Dielectric properties of bismuth-containing pyrochlores:A comparative analysis

The comparative analysis of the dielectric properties of bismuth-containing pyrochlores with different manifestation of atomic order/disorder was carried out.We examined the dielectric properties(including behavior in electric fields)of two pyrochlore compounds:BZN(presumably a composition close to Bi_(1.5)Zn_(0.5)Nb_(1.5)O_(6.5))ceramics with chemical disorder in both A and B cation sublattices and Bi_(2)Ti_(2)O_(7)single crystal with fully chemical ordered structure.The fundamental differences between the dielectric properties of the BZN ceramics and Bi_(2)Ti_(2)O_(7)single crystal were shown.In particular,in the dielectric relaxation behavior(which cannot be described via Arrhenius law in the Bi_(2)Ti_(2)O_(7))or in the influence of the electric fields on the dielectric permittivity(split-ting of the field-cooled and zero-field-cooled behaviors was observed for Bi_(2)Ti_(2)O_(7)below estimated freezing temperature).The results of this study highlights the special role of Bi_(2)Ti_(2)O_(7)as a candidate material for studying aspects of geometric frustration related with pyrochlore structure in non-magnetic medium and specifies the future directions of research.

High-entropy pyrochlores with low thermal conductivity for thermal barrier coating materials

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides(La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores(5RE1/5)2 Zr2O7 have been formed after heated at 1000 ℃. The(5RE1/5)2Zr2O7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the(5RE1/5)2Zr2O7 high-entropy ceramics are below 1 W·m–1·K–1 in the temperature range of 300–1200 ℃. The(5RE1/5)2Zr2O7 can be potential thermal barrier coating materials.

Native point defects and oxygen migration of rare earth zirconate and stannate pyrochlores

Various excellent properties of rare earth zirconate and stannate pyrochlores are close related with their native point defects.First-principles calculations are performed to systematically investigate the point defect mechanism and the oxygen diffusion behavior of A_(2)B_(2)O_(7)(A=La,Ce,Pr,Nd,Pm,Sm,Eu,Gd;B=Zr,Sn).The possible defect complexes and their associated reactions under stoichiometric and nonstoichiometric conditions are explored.The O Frenkel pairs are the most stable defect structure in stoichiometric zirconates,whereas the cation antisite defects are the predominant one in stoichiometric stannates.In the case of BO_(2) excess zirconates and stannates,the BA cation antisite defect with the A vacancy and/or the oxygen interstitial is energetically favorable,whereas the ABantisite defect together with the oxygen vacancy and/or the A interstitial is preferable under the A_(2)O_(3) excess condition.Meanwhile,the maximum point defect concentrations of zirconates are much higher than those of stannates.Furthermore,the oxygen migration barriers are similar in these compounds,ranging in 0.68 eV–0.80 eV.The predicted point defects and oxygen diffusion mechanisms play the critical role in their engineering applications and are expected to guide the future property improvement of pyrochlores through the control of point defects and/or composition.

Synthesis of hierarchical mesoporous Ln_(2)Ti_(2)O_(7)(Ln=Y,Tb-Yb)pyrochlores and uranyl sorption properties

Mesoporous structured metal oxides exhibit many active applications.However,the synthesis of crystalline metal oxides with a ternary composition while maintaining satisfactory pore features is challeng-ing.Typically,high temperatures are required which inhibit control of pore structure properties including surface area,pore volume,and pore size.Herein,the synthesis of ternary metal oxides Ln_(2)Ti_(2)O_(7)pos-sessing pyrochlore crystal structure is achieved using a novel technique which combines‘soft’and hard colloid templating strategies.The formed materials are of submicron size and composed of∼25-30 nm product‘building blocks’with good chemical and phase stability.The polycrystalline powders have a high specific surface area(up to 70 m^(2)·g^(-1))and pore volume(∼0.35 cm^(3)·g^(-1))which result in a good adsorp-tion capacity(U uptake closing to 60 mg·g^(-1)).Remarkably,the material exhibits a significant portion of mesopores(mainly 10-40 nm)which facilitate fast adsorption of the cations due to high accessibility.The synthetic methodology described herein produces highly homogenous powders and can be applied to other compositions and structures.

Hydrothermal synthesis, characterization and optical properties of La_2Sn_2O_7:Eu^(3+) micro-octahedra

Pyrochlore structure La2Sn2O7：Eu3＋ microcrystals with uniform octahedron shape were successfully synthesized via a hydrothermal route at 180 °C for 36 h. The crystal structure, particle size, morphologies, and optical properties of the as-synthesized products were investigated by XRD, TEM, SEM, EDS, FT-IR, Raman spectroscopy and PL. The effects of pH of precursor solution, precursor concentration, reaction temperature, and time were investigated. The results reveal that pH of the precursor solution not only plays an important role in determining the phase of the as-synthesized products, but also has a significant influence on the morphologies of the samples. High-quality and uniform octahedrons with an average size of about 700 nm could be easily obtained at the pH value of 12. The possible formation mechanism of octahedral-like La2Sn2O7：Eu3＋ microcrystals was briefly proposed. The photoluminescence spectra show that La2Sn2O7：Eu3＋ micro-octahedra display stronger emission in the range of 582-592 nm compared with the samples with other shapes.

Synthesis, Characterization and Luminescent Properties of La2Zr2OT：Eu3＋ Nanorods

The Eu3＋-doped La2Zr207 phosphor with rod-like morphology was successfully synthesized by conventional solid state reaction and hydrothermal method. X-ray diffraction patterns, transmission electron microscopy, and photoluminescence spectra were employed to charac- terize its structure and morphology as well as luminescent properties. The results indicated that the red-emitting phosphor La2Zr207：Eu3＋ had well crystallized and belonged to the cubic structure with space group of Fd3m. The as-obtained product mainly appeared as straight nanorods with an average diameter of 47 nm and length of 50-700 nm. The pos- sible growth mechanism was also discussed. It was found that under blue excitation with a wavelength of 466 nm, the La2Zr2OT：Eu3＋ phosphor exhibited a characteristic red emission at 616 nm that was attributed to the hypersensitive 5D0--＊TF2 electric dipole transition of Eu3＋ ions. Meanwhile, it was more interesting to note that the emission of 5D1--＊TFj （J=0, 1, 2） transitions and the splitting patterns of 5D0---＋TFJ （J--l, 2, 4） transitions of Eu3＋ ions can be observed in the luminescent spectra of La2Zr207：Eu3＋. It was demonstrated that Eu3＋ preferred to occupy a low symmetry site.

First-principles calculation of structural and electronic properties of pyrochlore Lu_2Sn_2O_7

A Density functional theory method within generalized gradient approximation has been performed to obtain the static lattice parameters, oxygen positional parameter, bond length and bond angle and electronic properties of ideal Lu2Sn207 pyrochlore. The results are in excellent agreement with available experimental measurements. Density of states （DOS） of this compound was presented and analysed. We also notice the presence of the hybridization between oxygen and Lu metal. The band structure calculations show that the compound has direct band gap of 2.67 eV at the F point in the Brillouin zone and this indicates that the material has a semi-conducting feature.

First-principles study of disordering tendencies in Gd_2B_2O_7(B=Ti,Sn,Zr) compounds

This paper performs the density functional theory calculations to obtain some factors influencing the response of pyrochlores Gd2B207 （B = Ti, Sn, Zr） to ion irradiation-induced amorphization. The 48f oxygen position parameter x, cohesive energy, bond type and defect-formation energy are discussed. The results show that parameter x can be used to indicate the disordering tendencies within a given pyrochlore family. Bond type, cohesive energy and defect-formation energies can be used to explain some experimental observations, but they are not determined exclusively by radiation ＂resistance＂ for a different pyrochlore family.

Thermal conductivity of (Sm_(1-x)La_x)_2Zr_2O_7 (x=0,0.25,0.5,0.75 and 1) oxides for advanced thermal barrier coatings

Pyrochlore oxides of general compositions, A2Zr2O7, where A is a 3＋ cation （La to Lu）, are promising candidate materials for applications as high temperature thermal barrier coatings because of their high melting points, high thermal expansion coefficients, and low thermal conductivities. In this study, oxides of Sm2Zr2O7, （Smo.75La0.25）2Zr2O7, （Sm0.5 La0.5）2 ZreO7, （Sm0.25La0.75）eZr2O7 and La2Zr2O7 were prepared by solid reactions at 1600℃ for 10 h using Sm2O3, La2O3 and ZrO2 as the reactants. The phase compositions of these ceramic materials were analyzed by X-ray diffractometer （XRD） and fourier transform infrared spectroscopy （FT-IR） methods, respectively. The microstructure was observed by scanning electron microscope （SEM）. The thermal conductivities of these ceramic materials were measured using laser-flash method. XRD and FT-IR results showed that pure ceramic materials with pyrochlore structure were prepared successfully. SEM results indicated that microstructures of these ceramic materials were dense and grain boundaries were very clean. The La2O3 doped Sm2Zr2O7 pyrochlores （Sm0.75 La0.25）2Zr2O7 and （Sm0.5 La0.5）2 Zr2O7 had lower thermal conductivity than the undoped Sm2Zr2O7. The thermal conductivity of （Sm0.25La0.75）2Zr2O7 was found to be lower than that of La2Zr2O7. The results showed that these ceramic materials had the potential to be used as candidate materials for TBCs.

Effect of rare earth doping on thermo-physical properties of lanthanum zirconate ceramic for thermal barrier coatings

The effect of rare earth doping on thermo-physical properties of lanthanum zirconate was investigated. Oxide powders of various compositions La2Zr2O7 were synthesized by coprecipitation-calcination method. High-temperature dilatometer, DSC, and laser thermal diffusivity methods were used to analyze thermal expansion coefficient （TEC）, specific heat, and thermal diffusivity. The results showed that CeO2 doped pyrochlores La2（Zr1.8Ce0.2）2O7 and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had higher TEC than La2Zr2O7 and La1.7Dy0.3Zr2O7. La2（Zr1.8Ce0.2）2O7, La1.7Dy0.3Zr2O7, and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had lower thermal conductivity than undoped La2Zr2O7. The Dy2O3, Nd2O3, and CeO2 codoped composition showed the lowest thermal conductivity and the highest TEC. Thermo-physical results also indicated that TEC of rare earth oxide doped La2Zr2O7 ceramic was slightly higher than that of conventional ZrO2-8Wt.% Y2O3 （8YSZ）, and its thermal conductivity was lower than that of 8YSZ.

Hydroxycalciopyrochlore, A New Mineral Species from Sichuan, China

Hydroxycalciopyrochlore,ideally （Ca,Na,U,□）2（Nb,Ti）2O6（OH）,cubic,is a new mineral species （IMA2011-026） within the pyrochlore supergroup that was found occurring at the Maoniuping mine,Mianning County,Xichang prefecture,Sichuan Province,southwest China.The mineral is found in an alkali feldspar granite rare-earth ore deposit （26-27 Ma）.Associated minerals include calcite,barite,celestine,albite,aegirine,aegirine-augite,fluorite,parasite-（Ce）,thorite,thorianite,zircon,galena,sphalerite,magnetite,and pyrite.Crystals occur mostly as octahedra,and less often as dodecahedra and tetrahexahedra or combinations thereof.Some occur with an allotriomorphic habit with a thick triangular tabular form.Crystals generally range from 0.1 to 1 mm in size.The mineral is brownishblack,greenish-black and black on fresh sections with a brown streak.The crystal is translucent,and has a greasy lustre on fresh sections.It is metamict without any observed parting or cleavage and with a conchoidal fracture.The Vickers microhardness is 572 kg/mm2 （5-6 on the Mohs hardness scale）.The density measured by hydrostatic weighing is 5.10（3） g/cm3.The strongest four reflections in the X-ray powder-diffraction pattern [d in（A） （Ⅰ） hkl] are：2.9657 （100） 2 2 2,1.8142 （34） 0 4 4,1.5463 （21） 2 2 6,2.5688 （18） 0 0 4.The unit-cell parameters are a =10.381（4） （A）,V=1118.7（7）（A）3,Z =8.The structure was solved and refined in the space group Fd3m with R =0.09.The empirical formula is （Ca0.74Na0.5sU0.40Ce0.05Fe0.02□0.21）2.00（Nb1.15Ti0.s0Ta0.03Al0.01Mg0.01）2.00O6.02 [（OH）1.01F0.09]1.10,on the basis of 2 atoms of B pfu; the simplified formula is （Ca,Na,U,□）2（Nb,Ti）2O6（OH）.Type material is deposited in the Geological Museum of China,Beijing,People＇s Republic of China,catalogue number M11800.

Methane dry reforming on Ni/La_2Zr_2O_7 treated by plasma in different atmospheres

A series of Ni/La2Zr2O7 pyrochlore catalysts prepared by impregnation method and treated by dielectric barrier discharge（DBD） plasma in different atmospheres and varied sequences were prepared and applied for dry reforming of methane（DRM）. It is found that all of the plasma treated catalysts show evidently improved activity and coke resistance in comparison with the non-plasma treated one. The best performance is achieved on Ni/La2Zr2O7–H2P–C,a catalyst treated in H2 plasma before calcination. TGA-DSC and SEM demonstrate that carbon deposition is significantly suppressed on all of the plasma treated samples. Moreover,XRD and TEM results testify that both Ni O and Ni sizes on the calcined and reduced samples treated by plasma are also decreased,which results in higher Ni metal dispersion on the reduced and used catalysts and enhances the interactions between Ni sites and the support. It is believed that these are the inherent reasons accounting for the promotional effects of plasma treatment on the reaction performance of the Ni/La2Zr2O7 pyrochlore catalysts.

Hydrothermal Synthesis and Characterization of Pyrochlore Titanate R_2Ti_2O_7(R=Gd^(3+),Tb^(3+),Dy^(3+))

Pyrochlore titanate oxides, R2Ti2O7（R=Gd3＋, Tb3＋, Dy3＋）, were synthesized under mild hydrothermal conditions. The crystal growth of pyrochlore titanate oxides and taking place of chemical reaction in the hydrothermal processing were sensitive to the alkalinity, temperature, reaction time, the nature of the rare earth ion and the composition of initial reaction mixture. The as-prepared samples were characterized by powder X-ray diffraction, scanning electron microscopy, Raman spectrum and variable temperature dc magnetic susceptibility（Superconductivity quantum interference device, SQUIDS）. The magnetic studies gave 7.29×10-23 A·m2/Gd3＋ and -8.28 K, 8.75×10-23 A·m2/ Tb3＋ and -19.7 K, and 8.85×10-23 A·m2/Dy3＋ and 0.84 K effective moments and Weiss constants for Gd2Ti2O7, Tb2Ti2O7 and Dy2Ti2O7, respectively.

Dry reforming of methane on active and coke resistant Ni/Y_2Zr_2O_7 catalysts treated by dielectric barrier discharge plasma

In this study, Ni/YZrOcatalysts prepared with impregnation method and treated by dielectric barrier discharge plasma(DBD) in different atmospheres have been investigated for methane dry reforming. It is revealed by H-TPR that plasma treatment can enhance the interaction between Ni O/Ni particles and the YZrOpyrochlore support. Therefore, catalysts with smaller Ni O and Ni grains sizes, higher metallic Ni active surface areas can be achieved, as evidenced by XRD, TEM and Hadsorption-desorption measurements. As a consequence, the plasma-treated catalysts show significantly improved activity, stability and coke resistance, as testified by the TEM and TGA-DSC results. Plasma treatment in H/Ar gas mixture is found to be the best condition to prepare Ni/YZrO, which can be used to obtain a catalyst with the highest activity, stability and most potent coke resistance. It is believed that the smaller Ni grain size and higher metallic Ni active surface area induced by plasma treatment are the inherent reasons accounting for the promoted reaction performance of the Ni/YZrOpyrochlore catalysts.

Hydrothermal synthesis and energy storage performance of ultrafine Ce2Sn2O7 nanocubes

Ultrafine cube-shape Ce2Sn2O7 nanoparticles crystallized in pure pyrochlore phase with a size of about 10 nm have been successfully synthesized by a facile hydrothermal method.Conditional experiments have been conducted to optimize the processing parameters including temperature,pH,reaction duration,precipitator types to obtain phase-pure Ce2Sn2O7.The crystal structure,morphology and sizes and specific surface area have been characterized by X-ray diffractometer(XRD),Raman spectrum,transmission electron microscope(TEM),high resolution transmission electron microscope(HRTEM),and Brunauer-Emmett-Teller(BET).The as-synthesized Ce2Sn2O7 ultrafine nanocubes have been evaluated as electrode materials for pseudo-capacitors and lithium ion batteries.When testing as supercapacitors,a high specific capacitance of 222 F/g at 0.1 A/g and a good cycling stability with a capacitance retention of higher than 86%after 5000 cycle have been achieved.When targeted for anode material for lithium ion batteries,the nanocubes deliver a high specific reversible capacity of more than 900 mA·h/g at 0.05C rate.The rate capability and cycling performance is also very promising as compared with the traditional graphite anode.

The Complex Magnetism in the Breathing Pyrochlore LiIn(Cr_(1-x)Rh_x)_4O_8

We perform a detailed investigation of the new ＇breathing＇ pyrochlore compound LiInCr4O8 through Rh substi- tution with measurements of magnetic susceptibility, specific heat, and x-ray powder diffraction. The antiferro- magnetic phase of LiInCr4O8 is found to be slowly suppressed with increasing Rh, up to the critical concentration of x = 0.1 where the antiferromagnetic phase is still observed with the peak in specific heat Tp = 12.5 K, slightly lower than Tp =14.3 K for the x = 0 compound. From tile measurements of magnetization we also uncover evidence that substitution increases the amount of frustration. Comparisons are made with the LiGayIn1-yCr4O8 system as well as other frustrated pyrochlore-related materials and comparable amounts of frustration are found. The results of this work show that the engineered breathing pyrochlores present an important method to further understand the complex magnetism in frustrated systems.

Phase and Rietveld Refinement of Pyrochlore Gd_2Zr_2O_7 Used for Immobilization of Pu(Ⅳ)

The phase of pyrochlore Gd_2Zr_2O_7 used for immobilization of Pu （Ⅳ） was investigated, tetravalent cerium was used as the simulacrum for plutonium with tetravalence, and the compounds in the system Gd_2Zr_2-_xCe_xO_7 （0.0≤x≤2.0） were synthesized via a high temperature solid reaction method with Gd_2O_3 and ZrO_2 powders being used as the starting materials. Based on the collected XRD data of the gained samples, the phase and microstructural change of compounds were calculated by means of rietveld structural refinement method. The experimental results indicated that the phases of compounds were changed from pyrochlore to fluorite-type phase with the increasing x. The linear relation between a and x was discovered in the range of fluorite-type phase, which accorded with a = 0.52748 ＋ 0.00825 x （0.2≤x≤2.0）, while V= 0.14668 ＋ 0.00711 x （0.2≤x≤2.0） was also achieved.

CMAS corrosion resistance characteristics of RE_(50)Ta_(x)Zr_(50-x)O_(175+0.5x)thermal barrier oxides in RE_(2)Zr_(2)O_(7)-RETaO_(4)systems

The corrosion resistance characteristics of RE-rich RE_(50)Ta_(x)Zr_(50-x)O_(175+0.5x)oxides in RE_(2)Zr_(2)O_(7)-RETaO_(4)systems to calcium-magnesium-alumino-silicate(CMAS)at 1300°C,and the influence of RE^(3+)and Ta^(5+)on chemical reactions and reactive crystallization of CMAS melts were investigated.The results show that following the thermochemical reactions,apatite,pyrochlore,reprecipitated fluorite and residual Yb(Y)TaO4phases were the predominant reaction products.Formation abilities of apatite and pyrochlore were found to be proportional to the ionic radius of RE^(3+).The increase of Ta^(5+)amount can decrease the number of available RE^(3+)to form apatite.Moreover,the resistance characteristic to CMAS corrosion in RE_(50)Ta_(x)Zr_(50-x)O_(175+0.5x)systems was decided by the combined action of apatite and pyrochlore phases.The cohesive mixture of apatite and pyrochlore phases can generate a dense layer near the reaction front,which had a positive effect on suppressing CMAS infiltration.The ability of the fluorite+RETaO4two-phase field was determined to be sufficient to mitigate CMAS corrosion.

Phase and Structure in the System Gd_(2-x)Eu_xZr_2O_7 (0.0≤x≤2.0)

Pyrochlore Gd2Zr2O7 are considered as an excellent candidate for treatment of radioactive waste forms, because of its superior physical, chemical and anti-irradiation properties. To investigate the phase and structure of pyrochlore Gd2Zr2O7 used for immobilizing Pu （Ⅲ）, trivalent europium was used as the simulacrum for plutonium with trivalence. The compounds of stoichiometry Gd2-xEuxZr2O7（0.0≤x≤ 2.0） synthesized by high temperature solid state reaction method, were analyzed with the help of XRD and Rietveld structural refinement method. The results indicated that the phases of compounds continuously kept the phase of pyrochlore under our experimental condition. The linear relation between a and x was discovered in the system of Gd2-xEuxZr2O7（0.0≤x≤ 2.0） at 1 773 K, which accorded with a = 10.538 41 ＋ 0.008 95 x, V = 1 170.373 32 ＋ 2.985 97 x.

Transfer learning aided high-throughput computational design of oxygen evolution reaction catalysts in acid conditions

Sluggish oxygen evolution reaction(OER)in acid conditions is one of the bottlenecks that prevent the wide adoption of proton exchange membrane water electrolyzer for green hydrogen production.Despite recent advancements in developing high-performance catalysts for acid OER,the current electrocatalysts still rely on iridium-and ruthenium-based materials,urging continuous efforts to discover better performance catalysts as well as reduce the usage of noble metals.Pyrochlore structured oxide is a family of potential high-performance acid OER catalysts with a flexible compositional space to tune the electrochemical capabilities.However,exploring the large composition space of pyrochlore compounds demands an imperative approach to enable efficient screening.Here we present a highthroughput screening pipeline that integrates density functional theory calculations and a transfer learning approach to predict the critical properties of pyrochlore compounds.The high-throughput screening recommends three sets of candidates for potential acid OER applications,totaling 61 candidates from 6912 pyrochlore compounds.In addition to 3d-transition metals,p-block metals are identified as promising dopants to improve the catalytic activity of pyrochlore oxides.This work demonstrates not only an efficient approach for finding suitable pyrochlores towards acid OER but also suggests the great compositional flexibility of pyrochlore compounds to be considered as a new materials platform for a variety of applications.