Sublattice Distortion Research Articles

Effect of nitrogen content on the structure and superconductivity of reactive sputtered NbTiN thin films

In this research, we have studied the structural and electrical properties of NbTiN films deposited on MgO and SiO2/Si substrates by reactive dc sputtering. The formation of stoichiometric NbTiN is very sensitive to N concentration and can be easily adjusted by changing the discharge current and Ar: N2 ratio along the current–voltage curves (IVCs) of the NbTi target. Excessive or insufficient N concentration in NbTiN leads to sublattice expansion or distortion, resulting in a decrease in critical temperature T c. At Ar: N2 ratio of 30:4 and discharge current of 2.2 A, T c as high as 15.8 K and 15.3 K has been obtained for 200 nm thick NbTiN/MgO and NbTiN/SiO2/Si samples, respectively. In addition, the critical density J c of the 4 μm-wide and 7 nm-thick NbTiN film grown on MgO substrate at 2 K reaches 19.2 MA cm−2, which is approximately twice as high as the 10.9 MA cm−2 of the same-sized NbTiN film grown on SiO2/Si substrate. Therefore, by further fine-tuning the N concentration in combination with the IVCs of the target, high-quality stoichiometric NbTiN can be obtained.

Short communication: The Raman fingerprint of U3O7

In this work, stoichiometric U3O7 (i.e., UO2.33) has been prepared, its stoichiometry and structure confirmed by TGA and XRD, respectively, and its Raman fingerprint described for the first time. To validate the analysis, this UO2.33 specimen has been compared with two less-oxidized samples, namely UO2.28 and UO2.31. In addition to the known Raman bands of UO2+x structures, new features obtained at 328 cm-1 (B1g stretching mode), 432 and 509 cm-1 (splitting of the T2g mode due to symmetry loss), and 671 cm-1 (likely indicative of a very substantial oxygen sublattice distortion) have been found to be distinctive of U3O7.

Yolk-Shell MnSe/ZnSe Heterostructures with Selenium Vacancies Encapsulated in Carbontubes for High-Efficiency Sodium/Potassium Storage.

The pursuit of high-performance batteries has propelled the investigation into advanced materials and design methodologies. Herein, the yolk-shell MnSe/ZnSe heterojunction encapsulated in hollow carbontubes (MnSe/ZnSe@HCTs) is prepared as a prospective electrode material for sodium/potassium batteries. The band structure in the heterojunction is methodically adjusted and regulated by intentionally utilizing Mn with unpaired electrons in the 3d orbital. The ZnSe shell confer effectively mitigates volumetric expansion challenges inherent in ions insertion/extraction processes and 1D carbontubular conductive substrate avert the aggregation of MnSe/ZnSe nanoparticles. Concurrently, the heterojunctions implantation induces sublattice distortion and charge redistribution, enriching active sites and regulating band structure. The selenium vacancies within these heterojunctions contribute to the provision of abundant active sites, thereby promoting efficient ions insertion/extraction. In sodium-ion batteries (SIBs), MnSe/ZnSe@HCTs present a superior capacity of 475mAhg-1 at 0.1Ag-1 and sustains a capacity of 408.5mAhg-1 even after 1000 cycles. In potassium-ion batteries (KIBs), MnSe/ZnSe@HCTs deliver a higher specific capacity of 422mAhg-1 at a current density of 0.1Ag-1 and maintain a high coulombic efficiency of 99% after 1000 cycles. The yolk-shell structured MnSe/ZnSe heterojunction demonstrates excellent electrode properties for high-performance sodium/potassium batteries, holding significant promise for future energy storage applications.

Impact of Crystallite Size on Structural, Optical and Magnetic Characteristics of La0.7Sr0.15Ca0.15MnO3 Nanocrystalline

Nanocrystalline La0.7Sr0.15Ca0.15MnO3 (LSCMO) manganites were prepared by the combustion process and heated to various annealing temperatures (TA) to get various sized crystallites. The X-ray diffraction (XRD) patterns provided evidence that a Rhombohedral structure with space group was formed. Additionally, an increase in the size of the crystallites was observed, from 15.64 to 36.78nm, as the temperature (TA) increased from 700℃ to 1300℃. The FESEM micrographs revealed that homogeneous with porosity. The FTIR spectra showed five absorption peaks. The Optical energy gap of LSCMO nanocrystalline is decreased from 3.51 to 3.28 eV as annealed temperature raised, reveals that the LSCMO nanoparticles are semiconductor in nature. Room temperature Raman spectra of LSCMO nanoparticles demonstrate a notable reliance on annealing temperature. When the Raman modes were analysed with respect to TA, it was observed that the Raman vibrational phonon mode below 200cm-1 (A1g) and four modes (Eg) in the range 200-800cm-1 displayed significant displacements and widening, which were associated with oxygen sublattice distortion. Considerable changes were observed in both the intensity and full width half maximum (FWHM) of the five Raman modes as the annealing temperature increased. Magnetic behaviour using M-H loop at room temperature were measured by the Vibrating sample magnetometer revealed that gradation of saturation magnetization as the function of annealing temperature. Hence there is a remarkable crystallite size effect on optical and magnetic properties of LSCMO nanocrystallites.

Influence of the octahedral cation on the evolution of lattice phonons in metal halide double perovskites: Raman spectroscopic investigation of Cs2B′B″Cl6 ( B′=Ag1−xNax ; B″=Bi1−xInx )

Vibrational dynamics in halide double perovskites govern several key aspects including carrier recombination and transport properties. Here, we present comprehensive vibrational studies investigated through micro-Raman spectroscopy to understand how octahedral cation substitution in a wide range of metal halide double perovskites ${\mathrm{Cs}}_{2}{B}^{\ensuremath{'}}{B}^{\ensuremath{''}}{\mathrm{Cl}}_{6}$ (${B}^{\ensuremath{'}}={\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}$; ${B}^{\ensuremath{''}}={\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}$) influence lattice vibrations. A significant enhancement in ${F}_{2g}$ mode intensity---a key factor in determining the cation ordering---is observed with ${\mathrm{Na}}^{+}$ substitution. In contrast to a generally observed trend, despite similar ionic sizes of ${\mathrm{Na}}^{+}$ and ${\mathrm{Bi}}^{3+}$, an increase in cationic ordering is observed as ${\mathrm{Na}}^{+}$ substitutes at ${\mathrm{Ag}}^{+}$ site in ${\mathrm{Cs}}_{2}{\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Bi}{\mathrm{Cl}}_{6}$ and ${\mathrm{Cs}}_{2}{\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{In}{\mathrm{Cl}}_{6}$. The ${F}_{2g}$ mode intensity depends on ${B}^{\ensuremath{'}}$-site cationic ordering (${\mathrm{Ag}}^{+}$ or ${\mathrm{Na}}^{+}$), while its vibrational energy is governed by the ${B}^{\ensuremath{''}}$-site cations (${\mathrm{Bi}}^{3+}$ or ${\mathrm{In}}^{3+}$). The symmetric stretching vibrations depicted by ${A}_{1g}$ mode are mainly influenced by $[{B}^{3+}\text{\ensuremath{-}}{X}_{6}]$ octahedra. The reduction in the linewidth of symmetric-stretching LO phonon mode (${A}_{1g}$) and the disappearance/diminishing of asymmetric-stretching vibrations (${E}_{\mathrm{g}}$) further substantiates the improved cationic ordering. The changes in the vibrational mode intensities with ${B}^{\ensuremath{'}}$-site substitution (${\mathrm{Ag}}^{+}, {\mathrm{Na}}^{+}$) and the appearance of distinct octahedral modes with ${B}^{\ensuremath{''}}$-site substitution (${\mathrm{Bi}}^{3+}, {\mathrm{In}}^{3+}$) allow us to disseminate different octahedral contributions to the vibrational dynamics in the lattice. Further, the vibrational analyses on double perovskites with different choices of ${B}^{\ensuremath{'}}$ and ${B}^{\ensuremath{''}}$ cations and $X$ anion reveal the origin of asymmetric stretching (${E}_{\mathrm{g}}$). This mode mainly prevails when sublattice distortions in the lattice exist. Thus, asymmetric-stretching mode can be a measure of sublattice distortion in the double perovskite, and a highly ordered system would exhibit very minimal or no asymmetric vibrations.

Growing, Structure and Optical Properties of LiNbO3:B Crystals, a Material for Laser Radiation Transformation

Physical and chemical properties have been studied in lithium niobate (LiNbO3, LN) crystals grown by Czochralski from a boron doped melt. Optical uniformity and optical damage resistance of LiNbO3:B crystals have been compared with control crystals of nominally pure congruent (CLN) and near-stoichiometric (NSLN K2O) composition. LiNbO3:B crystals structure has been studied. Studied LiNbO3:B crystals have been grown from differently synthesized charges. The charges have been synthesized from a mixture Nb2O5:B-Li2CO3 using homogeneously doped Nb2O5:B precursor (sample 1, (B) = 0.0034 wt% in the charge) and by a direct solid phase synthesis from Nb2O5-Li2CO3-H3BO3 mixture (sample 2, (B) = 0.0079 wt% in the charge). Only traces of boron (10-5-10-4 wt%) have been detected in the samples. We have established that concentration of anti-site defects NbLi is lower in both LiNbO3:B than in CLN crystals. XRD analysis has confirmed that B3+ cations localize in faces of tetrahedral voids O4 of LN structure. The voids act as buffers at the anion sublattice distortion. Sample 1 has been shown to have a structure closer to NSLN K2O crystal than sample 2. We have also shown that the chemical purity of LN crystal increases compared to the melt purity because boron creates strong compounds with impurities in the melt system Li2O-Nb2O5-B2O3. Metals impurities thus stay in the melt and do not transfer to the crystal.

Manipulating the sublattice distortion induced by Mn2+doping for boosting the emission characteristics of self-trapped excitons in Cs4SnBr6

Mn2+doping is beneficial for boosting the emission of self-trapped excitons in Cs4SnBr6. The Mn2+-doped Cs4SnBr6exhibits an enhanced photoluminescence quantum yield of up to ∼75%, a broadened emission spectrum, and improved thermal stability.

Defect redistribution along grain boundaries in SrTiO3 by externally applied electric fields

During thermal annealing at 1425 °C nominal electric field strengths of 50 V/mm and 150 V/mm were applied along the grain boundary planes of a near 45° (100) twist grain boundary in SrTiO3. Electron microscopy characterization revealed interface expansions near the positive electrode around 0.8 nm for either field strength. While the interface width decreased to roughly 0.4 nm after annealing at 50 V/mm, the higher field strength caused decomposition of the boundary structure close to the negative electrode. Electron energy-loss and X-ray photoelectron spectroscopies demonstrated an increased degree of oxygen sublattice distortion at the negative electrode side, and enhanced concentrations of Ti3+ and Ti2+ compared to bulk for both single crystals and bicrystals annealed with an external electric field, respectively. Oxygen migration due to the applied electric field causes the observed alteration of grain boundary structures. At sufficiently high field strength the agglomeration of anion vacancies may lead to the decomposition of the grain boundary.

Factors influencing the structure of the complex-defects in AF2: RE3+ (A= Ca, Sr and Ba): A first-principles study

As aliovalent doping materials, RE3+ doped AF2 (A = Ca, Sr and Ba), which show not only high thermal conductivity and low nonlinear refractive index ascribed to fluorides, but also unique spectroscopic properties related to the RE3+ ions, are promising ultra-strong and ultra-fast laser crystal candidates. It is noted that the stability of the C3v complex-defect, in which Fi− compensates at the next-nearest interstitial site of RE3+, increases with the ratio of the lattice parameter to the dopant ionic radius. In this work, systematic first-principles calculations are performed to resolve factors influencing the structure of the RE-Fi monomers. The formation energy, the reorientation energy and the local structure of the monomers are obtained, and the Coulomb interaction and the sub-lattice distortion are quantified. It is concluded that the sub-lattice distortion determines the defect structure even though the Coulomb interaction is the fundamental basis for the complex-defect formation in aliovalent doping materials. On this basis, the cluster aggregation process in AF2: RE3+ are elucidated and the octahedral arrangement of the hexamer is specially explained. Our calculation results and analytical methods not only provide new insights for the defect formation mechanism in AF2: RE3+, but also guide the improvement of aliovalent doping materials.

Boosting Near‐Infrared Luminescence of Lanthanide in Cs2AgBiCl6 Double Perovskites via Breakdown of the Local Site Symmetry

AbstractCurrently, lanthanide (Ln3+)‐doped near‐infrared (NIR)‐emitting double perovskites (DPs) suffer from low photoluminescence quantum yield (PLQY). Herein, we develop a new class of NIR‐emitting DPs based on Ln3+‐doped Cs2(Na/Ag)BiCl6. Benefiting from the Na+‐induced breakdown of local site symmetry in the Cs2AgBiCl6 DPs, effective NIR emissions of Ln3+ are realized through Bi3+ sensitization. Specifically, 7.3‐fold and 362.9‐fold enhanced NIR emissions of Yb3+ and Er3+ are achieved in Cs2Ag0.2Na0.8BiCl6 DPs relative to those in Na‐free Cs2AgBiCl6 counterparts, respectively. The optimal absolute NIR PLQYs for Yb3+ and Er3+ in Cs2Ag0.2Na0.8BiCl6 DPs are determined to be 19.0 % and 4.3 %, respectively. Raman spectroscopy and first‐principles density functional theory calculations verify the sublattice distortion in Cs2(Na/Ag)BiCl6 DPs via Na+ doping. These findings provide fundamental insights into the design of efficient NIR‐emitting Ln3+‐doped DPs for versatile optoelectronic applications.

Boosting Near-Infrared Luminescence of Lanthanide in Cs2 AgBiCl6 Double Perovskites via Breakdown of the Local Site Symmetry.

Currently, lanthanide (Ln3+ )-doped near-infrared (NIR)-emitting double perovskites (DPs) suffer from low photoluminescence quantum yield (PLQY). Herein, we develop a new class of NIR-emitting DPs based on Ln3+ -doped Cs2 (Na/Ag)BiCl6 . Benefiting from the Na+ -induced breakdown of local site symmetry in the Cs2 AgBiCl6 DPs, effective NIR emissions of Ln3+ are realized through Bi3+ sensitization. Specifically, 7.3-fold and 362.9-fold enhanced NIR emissions of Yb3+ and Er3+ are achieved in Cs2 Ag0.2 Na0.8 BiCl6 DPs relative to those in Na-free Cs2 AgBiCl6 counterparts, respectively. The optimal absolute NIR PLQYs for Yb3+ and Er3+ in Cs2 Ag0.2 Na0.8 BiCl6 DPs are determined to be 19.0 % and 4.3 %, respectively. Raman spectroscopy and first-principles density functional theory calculations verify the sublattice distortion in Cs2 (Na/Ag)BiCl6 DPs via Na+ doping. These findings provide fundamental insights into the design of efficient NIR-emitting Ln3+ -doped DPs for versatile optoelectronic applications.

Bimetallic heterojunction of CuSe/ZnSe@Nitrogen-doped carbon with modified band structures for fast sodium-ion storage

Transition metal selenides have attracted extensive attention for sodium-ion batteries (SIBs) by virtue of high capacity and intrinsic safety. However, mono-metallic selenides suffer from the low conductivity and sluggish kinetics for Na+ ions transfer. Herein, bimetallic selenide (CuSe/ZnSe@NC) is constructed with modified band structure to boost the fast Na+ ions diffusion. Particularly, the implantation of heterojunction triggers the sublattice distortion and charge redistribution, which is beneficial to provide abundant active sites and regulate band structure. As expected, bimetallic CuSe/ZnSe@NC delivers the specific capacities of 411.5 mA h g−1 after 1000 cycles at 1 A g−1 and 361.8 mA h g−1 at 5 A g−1, indicating the superior cycle and rate performance than that of mono-metallic selenides. Meanwhile, in-situ XRD, TEM, and EIS further reveal the high reversibility and the conversion and alloying mechanisms of bimetallic CuSe/ZnSe@NC for SIBs. Moreover, first-principles calculations (DFT) further confirm that the fast Na+ ions diffusion is attributed to the optimized band structure and the charge rearrangement. Therefore, bimetallic heterojunctions not only combined the multifunctional properties, but also exhibited unique physicochemical properties that transcend mono-metallic selenides.

Another view of oxygen in cathodes for high energy batteries

The concept of utilizing anionic redox reactions to achieve high energy batteries has been revived for almost a decade by now; however, both its fundamental mechanism and practical implementation remain elusive and highly debated. In this issue of <i>Joule</i>, Weixin Song, Peter D. Nellist, and co-workers utilized electron ptychography to image the oxygen sublattice distortion of a Li-rich material that is known to involve oxygen redox reactions. Through the dose-efficient ptychography reconstruction with picometer resolution, they provide a beautiful demonstration of detecting the atomic placements of both the heavy, i.e., transition metals, and light, i.e., O and Li, elements of the Li-rich electrode at different electrochemical states, inspiring valuable discussions on the oxygen activities in battery cathodes.

Understanding of the bond covalency nature with ionic electronegativity in perovskite cuprates La2CuMO6-x (M = Ti, Mn, Ru)

A bond valence sums (BVS) modified ionic electronegativity (EN) scale was suggested for better accommodating the double perovskite La2CuMO6-x (M ​= ​Ti, Mn, Ru) with mixed bonding nature. Both the M ​− ​O bond covalency fraction (f) and the cooperative B-site sublattice distortion are highlighted to connect structure and physical properties. Particularly, the Ti-based compound hosts the largest M/O ionic EN difference and hence yields the smallest f value while the Ru-based analogue exhibits the opposite result. As the consequence, robust spin disordering accompany with poor dc conductivity is convinced in the Ti-based sample owing to its featuring B-site ionic arrangement and profound suppression of 3d(Ti)-2p(O) orbital hybridization. For the Ru-based sample, strong ferromagnetic spin frustration and low charge carrier hopping potential might be essentially ascribed to its large f value through enhancing orbital degeneracy between 4d(eg)-2p σ and 4d(t2g)-2p π bonding states. The Mn-based sample with the moderate f value exhibits the similar magnetic and electric behaviors as those of Ru-based sample. Both the d(M)-p(O) orbital hybridization and the M ​− ​O bond covalency could be tuned by rationally selecting the M cation with proper ionic EN value. This work provides a deep understanding of the bond covalency nature in perovskite cuprates and might help to develop novel electronic functional materials.

Sublattice Distortion Enabled Strong Interplay between Phonon Vibrations, Electron-Phonon Coupling, and Self-Trapped Excitonic Emissions in Cs2Ag1-xNaxBiCl6 Double Perovskites.

Sublattice distortion resulting from alloying compositionally distinct double perovskites is shown to influence photoluminescence emission in Cs2Ag1-xNaxBiCl6 (0 < x < 1). The end members show negligible photoluminescence, whereas interestingly the alloys exhibit broad photoluminescence. These emissions are attributed to self-trapped excitons (STE) resulting from sublattice distortions arising due to the mismatch in [AgCl6]5- and [BiCl6]3- octahedra. Change in sublattice distortions plays significant role in the formation and recombination of STEs. The STE emission intensity and quantum yield greatly depend on x, with highest intensity observed for x = 0.75, consistent with a large change in sublattice found at this x. Variation in photoluminescence properties with composition follows a similar trend as that of bandgap and phonon vibrational changes observed due to sublattice distortion. Temperature-dependent phonon vibrations and photoluminescence studies reveal a giant electron-phonon coupling. A strong synergy between STE emissions, electron-phonon coupling, bandgap, and phonon vibrations in double perovskites with sublattice distortions is demonstrated.

Bandgap engineering and sublattice distortion driven bandgap bowing in Cs2Ag1-xNaxBiCl6 double perovskites

Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag+ at B′ site gets replaced with Na+ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x &amp;gt; 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]5− and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.

Stability of the peroxide group inBaO2under high pressure

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable $\mathrm{Ba}{\mathrm{O}}_{2}$ phase at high pressure up to 130 GPa found via a first-principles computational structure search and high-pressure experimental investigations. The identified monoclinic structure (space group $C2/m$) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure $Cmmm$ structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, this phase retains the peroxide group and, interestingly, exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron x-ray diffraction (XRD) measurements could not distinguish between $Cmmm$ and $C2/m\mathrm{Ba}{\mathrm{O}}_{2}$ definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively like that predicted theoretically due to the $Cmmm$ to $C2/m$ phase transition, thus supporting the predicted transformation.

(Invited) Functional Defects By Design—a High-Throughput Approach to Energy Materials Discovery

Defects and impurities introduce localized heterogeneities in solids and decisively control the behavior of a wide range of energy technologies. Fuel cell materials, especially proton conducting fuel cells, are a quintessential example in this regard. Designing and developing solid oxide materials that can selectively transport protons will enable us to develop the next generation proton conducting solid oxide fuel cells. Protons require less activation energy compared to oxygen ions which results in a lower operating temperature, higher operating efficiency and better material reliability. In this work [1,2,3,4] we focus obtaining fundamental insights on how properties of host material structure along with dopants, disorder and strain influences proton transport properties in solid oxides by coupling high-throughput computations with functional imaging, neutron spectroscopy and transport measurements.We initially focus on the perovskite family of compounds (such as doped BaZrO3). We benchmark our calculations against a wide range of experimental measurements such as kelvin probe force microscopy (KPFM), inelastic neutron scattering (INS) and atom probe tomography (APT). To obtain better insights on why certain cubic perovskite/dopant combinations are better at conducting protons compared to others, we developed a high-throughput framework to perform ab initio calculations. The high-throughput framework can scale massively to tens of thousands of nodes to fully exploit the computational capability of the Oak Ridge Leadership Computing facility. We employ this approach to calculate proton transport properties in several cubic perovskite materials with different host atoms and dopants. The results obtained from these calculations enables us to obtain better insights on how material structure – such as atomic properties (electronegativity, ionic radius) and lattice properties (sub-lattice distortion) influences proton transport. The results obtained from this high-throughput analysis is being employed to develop a machine learning framework to predict structure-property correlations on a larger set of perovskites materials. Finally, we explore the role of disorder on proton transport by studying for example fluorite based lanthanum tungstate materials. [1] “Defect Genome of Cubic Perovskites for Fuel Cell Applications”, Journal of Physical Chemistry C, 121, 26637 (2017)[2] “The Influence of Local Distortions on Proton Mobility in Acceptor Doped Perovskites”, Chemistry of Materials, 2018, 30 (15), pp 4919–4925[3]“The Influence of the Local Structure on Proton Transport in a Solid Oxide Proton Conductor La0.8Ba1.2GaO3.9”, J. Mat. Chem. A, 5, 15507 (2017)[4] “Influence of Non-Stoichiometry on Proton Conductivity in Thin Film Yttrium-doped Barium Zirconate”, ACS Appl. Mater. Interfaces, 2018, 10 (5), pp 4816–4823

Temperature Dependent Raman Spectroscopic Study of the Fe Doped La0.67Sr0.33MnO3 Prepared Using Ball Milling Method

Polycrystalline samples of La0.67Sr0.33Mn0.65Fe0.35O3 (LSMFO) were synthesized using the standard ball mill method with different calcination temperatures ranging from 800 to 1100°C for 7 h. The phase purity of these samples was confirmed using X-ray diffraction (XRD) patterns. All samples were found to have rhombohedral crystal structure with $$R\bar {3}c$$ space group. The lattice parameters, cell volume, bond angle and bond length have been obtained using the Rietveld refinement by FullProf software. The average crystallite size calculated using the Debye-Scherrer formula was found between 27 and 60 nm. Surface morphology of the prepared samples has been examined using a scanning electron microscope (SEM). SEM images show the formation of well-arranged grain sizes distributed from 240 to 400 nm, much larger than one estimated using the Scherrer formula. All tiny particles are highly agglomerated with the increasing temperature and porosity decreases with increasing temperature. An analysis of the frequency and peak broadening of Raman modes as a function of temperature clearly shows the significant temperature effect on the A1g and Eg modes of LSMFO. The shifts and broadening of the A1g and Eg modes are discussed in light of the oxygen sublattice distortion. Our study shows the reduction in distortion with increasing calcination temperature, which suggests a decrease in the JT effect.

Origin of Ferroelectricity in Epitaxial Si-Doped HfO2 Films.

HfO2-based unconventional ferroelectric materials were recently discovered and have attracted a great deal of attention in both academia and industry. The growth of epitaxial Si-doped HfO2 films has opened up a route to understand the mechanism of ferroelectricity. Here, we used pulsed laser deposition to grow epitaxial Si-doped HfO2 films in different orientations of N-type SrTiO3 substrates. Polar nanodomains can be written and read using piezoforce microscopy, and these domains are reversibly switched with a phase change of 180°. Films with different thicknesses displayed a coercive field Ec and a remnant polarization Pr of approximately 4-5 MV/cm and 8-32 μC/cm2, respectively. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) results identified that the as-grown Si-doped HfO2 films have strained fluorite structures. The ABAB stacking mode of the Hf atomic grid observed by HRTEM clearly demonstrates that the ferroelectricity originates from the noncentrosymmetric Pca21 polar structure. Combined with soft X-ray absorption spectra, the results showed that the Pca21 ferroelectric crystal structure manifested as an O sublattice distortion by the effect of the interface strain and Si dopant interactions, resulting in a nanoscaled ferroelectric ordered state because of further crystal splitting.