Structure Of LaAlO3 Research Articles

Effect of calcination temperature on structural, optical, and morphological properties of RAlO3 (R = La, Sm) perovskite oxides

RAlO3 (R = La, Sm) have attracted the research community due to their interesting optoelectronic properties and viable applications. The solution combustion method allows for a faster process and lower calcination temperature than the traditional solid-state method and is an economical alternative to wet chemical synthesis for producing RAlO3. This work explores the thermal, structural, morphological, optical, and electrical properties of RAlO3 (R = La, Sm) synthesised by the solution combustion method using urea as fuel. Thermogravimetric analysis—differential thermal analysis (TGA-DTA) showed the crystallization temperatures of lanthanum aluminate (LaAlO3) and samarium aluminate (SmAlO3) at 864 and 887 °C, respectively. The x-ray diffraction (XRD) and Rietveld analysis revealed the structure of LaAlO3 as rhombohedral (R-3c) and SmAlO3 as orthorhombic (Pbnm). The LaAlO3 and SmAlO3 samples calcinated at 800 °C showed crystallite size (D) of 19.26 nm and 19.06 nm, respectively. The field emission scanning electron microscopy (FE-SEM) images show a highly porous and large sheet-like morphology with voids and cracks. High resolution transmission electron microscopy (HR-TEM) images and the selective area electron diffraction (SAED) pattern show crystalline nature and the indexed planes agreed with the XRD results. The LaAlO3 and SmAlO3 samples had specific surface areas of 16.374 and 12.953 m2 g−1, respectively. The TGA-DTA results were affirmed by Fourier transform infrared spectroscopy (FT-IR) results which showed only the presence of metal-oxide bonds for materials annealed at and above 800 °C. These results were further validated by the electron dispersive x-ray spectroscopy (EDX) and the x-ray photoelectron spectroscopy (XPS) showing no additional peaks. The band gap of 5.08 and 4.82 eV were calculated from ultraviolet-visible spectroscopy (UV–vis) for LaAlO3 and SmAlO3, respectively. The results imply that the solution combustion technique using urea as fuel is a viable route for synthesising RAlO3.

Multiscale structure of LaAlO3 from single-crystal X-ray diffraction.

A domain-resolved synchrotron single-crystal X-ray diffraction study of a LaAlO3 pseudo-merohedral twin crystal was successfully carried out in combination with powder diffraction data from the same sample. Multiscale structure information ranging from micro- to nano- to atomic scale was determined from one single crystal. There is almost no change of domain ratios at temperatures of less than 400 K indicating no movement of the domain wall. The changes in domain ratio indicating domain-wall movement were observed in the temperature range of 450 to 700 K, which is consistent with the result of the previous mechanical measurement. It is also found that the ratio of four twin components becomes equal (25%), just below phase transition temperature. These findings are important for domain engineering and theoretical studies related to LaAlO3. The temperature dependence of domain ratio was preserved in the heating and cooling cycle except for the first heating process to 840 K. Therefore, the domain structure after heating to 840 K is intrinsic to the crystal. Accurate structure parameters were determined through unit-cell parameter calibration and domain-resolved structure analysis. The method for calibration of unit-cell parameters from twin crystal data was derived and used to solve the inconsistent unit-cell parameters between single crystal and powder data in the present and previous studies.

RKKY Interaction in Spin Valve Structure with LaAlO<sub>3</sub> Spacer

RKKY interaction is an indirect interaction between localized magnetic moments mediated by conduction electrons. RKKY exchange interaction is mostly found in multilayer involving conductors as the non-magnetic spacer. However, recent experiment shows that LaAlO3 spacer can intermediate long-range exchange interaction even though it is an insulator. Here we study the mechanism of RKKY interaction in LaAlO3. To study the effect of charge transfer, we perform a density functional theory (DFT) approach to obtain the band structure of LaAlO3 with charge doping. Undoped LaAlO3 has an indirect band gap around 3 eV with peak valence at R symmetry point and lowest conduction point at Γ point. We show that the conducting characteristic arises from Fermi energy shifts via charge doping. By adding charge doping, we show that LaAlO3 can have conductor characteristics, in agreement with the charge transfer experiment on LaAlO3. The spin-polarized conduction electron is measurable as additional magnetization of the system. Furthermore, the conducting LaAlO3 is able to mediate RKKY interaction in spin valve structure.

Effect of zinc doping on electrical properties of LaAlO3 perovskite

New solid solution with the general formula of LaAl1-xZnxO3-1/2x was prepared by a solid-state reaction route. According to XRD, the crystal structure of LaAlO3 is rhombohedral, while the solid solution possesses cubic symmetry. Homogeneity region of the solid solution LaAl1-xZnxO3-1/2x was narrow and limited to the maximum concentration of 5 mol. %. Computer simulations using crystalochemistry and density functional theory approaches showed that LaAlO3 has high energy barriers for O2–-ion transport (2.79 eV). These results are in good agreement with the low values of electrical conductivity obtained experimentally. The electrical conductivity of LaAl1-xZnxO3-1/2x was measured by impedance spectroscopy in the temperature range of 200–1000 °C. The partial substitution of Al3+ by Zn2+ was found to increase the electrical conductivity by ~2 order of magnitude. The electrical conductivity of doped phase LaAl0.95Zn0.05O2.975 as a function of oxygen partial pressure was measured, and the partial contributions (oxygen-ionic and electronic) were determined. It was found that the sample has mixed ionic and p-type electronic conductivity, while the electronic contribution increases with the rise of the temperature.

Tunable white light emission from Dy3+/Eu3+ doped LaAlO3 nanophosphors via hydrothermal method

A sequence of Dy3+/Eu3+ co-doped LaAlO3 nanophosphors are synthesized by hydrothermal technique and their phase purity, particle nature and luminescence characteristics are investigated. All the prepared samples possess single-phased perovskite type rhombohedral structure of LaAlO3 and consist of polygonal natured particles with the mean size of 30 nm. Under the excitation with 351 nm, LaAlO3: Dy3+ nanophosphors shown characteristic emission peaks of Dy3+ ion and are positioned at 485, 576 and 665 nm. LaAlO3: 9 mol% Dy3+, y mol% Eu3+ nanophosphors exhibited emission bands of both Dy3+ and Eu3+ ions and are noticed at 485, 576, 590, 620 and 696 nm under 351 nm excitation. It can be perceived that the Dy3+ integrated luminescence intensity is reduced and the average lifetime of Dy3+ is decreased with the enhancing Eu3+ content, which is owing to energy migrates from Dy3+ ion to Eu3+ ion. Moreover, CIE coordinate values of LaAlO3: 9 mol% Dy3+, y mol% Eu3+ nanophosphors are altered from the bluish-white to white light and ultimately to the orange-white light location with increasing the Eu3+ ion content. This nature of color adjustable nanophosphors shows the excellent potential advantages in the fields of white light luminescent materials, optoelectronic devices and color displays.

Tailoring electronic and optical properties of LaAlO3 by Cu inclusion: a DFT study

First principles calculations in the context of density functional theory (DFT) provides a platform to investigate the material’s properties in a way to explain the physics of solids. Electronic, structural, and optical properties for pure and Cu-doped LaAlO3 are investigated. The study reveals that the band structure of LaAlO3 after the inclusion of Cu at the La site is affected significantly. Due to the substitution of Cu, there is a decrease in the band gap from 3.02 eV to 2.95 eV. Furthermore, the partial density of states of LaAlO3 changes significantly after Cu doping, which has confirmed the doping effect on intrinsic LaAlO3. The p and d states play a vital role on top of the valence band. The range in which the top of a valence band lies is from 0 eV to –9 eV having a significant effect and it is because of Cu-d states. The optical properties of both pure and doped systems were analyzed. Redshift was observed as absorption edge shifted from 4.5 eV to 4.0 eV and even the refractive index reduces from 2.25 to 1.0, which means the Cu inclusion significantly play its role to tailor optical properties. Hence Cu-doping affects the properties of LaAlO3 and makes it a suitable candidate for use in different optical devices.

A first principles study of the phonon anharmonicity, electronic structure and optical characteristics of LaAlO3

The way elementary excitations work together with their couplings and interact as condensed matter systems is very important when designing optimum energy-conversion devices. We investigated the electronic structure of LaAlO3, and we show that the bandgap insulator of LaAlO3 obtained theoretically by the hybrid functional HSE06 is an indirect 5.649eV that show a very good agreement with experimental data. The lattice constant is obtained exactly as experiment. In thermos-electric materials, the concept of conversion-efficiency (heat to electricity) is improved instantly by suppressing the phonon quasi-particles propagations that are responsible for draft macroscopic thermal transport. The material presented here for thermo-electric conversion-efficiency of cubic perovskite LaAlO3, show that it has an ultralow thermal-conductivity, while the formalism to its strong phonon scattering interactions resides mostly unclear. From the bases of Ab-initio simulations, the 4-dimensional phonon-dispersion surfaces of the cubic perovskite LaAlO3, have been mapped and we found that the origins of the ionic potential an-harmonicity being responsible for the unique behaviour and properties of LaAlO3. It is investigated that these phonon scattering arise solely from the LaAlO3 unstable electronic-structure, with its orbital interactions resulting to lattice instability similar to the ferroelectric instabilities. Our results show a microscopic insight bonding electronic-structure and phonon an-harmonicity in LaAlO3, and provides some new picture the way interactions happen between phonon–electron and phonon–phonon this lead to understand the concept of ultralow thermal-conductivity. Ab-initio calculations was performed on cubic perovskite LaAlO3 to obtain the phonon density of states (DOS) from 50 K to 5000 K, we find that the anharmonic behaviour starts around temperature limits of 500 K. The computed optical spectra were obtained using both the Beth Slapter Equation BSE and compared with the perturbed method using HSE06, optical spectra show that the inter-band transition occur precisely from the O-valence bands to the La-conduction bands throughout the low energy area. The energy-loss spectrum, optical conductivity and reflectivity and the refractive index are computed from first principles by using HSE06 hybrid functional. The optical band gap of material shows about 6.21 eV, which agrees with some cited experimental measurements.

Luminescent properties of dysprosium(III) ions in LaAlO3 nanocrystallites

The absorption and emission spectra as well as decay time profile of Dy3+ ions in LaAlO3 nanocrystals were analyzed. The crystal structure of LaAlO3 was confirmed from XRD measurement. The emission peaks from blue to red came from main emitting level of dysprosium 4F9/2 to the ground and other excited levels of Dy3+ ions. Cross relaxation process led to non-radiative quenching of luminescence, so that the lifetime of the 4F9/2 energy level ions decreased with increasing amount of doped Dy3+ ions. The cross relaxation transfer rates were experimentally determined as a function of Dy3+ concentration.

Atomic and Electronic Structure of LaAlO<sub>3</sub> and LaAlO<sub>3</sub>:Mg from First-Principles Calculations

In the present paper, the lattice structure, band structure and density of state of LaAlO3 and LaAlO3:Mg are calculated by first-principle method based on density functional theory. Firstly, we select the different cutoff energy and k-point grid in the calculations, and obtain the most stable geometry structure of single crystal LaAlO3. The calculated lattice parameters are a=b=5.441 Å, c=13.266 Å, which matches with experimental values. To deeply understand the electronic structure of LaAlO3, a 2×1×1 super-cell structure is established and the doping concentration of Mg at Al sites is 25%. From the band structure and density of states, it can be seen that LaAlO3 has a direct band gap Eg=3.6 eV. However, LaAlO3:Mg has a larger band gap Eg=3.89 eV and the Fermi level enters into the valence band, which indicates the holes are introduced. The calculated results show that the conductivity of LaAlO3:Mg is better than pure LaAlO3, which is in good agreement with experimental results.

X-ray truncation rod analysis of the reversible temperature-dependent [001] surface structure of LaAlO3

A reversible change in the [001] surface structure of ${\mathrm{LaAlO}}_{3}$ has been studied using x-ray truncation rod analysis. Crystal truncation rod profiles in the $00L,$ $10L,$ and $20L$ directions have been recorded on [001]-oriented single crystals at both room temperature and approximately 670 K under UHV conditions. Modeling of these data has enabled a detailed picture of the surface structure at both room and high temperature to be determined. At room temperature the surface is found to be terminated by aluminum, and a fairly minor structural rearrangement relative to the ideally terminated surface is found to occur, involving a relaxation of the surface oxygen atoms away from the surface and a small relaxation of the aluminum atoms into the surface. At the higher temperature a much more significant and reversible structural transformation occurs involving the top four atomic surface layers. This is primarily characterized by a substantial vertical movement of the top surface oxygen atoms and a concomitant relaxation of the aluminum atom into the surface, together with appreciable lateral movements of the oxygen atoms in the top two layers. The lanthanum and oxygen atoms in the third and fourth atomic layers also show clearly discernible movements.

A powder diffraction study of the phase transition in LaAlO3

The structure of LaAlO3 has been investigated around the phase transition at Tc ≈ 800 K by neutron powder diffraction in vacuum and by X-ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in frame of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room temperature structure is a rhombohedrally distorted perovskite structure, space group R-3c , which undergoes a transition to the ideal perovskite structure, space group Pm-3m , at high temperatures. The order parameter is a rotation of the O6-octahedron described by one x(O)-parameter. This parameter and the spontaneous strain (c/a-√6), as well as the relevant atomic displacement parameter Uop 11(O) in the order parameter system, show a critical behaviour in agreement with a second order phase transition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of the transition temperature: the metric becomes cubic roughly 30 K below the proper Tc. This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.

Transport and Magnetic Properties of the Perovskites La1-yMnO3 and LaMn1-zO3

Single-phase samples of La1-yMnO3-(3y/2)+δ with 0 ≤ y ≤ 0.09 and LaMn1-zO3-(3z/2)+δ with 0 ≤ z ≤ 0.09 have been prepared via the citrate method with subsequent heating at different temperatures and partial pressures of oxygen. Oxygen parameters 0 ≤ δ ≤ 0.18 were obtained. At room temperature, all compositions y and z have the orthorhombic structure of GdFeO3 (Pbnm) for small values of δ and the rhombohedral structure of LaAlO3 (R3c) for larger values of δ. Below a temperature Tc, all samples show a weak ferromagnetism at δ = 0 that increases with δ, samples with y = 0 approaching a full ferromagnetic alignment of all Mn moments in the range 0.10 < δ < 0.13. At smaller δ, the orthorhombic samples contain randomly distributed superparamagnetic clusters that couple via an antiferromagnetic matrix to form a spin glass below Tc. At higher values of δ, a sharp increase of Tc to a value greater than the Weiss temperature θp for the paramagnetic phase indicates a first-order transition on lowering T through Tc f...

Low‐Temperature Chemical Synthesis of Lanthanum Monoaluminate

One of the promising candidates for ferroelectric substrate materials, lanthanum monoaluminate (LaAlO3), was successfully synthesized by two separate chemical powder preparation techniques: (i) homogeneous precipitation from aqueous solutions containing urea (CH4N2O) in the presence of nitrate salts, and (ii) self‐propagating combustion synthesis from aqueous solutions containing CH4N2O and the respective nitrate salts of lanthanum and aluminum. The submicrometer, spherical‐like particles of the precursors were completely converted to pure LaAlO3 at 850C in the homogeneous precipitation route, and the same conversion temperature was observed to be 750C, which becomes the lowest temperature ever reported for the powder synthesis of a pure LaAlO3 phase. The materials were characterized by powder X‐ray diffractometry, simultaneous thermogravimetric/differential thermal analysis, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy. Structural refinements by Rietveld analysis showed that LaAlO3 was isostructural with BaTbO3 and had the space group R‐3c, in contrast to the previously assumed space group of R‐3m for this phase. The atomic positions in the structure of LaAlO3 were refined and presented for the first time, with respect to the present space group.

INVESTIGATION OF LaAlO3 CRYSTAL STRUCTURE

X-ray diffraction, neutron scattering and thermal analysis were used to determine the room temperature crystal structure of LaAlO3 and its phase transition. Two different models were calculated and compared. Tht space group of room temperature LaAlO3 is determined as R3C (a= 5.3648 ± 2?, c = 13.1113±3?). The influence of orientation of AlO6 octahedron or room temperature LaAlO3 crystal on the phase transition was studied.