The influence of defects on ferroelectric and pyroelectric properties of Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Relaxor-based ferroelectric single crystals 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-0.28PT) were grown by a modified Bridgman technique. The direct current (dc) conductivity was investigated and corresponding conduction mechanisms were discussed. V^″Pb−VO defects are dominant from 245 °C to 650 °C. The ferroelectric properties of [1 1 1]-oriented PMN-0.28PT were systematically investigated, with the coercive field (E_c) of 5.2 kV cm⁻¹ and remnant polarization (P_r) of 37.8 μC cm⁻² at room temperature. Moreover, the dielectric and pyroelectric performances of PMN-0.28PT were measured and the integrated pyroelectric performances greatly enhanced after annealing in oxygen at 500 °C for 20 h. This is due to the decrease of oxygen vacancies in the single crystals when being annealed in the oxygen-rich atmosphere. These make [1 1 1]-oriented PMN-0.28PT crystals a promising candidate for infrared detectors and thermal imagers used at room temperature.     

Enhanced reduction properties of mesostructured Ce0.5Zr0.5O2 solid solutions

Mesostructured Ce_0.5Zr_0.5O₂ solid solutions (M-CZ) were hydrothermally synthesized using Gemini surfactant as the template. X-ray diffraction (XRD), small-angle X-ray diffraction (SAXRD), N₂ adsorption–desorption isotherms and high-resolution transmission electronic microscopy (HRTEM) were adopted to characterize the samples. The product had a surface area of 123.5 m² g⁻¹ with maximum oxygen storage capacity (OSC) of 0.58 mol O₂/mol Ce. Oxygen anions in the M-CZ can be repeatedly released and resumed during the redox recycles. Reduction of Ce⁴⁺ to Ce³⁺ or lower valence and Zr⁴⁺ to Zr³⁺ were fulfilled with an obvious color change during the temperature programmed reduction (TPR) process, while the crystal structure of the product remained unchanged even after severe reduction. The mesostructure of the product can improve the reductive ability of Ce⁴⁺ and Zr⁴⁺ cations, which was beneficial to the enhancement of OSC.     

Comparative electrochemical treatments of two chlorinated aliphatic hydrocarbons. Time course of the main reaction by-products

Acidic aqueous solutions of the chlorinated aliphatic hydrocarbons 1,2-dichloroethane (DCA) and 1,1,2,2-tetrachloroethane (TCA) have been treated by the electro-Fenton (EF) process. Bulk electrolyses were performed at constant current using a BDD anode and an air diffusion cathode able to generate H₂O₂ in situ, which reacts with added Fe²⁺ to yield OH from Fenton's reaction. At 300 mA, almost total mineralization was achieved at 420 min for solutions containing 4 mM of either DCA or TCA. Comparative treatments without Fe²⁺ (anodic oxidation) or with a Pt anode led to a poorer mineralization. The better performance of the EF process with BDD is explained by the synergistic action of the oxidizing radicals, BDD(OH) at the anode surface and OH in the bulk, and the minimization of diffusional limitations. The decay of the initial pollutant accomplished with pseudo first-order kinetics. Chloroacetic and dichloroacetic acids were the major by-products during the degradation of DCA and TCA, respectively. Acetic, oxalic and formic acids were also identified. The proposed reaction pathways include oxidative and reductive (cathodic) dechlorination steps. Chlorine was released as Cl⁻, being further oxidized to ClO₃⁻ and, mostly, to ClO₄⁻, due to the action of the largely generated BDD(OH) and OH.     

Preparation and characterization of NaSm9(SiO4)6O2 powders with infrared absorptive properties

NaSm₉(SiO₄)₆O₂ powders were synthesized by mild hydrothermal method at 180 °C for 24 h. The infrared optical properties and structure of the obtained powders were characterized. There existed two narrow and sharp absorptive bands near 943 cm^− 1 (10.6 μm). The band at 938 cm^− 1 was assigned to the stretching vibrations of SiOSm groups connecting to Q¹ species and the band at 989 cm^− 1 was attributed to the stretching vibrations of SiOSm groups linking with Q⁰ species. The reflectivity was lower than 1% from 900 to 1200 nm and reached the minimum of 0.46% at 1073 nm. The prepared powders exhibit potential to act as a new kind of absorptive material for the infrared light of 10.6 μm and 1.06 μm.     

Bonding structure and optical properties of a-CNx:H films deposited in CH4-NH3 system

Hydrogenated amorphous carbon nitride (a-CN_x:H) films were deposited by plasma enhanced chemical vapor deposition (PECVD) in CH₄-NH₃ system. The chemical composition and bonding configuration were investigated by XPS and FTIR. The results indicated that both sp²CN and sp³CN bonds generally increased with the increase of the nitrogen concentration, and the N atoms bonded to C atoms through CN, CN and CN bonds. Remarkably, for FTIR spectra, two peaks (2125 and 2200 cm⁻¹) were obviously observed, corresponding to CN bond which was found to predominantly exist in the isonitrile structure. As more nitrogen atoms were incorporated, the optical band gap was found to vary from 1.8 to 2.5 eV. Finally, the conduction mechanisms were discussed at low and high temperature, respectively.     

CO and NH3 sensor properties and paramagnetic centers of nanocrystalline SnO2 modified by Pd and Ru

Nanocrystalline gas sensitive materials based on tin dioxide modified by Pd or Ru were synthesized via wet chemical route. The interaction of modified materials with CO and ammonia was studied by in situ DC-conductivity measurements and ex situ EPR spectroscopy. Modification by Pd yields the material highly sensitive to CO in low temperature region, while Ru-modified SnO₂ is very sensitive to NH₃ at raised temperature. We have detected that O₂⁻ and OH radicals are the main spin centers in unmodified nanocrystalline tin dioxide. The modification of tin dioxide by Pd and Ru is accompanied by formation of new spin centers in the samples: Pd^+ 3 and Ru^+ 3. The concentration of these paramagnetic species on the materials interacting with CO and ammonia gases decreased because of their transition to the diamagnetic state Pd^+ 2, Pd⁰ and Ru^+ 4, respectively.     

Synthesis and structural refinement of polycrystalline ceramic powder Pr0.1Ca0.9TiO3

Perovskite structure-based ceramic precursors have a characteristic property of substitution in the ‘A’ site of the ABO₃ structure. This makes them a potential material for nuclear waste management in synthetic rock (Synroc) technology. In order to simulate the mechanism of rare earth fixation in perovskite, Pr_xCa_1−xTiO₃ (where x = 0.1) has been synthesized through ceramic route by taking calculated quantities of oxides of Ca, Ti and Pr as starting materials. The ceramic phase has been characterized by its powder diffraction pattern. The Rietveld analysis of the X-ray diffraction data has been carried out using GSAS software to achieve the convergence which gives the R_p = 5.74% and R_wp = 8.17%. The (h, k, l) values for different lattice planes have been calculated. The praseodymium substituted perovskite crystallizes in orthorhombic symmetry with space group: Pbnm, Z = 4. The unit cell parameters at room temperature are a = 5.39609(31) Å, b = 5.44869(30) Å and c = 7.6565(5) Å. The calculated and observed values of the corresponding intensities, 2θ and density of the polycrystalline powder show good agreement. GSAS-based calculation for bond distances TiO, CaO and bond angles OTiO, OCaO has been reported.     

Raman spectroscopic study of structure of WO3La2O3B2O3 glasses with no color and crystallization of LaBWO6

Glasses with the nominal compositions of xWO₃25La₂O₃(75 − x)B₂O₃ (mol%) with x = 15, 25, and 50 were prepared using a conventional melt quenching method, and their structure and crystallization behavior were examined from Raman scattering spectra and X-ray diffraction analyses. The glasses are colorless in the visible light region and give the optical band gap energy of 3.49-3.61 eV. The glass transition and crystallization temperatures and the thermal stability against crystallization decrease with increasing WO₃ content. The strong Raman bands at 840 and 940-960 cm⁻¹ suggest that the main coordination state of W⁶⁺ ions in the glasses is isolated (WO₄)²⁻ tetrahedral units. The formation of WO₆ octahedral units is also suggested in the glasses with high WO₃ contents. The main crystallization mechanism in the glasses is the surface crystallization, and the glass of 50WO₃25La₂O₃25B₂O₃ shows the crystallization of LaBWO₆ single phase. The present study proposes that WO₃La₂O₃B₂O₃ glasses and crystallized glasses are very interesting as optical functional materials.     

(Sr6N)[CoN2][CN2]2: The first low-valency nitridometalate carbodiimide

The first nitridocobaltate carbodiimide (Sr₆N)[CoN₂][CN₂]₂ has been synthesized from the elements Sr, Co, and graphite powder and NaN₃ (as a nitrogen source). The crystal structure was determined from X-ray single-crystal diffraction data as orthorhombic (space group P2₁2₁2, No. 18, a=9.8807(6) Å, b=14.6474(9) Å, c=3.8569(3) Å, V=558.2 Å³, Z=2, R₁=0.0265, wR₂=0.0383). (Sr₆N)[CoN₂][CN₂]₂ is the first low-valency 3d-transition nitridometalate containing additional [CN₂]²⁻ groups. The crystal structure can be described as an array of rocksalt-like columns of Sr and N linked via common corners and connected by [NCN]²⁻ and [Co^IN₂]⁵⁻ units located within structural channels running along [0 0 1]. The magnetic susceptibility follows the Curie-Weiss law with an effective moment of 3.26 μ_B consistent with two unpaired spins (d⁸, Co^I). The compound is a bad metallic conductor with a resistivity of order 1 mΩ cm at 300 K. Vibrational spectroscopic data support the existence of carbodiimide [NCN]²⁻ species. The Co K-edge X-ray absorption spectra (XAS) of Ca₅[CoN₂]₂ and (Sr₆N)[CoN₂][CN₂]₂ confirm the presence of Co^I within the complex anions.