Structural chemistry and physicochemical properties of perovskite-like materials

New silicon substituted BiMeVO x : synthesis and study of structural properties in relation to ionic conductivity

Partial substitution of vanadium with silicon in the compound Bi₄V₂O₁₁, which belongs to the Aurivillius family, leads to the creation of a solid solution Bi₄V_2-x Si _x O_11-δ (0 ≤ x ≤ 0.4). The compound with x = 0.1 turns out to be a monoclinic α-form of Bi₄V₂O₁₁, while the compounds with x = 0.2 and x = 0.3 are orthorhombic β-polymorphs, and the compound with x = 0.35 is of tetrahedral γ-polymorph. Electrochemical Impedance Spectroscopy has been used to measure the ionic conductivity of doped samples. The ceramic sample with x = 0.1 has the highest ionic conductivity values.

Down-conversion luminescence and temperature sensing characteristics of LaSrGaO4 :Er3

Erbium(III) ion (Er³⁺ ) has abundant energy levels that can emit light covering a quite broad wavelength range in many hosts. Here we synthesized LaSrGaO₄ :Er³⁺ phosphors by a high-temperature solid-state method. Upon excitation at the ultraviolet (UV) band, LaSrGaO₄ :Er³⁺ phosphors could emit green, red and near-infrared emission simultaneously. The temperature dependent emission characteristics of the as-prepared samples was then studied and two kinds of luminescent ratiometric thermometry were constructed. The first one is on the basis of two green emission bands that stems from the ² H_11/2  → ⁴ I_15/2 and ⁴ S_3/2  → ⁴ I_15/2 transitions of Er³⁺ . The intensity ratio between these two emission bands was found to follow well with the Boltzmann distribution, and its maximum relative sensitivity was calculated to be 0.84% K^-1 at 299 K. The other one depends on the ⁴ F_9/2  → ⁴ I_15/2 transition of Er³⁺ and self-luminescence of the host LaSrGaO₄ , considering that these two emission lines have different temperature response. The relative sensitivity of this type of luminescence intensity ratio (LIR) thermometry could reach 1.86% K^-1 at 299 K, we have successfully developed materials with one of the largest relative sensitivities to date, which provides some basis for the subsequent development of a new type of non-contact temperature sensor.

Self-Luminescence of Perovskite-Like LaSrGaO4 via Intrinsic Defects and Anomalous Luminescence Analysis of LaSrGaO4:Mn2

A series of deep red phosphors with perovskite-like oxide LaSrGaO₄ as host are synthesized by a high temperature solid state method, and the luminescence properties and mechanisms have been investigated in detail. LaSrGaO₄ presents self-luminescence at 722 nm, and it is proved that the self-luminescence comes from two kinds of electronic defects and three kinds of vacancy defects, which are anti-occupation defects La _Sr^•, strontium gap defects Sr _i^••, the oxygen interstitial defects O _i^″, substitution defects Sr _La^', and strontium vacancy defects V _Sr^″. In addition, when Mn²⁺ ions are doped in LaSrGaO₄, interestingly, the shape of the emission spectra of LaSrGaO₄:Mn²⁺ is the same as that of LaSrGaO₄, and the emission intensities are enhanced greatly. Luminescence of Mn²⁺ ions has been confirmed by doping Mg²⁺ into LaSrGaO₄ and measuring the lifetimes of host LaSrGaO₄, LaSrGaO₄:Mg²⁺, and LaSrGaO₄:Mn²⁺ for comparison. The mechanisms of host self-luminescence and Mn²⁺ luminescence are discussed by detecting the luminescence centers with the low temperature spectra, calculating the forbidden bandwidth with the diffuse reflectance spectra, and calculating the trap depths with the thermoluminescence spectra and further depicted by establishing the transition model. LaSrGaO₄:Mn²⁺ can emit strong deep red light about 722 nm, so the phosphor will have a good application prospect in the field of plant lighting.

Fe–Zr–O catalyzed base-free aerobic oxidation of 5-HMF to 2,5-FDCA as a bio-based polyester monomer

An environment-friendly and economical route for 5-hydroxymethylfurfural (HMF) aerobic oxidation to 2,5-furandicarboxylic acid (FDCA) in an ionic liquid (IL)-promoted base-free reaction system was reported using Fe–Zr–O as a catalyst.

Perovskite-type catalytic materials for environmental applications

Perovskites are mixed-metal oxides that are attracting much scientific and application interest owing to their low price, adaptability, and thermal stability, which often depend on bulk and surface characteristics. These materials have been extensively explored for their catalytic, electrical, magnetic, and optical properties. They are promising candidates for the photocatalytic splitting of water and have also been extensively studied for environmental catalysis applications. Oxygen and cation non-stoichiometry can be tailored in a large number of perovskite compositions to achieve the desired catalytic activity, including multifunctional catalytic properties. Despite the extensive uses, the commercial success for this class of perovskite-based catalytic materials has not been achieved for vehicle exhaust emission control or for many other environmental applications. With recent advances in synthesis techniques, including the preparation of supported perovskites, and increasing understanding of promoted substitute perovskite-type materials, there is a growing interest in applied studies of perovskite-type catalytic materials. We have studied a number of perovskites based on Co, Mn, Ru, and Fe and their substituted compositions for their catalytic activity in terms of diesel soot oxidation, three-way catalysis, N2O decomposition, low-temperature CO oxidation, oxidation of volatile organic compounds, etc. The enhanced catalytic activity of these materials is attributed mainly to their altered redox properties, the promotional effect of co-ions, and the increased exposure of catalytically active transition metals in certain preparations. The recent lowering of sulfur content in fuel and concerns over the cost and availability of precious metals are responsible for renewed interest in perovskite-type catalysts for environmental applications.

Sol-gel preparation and characterization of non-substituted and Sr-substituted lanthanum cobaltates

This paper reports on the results concerning the sol-gel preparation and characterization of Sr-substituted perovskite lanthanum cobaltates La1−xSrxCoO3−δ (x = 0.0, 0.25, 0.5 and 0.75). The metal ions, generated by dissolving starting materials in diluted acetic acid were complexed by 1,2-ethanediol to obtain the precursors for the non-substituted and Sr-substituted LaCoO3. The influence of the synthesis temperature, heating time and the amount of substituent on the phase purity of La1−xSrxCoO3−δ were investigated. The phase transformations, composition and micro-structural features in the gels and polycrystalline samples were studied by thermal analysis (TG/DTA), infrared spectroscopy (IR), powder X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM).

Classification of perovskites with supervised self-organizing maps

In this work supervised self-organizing maps were used for structural classification of perovskites. For this purpose, structural data for total number of 286 perovskites, belonging to ABO3 and/or A2BB'O6 types, were collected from literature: 130 of these are cubic, 85 orthorhombic and 71 monoclinic. For classification purposes, the effective ionic radii of the cations, electronegativities of the cations in B-position, as well as, the oxidation states of these cations, were used as input variables. The parameters of the developed models, as well as, the most suitable variables for classification purposes were selected using genetic algorithms. Two-third of all the compounds were used in the training phase. During the optimization process the performances of the models were checked using cross-validation leave-1/10-out. The performances of obtained solutions were checked using the test set composed of the remaining one-third of the compounds. The obtained models for classification of these three classes of perovskite compounds show very good results. Namely, the classification of the compounds in the test set resulted in small number of discrepancies (4.2-6.4%) between the actual crystallographic class and the one predicted by the models. All these results are strong arguments for the validity of supervised self-organizing maps for performing such types of classification. Therefore, the proposed procedure could be successfully used for crystallographic classification of perovskites in one of these three classes.