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The calculated vibrational patterns of the optical phonons in LaAlO 3. One oxygen octahedron is drawn only. The hexagonal c axis parallel to C 3 ) is vertical. Due to the similarity of the vibrational shapes of the pair modes, originated from triply degenerated modes in the cubic structure, the vibrational pattern of the nondegenerated A-type modes is drawn only.
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The Raman and infrared phonons of isostructural rhombohedral LaMnO3 and LaAlO3 are studied at room temperature. The experimental spectra are compared with the prediction of lattice-dynamical calculations and the lines observed are assigned to definite atomic vibrations. It is shown that the Raman mode of A1g symmetry in LaAlO3 and LaMnO3 (at 123 cm...
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Context 1
... values of model parameters listed in Table III Table I. The corre- sponding vibrational patterns are drawn in Fig. ...
Context 2
... LaMnO 3 Table I we can reanalyze and inter- pret available experimental data see Fig. 4 and Table IV. According to the symmetry, the following correlations hold between the long-wavelength IR-active 3A 2u and 5E u modes of the rhombohedrally distorted R3 ¯ c perovskites and phonons of the cubic Pm3m perovskite structure compounds see Table I and Fig. ...
Citations
... In the Raman spectra of LAO, the A 1g mode at 132 cm −1 is due to the rotation of oxygen octahedra; for example, the mode at 163 cm −1 is due to La vibration, that at 463 cm −1 is due to oxygen-bending vibration and that at 691 cm −1 is attributed to oxygen-stretching vibration. The most intense A 2g mode at 741 cm −1 arises due to the outward stretching of oxygen in AlO 6 octahedra [38,39]. The Raman spectra of bare and air-annealed LAO substrates are similar, confirming that both substrates have the same space group symmetry R3C. ...
In recent years, thin films of weak ferromagnetic materials have been in huge demand, however, probing its magnetic character has been ambiguous due to contribution from the underlying substrates. In the present study, we have analyzed the magnetic properties of the commonly used single crystal SrTiO 3 (100) and LaAlO 3 (100) substrates and have performed a time dependent annealing protocol in vacuum and ambient oxygen pressure for mitigating the intrinsic weak ferromagnetic contribution from these substrates arising due to presence of disorder or defects like vacancy in the pristine substrates. It is shown that after a proper air annealing, the substrates' magnetic background turns to diamagnetic. When such air annealed diamagnetic substrates are used for the deposition of low thickness films carrying low magnetic moments like SrRuO 3 thin film and SrMnO 3 thin film, their magnetic transitions are explicitly observed. The proposed annealing protocols certainly help improve the signal from weak magnetic samples, which allows us to analyze the film's magnetic properties without worrying about the substrate contribution.
... Group theory provides 24 Raman active modes (7A g + 5B 1g + 7B 2g + 5B 3g ) in a LaMnO 3 orthorhombic crystal structure [44]. At the same time, the heating that occurs from laser radiation results in a structural transformation toward the rhombohedral phase in which only five Raman modes (A 1g + 4E g ) are active [45]. The results obtained for La 0.9 A 0.1 Mn 0.9 Cr 0.1 O 3 (A: Li, Na, K) powders and ceramics are consistent with previous studies of similar perovskite structures, such as LaMnO 3 [45]. ...
... At the same time, the heating that occurs from laser radiation results in a structural transformation toward the rhombohedral phase in which only five Raman modes (A 1g + 4E g ) are active [45]. The results obtained for La 0.9 A 0.1 Mn 0.9 Cr 0.1 O 3 (A: Li, Na, K) powders and ceramics are consistent with previous studies of similar perovskite structures, such as LaMnO 3 [45]. However, in our study, we observed only three E g modes ( Figure 2) and a shift of the Raman peak position to higher frequencies with the decrease in the atomic nucleus size of the doping alkali metals ( Table 2). ...
Nanocrystalline La0.9A0.1Mn0.9Cr0.1O3 (A: Li, K, Na) powders have been synthesized by combustion method. The powders were used to prepare ceramics by high-pressure low-temperature sintering technique. For all samples the structure, elemental composition and morphology were studied using X-ray diffraction (XRD), Raman spectroscopy, Energy-Dispersive X-ray Spectroscopy (EDS) and Scanning electron microscopy (SEM). Magnetic properties were studied using magnetometry methods and the valency changes of the cations after alkali ions doping were studied using X-ray photoelectron spectroscopy (XPS). The influence of the sintering pressure on the structural and magnetic properties of the manganites doped with different alkali ions and chromium was also investigated. Magnetization properties were studied as a function of sintering pressure and type of the dopant. Chemical doping with alkali ions as well as external pressure significantly changed the magnetic properties of the compounds. It was found that the magnetic properties of the manganites could be predictably modified through the use of a suitable dopant element.
... 104 Raman spectra of LaAlO 3 exhibit five Raman-active modes that are related to the lattice vibration of AlO 6 octahedra, La, and oxygen (bending). 105 The filled circles in Fig. 6 depict the Raman spectra of pure LaAlO 3 in which the modes at 36 cm −1 (E g ) and 121 cm −1 (A 1g ) indicate the lattice vibration related AlO 6 octahedral tilting. 106 The modes around 150 cm −1 exhibit the lattice vibrations of La. 106 In the same Fig. 6, open square data indicate the Raman spectra of Mg doped at Al-site in LaAlO 3 . ...
This Tutorial provides a fundamental discussion on the lattice dynamics of physical systems introduced with disorder and, hence, the importance of Raman spectroscopy (RS) technique to probe these impacts. The article first discusses, analytically, the impact of disorder on the symmetry allowed phonon modes of the system by considering the finite probability of discrete-continuum interference in terms of electron–phonon interactions in the system, thereby briefly discussing the relevant experimental reports, followed by providing an ephemeral description on the loss of translational symmetry in the lattice environment under the strain field generated due to disorder and its consequence as relaxation of the [Formula: see text] selection rule in terms of RS; thus, correlating these discussions with the observation of the symmetry-forbidden disorder induced phonon modes. The same is also elaborated with the experimental reports on various systems of ABO 3 and AO 2 kinds, where A and B are cations that exhibit the occurrence of disorder induced phonon modes in the respective Raman spectra because of the disorder introduced into the host lattice, and which is emphasized to be not originating due to any structural phase transitions.
... Similar to the synthesized REFeO 3 NPs, 24 Raman active modes are expected for manganites with Pnma symmetry, while only five Raman active modes (A 1g + 4E g ) are predicted for the rhombohedral R-3c symmetry of LaMnO 3 . [23] All Raman active vibration modes corresponding to the orthorhombic structure of NdMnO 3 , SmMnO 3 , and GdMnO 3 were evidenced and in excellent agreement with those previously reported in the literature. [24] Moreover, the Raman spectra of the studied RE-manganites exhibit similar trends as those of RE-ferrites with increasing r RE3 + . ...
The main limitations of current methods for synthesizing perovskite oxide (ABO3) nanoparticles (NPs), e.g., the high reagent costs and sophisticated equipment, the long time and high‐temperature processing, or multiple post‐processing and thermal treatment steps, hamper their full study and potential application. Here, we use a facile low temperature (50 °C) chemical bath synthesis and only one annealing step to successfully produce high phase purity and crystalline quality nano‐shaped rare‐earth‐based REMO3 NPs (RE=La, Nd, Sm, Gd; M=Fe, Mn, Al). We also show the versatility of this approach by fabricating La0.7Sr0.3MnO3 solid solution and non‐RE‐based BiFeO3 perovskite. To assess the potential of the as‐prepared REFeO3 and REMnO3 NPs, they are used for photocatalytic degradation of the norfloxacin antibiotic and show high efficiency. We believe this easy, robust, versatile, and general route for synthesizing ABO3‐based NPs can be further explored in the vast perovskite family and beyond.
... The change of micro-crystal structure in Cu 2+ ion doping into hematite film was confirmed from the Raman spectra as presented in Fig. 5. Hematite has the corundum structure, and it belongs to the D3d point group. Only two A1g modes and five E g modes are Raman active, while two A2u modes and four E u modes are infrared active only [28][29][30][31] . For a molecule to be Raman-active, there must be a change in the polarizability of the molecule while IR-active modes are induced by dipole moment change within the molecule. ...
The concentration of guest elements (dopants) into host materials play an important role in changing their intrinsic electrical and optical properties. The existence of hetero-element induce defect in crystal structure, affecting conductivity. In the current work, we report Cu ²⁺ ion into hematite in the defectronics point of view and their photoelectrochemical properties. Crystal distortion in the structure of hematite is observed as the amount of dopant increases. Among 1, 3 and 5 mol% of Cu ²⁺ doped hematite, the existence of 1 mol% of Cu ²⁺ ion into hematite crystal structure produce photocurrent value of 0.15 mA/cm ² , IPCE value of ~ 4.7% and EIS value of ~ 2000 Ω/cm ² as best performances. However, further increasing dopants increases the number of interstitial defects, which cause the deformation of intrinsic lattice structure.
... Figure 2b shows the Raman patterns of La1-xCexMnO3. The strong peak of 650 cm −1 appears for all the samples and is assigned to the LaMnO3 perovskite structure [26]. For the Ce doped samples, La0.9Ce0.1MnO3 ...
In this paper, Ce-doped La1-xCexMnO3 perovskite catalysts are prepared by the sol–gel method, and the promotion effect of Ce doping on LaMnO3 catalysts for CO oxidation is investigated. The catalysts are characterized by X-ray diffractograms, Raman, N2 physisorption isotherms, temperature-programed reduction with H2, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show that the Ce doping greatly improves the catalytic activity of LaMnO3 for CO oxidation. Among the La1-xCexMnO3 catalysts, La0.8Ce0.2MnO3 shows the best CO catalytic activity, with 100% CO conversion obtained at 180 °C. The characteristic results show that the LaMnO3 perovskite phase exists in all Ce-doped catalysts, and the CeO2 crystalline phase begins to appear at x ≥ 0.1. The high activity of La0.8Ce0.2MnO3 for CO oxidation could be that: (1) it possesses large surface area (25.8 m2/g) to contact with reactants; (2) it has a high surface Ce3+/(Ce3+ + Ce4+) ratio of 0.27, which means high content of oxygen vacancies used for O2 adsorption and activation; and (3) it exhibits strong reducibility that is beneficial to CO activation.
... However, only one mode from each group of vibrations can be observed in the experimental spectrum. Based on the shell model calculation for rhomboidal LaMnO 3 [26], the Raman peak at 642 cm − 1 (see Fig. 2(a)) is assigned to the E g mode with a predicted frequency of 646 cm − 1 , which is related to the out-of-phase stretching oxygen vibration. Also, there is a typical reflection IR band around 590 cm − 1 (see Fig. 2(b)), which is due to the internal change in the Mn-O bond length (d Mn-O ) [27,28]. ...
In this work, silver-doped La0.85-xSr0.15AgxMnO3 (LSAMO, 0.05 ≤ x ≤ 0.20) polycrystalline ceramics were prepared by the sol-gel method. The experimental characterization of ceramics revealed that Ag ions were bounded with the lattice matrix. The resistivity and the corresponding peak temperature coefficient (TCR) of LSAMO ceramics were systematically tuned by changing the Ag dopant content. Adjusting the proportion of Ag enabled one to vary the metal-insulator transition temperature (TMI) for LSAMO specimens in a wide range, and their peak resistivity temperature (Tp) was increased from 299.6 to 335.7 K. At the same time, the peak TCR value achieved its maximum of 15.2% K⁻¹ at the doping molar ratio x of 0.15 and the peak resistivity temperature (Tk) of 294.1 K. In addition, the electrical transport was described in the context of the small polaron hopping (SPH) model and the phenomenological percolation model (PP) over the metal-insulator transition region. Under the combined action of the Jahn-Teller (JT) effect, the double exchange (DE) mechanism and the PP model, LSAMO ceramics possessing high TCR at room temperature were obtained by varying the amount of Ag doping. The observed properties suggest LSAMO material can be used in advanced uncooled infrared bolometers.
... The Raman profile has a shift at 643 cm À 1 correspond-ing to MnÀ O bond. There are two small humps observed at 236 and 393 cm À 1 which are ascribed to MnÀ O and MnÀ OH. [18] It can be seen that there is an asymmetry in the Raman shift centered at 636 cm À 1 . This asymmetric broadening of the Raman shift is usually observed for the material with defects in them. ...
Development of cost‐effective efficient bifunctional electrocatalysts is indispensable for commercialization of metal‐air batteries. Here, vacancy‐induced nanostructured LaMnO3 perovskite was generated by a facile hydrothermal method and explored as a bifunctional air‐breathing electrode for rechargeable Lithium‐Air battery. Materials characterizations confirmed the formation of the nanostructured LaMnO3 with anion and cation vacancies. The LaMnO3 electrocatalyst on rotating ring disc electrode study showed excellent oxygen reduction reaction and oxygen evolution reaction activities in 0.1 M KOH electrolyte. The oxygen vacancies associated with the LaMnO3 acted as catalyst centre and promoted reaction rate for the ORR and OER. Laboratory prototype CR‐2032 air‐coin cell was fabricated using the LaMnO3 as air‐breathing electrode, resulted open circuit voltage 3.3 V for more than 5 days and high discharge capacity of 3100 mA h g⁻¹ at current density of 50 mA g⁻¹. Further, once charged air‐coin cell could power an LED for more than 24 h continuously.
... 7-10 for KLTN1, KLTN2, KNTN, and KTN, respectively. This Raman peak, displaying features of a soft mode, is identified in agreement with similar perovskitic crystals [19,[53][54][55][56]. Notably, the behavior of this Raman mode is correlated to local polarized regions that display relaxation [19]. ...
We investigate samples of compositionally disordered potassium-based perovskite single crystals with different composition and stoichiometry. The dielectric and Raman response is inspected over the nominal cubic-to-tetragonal long-range phase transition. The comparison between results shows that the occurrence of the phase transition is controlled by the temperature evolution of polar nanoregions (PNRs). We are able to correlate PNR order, formation, percolation, and freezing to the characteristic temperatures of each macroscopic crystal. The onset of the phase transition is found to occur when PNRs undergo a percolative ordering process, in correlation to the sudden arising of a specific high-frequency Raman mode.
... The blue line in Fig. 4(b) shows the model behavior using our calculated values of a, b and band gap (see SM [29] for details), and the experimental dielectric permittivity r = 25. This hardness of the polar mode [33] is of course favorable for the production of the interfacial two-dimensional electron gas, the formation of which is a lower energy process than the polarization of LaAlO 3 [30]. Conversely, we anticipate that the formation of a conducting two-dimensional electron gas at a KTaO 3 -SrTiO 3 interface will be less favorable [34]. ...
We demonstrate the existence of a polarizing field in thin films of insulators with charged ionic layers. The polarizing field derives from the same physics as the well-known depolarizing field that suppresses ferroelectric polarization in thin-film ferroelectrics, but instead drives thin films of materials that are centrosymmetric and paraelectric in their bulk form into a noncentrosymmetric, polar state. We illustrate the behavior using density-functional computations for perovskite-structure potassium tantalate, KTaO3, which is of considerable interest for its high dielectric constant, proximity to a quantum critical point, and superconductivity. We then provide a simple recipe to identify whether a particular material and film orientation will exhibit the effect and develop an electrostatic model to estimate the critical thickness of the induced polarization in terms of basic material parameters. Our results provide practical guidelines for exploiting the electrostatic properties of thin-film ionic insulators to engineer novel functionalities for nanoscale devices.
