A-site Disorder Research Articles

Effect of A-site disorder and reduced grain size on low temperature resistivity minima and magnetoresistance behaviour in Nd0.7-xGdxSr0.3MnO3 (0 ≤ x ≤ 0.3)

ABSTRACT The present work provides a detailed investigation of the electronic- and magneto-transport properties of bulk and nanostructured Nd0.7-xGdxSr0.3MnO3 (x = 0, 0.1, 0.2 and 0.3) polycrystalline manganites. We have specially addressed the issue of the observed minimum in the temperature dependent resistivity of these manganites in the low temperature regime (<50 K). It is observed that increase of A-site disorder due to progressive substitution of Gd and decrease of grain size enhances the observed resistivity minima. Different theoretical models have been employed to analyze the resistivity data in the low temperature regime. It has been established that the quantum interference effect due to electron–electron interaction, most likely, is the dominant factor of the observed resistivity minimum and upturn in resistivity in the lower temperature regime. The effect of phase separation is also found to play a key role in the electronic-transport of the materials. Further the magnetoresistance is found to increase with the increase of Gd concentration as well as with the decrease of grain size in the low temperature regime. The origin of magnetoresistance has been critically analyzed.

Effect of sintering temperature on microstructure and electrical properties of Na0.5Bi0.47Sr0.02TiO3-δ ceramics

In this investigation, a series of Na0.5Bi0.47Sr0.02TiO3-δ samples, sintered from 1000 to 1100 °C, were synthesized utilizing the solid-state method. The XRD results indicate that all samples exhibit a perovskite phase, with the exception of those sintered at 1100 °C, which display no discernible impurity peaks. The SEM results indicate that the average grain size exhibits an increase proportionate to the sintering temperature, which attributes to the variations in the actual Na/Bi ratio. From the study of grain conductivity we can obtain that the dominant factor affecting grain conductivity is the activation energy. At 1100 °C, the oxygen vacancy concentration emerges as the primary determinant of grain conductivity. It should be noticed that the phase transition temperature of the sample increases (350 °C→400 °C) with the increase of sintering temperature, which is due to the local A-site disorder aggravated by high temperature sintering. Therefore, reducing the sintering temperature is beneficial to improve the grain conductivity. The grain boundary conductivity initially rises, then diminishes with increasing sintering temperature. It is evident that the enhanced macroscopic grain boundary conductivity stems from a marked reduction in space charge potential within the 1000–1025 °C temperature range. Within the 1050 °C−1100 °C temperature range, the space charge potential barely changed, the significant decrease of intrinsic grain boundary conductivity can be attributed to the surge in the coverage ratio of grain boundary impurities and the number of grain boundary impurities increased caused by the rapid reduction of grain boundary area and thermal evaporation. The research on electrical performance of NBT-based oxygen-ion conductors will promote the application of NBT-based oxygen ion conductors in solid oxide fuel cells, oxygen sensors and other fields.

Effect of A-site disorder on the electronic and magnetic properties in double perovskite SrCaCoTeO6

Herein, the effect of A-site disorder on the electronic structures and magnetic properties in double perovskite SrCaCoTeO6 was systemically investigated using the first-principles calculations. Our results indicate that its magnetic coupling is significantly influenced by the A-site disorder, i.e., the ground magnetic properties vary with different cationic arrangement. However, this effect can be negligible on the electronic structure. Its semiconducting nature does not alter despite of different structures. Further analysis confirms that the B-site Co2+ is magnetic cation in the high-spin state with 3d7 (t2g5eg2) electronic configuration, while Te at B′-site is nonmagnetic with no contribution to the magnetism.

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

A-site disorder induced ferroelectric to relaxor phase transition of Na(Nb0.75Ti0.25)O3 ceramics by Bi3+ for ultra-high energy storage properties

The present work introduces a new insight into effect of A-site disorder to develop the energy storage properties of Na(Nb0.75Ti0.25)O3 ceramic. Lead-free (Na1-xBix)(Nb0.75Ti0.25)O3 (x = 0.0, 0.05 and 0.1) (abbreviated as NNTBx) ceramics were synthesized using solid-state reaction technique. The introduction of Bi3+ shown a significant effect into the crystal structure, dielectric, ferroelectric and energy storage properties of NNT ceramics. The addition of Bi2O3 induced ferroelectric to relaxor phase transition with psedu-cubic crystal structure at 0.1. Significant inhibits of grain size to sub-micrometer has been achieved at 0.1 sample due to lower ionic radius of Bi+3 (1.34 Å, CN=12) to the ionic radius of Na1+ (1.39 Å, CN=12). Meanwhile, the dielectric constant increased from 300 to 1500 when the Bi3+-content increased from 0.0 to 0.1. Effectively increasing in maximum polarization (Pmax) and significant reducing in remnant polarization (Pr) resulting in increasing ΔP (Pmax – Pr) has been obtained at 0.1 content of Bi2O3. This enhancement is attributed to hybridization between Bi3+ 6Pand O2− 2P instead of Na1+ 2S and O2− 3S orbitals. On the other hand, the substitution of monovalent (Na1+) by trivalent (Bi3+) lead to create sodium vacancies into the A-sites of NNT lattice subsequently reducing the Pr value. The tolerance factor (τ) decreased from 0.955 to 0.953 when Bi2O3 increases from 0.0 to 0.1 indicate increasing the cations disorder, charge misfit and relaxor degree of NNT ceramics. The sample with 0.1 of Bi2O3 demonstrate remarkably high energy storage density (Wrec = 17.5 J/cm3), and high relatively energy storage efficiency (n = 80 %) at ultra-high dielectric breakdown strength (Eb = 700 kV/cm). Partially, (NNTB0.1) exhibit an outstanding stability of energy storage properties in terms of temperature (25 to 150 °C) and frequency (2–50 Hz). The variation of Wrec and n were observed about 1 % within the whole range of applied temperature and frequency. These results imply the high potential of (Na0.9Bi0.1)(Nb0.75Ti0.25)O3 ceramic in temperature-stable advanced pulsed power capacitor applications.

Investigation of the impact of A-site cation disorder on the structure, magnetic properties, and magnetic entropy change of trisubstituted divalent ions in La0.7(Ba,Ca,Sr)0.3MnO3 manganite.

This study investigates the effect of A-site disorder, characterized by the average ionic radius (〈rA〉) and the cation mismatch (σ2), on the structural, magnetic, critical behavior, and magnetic entropy changes in La0.7(Ba,Ca,Sr)0.3MnO3 manganites with trisubstituted Ba, Ca, and Sr. The sol-gel method was used to prepare polycrystalline samples. All series of compounds crystallize in rhombohedral symmetry with the R3̄c space group. A linear relationship between lattice parameters, unit cell volume, and 〈rA〉 was observed. This reveals an unusual behavior in the correlation between 〈rA〉 and σ2 concerning magnetic properties, which is attributed to the complex simultaneous trisubstitution of divalent ions. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were utilized to validate the chemical composition of compounds. All the samples crystallized in rhombohedral symmetry, and the lattice parameters increased continuously with increasing 〈rA〉. A-site disorder causes distortions in the Mn-O bond length and Mn-O-Mn bond angle in the MnO6 octahedral structure, which influences the double-exchange interaction and electronic bandwidth (W). The Curie temperature (TC) increases linearly with increasing W. The critical behavior around TC for all the samples was investigated by determining the values of the critical exponents (β, γ, and δ) using the modified Arrott plot (MAP) method. The estimated critical exponents show that the unconventional model establishes a short-range ferromagnetic order. The maximum magnetic entropy change (-ΔSM) was obtained with the lowest 〈rA〉 and σ2 value. The analysis of the critical behavior and universal curve indicates a second-order phase transition (SOPT) nature for all samples.

A-site K-doped lanthanum manganite nanocrystalline La0.67Ba0.33MnO3 for room-temperature micro-scale magnetic cooling

Bulk nanocrystalline La0.67Ba0.33−xKxMnO3 (with x = 0, 0.05, 0.1, and 0.2) manganites have been prepared by the modified sol-gel method (Pechini). The single-phase rhombohedral crystal structure with the R-3c (no. 167) space group was verified by X-ray diffraction (XRD) and sustained by Rietveld refinement. As follows from the results of XRD structural analyses, the increase in K-doping triggers an increase in the distortion of the MnO6 octahedra, which eventually causes the narrowing of the eg bandwidth. Mn is in a mixed valence state of Mn4+/Mn3+ as inferred by X-ray photoelectron spectroscopy. Magnetic measurements confirm that the Curie temperature decreases from 348 K for La0.67Ba0.33MnO3 to 316 K for La0.67Ba0.13K0.20MnO3. The increasing of the Mn4+ ion concentration at the B-site sublattice and A-site ionic disorder (σ2) breaks up the double exchange interaction between the Mn3+ and Mn4+ ions. The ferromagnetic to paramagnetic second-order magnetic phase transition at TC is also confirmed by electron paramagnetic resonance. According to magnetic field-dependent magnetization isotherms at different temperatures, La0.67Ba0.13K0.20MnO3 shows a relatively large magnetocaloric effect (1400 mJ cm−3 K−1 at 316 K under 5 T applied magnetic field), which raises the possibility of using this material for room-temperature micro-scale magnetic cooling.

Effect of A-site ionic disorder on structure, magnetic and magnetocaloric properties of La0.85-xNdxAg0.15-yKyMnO3 manganites

In this paper, the effect of A-site ionic disorder (σ2) on the structure, magnetic and magnetocaloric properties in La0.85-xNdxAg0.15-yKyMnO3 have been investigated. X-ray diffraction analysis using Rietveld refinement shows that the crystal structure of the perovskite manganites changes from rhombohedral (space group R3 ͞C) to orthorhombic (space group Pnma) structure with the increasing in σ2. Magnetization measurements showed that, upon increasing σ2, ferromagnetic double exchange interaction of the perovskite manganites diminishes, and as a result, Curie temperature (TC) of the samples decrease linearly. The inverse of susceptibility shows a downturn with decreasing temperature before TC indicating the existence of the Griffiths phase for La0.62Nd0.23Ag0·099K0·051MnO3 (σ2 = 0.004). The magnetic phase transitions of all the samples are belongs to second order transition.

A-Site Cation Disorder in SrxBa1-xNb2O6 (x = 0.25, 0.33, 0.50, 0.61) Studied by High-Resolution Resonant X-ray Powder Diffraction.

The dielectric and ferroelectric properties of SrxBa1-xNb2O6 (SBN, 0.2 < x < 0.8) are known to be affected by the Sr fraction and can be further controlled by various quenching schemes. Changes in A-site cation configuration are believed to be linked to these changes in properties. In this work, we study the A-site cation disorder in SBN by the use of high-resolution resonant X-ray powder diffraction. The experimental results show that the larger Ba2+ is found exclusively on the larger A2 site, while Sr2+ is found on both the A1 and A2 sites, with an increasing amount on A2 with an increasing Sr fraction. At elevated temperatures, a small migration of Sr2+ from A1 to A2 is observed for SBN50 and SBN61. Linking this change in occupancies to changes in the average cation size on the A1 and A2 sites allows for rationalization of the property changes observed for quenched samples. Furthermore, SBN25 is shown to deviate from the tetragonal P4bm structure and is found to be orthorhombic with a Cmm2 structure.

Chemical durability of multicomponent lanthanide zirconate solid solutions

Multicomponent zirconate solid solutions A2Zr2O7 (A= (Sm,Yb), (Sm,Yb,Gd), (Sm,Yb,Gd,Er), and (Sm,Yb,Gd,Er,Dy)) were synthesized through spark plasma sintering and their chemical durability was investigated via semi-dynamic leaching tests in pH = 1 nitric acid at 90 °C. Incongruent dissolution occurred for all samples with the preferential elemental release of zirconium in all A2Zr2O7 except for high entropy (Sm,Yb,Gd,Er,Dy)Zr2O7 which displays a congruent release of the A-site elements and Zr. A strong positive correlation was found between the lanthanide elemental release rates and the A-site cation size disorder, whereas a negative correlation was observed between the elemental release rates and mixing entropy.

Effect of sintering time on electrical, ferroelectric, and piezoelectric performances of microwave synthesized sodium bismuth titanate ceramics

Herein, we report the synthesis of sodium bismuth titanate (Na0.5Bi0.5TiO3/NBT), a lead-free ceramic prepared via a cost and time-effective microwave synthesis route. This work presents the investigation of microstructural, electrical, and optical characteristics of NBT ceramics. The formation of a single-phase NBT sample sintered for 30 and 40 min (denoted as NBT 30 and NBT 40) is confirmed by Rietveld refinement and Raman measurement. FTIR spectroscopy is used to characterize the single-phase nature and to confirm the presence of desired vibrational spectra in the perovskite structure. Tightly packed micron-sized grains are observed from the field emission scanning electron microscopy. Due to the A-site disorder present in the lattice, two diffuse types of dielectric anomalies are present in the temperature-dependent dielectric plot for both samples. The maximum dielectric constant values of 1475 and 1628 are found for NBT 30 and NBT 40 samples at the transition temperature. From temperature-dependent AC conductivity measurement, NBT 40 samples show a higher value of conductivity in comparison to NBT 30. The PE hysteresis loops confirm the ferroelectric character present in both samples. An improved piezoelectric coefficient is observed with increasing sintering time and is found to be 80 pCN−1 for NBT 40. The optical bandgap of materials is 3.03 and 3.1 eV for NBT 30 and NBT 40, respectively.

Effects of Li substitution on structural, magnetic properties and electroresistance behaviour in La0.8Na0.2-xLixMnO3 monovalent-doped manganite

The effects of Li substitution on structural, magnetic properties, and electroresistance (ER) behaviour in La0.8Na0.2-xLixMnO3 (x = 0 – 0.15) manganites prepared via the conventional solid-state method were reported. All samples exhibited a metal–insulator transition behaviour with a decrease in the metal–insulator transition temperature, TMI from 270 K (x = 0) to 255 K (x = 0.15) and reduction in Curie temperature, TC from 272 K (x = 0.10) to 235 K (x = 0.15). All samples exhibited a reduction in resistivity under higher applied current of 5 mA than that of 1 mA, which led to an ER effect. Large ER value of −59 % is achieved for sample × = 0.15 at 300 K, thereby indicating its potential application for spintronic-based devices. The competition between the occurrence of lattice distortion and the presence of magnetic inhomogeneity induced by A-site disorder (σ2) is suggested responsible for the observed ER effect.

Mitigation of Thermal Instability for Electrical Properties in CaZrO3-Modified (Na, K, Li) NbO3 Lead-Free Piezoceramics.

Lead-free ceramics 0.96(Na0.52K0.48)0.95Li0.05NbO3-0.04CaZrO3 (NKLN-CZ) are prepared by using the solid-state procedure and two-step synthesis technique. The crystal structure and thermal stability of NKLN-CZ ceramics sintered at 1140-1180 °C are investigated. All the NKLN-CZ ceramics are ABO3-type perovskite phases without impure phases. With the increase in sintering temperature, a phase transition occurs in NKLN-CZ ceramics from the orthorhombic (O) phase to the concomitance of O-tetragonal (T) phases. Meanwhile, ceramics become dense because of the presence of liquid phases. In the vicinity of ambient temperature, an O-T phase boundary is obtained above 1160 °C, which triggers the improvement of electrical properties for the samples. The NKLN-CZ ceramics sintered at 1180 °C exhibit optimum electrical performances (d33 = 180 pC/N, kp = 0.31, dS/dE = 299 pm/V, εr = 920.03, tanδ = 0.0452, Pr = 18 μC/cm2, Tc = 384 °C, Ec = 14 kV/cm). The relaxor behavior of NKLN-CZ ceramics was induced by the introduction of CaZrO3, which may lead to A-site cation disorder and show diffuse phase transition characteristics. Hence, it broadens the temperature range of phase transformation and mitigates thermal instability for piezoelectric properties in NKLN-CZ ceramics. The value of kp for NKLN-CZ ceramics is held at 27.7-31% (variance of kp < 9%) in the range from -25 to 125 °C. The results indicate that lead-free ceramics NKLN-CZ is one of the hopeful temperature-stable piezoceramics for practical application in electronic devices.

Prediction of lattice distortion and mechanical behavior of tetragonal phase (Bi0.2Na0.2Ba0.2Sr0.2Ca0.2)TiO3 high-entropy perovskite with A-site disorder from first-principles calculations

High-entropy perovskite ceramics have drawn widespread attention as their excellent physical properties. Herein, the high-entropy (Bi0.2Na0.2Ba0.2Sr0.2Ca0.2)TiO3 (BNBSCT) perovskite is selected as case study. The distorted structure and mechanical behavior as well as the effect of pressure are comprehensively studied through implementing first-principles calculations in conjunction with special quasi-random structure. The results reveal the atomic random distribution and size different cause serious lattice distortion within the BNBSCT structure, and the pseudo-intralayer distortion is significantly smaller than the interlayer distortion. Further studies of the mechanical properties indicate that the high-entropy BNBSCT perovskite is mechanical stable, especially larger elastic parameters imply the larger resistance against elastic deformation in comparison with Bi0.5Na0.5TiO3. Compared to the rule of mixture, the higher resistance to volume deformation and crack propagation of BNBSCT suggests the strengthening of high-entropy perovskite presumably due to lattice distortion in structure. Increasing pressure leads to decrease in lattice parameter and volume of structure, then increases the degree of lattice distortion, elastic constants and elastic moduli, as well the degree of lattice distortion. Moreover, the ductility and fracture toughness are also enhanced, though the elastic anisotropy is slightly larger. This work provides a new insight into the high-entropy perovskite under high pressure, which will contribute to the further research and application of high-entropy materials.

Lattice disorder effect on the structural, ferroelectric and electrocaloric properties of (Ba, Sr, Ca)TiO3 ceramics

We report the effect of A-site disorder σ2 on the structural, dielectric, ferroelectric (FE) and electrocaloric (EC) properties of lead-free Ba1-x-ySrxCayTiO3 (0.025 ≤ x ≤ 0.3, 0.275 ≤ y ≤ 0.113) perovskites. We have synthesized a series of samples with the same A-site cationic mean radius but different A-site ionic radii variance (i.e., A-site disorder) σ2. The tetragonal distortion and FE Curie temperature TC are found to decrease with decreasing A-site disorder σ2. The temperature of the maximum EC effect decreases from ~120–27 °C with decreasing disorder σ2. The highest adiabatic temperature change ΔT obtained using both the direct and indirect methods reaches ~0.8 K for the sample with optimized A-site disorder σ2. Our results suggest that, in addition to the lattice distortion, the lattice disorder might be a new and effective parameter for promoting the EC effect.

High-entropy (Ca0.2Sr0.2Ba0.2La0.2Pb0.2)TiO3 perovskite ceramics with A-site short-range disorder for thermoelectric applications

• High-entropy (Ca 0.2 Sr 0.2 Ba 0.2 La 0.2 Pb 0.2 )TiO 3 thermoelectric ceramic was prepared. • Low κ of 1.75 W/m•K and high | S | of 272 μV/K at 1073 K were reported. • Long-range structural order and short-range chemical disorder lead to suppression κ . • Pave a way of effectively reducing κ through high-entropy engineering to improve ZT . Novel high-entropy perovskite-type (Ca 0.2 Sr 0.2 Ba 0.2 La 0.2 Pb 0.2 )TiO 3 (CSBLP) ceramics with cubic structure of Pm-3m space group were successful prepared by solid-state reaction method. Results of XRD, SEM-EDS, HRTEM confirmed a homogeneous distribution and equimolar arrangement of multicomponent cations on the A-site. The high-entropy CSBLP ceramics showed long-range structural order and short-range chemical disorder with widely dispersed nanoscale grains varying sizes of 4-6 nm in the microstructure. Because of increased configurational entropy, the CSBLP ceramic has an enhanced Seebeck coefficient ( | S | = 272 μV/K at 1073 K) and low thermal conductivity ( κ =1.75 W/m•K at 1073 K) when annealed at 1300 °C. This work demonstrates the possibility of effectively reducing thermal conductivity and improving the performance of thermoelectric oxides through high-entropy composition design.

Oxygen Release and Incorporation Behaviors Influenced by A-Site Cation Order/Disorder in LaCa2Fe3O9 with Unusually High Valence Fe3.67+

Fully oxygenated perovskites with the same chemical composition crystallize in different polymorphs, o-LaCa2Fe3O9 with A-site triple-layer order and d-(LaCa2)Fe3O9 with A-site disorder. Both compounds contain unusually high valence Fe3.67+ and, when they are heated, release oxygen to reduce such an unusual valence state. Thermogravimetry analysis revealed that the ordered/disordered arrangements of the A-site cations in the perovskite structures strongly influence the stability of unusually high valence Fe ions at the B-site. The A-site-ordered o-LaCa2Fe3O9 releases its oxygen topotactically and selectively from the FeO6 octahedra between the Ca layers above 400 °C, and the released oxygen is not incorporated back on cooling in air, Ar, or O2 atmospheres. On the other hand, oxygen in d-(LaCa2)Fe3O9 is released from and incorporated into the rigid octahedra reversibly when the compound is heated in air or O2. More importantly, the oxygen-deficient d-(LaCa2)Fe3O8 obtained by heating d-(LaCa2)Fe3O9 in Ar incorporates extra oxygen to increase the valence state of Fe in an unusually high value even under ambient conditions. Once the A-site cation disorder structure framework is established, unusually high valence states, which usually require extreme conditions, can be stabilized by incorporating extra oxygen into the structure even under ambient conditions.

Microstructure and ferroelectric properties of high-entropy perovskite oxides with A-site disorder

High-entropy oxides with complex compositions can be designed as new ferroelectric materials with interesting physical consequences. Here, a series of high-entropy perovskite ceramics (Bi0.2Na0.2Ba0.2Sr0.2Ca0.2TiO3, Bi0.2Li0.2Ba0.2Sr0.2Pb0.2TiO3, Bi0.2Na0.2Ba0.2Sr0.2Pb0.2TiO3, Bi0.2K0.2Ba0.2Sr0.2Pb0.2TiO3, and Bi0.2Ag0.2Ba0.2Sr0.2Pb0.2TiO3) was proposed, which selected various elements to diminish the formational enthalpy and thus to achieve a single-phase structure. Detailed crystal structure and microstructure characterizations indicated that the high-entropy perovskites exhibited a single tetragonal phase with excellent chemical homogeneity. The equiatomic ratios of the A-site cations in perovskites could be used to maximize the entropy stabilization effect and effectively disordered the symmetry of the crystal structure. A robust ferroelectric polarization reaching 20 μC/cm2 under 50 kV/cm was achieved in Bi0.2Na0.2Ba0.2Sr0.2Pb0.2TiO3 high-entropy ferroelectrics. This work provides an effortless approach to discover new high-entropy ferroelectrics in materials with unexplored compositional complexity and gives additional opportunities to design and tailor the functional properties in entropy-stabilized ferroelectrics.

An Sr doping 0.65(Bi0.5Na0.5) TiO3-0.35 (Sr0.7+x + Bi0.2) TiO3 ceramic with tunable crystal structures and energy storage performances

A group of 0.65(Bi0.5Na0.5)TiO3 -0.35(Sr0.7+x + Bi0.2)TiO3 (BNT-S0.7+xBT) composite ceramic pellets are synthesized using a traditional solid sintering method, where a tunable x, the changeable volume of Sr, is to tailor energy storage through the adjustments of the A-site stoichiometry in BNT-S0.7+xBT. We find that a small excess of Sr2+ ions will result in an extensively tuning on the crystal grain size and even contribute to the A-site disorder and charge fluctuation of BNT-S0.7+xBT. As such, the BNT-S0.7+xBT exhibits a minimum average grain size and a highly compact crystal morphology, and thus, BNT-S0.75BT ceramic exhibits a high dielectric constant (εr) of about 5100 at 110 °C. Meanwhile, a relatively thin polarization–electric field (P–E) loop with a high maximum polarization of 42 μC/cm2 and a low remnant electric polarization of 5 μC/cm2 are obtained in a BNT-S0.75BT pellet under 100 kV/cm, corresponding to an energy density of 0.98 J cm−3 and a good η of 70.7%. Attractively, the maximum polarization (Pm) of BNT-S0.75BT ceramic at 25–100 °C hardly decreases, implying excellent temperature stability of polarization performances under high electric field of 100 kV/cm, which favors the energy storage of relaxor ferroelectric ceramics and is valuable to a supercapacitor serving at evaluated high temperature.

Systematic analysis of metamagnetic transitions in A-site disordered Sm0.5(Ca0.5−ySry)MnO3 compounds: A combined experimental and model Hamiltonian study

We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline Sm0.5(Ca0.5−ySry)MnO3(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5) compounds. The critical field (Hcr), required for the ultra-sharp jump, decreases with the increase of ‘Sr’ concentration, i.e. with an increase of average A-site ionic radius 〈rA〉. In addition, The magneto-transport data indicate that the phase separation increases with the increase of 〈rA〉, i.e. with ’y’. Moreover, the dependency of Hcr with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. In parallel with experiments, we have carried out numerical calculations to understand the role of A-site disorder in the metamagnetic transition. Our two-band double exchange model Hamiltonian calculations show that the ‘Sr’ doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. Overall, our combined study comprising of several materials with different composition of A-site reveal the fact that how and why certain A-type disorder produces prominent metamagnetic transition.