Synthesis,photophysical properties,and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles

Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (perovskite, defect pyrochlore) nanoparticles prepared by a hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO₃·2H₂O) markedly affect the crystal structure of sodium tantalum oxide. At low NaOH concentration and high bismuth precursor (NaBiO₃·2H₂O) content, Bi doped Na₂Ta₂O₆ (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO₃ (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na₂Ta₂O₆) phase was formed and stabilized by the presence of dopant precursor (NaBiO₃·2H₂O). The chemical analysis of the samples confirmed the doping of Bi³⁺ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO₃ samples showed better performance for the photocatalytic degradation of rhodamine B than that of Bi doped Na₂Ta₂O₆, under visible light irritation (λ>420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO₃ based compounds, synthesized by the hydrothermal method.     

Influence of Cr3+ doping on photoelectronic processes of La2Ti2O7 pyrochlore and La23TiO3 perovskite anodes

Photoelectrical properties of La₂TiO₂O₇ and La_{$\text{2}\text{3}$}TiO₃ anodes undoped and Cr³⁺—doped have been investigated. The onset of photocurrent for both the undoped and Cr³⁺ doped pyrochlore is at 400 nm while for undoped perovskite it is at 420 n.m. A shift of the spectral photoresponse down to 550 n.m. is observed for the Cr-doped perovskite. The experimental data have been analyzed according to the Schottky barrier model of the semiconductor-electrolyte junction. This analysis has allowed us to propose a photoconduction mechanism.     

Phase evolution studies of sol�Cgel derived lead zirconate titanate (PZT) nanopowder using X-ray diffraction and X-ray photoelectron spectroscopy

We report here the formation of single phase lead zirconate titanate (PZT) nanopowder with composition Pb(Zr_0.52Ti_0.48)O₃ and average crystallite size 12?C20?nm, synthesized by sol?Cgel process. The phase evolution of PZT gel powder, heat treated at temperatures 550, 650 and 800°C was monitored by X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS). The high resolution XPS spectra of Pb4f, Zr3d, Ti2p and O1s show that PZT with pure perovskite structure is obtained at 800°C while at lower temperatures pyrochlore phase co-exists with perovskite phase. The XRD results also support this analysis. We have also identified the pyrochlore phase using XPS by analyzing the corresponding variations in the FWHM values, peak positions and the separation between the spin doublets of Pb, Zr and Ti associated with it. The composition of the final powder obtained with pure perovskite structure is calculated and is close to the designed value.     

Excitation and Emission Spectra of Cs2NaLnCl6 Crystals Using Synchrotron Radiation

ABSTRACT

The visible emission and vacuum ultraviolet excitation spectra of the series Cs₂NaLnCl₆ (Ln = Y, Nd, Sm, Eu, Tb, Er, Yb) and Cs₂NaYCl₆:Ln³⁺ (Ln = Sm, Er) have been recorded using synchrotron radiation at room temperature, and in some cases at 10 K. The excitation spectra comprise features associated with charge transfer, excitation from the valence to conduction band, and impurity bands. No d–f emissions were observed for these Ln³⁺ ions, so that the emission bands comprise intraconfigurational 4f ^N –4f ^N transitions and impurity bands, whose natures are discussed. Theoretical simulations of the f–d absorption spectra have been included. The comparison with data from the synchrotron at Desy enables a comprehensive account of the ground (or vibrationally excited ground for Ln²⁺) states of the Ln³⁺ 4f ^N , Ln³⁺ 4f ^N?15d, and Ln²⁺ 4f ^N+1 configurations relative to the valence and conduction bands of Cs₂NaLnCl₆, for which the band gaps are between 6.6 and 8.1 eV.     

Charge transfer transitions in the transition metal oxides ABO4:Ln and APO4:ln (A=La, Gd, Y, Lu, Sc; B=V, Nb, Ta; Ln=lanthanide)

We have compiled and analyzed optical and structural properties of lanthanide doped non-metal oxides of the form APO₄:Ln³⁺ with A a rare earth and of transition metal oxides with formula ABO₄:Ln³⁺ with B a transition metal. The main objective is to understand better the interrelationships between the band gap energy, the O²⁻→Ln³⁺ charge transfer energy, and the Ln³⁺→B⁵⁺ inter-valence charge transfer energy. Various models exist for each of these three types of electron transitions in inorganic compounds that appear highly related to each other. When properly interpreted, these optically excited transitions provide the locations of the lanthanide electron donating and electron accepting states relative to the conduction band and the valence band of the hosting compound. These locations in turn determine the luminescent properties and charge carrier trapping properties of that host. Hence, understanding the relationship between the different types of charge transfer processes and its implication for lanthanide level location in the band gap is of technological interest.     

Effect of LaNiO3 buffer layers on the structure and electrical properties of sol–gel-derived Pb(Mg1/3Nb2/3)O3PbTiO3 thin films

Pb(Mg_1/3Nb_2/3)O₃PbTiO₃ (PMNT) thin films on Pt/TiO₂/SiO₂/Si substrates with and without a LaNiO₃ (LNO) buffer layer have been prepared using a sol–gel method. Structures and electrical properties of these two films have been investigated and compared. Highly (111)-oriented PMNT thin films with a certain amount of pyrochlore phase are obtained on bare Pt electrodes. On the contrary, (100)-oriented PMNT thin films with pure perovskite phase are formed on Pt electrodes with a LNO buffer layer. Cracks are found in the former but not in the latter. The dielectric constant of PMNT thin films on LNO-buffer Pt electrodes is larger than that on bare Pt electrodes. A great lowering of the leakage current is observed in the films with a LNO buffer layer. The improvement in the electrical properties is attributed to both the elimination of cracks and the suppression of pyrochlore phase in the films. PACS 77.84.Dy; 77.80.-e; 77.22.Gm     

Effect of buffer layers on the property of Pb(Zr,Ti)O3-Pb(Mn,W,Sb,Nb)O3 thin films grown by pulsed laser deposition

New ferroelectric Pb(Zr,Ti)O₃-Pb(Mn,W,Sb,Nb)O₃ (PZT-PMWSN) thin film has been deposited on a Pt/Ti/SiO₂/Si substrate by pulsed laser deposition. Buffer layer was adopted between film and substrate to improve the ferroelectric properties of PZT-PMWSN films. Effect of a Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) buffer layers on the stabilization of perovskite phase and the suppression of pyrochlore phase has been examined. Role of buffer layers was investigated depending on different types of buffer layer and thickness. The PZT-PMWSN thin films with buffer layer have higher remnant polarization and switching polarization values by suppressing pyrochlore phase formation. The remnant polarization, saturation polarization, coercive field and relative dielectric constant of 10-nm-thick PLT buffered PZT-PMWSN thin film with no pyrochlore phase were observed to be about 18.523 μC/cm², 47.538 μC/cm², 63.901 kV/cm and 854, respectively.     

Excimer laser-induced transformation in laser ablated Pb(Zr0.52Ti0.48)O3 amorphous thin films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) amorphous thin films were deposited on Si substrates at room temperature and 573?K by pulsed laser deposition. The as-deposited films were subsequently annealed at various laser power densities using a KrF pulsed excimer laser irradiation to induce the phase transformation from amorphous to ferroelectric perovskite structure. Structural analysis shows the possibility of transformation from pyrochlore to perovskite transformation when irradiated above a laser power density of 1.4?MW/cm², which is in agreement with the thermal simulation. The surface quality of the PZT films deposited on 573?K is remarkably superior to that deposited at room temperature due to the enhanced thin structure and composition homogeneity. Almost all the pyrochlore phase transformed into perovskite structure after annealing at 2.8?MW/cm² for 120?s for both PZT films deposited at room temperature and 573?K, respectively. P-E hysteresis measurement of the laser-treated PZT shows relatively low remnant polarization P _r of about 1.2?μC/cm².     

Preparation and dielectric properties of the KBiScNbO6 double perovskite

Ceramic KBiScNbO₆ compound was synthesized for the first time. X‐ray diffraction analysis revealed that at room temperature the samples consist of a mixture of the perovskite and pyrochlore phases. The dominant phase is the perovskite one. This phase can be described as distorted pseudocubic Pm3m structure with unit‐cell parameter a_c = 4.069 Å. Rhombohedral distortion leading the polar R3m phase is assumed to be one of possible structural distortions. Dielectric measurements of the compound under study demonstrated anomalies associated with relaxor ferroelectric behavior (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of lithium ion-conducting ceramics based on rare-earth titanates

The properties of complex oxides with perovskite structure of the Ln_2/3−XM_3XTiO₃ type (where Ln = rare-earth element and M = Li, Na, K) have been investigated. It has been shown that lithium - containing perovskites possess cationic conductivity, with values depending on the size of conducting channels and the concentration of vacant sites in the rare-earth ion sublattice. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Characterization of CaTi0.9Fe0.1O3/La0.98Mg0.02NbO4 composite

A composite of CaTi_0.9Fe_0.1O₃ and electrolyte material, i.e. magnesium doped La_0.98Mg_0.02NbO₄ was prepared and studied. The phase content and the sample microstructure was examined by an X-ray diffraction method and scanning electron microscopy. EDS measurements were done both for composite samples and the diffusion couple. The electrical properties were studied by four terminal DC method. The high-temperature interaction between the two components of the composite has been observed. It has been suggested that lanthanum diffused into the perovskite phase and substituted for calcium whereas calcium and niobium formed the Ca₂Nb₂O₇ pyrochlore phase. At 1500°C very large crystallites of the pyrochlore were observed. Regardless of strong interaction between the composite components, its total conductivity was weakly dependent on the sintering temperature.     

KTa0.4Nb0.6O3 nanoparticles synthesized through solvothermal method

Potassium tantalate niobate (KTa_0.4Nb_0.6O₃, KTN) nanoparticles of perovskite structure were successsfully synthesized by a solvothermal method. The KTN nanoparticles synthesized at 250 °C for 8 h with 1 to 4 M KOH concentration using isopropyl alcohol [(CH₃)₂ CHOH] as the solvent was composed of a single phase of cubic perovskite structure. Futhermore, the KTN powers synthesized at the same conditions besides of using (CH₃)₂CHOH/H₂O as a solvent compose of a single phase of tetragonal perovskite structure. The nanoparticles exhibit a mixture of cubic and prism-like shapes with lengths of 100 nm to 500 nm and average cross sections of 200×200 nm². The solvent dependence of the powder formation is discussed. X-ray diffraction and electron diffraction results show that the powders have the needed tetragonal perovskite structure. The band gap of KTN nanoparticles is determined to be 3.26 eV from the optical absorption spectra.      

Luminescence of europium and ytterbium ions in strontium hexaborate

We have studied the luminescent properties of Eu^2+/3+ and Yb²⁺ ions in strontium hexaborate SrB₆O₁₀ for excitation in the 120–400 nm region. The luminescence spectra of Ln²⁺ ions in SrB₆O₁₀ consist of overlapping bands in the 370–520 nm region, due to 5d → 4f transitions at several nonequivalent centers. In the excitation spectra, besides the bands associated with 4f → 5d transitions in the Ln²⁺ ions, we also observe a band in the 135–160 nm region due to the transitions O(2p) → B(2s,2p) within the borate anions. The luminescence of the Eu³⁺ ions is excited most efficiently in the region of the Eu³⁺ charge transfer band (λ_max = 226 nm). The results obtained are compared with data for Ln in other strontium borates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 770–774, November–December, 2006.     

Electronic and magnetic structure studies of double perovskite Sr2CrReO6 by first-principles calculations

First-principles calculations have been performed to study the electronic band structure and ferromagnetic properties of the double perovskite Sr₂CrReO₆. The density of states (DOS), the total energy, and the spin magnetic moment were calculated. The calculations reveal that the Sr₂CrReO₆ has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 1.0 μ_B, in good agreement with the experimental value. By analysis of the band structure, we propose that the ordered double perovskite Sr₂CrReO₆ is a strong candidate for half-metallic ferromagnet.     

The influence of localized states on the optical absorption and carrier transport properties of acylamino hybrid perovskites with tunable electronic structures

Organic-inorganic hybrid perovskite solar cells have excellent optoelectronic properties, but their low thermal and chemical stabilities limit their commercial applications. In this paper, a new type of organic-inorganic hybrid perovskite is proposed. Malondiamide (MA,CH₂(CONH₂)₂) and propionamide (PA, CH₃CH₂CONH₂) were used as organic layers, with Pb-I octahedral inorganic layers to form quasi three-dimensional (3D) perovskites. The crystal structure, stability, electronic structure, and optical properties of MAPbI₄ and PAPbI₄ perovskites were investigated, and the results showed that there were localized states that corresponded to the number of acyl groups in the two perovskites. Energy band calculations showed that the localized states of the two perovskites rose above the bottom of the conduction band. This can be used to regulate the band gap of the two perovskites, which affects the electronic properties and optical absorption characteristics of the two perovskites. Compared with PAPbI₄, MAPbI₄ has a lower formation energy, lower band gap, lower effective mass of electrons and holes, wider energy range, and larger absorption coefficient, which indicates that MAPbI₄ is more suitable for use in solar cells. This study provides guidance for obtaining efficient and stable photovoltaic materials.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.     

Studies of Fe–Co based perovskite cathodes with different A-site cations

Iron–cobalt based perovskite cathodes with different A-site cations ((Ln_0.6Sr_0.4)_0.99Fe_0.8Co_0.2O_3−δ, where Ln is La, Pr, Sm or Gd) have been synthesised, characterised by a powder XRD, dilatometry, 4-point DC conductivity measurements, and electrochemical impedance spectroscopy (EIS) on cone shaped electrodes. In addition to this scanning electron microscopy (SEM) was used to characterise the bars. XRD revealed that only the La-containing perovskite was hexagonal. The Pr and Sm perovskites were orthorhombic. The gadolinium-based perovskite was a two phase system consisting of an orthorhombic and a cubic perovskite phase. The thermal expansion coefficient (TEC) increased systematically with a decrease in the size of the A-site cation until the gadolinium-containing perovskite where the TEC decreases abruptly. The total electric conductivity was the highest for the La-based perovskite and the lowest for the Gd-based perovskite as determined by 4-point DC conductivity measurements on bars. A clear correlation between the size of the A-site cation and the electrochemical performance was revealed, as the area specific resistance (ASR) was the lowest for the compounds with the smallest A-site cation. This might be explained on the background of the creation of a two-phase structure with a unique microstructure when the size of the A-site cation is lowered, or that one of the phases has a high electro-catalytic activity towards the electrochemical reduction of oxygen.     

The ac conductivity and complex impedance of CuO-doped (Ba0.5Sr0.5)TiO3 ceramic

The AC conductivity and complex impedance spectroscopy of CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic were investigated. X-ray diffraction analysis showed that CuO-doped (Ba_0.5Sr_0.5)TiO₃ has a perovskite structure without any pyrochlore phase. Frequency dependent dielectric permittivity was discussed at a different temperature range. The activation energy was calculated and discussed through the Arrehnius equation from the ac conductivity with different frequency plots. CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramics have a negative temperature coefficient of resistivity. Dependence of impedance spectroscopy on frequency and temperature showed that the conduction process in the CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic follows the thermally activated conduction mechanism.     

Pulsed laser deposition of Pb(Zr0.52Ti0.48)O3 thin film on cobalt ferrite nano-seed layered Pt(111)/Si substrate: effect of oxygen pressure

The effect of oxygen pressure during pulsed laser deposition of Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films on CoFe₂O₄ nano-seed layered Pt(111)/Si substrate was investigated. The PZT film deposited at oxygen pressure lower than 25 mTorr is identified as both perovskite and pyrochlore phases and the films deposited at high oxygen pressure (50-100 mTorr) show the single-phase perovskite PZT that has a perfect (111)-orientation. In addition, the film deposited at P_O2 of 50 mTorr has a uniform surface morphology, whereas the film deposited at P_O2 of 100 mTorr has a non-uniform surface morphology and more incompacted columnar cross-section microstructure. The polarization of film deposited at 100 mTorr is higher than that deposited at 50 mTorr, but shift of the hysteresis loop along the electrical field axis in the film deposited at P_O2 of 100 mTorr is larger than that of the film deposited at P_O2 of 50 mTorr.     

Preparation,characterization, and conductivity of YLnTiZrO7 (Ln = La,Nd, Sm,and Eu)

The pyrochlore oxide of composition YLnTiZrO₇ (Ln?=?La, Nd, Sm, and Eu) was prepared by sol–gel method. All the samples were characterized by powder X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffused reflectance spectroscopy, and impedance spectroscopy. The powder XRD and Raman studies reveal that these samples were crystallized in cubic lattice with pyrochlore structure. The Rietveld analysis of the samples was carried out to obtain the unit cell parameters and reliability factors. The broad Raman bands observed for all the samples are due to cation/anion disorder in the lattice and nanosize. The XPS analysis of the samples shows the characteristic peaks belonging to Y³⁺, Ln³⁺ (Ln?=?La, Nd, Sm, and Eu), Ti⁴⁺, and Zr⁴⁺. Electrical conductivity of YLaTiZrO₇ (YLTZ) and YEuTiZrO₇ (YETZ) samples was calculated from the impedance as a function of frequency and temperature. These samples have shown conductivity of the order of 10^?5 scm^?1 at 500 °C.