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Effect of Defects on Spontaneous Polarization in Pure and Doped LiNbO3: First-Principles Calculations
摘要： Numerous studies have indicated that intrinsic defects in lithium niobate (LN) dominate its physical properties. In an Nb-rich environment, the structure that consists of a niobium anti-site with four lithium vacancies is considered the most stable structure. Based on the density functional theory (DFT), the specific configuration of the four lithium vacancies of LN were explored. The results indicated the most stable structure consisted of two lithium vacancies as the first neighbors and the other two as the second nearest neighbors of Nb anti-site in pure LN, and a similar stable structure was found in the doped LN. We found that the defects dipole moment has no direct contribution to the crystal polarization. Spontaneous polarization is more likely due to the lattice distortion of the crystal. This was verified in the defects structure of Mg2+, Sc3+, and Zr4+ doped LN. The conclusion provides a new understanding about the relationship between defect clusters and crystal polarization.
关键词： defects,lithium niobate,dipole moment,spontaneous polarization
更新于2025-09-23 15:22:29
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Effect of doping by enantiomers with the different absolute configuration and phase sequence on mesomorphic, helical and electro-optical properties of highly tilted chiral anticlinic mixture
摘要： The main objective of the present work is to contribute to better understanding of the effect of the absolute configuration and phase sequences of dopants on the self-assembling and physicochemical properties as well as on the electro-optical parameters of a specific liquid crystalline mixture with a broad temperature range of the antiferroelectric phase. In order to establish this effect, the mesomorphic behaviour, helical pitch length and its handedness, spontaneous polarization and tilt angle, as well as electro-optical performance for all design mixture, are determined. The results show a broad temperature range of the antiferroelectric phase and very low melting points in all mixtures. Mixtures doped with (R) enantiomers exhibit longer helical pitch lengths, lower values of spontaneous polarization and rotational viscosity, comparable values of a tilt angle but also longer switch off times in comparison to similar mixtures doped with (S) enantiomers. The doping by the enantiomers exhibiting isotropic-ferroelectric-antiferroelectric phase sequence reveals the most significant impact on properties of the base mixture. The knowledge gained will very advantageous while designing new multicomponent liquid crystalline mixtures exhibiting the chiral tilted smectic mesophases for application in surface stabilized modes with passive and active matrix drive for utilization in photonics and optoelectronics.
关键词： rotational viscosity,multicomponent mixtures,antiferroelectric liquid crystals,spontaneous polarization,helical pitch,tilt angle,(S) and (R) enantiomers
更新于2025-09-23 15:21:01
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Inorganic photovoltaic cells based on BiFeO <sub/>3</sub> : spontaneous polarization, lattice matching, light polarization and their relationship to photovoltaic performance
摘要： Inorganic ferroelectric perovskite oxides are more stable than hybrid perovskites. However, their solar energy harvest efficiency is not so good. Here, by constructing a series of BiFeO3-based devices (solar cells), we investigated three factors that influence the photovoltaic performance, including spontaneous polarization, terminated ions species in the interface between BiFeO3 and the electrode, and polarized light irradiation. This work was carried out in the framework of density functional theory combined with non-equilibrium Green’s function theory under built-in electric field or finite bias. The results showed that 1) the photocurrent is larger only under a suitable electronic band gap rather than larger spontaneous polarization; 2) the photocurrent reaches the largest in Bi3+ ions terminated interface than in the case of Fe3+ or O2- with SrTiO3 electrode; 3) the photocurrent could be largely enhanced if the polarized direction of the monochromatic light is perpendicular to the spontaneous polarization direction. The results would deepen the understanding of some experimental results of BiFeO3-based solar cells.
关键词： BiFeO3,solar cells,spontaneous polarization,polarized light,photovoltaic performance
更新于2025-09-23 15:19:57
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Effect of fluorinated achiral chain length on structural, dielectric and electro-optic properties of two terphenyl based antiferroelectric liquid crystals
摘要： Two antiferroelectric liquid crystals of different fluorinated chain length have been studied. Although both exhibit antiferroelectric (SmCA*), ferroelectric (SmC*) and paraelectric (SmA*) phases, melting point decreases with increasing achiral chain length but clearing point shows opposite trend. Thermal ranges of SmCA* and SmC* phases exhibit odd-even effect like the dipole moments and spontaneous polarization. Layer spacings, average intermolecular distances and correlation lengths across the smectic planes also increase with chain length. Temperature dependence of layer spacings, x-ray tilt, optical tilt and spontaneous polarization suggest tricritical nature of SmC*–SmA* transition. Dielectric increments decrease with increased achiral chain. Both soft mode and Goldstone mode critical frequencies decrease with decreasing temperature in the lower derivative but opposite behaviour is observed in the higher derivative. Both the compounds show Curie–Weiss behaviour in soft mode near SmC*–SmA* transition temperature (Tc). Goldstone mode critical frequency is much higher in the higher derivative. Both in-phase and anti-phase antiferroelectric modes are observed in SmCA* phase in the higher derivative but in the lower derivative only anti-phase antiferroelectric mode is observed. Optical tilts suggest orthoconic nature of the SmCA* phase of both the compounds. Ferroelectric-antiferroelectric transition temperature decreases with increasing ac field in both compounds.
关键词： Dielectric increment,Switching time,Spontaneous polarization,Orthoconic antiferroelectric liquid crystal,Critical frequency
更新于2025-09-11 14:15:04
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Fast and Simultaneous Determination of Soil Properties Using Laser-Induced Breakdown Spectroscopy (LIBS): A Case Study of Typical Farmland Soils in China
摘要： This work provides a new paradigm for designing a laser crystal requiring a broad luminescent spectrum through spontaneous polarization and local structural disorder. The grown complex ferroelectric crystals Nd-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Nd:PMNT) have a broad emission spectrum with a full width at half-maximum value of 30 nm, much larger than typical Nd laser crystals (Nd:YAG ≈ 0.8 nm). The internal mechanism of spectral broadening in complex ferroelectric crystals was analyzed using the con?guration coordinate. The shift of the peak site in the 4F3/2 → 4I11/2 transition with temperature was a?ected by structural changes, consistent with the change in dielectric and ferroelectric properties. The family of complex ferroelectric crystals could be an abundant treasure for Nd femtosecond laser crystals requiring a broad luminescent spectrum.
关键词： local disorder,spontaneous polarization,ferroelectric crystals,Pb(Mg1/3Nb2/3)O3,Nd-doped,broad bandwidth emission
更新于2025-09-11 14:15:04
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Composition-dependent xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 bulk ceramics for high energy storage applications
摘要： This work reports the composition dependent microstructure, dielectric, ferroelectric and energy storage properties, and the phase transitions sequence of lead free xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 [xBZT-(1-x)BCT] ceramics, with x = 0.4, 0.5 and 0.6, prepared by solid state reaction method. The XRD and Raman scattering results confirm the coexistence of rhombohedral and tetragonal phases at room temperature (RT). The temperature dependence of Raman scattering spectra, dielectric permittivity and polarization points a first phase transition from ferroelectric rhombohedral phase to ferroelectric tetragonal phase at a temperature (TR-T) of 40 0C and a second phase transition from ferroelectric tetragonal phase - paraelectric pseudocubic phase at a temperature (TT-C) of 110 0C. The dielectric analysis suggests that the phase transition at TT-C is of diffusive type and the BZT-BCT ceramics are a relaxor type ferroelectric materials. The composition induced variation in the temperature dependence of dielectric losses was correlated with full width half maxima (FWHM) of A1, E(LO) Raman mode. The saturation polarization (Ps) ≈ 8.3 μC/cm2 and coercive fields ≈ 2.9 kV/cm were found to be optimum at composition x = 0.6 and is attributed to grain size effect. It is also shown that BZT-BCT ceramics exhibit a fatigue free response up to 105 cycles. The effect of a.c. electric field amplitude and temperature on energy storage density and storage efficiency is also discussed. The presence of high TT-C (110 0C), a high dielectric constant (εr ≈ 12285) with low dielectric loss (0.03), good polarization (Ps) ≈ 8.3 μC/cm2) and large recoverable energy density (W = 121 mJ/cm3) with an energy storage efficiency (η) of 70 % at an electric field of 25 kV/cm in 0.6BZT-0.4BCT ceramics make them suitable candidates for energy storage capacitor applications.
关键词： spontaneous polarization,Raman spectroscopy,dielectric constant,lead free ceramics,Energy storage capacitors,fatigue
更新于2025-09-09 09:28:46
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Ferroelectric Polarization Promoted Bulk Charge Separation for Highly Efficient CO2 Photoreduction of SrBi4Ti4O15
摘要： Fast recombination of photogenerated charge carriers in bulk remains the major obstacle for photocatalysis nowadays. Developing ferroelectrics directly as photoactive semiconducting catalysts may be promising in view of the strong ferroelectric polarization that induces the anisotropic charge separation. Here, we report a ferroelectric layered perovskite SrBi4Ti4O15 as a robust photocatalyst for efficient CO2 reduction. In the absence of co-catalysts and sacrificial agents, the annealed SrBi4Ti4O15 nanosheets with the strongest ferroelectricity cast a prominent photocatalytic CO2 reduction activity for CH4 evolution with a rate of 19.8 μmol·h-1·g-1 in the gas-solid reaction system, achieving an apparent quantum yield (AQY) of 1.33% at 365 nm, outperforming most of the reported photocatalysts. The ferroelectric hysteresis loop, piezoresponse force microscopy (PFM) and ns-level time-resolved fluorescence spectra uncover that the outstanding CO2 photoreduction activity of SrBi4Ti4O15 mainly stems from the strong ferroelectric spontaneous polarization along [100] direction, which allows efficient bulk charge separation along opposite direction. DFT calculations also disclose that both electrons and holes show the smallest effective masses along a axis, verifying the high mobility of charge carriers facilitated by ferroelectric polarization. This study suggests that the traditionally semiconducting ferroelectric materials that have long been studied as ferro/piezoelectric ceramics now may be powerfully applied in the photocatalytic field to deal with the growing energy crisis.
关键词： CO2 photoreduction,ferroelectric perovskite,spontaneous polarization,SrBi4Ti4O15 nanosheets,piezoelectric-catalysis
更新于2025-09-09 09:28:46