH3 (MA) show different photocatalytic activities for toluene oxidation and the photocatalytic performance is enhanced when increasing the amount of Cs. The octahedron distortion caused by A‐site cations can change the electronic properties of X‐site ions and further affect the electron transfer from toluene molecules to Br sites. After the regulation of A‐site cations, the photocatalytic activity is higher with A3Sb2Br9 NPs than that with classic photocatalysts (TiO2, WO3, and CdS). The main active species involved in photocatalytic oxidation of toluene are photogenerated holes (h+) and superoxide anions (.O2−). The octahedron distortion by A‐site cations affecting photocatalytic activity remains unique and is also a step forward for understanding more about halide‐perovskite‐based photocatalysis. The relationship between octahedron distortion and photocatalysis can also guide the design of new photocatalytic systems involving other halide perovskites.

High-pressure phase transformation of carbonate malachite Cu2(CO3)(OH)2 driven by [CuO6] regularization and [CO3] rotation

High-pressure synchrotron X-ray diffraction and infrared absorption spectroscopy have been employed to study the crystal chemistry and phase transitions in an [OH]-bearing carbonate, malachite Cu2(CO3)(OH)2, to determine the effect of [OH] on the stability of carbonate. We found that the crystal structure of malachite is stabilized by a high degree of [CuO6]-octahedron distortion, as is manifested by large variations in Cu–O bond lengths resulting from oxygen atoms that connect to hydrogen at crystallographically different sites. External pressure offsets the effect of hydrogen bond, promotes [CuO6] compression and regularization and accordingly [CO3] rotation. Rotation of [CO3]-triangles, in turn, assists in a conversion in the crystal orientation of the [CuO6] structural unit. During compression to above ~6 ​GPa, malachite begins to turn into the rosasite lattice, accompanied with a jump in density of 3.3%. Rosasite is characterized with a hardened lattice and preserves to the maximum pressure (18.2 ​GPa) of the present study. Phase transformation mechanism of malachite to rosasite is different from that of carbonates, with the latter being driven by an almost uniform compression of [MO6]-octahedron (M ​= ​Ca, Cd, Mn, Fe, Zn, Mg, etc.) and rotation/translation of [CO3]-triangle under pressure.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity.

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN2O4} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ0H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm2 at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Rearranging cations on B sites to modify luminescence in layered-perovskite-like La3Ti2TaO11:Eu3+ ceramic phosphors

Compounds with the layered perovskite-like structure are well-known engineering materials because of the multifunctionality. In this work, a novel La3-xEuxTi2TaO11 (0 ≤ x ≤ 0.45) solid-solution with a typical layered perovskite-like structure was synthesized via the traditional solid-state reaction ceramic technique. The optimal emission happened in La3-xEuxTi2TaO11 (x = 0.3). To modify the luminescence properties, La2.7Eu0.3Ti2-5y/4Ta1+yO11 (y = 0.04, 0.05, 0.08, 0.1, 0.12, 0.14) phosphor was developed via stoichiometric alteration of (Ta5+→Ti4+) in “B” sites. The research motivation is on the fact that the framework distortion due to cation-disorders is an efficient method to modify the luminescence and improve the thermal stability of a rare earth ion (RE)-activated phosphor. The phase formations were conducted by XRD measurements and Rietveld refinements. The morphology was characterized by SEM and EDS. The luminescence properties such as luminescence, emission excitation wavelength, and decay lifetimes showed a dependence on the distortion of the framework. Cation alteration of (Ta5+→Ti4+) in La3-xEuxTi2TaO11 (x = 0.03–0.45) greatly modified the red-luminescence activities, such as improved efficiency, pure red chromaticity, and thermal stability. The excess of Ta5+ on Ti4+ sites bring about the distortion of octahedron surrounding resulting in intensive changes in the crystal-field environment around Eu3+ activators. The framework distortion due to substitutions of multiple cations is effective to enhance the emission efficiencies and thermal stability of RE3+-activated phosphors. The cation-disorder was discussed via luminescence features in 5D0→7F0 transition region. The results are helpful to introduce Eu3+ as a probe for rare-earth dopant site microstructure in phosphor materials.

Structure optimization of CH3NH3PbI3 by higher-valence Pb in perovskite solar cells with enhanced efficiency and stability

The crystal structure of perovskite has a significant influence on the photovoltaic performance and stability of perovskite solar cells (PSCs). Pb4+ is introduced into CH3NH3PbI3 (MAPbI3)-based PSCs by participating the octahedral [PbI6]4- structure, then to induce the formation of stronger Pb-I bond and reduce [PbI6]4- octahedron distortion, which would improve structural symmetry, decrease the degree of disorder and be beneficial to the crystallization of perovskites. Synchrotron based X-ray absorption fine spectroscope (XAFS) revealed that the existence of higher valence state lead could be realized by doping Pb4+ directly or induced by electron withdrawing group, in consequence the [PbI6]4- octahedral structure becomes more stable. Grazing incidence X-ray diffraction (GIXRD) especially demonstrates that Pb4+ with appropriate proportion may well replace part of Pb2+ to form an uniform phase in the primal perovskite structure to improve the crystallization on the surface and homogeneous out-of-plane (OOP) ordered crystal accumulation in the bulk, which is also important for improving the efficiency and stability of PSCs. As a result, a power conversion efficiency (PCE) exhibits a 42.1% increase with the doping of 0.03% PbF4 and 0.075% PCBM compared with a pristine device and its stability improves markedly after 30 days of storage in ambient atmosphere.

The relationship between enhanced dielectric property and structural distortion in Ca doped Ba2NaNb5O15 tungsten bronze ceramics

The relationship between enhanced dielectric property and structural distortion in tungsten bronze structure ceramics was discussed in this work. The ceramics with the composition of (Ba1-xCax)2NaNb5O15 (x = 0, 0.1, 0.2, 0.3, 0.4) were fabricated via conventional solid-state method. All ceramics were pure without secondary phase and the distinct lattice distortion in structure was testified by Rietveld XRD refinement. Compared with the un-doped composition, the maximum polarization and energy storage density were strongly enhanced according to the ferroelectric property measurements, which were contributed to the distortion of NbO6 octahedron induced by the variation of ionic radius. The actually pulsed charge-discharge property of x = 0.3 ceramic was tested, whist excellent power density (PD = 35.106 MV/cm3) and discharge energy density (Wd = 0.29 J/cm3) were obtained at 100 °C under 120 kV/cm, revealing the potential for application of Ba2NaNb5O15 system-based ceramics in harsh environment.

Role of charge doping and distortions on the structural, electrical, and magnetic properties of modified CuFeO2 compounds

Single phase CuFeO2, CuFe0.96M0.03V0.01O2 (M = Ti, Mn, and Ga), and CuFe0.96V0.04O2 compounds with space group R3¯m were prepared through a newly developed solid-state technique using annealing under high vacuum. Raman studies demonstrate, for the first time, a minor shift in Ti and Mn-doped samples as well as the Jahn–Teller effect induced distortions around 500 cm−1 along with well-documented peaks of Eg and A1g modes. Mössbauer studies confirm the presence of iron in high spin Fe3+ and quadruple splitting values endorsing the presence of octahedron distortions. An increase in the electrical conductivity at low temperatures is observed due to the enhanced charge doping with minor variations related to local distortions. Low-temperature magnetic studies of pristine and Ti-doped samples demonstrate paramagnetic nature up to 25 K. However, on one hand, the pristine sample shows a sharp rise in magnetization values at low applied fields and on the other side, Ti-doped samples exhibit nearly linear behavior. Dielectric measurements confirm weakening of electrical ordering near the magnetic transition in distorted Ti-doped samples, compared to pure CuFeO2 sample. Level of charges along with its local distortions affected mobility has significant effects on transport and multiferroic nature of these samples.

Pressure induced band gap narrowing and phase transitions in Dy2Ti2O7

An isostructural phase transition and a cubic to tetragonal phase transition in spin-frustrated pyrochlore Dy2Ti2O7 (DTO) at 35 GPa and 46 GPa, respectively, were detected by x-ray diffraction (XRD) and Raman spectroscopy studies at room temperature under high pressure. The band gap of DTO gradually increased with the increasing pressure up to 20 GPa, after which it narrowed significantly around 35 GPa, and further manifested with a slight discontinuity around 46 GPa. Below 35 GPa, the evolution of band gap with pressure maintained a close relationship with the Ti–O bond length and the distortion of TiO6 octahedron. The remarkable narrowing of the band gap around 35 GPa implied the changes of the band structure and confirmed the isostructural phase transition. These findings provide an in-depth understanding of the evolution of the structure and the band gap of DTO under high pressure.

Atomic-environment-dependent thickness of ferroelastic domain walls near dislocations

Domain walls are of increasing interest in ferroelectrics because of their unique properties and potential applications in future nanoelectronics. However, the thickness of ferroelastic domain walls remains elusive due to the challenges in experimental characterization. Here, we determine the atomic structure of ferroelastic domain walls and precisely measure the polarization and domain wall thickness at picometer scale using annular bright field imaging from an aberration-corrected scanning transmission electron microscope. We find that the domain wall thickness in PbZr0.2Ti0.8O3 and PbTiO3 thin films is typically about one unit cell, across which the oxygen octahedron distortion behavior is in excellent agreement with previous first-principles calculations. Remarkably, wider domain walls about two unit cells in thickness are also observed for those domains walls are coupled with dislocations and underwent a compressive strain. These results suggest that the thickness of ferroelastic domain walls highly depends on their atomic environments. This study can help us to understand the past debatable experimental results and provide further insights into control of domain wall thickness via strain engineering for their possible applications in nanoelectronics.

Structural-distortion modes and transport properties of La0.5Ca0.5MnO3 by co-doping Dy3+ and Sr2+ ions

La0.5-xDyxCa0.5-ySryMnO3 (x=0, 0.05, 0.08, 0.1, 0.14, 0.16, 0.2) polycrystalline samples were prepared by high-temperature solid-state reaction. The A-site disordering is increased with the doping concentration x, and meanwhile the average ionic radius of A-site is kept constant. X-ray diffraction and Raman spectroscopy were combined to investigate the microstructural distortion of MnO6 octahedron. Results show that the rotational vibration (RV) is the dominant distortion mode. With the A-site disordering increasing, RV mode is obviously hardened, and the Jahn-Teller distortion is enhanced. Properties investigation shows that the insulation is strengthened because of FM double exchange effect suppressed with A-site disordering increasing, and the magnetoresistance effect is weakened. A relatively significant magnetoresistance effect was observed in the low temperature region for the high-doped concentration sample, which may be related to the strong magnetic moment of Dy3+.