Formation Of Pyrochlore Phase Research Articles

Synthesis OF Bi<sub>1.5</sub>CoSb<sub>1.5</sub>O<sub>7</sub> pyrochlore under hydrothermal conditions and its catalytic properties in the CO oxidation

The pyrochlore solid solution region in the Bi2O3–CoO–Sb2O5 system is determined. A previously unknown ternary oxide Bi3Co2/3Sb7/3O11 with cubic KSbO3 structure (sp. gr. Pn-3, a = 9.5801(1) Å, wR =0.0132) is found. An isothermal section of the system is constructed in the region of CoO-Bi3SbO7-CoSb2O6-BiSbO4 at 650°C. It is shown that cobalt state in pyrochlore crystal lattice is Co2+. For pyrochlore composition Bi1.5CoSb1.5O7, the synthetic methods under hydrothermal conditions of both without microwave-assisted treatment and with it have been developed. Based on the data of X-ray diffraction analysis, local energy-dispersive X-ray analysis, scanning microscopy and IR spectroscopy, a mechanism for the pyrochlore phase formation under hydrothermal condition is proposed. The dispersed Bi1.5CoSb1.5O7 samples (SCD = 30 nm) are synthesized and the prospects of their use as catalysts for CO oxidation are shown.

Stabilizing Perovskite Pb(Mg0.33Nb0.67)O3-PbTiO3 Thin Films by Fast Deposition and Tensile Mismatched Growth Template.

Because of its low hysteresis, high dielectric constant, and strong piezoelectric response, Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) thin films have attracted considerable attention for the application in PiezoMEMS, field-effect transistors, and energy harvesting and storage devices. However, it remains a great challenge to fabricate phase-pure, pyrochlore-free PMN-PT thin films. In this study, we demonstrate that a high deposition rate, combined with a tensile mismatched template layer can stabilize the perovskite phase of PMN-PT films and prevent the nucleation of passive pyrochlore phases. We observed that an accelerated deposition rate promoted mixing of the B-site cation and facilitated relaxation of the compressively strained PMN-PT on the SrTiO3 (STO) substrate in the initial growth layer, which apparently suppressed the initial formation of pyrochlore phases. By employing La-doped-BaSnO3 (LBSO) as the tensile mismatched buffer layer, 750 nm thick phase-pure perovskite PMN-PT films were synthesized. The resulting PMN-PT films exhibited excellent crystalline quality close to that of the STO substrate.

Achieving high energy storage performance in Na0.5Bi0.5TiO3‒based ceramics via Li+ substitution

In the search for dielectric material with high energy storage density, a ceramic system of 0.8Na0.5Bi0.5TiO3–0.2K1-xLixTaO3 (NBT-KLT-x, x = 0.1–0.5) is proposed in this work. The addition of Li2CO3 is conducive to the decrease of sintering temperature of ceramics. Moreover, the XRD diffraction data of sintered ceramics indicate that pure perovskite phase showing the pseudo-cubic structure can be achieved when x = 0.1–0.3 while the excessive addition of Li2CO3 leads to the formation of pyrochlore phase. Based on the Rietveld refinement and Raman results, the Li+ doping in the NBT-KLT-x solid solutions (x = 0.1–0.3) can improve the phase content of P4bm, induce the lattice expansion and structural change towards a higher-symmetry phase. TEM observation also demonstrates the reduction in the size of polar nanoregions. With increasing x from 0.1 to 0.5, both the oxygen vacancy concentration and grain size display a tendency to decrease first and then increase. For the x = 0.2 ceramics with the lowest oxygen vacancy concentration and grain size, the largest electrical breakdown strength and lowest polarization hysteresis are achieved, giving an excellent recoverable energy storage density of 4.86 J/cm3 under 32 kV/mm, accompanied by a high efficiency of 90.46 %. Additionally, the recoverable energy density achieves a high level of ∼3.5 J/cm3 and energy storage efficiency is larger than 80 % over 28–160 °C, suggesting the outstanding temperature stability.

A comparative study on pyrochlore phase formation in La2Zr2O7 in microscopic and macroscopic scale

A comparative study on pyrochlore phase formation in La2Zr2O7 in microscopic and macroscopic scale

Features of the Phase Formation of Cr/Mn/Fe/Co/Ni/Cu Codoped Bismuth Niobate Pyrochlore

The phase formation process of Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Nb2O9+Δ containing 3d-ions of transition elements in equimolar quantities was studied in a wide temperature range (400–1050 °C). The complex oxide crystallizes in the structural type of pyrochlore (sp. gr. Fd-3m:2, a = 10.4937(2) Å). The investigation of the multi-element pyrochlore phase formation process showed that the synthesis goes through a series of successive stages, during which the transition from Bi-rich to Bi-depleted compounds takes place. The predecessor of the pyrochlore phase is bismuth orthorhombic modification orthoniobate (α-BiNbO4) with an equimolar ratio of Bi(III)/Nb(V) ions. The pyrochlore phase is formed as a result of bismuth orthoniobate doping with transition element ions. The complex oxides Bi14CrO24, Bi25FeO40, BiNbO4, and Bi5Nb3O15 appeared as intermediate phases during the synthesis. The interaction between the initial oxide precursors is fixed at temperatures above 500 °C. The phase transition of α-Bi2O3 into β-Bi2O3 near 500 °C is observed. Varying the heat treatment duration at each synthesis step did not qualitatively change the phase composition of the sample but had an effect on the quantitative phase ratio. Phase-pure pyrochlore of the given composition by solid-phase synthesis method can be obtained at a temperature no lower than 1050 °C. Ceramics are characterized by low-porous dense microstructure with blurred outlines of grain boundaries.

Low-, medium-, and high-entropy pyrochlores in (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions, their optical and dielectric properties

The pyrochlore phase formation in the (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions with different element fractions in the A sites of the A2B2O6O' structure was studied. The low- and medium-entropy pyrochlores were formed after calcination of the batch at 1000–1150 °C. The high-entropy pyrochlore composition was detected during the investigations. Its stability by ab initio calculation was proven. The main impurities (Eu0.55La0.35Bi0.04)NbO4-δ and Na0.65MxNbO3-δ were identified in the initial medium- and high-entropy compositions as more stable phases. The samples belong to wide band gap semiconductors (Egdir = 3.2–3.6 eV, Egindir = 2.9–3.0 eV). The dielectric parameters calculated from impedance spectra depend on polarizability created by different types of cations in the A2O' sublattice. The dielectric permittivity (ε' = 67–162, at 25 °C and 1 MHz) decreases with decreasing polarizability of the system, while the tangent loss remains equally low (tanδ = 0.002–0.003 up to 240 °C). The presence of impurity phases in the initial high-entropy pyrochlores adversely affected the dielectric properties. Two mechanisms of conductivity were distinguished.

Design and synthesis of high-entropy pyrochlore ceramics based on valence combination

In this paper, we report the design and synthesis of La3+ based high-entropy pyrochlore ceramics (HEPCs) with five different B-site cations using the valence combination strategy. The factors affecting the formation of pyrochlore phase have been investigated. The result reveals that both valence disorder and size disorder can significantly affect the formability of pyrochlore phase, and the formability increases as the two disorders decrease. Following this guideline, a set of new HEPCs with single phase have been successfully synthesized.

Evaluation of mobility range of charge carriers in Nd-substituted

Here we report tuning of structural parameters of La2Sn2O7 by Nd substitution at La-site in a quest to effectively control the dielectric response as a small variation in lattice parameters can result in large dielectric fluctuations. For this purpose, a series of Nd-substituted La2Sn2O7 was synthesized using sol ̶ gel auto-combustion route. The formation of stable cubic pyrochlore phase with consistent variation in the lattice parameters was observed in structural analysis. The morphological investigations revealed a spherical shape of La2Sn2O7 particles which transformed to diamond like shape with a systematic variation in grain size by increasing the Nd contents. The variation in grain size dominantly affected the dielectric dispersion in the vast frequency range. Less energy loss and improved dielectric properties make La1.2Nd0.8Sn2O7 a potential candidate for utilization in energy storage devices. The complex modulus analysis provided an insightful view which helped to distinguish the mobility range of charge carriers in different frequency ranges.

Fabrication and properties of transparent Tb2Ti2O7 magneto-optical ceramics

Transparent Tb2Ti2O7 magneto-optical ceramics were fabricated from co-precipitated nano-powders by vacuum pre-sintering with hot isostatic pressing (HIP) post-treatment. The formation of pyrochlore phase, decomposition of the precursor, and the morphology of powders calcined at different temperatures were investigated. The in-line transmittance of Tb2Ti2O7 ceramics, which were pre-sintered at 1350 ℃ for 2 h with HIP post-treatment at 1450 ℃ for 3 h and subsequently annealed at 800 ℃ for 20 h in NH3 atmosphere, reaches 65.5 % at 1064 nm. The Verdet constant of Tb2Ti2O7 ceramics is −229.0 ± 0.6 rad·T−1 m−1 at the wavelength of 633 nm, which is 71 % higher than that of the commercial Tb3Ga5O12 crystals. Tb2Ti2O7 magneto-optical ceramics show a promising application for Faraday rotators.

Effect of oxygen vacancies, lattice distortions and secondary phase on the structural, optical, dielectric and ferroelectric properties in Cd-doped Bi2Ti2O7 nanoparticles

(Bi1-xCdx)2Ti2O7 (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) nanoparticles are synthesised using the co-precipitation method. The prepared samples show pyrochlore phase formation except for x = 0.02 and 0.08, where Bi4Ti3O12 appears as a secondary phase. The crystallite size and unit cell volume decrease while the strain and dislocation density increase with an increase in Cd-doping. The vibrational bands corresponding to Cd−O and CO are red-shifted, while the HOH bending band is blue-shifted with increasing Cd-doping. The band gap energy for x = 0.00 is found to be 1.78 eV. It increases with an increase in ‘x’ up to 0.06 and then decreases with a further increase in ‘x’ except for x = 0.02 and x = 0.08. The impedance data show non-Debye type relaxation. The pyrochlore phase is found to be non-ferroelectric. However, the samples with x = 0.02 and x = 0.08 having the secondary phase (Bi4Ti3O12) are ferroelectric.

Effect of temperature on phase evolution in Gd2Zr2O7: A potential matrix for nuclear waste immobilization

The dependencies of phase evolution and thermodynamic properties of Gd2Zr2O7 with temperature of preparation have been reported. A series of samples with composition Gd2Zr2O7 have been prepared. The effect of temperature on the phases has been inferred from the powder XRD and Raman spectroscopy. High temperature favors the formation of pyrochlore phase compared to the defect fluorite type phase. The evolution of phase was further quantitatively explained by measuring the formation energy employing high temperature calorimeter. The samples with predominantly pyrochlore phase have higher ΔfH°298 and thermodynamically stable. Also, it is observed that the pyrochlore type phase has larger formation energy compared to fluorite type phase and hence the fluorite to pyrochlore phase transformation is driven by enthalpy. On increasing the annealing temperature more disorder fluorite structure is formed and once the defect concentrations exceed a critical value they order to form the thermodynamically stable pyrochlore structure.

Formation of Perovskite and Pyrochlore Phases during Mechanochemical Synthesis of Lead Ferroniobate

We have studied the effect of mechanical activation in lead ferroniobate synthesis on the formation of perovskite and pyrochlore phases during both mechanochemical synthesis and subsequent firing. It has been shown that, during mechanical activation, the first to form is the perovskite structure. During subsequent sintering of the material, the pyrochlore structure begins to form as well. As the firing temperature is raised to 650–750°C, the perovskite structure is again formed. We provide explanation for this process.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study.

The photocatalytic properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi2Ti2O7) and 550 °C (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 phase in comparison with the Bi2Ti2O7 phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi2Ti2O7) and -27 mV (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical Eg values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi2Ti2O7 shows the highest activity in comparison with Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Structural and electrical characteristics of nanocrystalline La2Sn2O7 pyrochlore

Nanocrystalline La2Sn2O7 ceramic with pyrochlore structural phase was prepared by the chemical co-precipitation method followed by conventional sintering. Structural analysis by X-ray diffraction, transmission electron microscopy, and Raman spectroscopy revealed the pyrochlore phase formation. The Nyquist impedance plots measured at different temperatures (550–700 °C) exhibited a single semicircular arc and the relaxation times corresponding to grain interior and grain boundary were found to lie close to each other. The dielectric relaxation behavior in La2Sn2O7 was found to be of non-Debye type. The activation energy for conduction of oxygen ions was determined to be 0.87 eV. Frequency and temperature-dependent ac conductivity spectra were analyzed based on the Universal dielectric response model given by Jonschers’ power law. The conduction mechanism in La2Sn2O7 was discussed by employing the jump relaxation model and correlated barrier hopping (CBH) model. The maximum barrier height for the mobile ions to overcome for hopping was found to be 0.62 eV. The insights into the frequency and temperature dependence of the dielectric permittivity and dielectric loss in La2Sn2O7 were discussed using the Maxwell–Wagner model. The electric modulus spectra of La2Sn2O7 were manifested by the Kohlrausch-Williams-Watts (KWW) function. Further, the non-Debye type of relaxation was explicitly confirmed by the β values. Additionally, the universal scaling behavior was also demonstrated in La2Sn2O7 by the electrical modulus and impedance characteristics.

Thermal analysis of the crystallization kinetics of lead zirconate titanate powders prepared via sol–gel route

The crystallization evolution of lead zirconate titanate (PZT) powders processed from two organic gel systems with nominal composition was investigated. The crystallization of the perovskite PZT form initiated as the calcining temperature reached 400 °C. The formation of PbO phase constituted seeding grains for PZT crystallization at such low temperature. The formation of lead-deficient fluorite or pyrochlore phases was completely avoided under judicious processing conditions. TGA/DTA analysis was carried out to evaluate the activation energy associated with the perovskite phase formation. The kinetic parameters of the PZT crystallization toward perovskite formation were followed by employing Kissinger and JMAK isoconversional kinetic models. The results revealed that the crystallization mechanism is related to the powder precursor system. The value of the activation energy for the perovskite formation was about 77 kJ mol−1 irrespective of the precursor system when the Kissinger model was applied.

Enhancement in the photocatalytic activity of Bi2Ti2O7 nanopowders synthesised via Pechini vs Co-Precipitation method

Abstract Bi2Ti2O7 nanopowders were synthesised via two different methods i.e. Pechini (P) and Co-Precipitation (C) method by annealing at 550 °C for 16 h. The effect of synthesis technique on photocatalytic activity of Bi2Ti2O7 nanopowders was extensively studied. Formation of pure Bi2Ti2O7 pyrochlore phase was observed in both samples. Various crystallographic parameters obtained by reitveld analysis of X-Ray Diffraction (XRD) spectra of both samples infer the formation of a highly loose crystallographic structure in P sample. The smaller crystallite size of P sample correlates with the observed blue shift in the Raman spectra, furthermore decreasing the particle size in P sample. P sample exhibits higher specific surface area, mean pore diameter and total pore volume which is inferred by Brunauer-Emmett-Teller (BET) analysis. Higher photocatalytic activity of P sample can be attributed to the formation of distorted crystallographic structure and higher specific surface area.

Uranium brannerite with Tb(III)/Dy(III) ions: Phase formation, structures, and crystallizations in glass

AbstractUranium brannerite phases with terbium(III) or dysprosium(III) ions have been investigated. The precursors with molar ratio of 0.5:0.5:2 (Ln: U: Ti with Ln = Tb or Dy) were prepared and calcined at 750°C in argon. Sintering the pelletized samples in argon at 1200°C led to the formation of pyrochlore phases with TiO2 rutile and U‐rich oxides while sintering in air led to the formation of brannerite phases with the nominal composition close to Ln0.5U0.5Ti2O6 together with trace amounts of TiO2 rutile and LnUO4. Incorporating an excess of TiO2 (20 wt%) and sintering at higher temperature (1300°C) resulted in no obvious change to the phase equilibrium. As designed, pentavalent uranium has been proven to be dominant in these brannerite phases with diffuse reflectance spectroscopy. The relationships between the cell parameters and the ionic radii of the A‐site cations have been explored and rationalized from the structure point of view for a range of titanate brannerite phases (ATi2O6). In addition, the crystallization of Ln0.5U0.5Ti2O6 brannerite in glass has been achieved via heat treatment at 1200°C and confirmed with X‐ray diffraction, scanning electron microscopy‐energy dispersive spectroscopy and transmission electron microscopy–selected area electron diffraction.

Specific structural features of LnZrOx (Ln: La, Sm) mixed oxides prepared by different methods

LnZrOx (Ln: La, Sm) mixed oxides of Ln: Zr = 1 were prepared by different methods (complex polymerized method, sorption of cations on starch from aqueous salt solution and conventional co-precipitation with additional redispersion of precipitate by ultra sound) and calcined at 700–1300 °C. Their specific structural features and changes were studied and discussed. Various characterization methods were used such as X-ray diffraction, Electron microscopy, Fourier-transform infrared and Raman spectroscopy, UV–Vis spectroscopy, X-Ray absorption fine structure and X-ray photoelectron spectroscopy.The formation of pyrochlore structure occurred at 1100–1300 °C from fluorite-like pseudocubic phase ZrO2 regardless the method of preparation. This phase had a block-like structure consisting of ZrO2 nanocrystals stabilized by Ln cations and residual anions such as hydroxyls and carbonates. The desorption of such anions with heating already started at 900 °C and lead to local changes of Zr cations coordination to octahedral and to the formation of pyrochlore nanodomains inclusions within fluorite-like phase. The increased cation mobility and further elimination of anions caused by further heating was accompanied by the formation of bulk pyrochlore phase at 1100–1300 °C. Even after calcination at 1300 °C the local microheterogeneity as well as defects were identified at domains boundaries or sintered microstructure. These specific features of the formed pyrochlores depend on the method of preparation.

Structural tuning of dielectric properties of Ce-substituted Nd2Zr2O7

Rare earth zirconates with pyrochlore type structure have shown a potential for practical applications due to their stable crystal structure with tunable lattice parameters. In this context, a series of Ce-substituted neodymium zirconates with general formula, Nd2Zr2−xCexO7 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) was synthesized using wet chemical sol–gel auto-combustion technique. Structural analysis confirmed the formation of stable pyrochlore phase with space group Fd3m. The scanning electron microscopic analysis revealed that the parent compound adopted a mixed morphology (rectangular, spherical, squared etc.) with sharp grain boundaries and considerable porosity. A significant agglomeration was observed with Ce-substitution at Zr-site which improved the density of material. The stoichiometric composition of the elements present in the samples was confirmed by energy dispersive X-ray spectroscopy. The response of the material to external electric field of varying frequency was studied through impedance spectroscopy which confirmed the active contribution of grains and grain boundaries in the form of electrical response of the material. The nature of mobility of charge carriers was checked by the interaction of external varying electric field with the electric dipole moments of the material. This whole study provides valuable information about the energy storage capacity of the material and thus determines the potential of the material to be used in energy storage devices.

A comparative study on macroscopic and nanoscale polarization mapping on large area PLD grown PZT thin films

By optimizing laser raster scanning conditions and controlling PbO evaporation, we have grown high quality large area PbZr.52Ti.48O3 (PZT) thin films using custom-built off-axis pulsed laser deposition (PLD) technique. The realization of high quality PZT thin films over a large area with uniform polarization is a challenging task due to the formation of non-ferroelectric or pyrochlore like PbTi3O7 phases at the processing temperature (>600°C). Our results show the polycrystalline nature of the sample with a uniform chemical surface composition. The macroscopic level of polarization mapping shows excellent saturated P-E loops with a narrow variation (6%) of the remnant polarization (2Pr) across the large area thin films. A comparative study of macroscopic and nanoscale level of mapping of polarization switching are performed, to demonstrate the existence of stoichiometry ferroelectric PZT thin films over a large area. The achievement of high quality large area PZT thin films with uniform polarization might be used for under-water SONAR devices and in power harvesting.