Pyrochlore-type Structure Research Articles

Synthesis and characterization of the RE2A2O7 oxides with an ultrahigh-entropy sublattice occupied by rare-earth elements

The possibility of fabricating novel high-entropy oxides (HEOs) with one ultrahigh-entropy sublattice consisting of 13 cations in RE2A2O7 (RE = [La, Sm, Nd, Gd, Dy, Yb, Y, Er, Eu, Tb, Ho, Lu, Tm], A = Zr, Hf, Ti, Sn, Ce or Pr) is demonstrated. All the samples exhibited a pure pyrochlore-type or defective fluorite crystalline structure with various degrees of cation disordering. Particularly, RE2Ti2O7 and RE2Sn2O7 exhibited a pure pyrochlore-type structure with high degree of ordering, while RE2Ce2O7 and RE2Pr2O7 demonstrated a single fluorite type HEO structure. The results pave the way to fabricate RE2A2O7 HEOs with an ultrahigh-entropy sublattice and potential application as the ceramic layer in thermal barrier coatings.

Phase relations in the La2O3-ZrO2-HfO2 system at 1250 °C and 1500 °C

In the present study, the phase equilibria in the ternary system La2O3-ZrO2HfO2 have been studied in the whole concentration range. In the course of the study, isothermal sections of the phase diagram of the system were constructed at temperatures of 1500 and 1250 °C. The obtained results indicate the no new phase formation in the studied system. It was found that in the La2O3-ZrO2HfO2 ternary system at temperatures of 1500 and 1250 °C, solid solutions are formed on the basis of monoclinic (M, space group P21/C) HfO2 and tetragonal (T, space group P42/nmc) modifications of ZrO2, solid solutions based on a hexagonal modification (A, space group P3m1) of La2O3, and an ordered phase with a pyrochlore-type structure of La2Zr2О7 (La2Hf2О7) (Py, space group Fd-3 m). The system studied is characterized by the formation of a continuous series of solid solutions with a pyrоchlore-type structure. A decrease in temperature from 1500 °C to 1250 °C leads to the shift of the three-phase region, containing solid solutions based on a pyrochlore-type structure of Ln2Zr2О7 (Ln2Hf2О7), monoclinic M-HfO2, and tetragonal T-ZrO2, towards the concentration area with higher ZrO2 content and also to an increase in the extent of the solid solution based on the monoclinic modification M-HfO2.

Nano-Sheets of CsNiVF6 Pyrochlore Electrocatalyst for Enhanced Urea Oxidation and Hydrogen Green Production Reactions

This study presents the successful synthesis of a cesium–nickel–vanadium fluoride (CsNiVF6) pyrochlore nano-sheet catalyst via solid-phase synthesis and its electrochemical performance in green hydrogen production through urea electrolysis in alkaline media. The physicochemical characterizations revealed that the CsNiVF6 exhibits a pyrochlore-type structure consisting of a disordered cubic corner-shared (Ni, V)F6 octahedra structure and nano-sheet morphology with a thickness ranging from 10 to 20 nm. Using the CsNiVF6 catalyst, the electrochemical analysis, conducted through cyclic voltammetry, demonstrates a current mass activity of ~1500 mA mg−1, recorded at 1.8 V vs. RHE, along with low-resistance (3.25 ohm) charge transfer and good long-term stability for 0.33 M urea oxidation in an alkaline solution. Moreover, the volumetric hydrogen production rate at the cathode (bare nickel foam) is increased from 12.25 to 39.15 µmol/min upon the addition of 0.33 M urea to a 1.0 KOH solution and at a bias potential of 2.0 V. The addition of urea to the electrolyte solution enhances hydrogen production at the cathode, especially at lower voltages, surpassing the volumes produced in pure 1.0 M KOH solution. This utilization of a CsNiVF6 pyrochlore nano-sheet catalyst and renewable urea as a feedstock contributes to the development of a green and sustainable hydrogen economy. Overall, this research underscores the potential use of CsNiVF6 as a cost-effective nickel-based pyrochlore electrocatalyst for advancing renewable and sustainable urea electrolysis processes toward green hydrogen production.

Geometrically frustrated Gd2Ti2O7 oxide: A comprehensive exploration of structural, magnetic, and magnetocaloric properties for cryogenic magnetic cooling applications

The magnetocaloric effect (MCE) has been extensively studied in various magnetic solids, aiming to both facilitate practical applications in magnetic cooling and deepen our understanding these materials’ intrinsic properties. This study presents a systematic investigation of Gd2Ti2O7 oxide, characterized by a geometrically frustrated magnetic structure, focusing on its structural and magnetic characteristics through experimental analysis and theoretical calculations. Emphasis is placed on its magnetic phase transition (MPT) and magnetocaloric properties, scrutinizing temperatures as low as 0.4 K and magnetic fields up to 16 T. The results reveal that Gd2Ti2O7 oxide possesses a crystalline cubic pyrochlore-type structure with an antiferromagnetic (AFM) semiconductor ground state. The material undergoes a first-order MPT in low field and low-temperature regimes, transitioning to a second-order MPT above 2 K up to 16 T. Evaluation of magnetocaloric parameters, including maximum magnetic entropy change, temperature-averaged entropy change, relative cooling power, and refrigerant capacity, indicates moderate values under low magnetic field changes, while significantly large values are achievable under high magnetic field changes. This finding suggests that potential cryogenic magnetocaloric materials (MCMs) with optimal performance should exhibit low MPT temperatures induced by magnetic frustration and possess a ground state with a sufficiently large magnetic moment induced by low magnetic fields, thus avoiding excessively strong AFM coupling. This study lays the groundwork for future cryogenic MCM design and provides valuable insights into the intrinsic properties of geometrically frustrated magnetic materials.

Composition and structure of tungsten antimony acid

Tungsten antimony acids (TAA) with the composition H(2)Sb(2)WXO6·nH2O (0 < x ≤ 1.45; 0 < n ≤ 2.0) have been synthesized by hydrolysis of antimony trichloride pre-oxidized with nitric acid in the presence of varying amounts of Na2WO4. To obtain TAA protonated forms, the samples were kept in a 96% solution of sulphuric acid, the precipitate was washed until reaction became neutral and dried in air. The amount of tungsten, antimony, and silver ions in TAA was determined using energy dispersive analysis. Changes in structural parameters upon doping of AA with tungsten ions were studied using a Bruker D8 ADVANCE X-ray diffractometer (CuKa1-radiation). The number of oxonium ions in TAA was determined by the substitutionof these ions by silver ions in equivalent amounts (Ag+-TAA forms). All obtained TAA samples and Ag+ TAA forms had a pyrochlore-type structure, space group symmetry Fd3m. Refinement of the arrangement of atoms in the structure using the Rietveld method showed that tungsten ions replace antimony ions and are statistically located in 16c, oxygen anions in 48f, and oxonium ions and water molecules in 16d and 8b positions, respectively. When tungsten ions were introduced into samples, the structural parameters of the resulting phases changed. There was a decrease in the unit cell parameter and the distance between antimony ions and oxygen anions, while an increase in the distance between oxonium ions and oxygen anions located in 48f positions was observed. This allowed the removal of a proton from oxonium molecules and its transport via a system of hydrogen bonds formed by water molecules

Synthesis, Characterization of Dy2NdSbO7/Bi2WO6 Heterojunction Photocatalyst and the Application for the Photocatalytic Degradation of Chlorpyrifos under Visible Light Irradiation

A groundbreaking photocatalytic nanomaterial, Dy2NdSbO7, was fabricated smoothly using the hydrothermal synthesis technique for the first time. Apart from that, Dy2NdSbO7/Bi2WO6 heterojunction photocatalyst (DBHP) was initially fabricated using the solvothermal fabrication technique. X-ray diffractometer, Fourier-transform infrared spectrometer, Raman spectrometer, UV-visible spectrophotometer, X-ray photoelectron spectrometer, inductively coupled plasma optical emission spectrometer, transmission electron microscope, and X-ray energy dispersive spectroscopy have been applied to evaluate and investigate the thetastructure, morphology, and physicochemical properties of synthesized samples. The results confirmed that the pyrochlore-type crystal structures of Dy2NdSbO7 belonged to the Fd3m space group with the cubic crystal system and the β-pyrochlore-type crystal structures of Bi2WO6 which belonged to the Pca21 space group with orthorhombic crystal system. Under visible light exposure for 155 min (VLP-155min) using DBHP in the capacity of the photocatalytic nanomaterial, the removal efficiency of chlorpyrifos (CPS) saturation reached 100%. Comparison of CPS removal efficiency after VLP-155min revealed that DBHP exhibited higher removal efficiency than Dy2NdSbO7, Bi2WO6, or N-doped TiO2 photocatalyst, with removal efficiency 1.15 times, 1.23 times, or 2.55 times higher, respectively. Furthermore, the oxidizing capability of free radicals was investigated using trapping agents. Results demonstrated that superoxide anions exhibited the strongest oxidative capability, followed by hydroxyl radicals and holes. The results presented in this study lay a robust groundwork for future investigations and advancements in the field of highly efficient heterostructure material. These findings have significant implications for the development of environmental remediation strategies and provide valuable insights into sustainable solutions for addressing CPS contamination.

Thermophysical Properties of Lanthanum and Samarium Zirconate—Hafnates

The synthesis and identification of lanthanum and samarium zirconate–hafnates of the pyrochlore structure type have been reported. The heat capacity of the samples in the temperature range 310–1380 K was measured by the differential scanning calorimetry method. The temperature dependences of the cubic lattice parameters were determined, and the thermal expansion coefficients were evaluated in the range 298–1273 K using high-temperature X-ray powder diffraction. The thermal diffusivity of the samples was measured by the laser flash method, and the temperature dependences of the thermal conductivity were calculated considering the porosity of the samples.

MECHANOCHEMICAL SYNTHESIS OF ION-EXCHANGE SILVER FORMS OF POLYANTIMONIC ACID

The possibility of synthesising ion-substituted forms of hydrated silver antimonates has been studied. For the synthesis, the method of mechanochemical activation of the components of an inorganic mixture consisting of polyantimonic acid (PAA) with composition and silver nitrate with the concentration range from 0.0 to 1.0 has been applied. The results of studies of the phase composition of the synthesized compounds and their structural features are presented. Using the Rietveld method, the parameters of the crystal lattice of PAA hydrated ionsubstituted silver forms with a pyrochlore-type structure have been refined. The model for the occupation of metal ions by crystallographic positions has been proposed: the framework of the structure of the compounds is formed by 16c- and 48f-positions, in which Sb5+ and O2- are statistically located; hydrated oxonium ions (H3O+) and silver ions occupy 16d- and 8b-positions respectively. It has been shown that the synthesis of silver forms of PAA is preferably carried out by the mechanochemical synthesis, which results in the complete substitution of proton groups by silver ions in the structure of the compounds.

ISOTHERMAL SECTION OF THE ZrO2–HfO2–Eu2O3 SYSTEM PHASE DIAGRAM AT 1100 °C

Phase equilibria and structural transformations in the ternary ZrO2–HfO2–Eu2O3 system at 1100 °C were studied by X-ray diffraction over the entire composition range. The samples of different compositions have been prepared from nitrate acid solutions by evaporation, drying, and calcinations at 1100 °C. Fields of solid solutions based on the cubic (F) modification with fluorite-type structure and tetragonal (T) and monoclinic (M) modifications of ZrO2 (HfO2), cubic (C) modification of Eu2O3, and an ordered intermediate phase with pyrochlore-type structure, Ln2Zr2O7 (Ln2Hf2O7)(Py), were established to exist in the system. The solubility of Eu2O3 in M-HfO2(ZrO2) is pretty low and approximately less than 1 mol%, which is confirmed by XRD. The boundaries of phase fields and lattice parameters of the phases were determined. The lattice parameters for F phase vary from а = 0.5305 nm in two-phase sample (C+F) containing 6.75 mol% ZrO2–8.25 mol% HfO2–85 mol% Eu2O3 to а = 0.5283 nm for sample containing 27 mol% ZrO2–33 mol% HfO2–40 mol% Eu2O3 and to а = 0.5275 nm in two-phase sample (Py+F) containing 29.25 mol% ZrO2–35.75 mol% HfO2–35 mol% Eu2O3, and from а = 0.5232 nm in two-phase sample (Py+F), containing 32.625 mol% ZrO2–39.875 mol% HfO2–27.5 mol% Eu2O3 to а = 0.5218 nm sample containing 33.75 mol% ZrO2–41.25 mol% HfO2–25 mol% Eu2O3 and to а = 0.5176 nm in two-phase sample (F+M) containing 40.5 mol% ZrO2–49.5 mol% HfO2–10 mol% Eu2O3 along the section Eu2O3–(55 mol% HfO2–45 mol% ZrO2). The fluorite-type structure (F) is in equilibrium with all phases that exist in the ternary ZrO2–HfO3–Eu2O3 system at 1100 °C and forms substitutional solid solutions with phases of the binary systems. In the ZrO2–HfO2–Eu2O3 system, an infinite series of solid solutions form from the Ln2Zr2O7 (Ln2Hf2O7) (Py) phase. The isothermal section of the ZrO2–HfO2–Eu2O3 phase diagram at 1100 °C contains one three-phase regions (T + M + F) and six two-phase regions (F + C, two-F + Py, F + M, F + T, T + M). No new phases were found in the ZrO2–HfO2–Eu2O3 system at 1100 °C and the nature of phase equilibria was determined by the constitution of the boundary binary systems.

Investigating photoluminescence in Gd3+ activated pyrochlore-type fluoro-aluminates

ABSTRACT Fluoro-aluminates with pyrochlore-type structures such as Cs2KAl3F12 & Cs2NaAl3F12 could have very useful industrial applications but despite of having much significance, very less exploration on these kinds of compounds has been accomplished. Although some experiments related to luminescence studies on Ce3+ activated phosphors have been achieved on these fluorides long ago, since then very less considerable work on investigating such compounds was done so far. Therefore further work has been carried out by adding rare-earth gadolinium (Gd) as an activator in these complexes. Gadolinium in its trivalent form showed encouraging luminescence results. Additionally, the effect of dopant concentration on photoluminescence intensities has been studied. Decay characteristics showed long lifetimes (in ms) that vary with Gd3+ concentrations. Also, respective values for quantum efficiency (Ø)% and non-radiative rates (A nR) have been evaluated. Phase-purity was confirmed with the help of XRD and Rietveld analysis.

Investigating electronic and optical properties of pyrochlore oxides Ta2A2O7 (A = Cd and Ge) for promising optoelectronic applications

Due to their stable crystal structure and controllable lattice parameters, rare earth zirconates with pyrochlore type structures have demonstrated a promise for usage in practical applications. We have investigated the electronic and optical properties of pyrochlore oxide compound named Ta2A2O7 (A = Cd and Ge) using density functional theory based on spin–orbit coupling and Hubbard (SOC + U) corrections to consider the existence electronic d orbitals of (A) and f elements (Ta). This study focuses on the characterization and investigation of pyrochlore oxides with the A2B2O7 composition, which have significant technological applications including quantum spin ices, solid oxide fuel cells, and nuclear waste forms. No evidence regarding magnetic moment is obtained from the first run with spin-polarization setup. Also, from the spectra of TDOS was showing a similar overall profile for both spin channels. The electronic properties (total and partial) densities of states as well as the optical properties such as the complex dielectric function, refractive index, absorption coefficient and reflectivity for wavelengths in the visible spectrum have been reported to explore the potential use of these compounds in technological applications. Consequently, the investigated compounds can be used in optoelectronic device fabrication.

ТЕРМОДИНАМИЧЕСКОЕ ИССЛЕДОВАНИЕ СЛОЖНЫХ ОКСИДОВ САМАРИЯ СО СТРУКТУРОЙ ПИРОХЛОРА

The results of a thermodynamic study of a group of inorganic compounds of the general formula Ln2M2O7, where M = Sn, Ti, are presented. Single-phase samples were obtained by reverse precipitation followed by high-temperature synthesis. Crystallographic parameters were calculated and their dependence on the radius of the element is shown. The temperature dependences of the heat capacity are experimentally studied in a wide temperature range. The thermodynamic properties of samarium compounds with a pyrochlore-type structure are compared.

DURABILITY IN WATER OF MATRICES FOR RARE-EARTH – ACTINIDE FRACTION OF HIGH-LEVEL RADIOACTIVE WASTE

The durability in waters of matrices for rare-earth – actinide fraction HLW has been studied at 25 (1 atm.), 200 or 240°C (pressure of saturated vapor). The samples composed of Nd2ZrTiO7 (cubic pyrochlore-type structure) or Nd4(Ti,Zr)9O24 (orthorhombic symmetry, no natural analogue), phases were obtained by induction melting in a “cold” crucible. Leaching rate of Nd (imitator of REE-MA fraction) on the 28th day of experiment was lower than 5 × 10–8 g/(cm2 day). These results confirmed a very high durability of the ceramic waste forms in hot underground waters.

Effect of Codoping on the Electrical Properties of Magnesium- and Copper-Containing Bismuth Niobate with Pyrochlore-Type Structure

Effect of Codoping on the Electrical Properties of Magnesium- and Copper-Containing Bismuth Niobate with Pyrochlore-Type Structure

Sub-Ångstrom-scale structural variations in high-entropy oxides.

High-entropy oxides (HEOs) are a special class of materials that utilize the concept of high-entropy alloys (HEAs) with five or more elements randomly distributing at a single sublattice in near-equiatomic proportions. HEOs have been attracting increasing attention owing to their many outstanding physical and chemical properties. However, unlike HEAs, for which local chemical compositions, order/disorder behaviors, and property-structure relationships have been comprehensively investigated, detailed information on the atomic-scale chemical and structural features and their correlations with functionalities in HEOs so far is still not sufficient. Herein, we select four typical HEOs with pyrochlore, spinel, perovskite and rock-salt type structures, and directly observe and quantify sub-Ångstrom-scale structure variations in different manners by means of advanced aberration-corrected scanning transmission electron microscopy techniques. Visualization and quantification of local structural variations and lattice distortions in the current work may show a valuable example for future investigations on local fluctuating structures and their relationships with properties in more systems of HEOs.

Effect of A-site deficiency on perovskite-type Mn4+-activated La5/3MgTaO6 red phosphor and green luminescence of the Mn2+ occupied six-coordinate site in Mg2LaTaO6

White light LEDs have a long life and high energy efficiency; however, their application is limited owing to the lack of red components. Herein, we use the solid-state reaction method to fabricate a new Mn-activated perovskite-type La5/3MgTaO6 phosphor that exhibits red emission under UV excitation at 365 ​nm. A-site deficiency in the perovskite-type structure plays a crucial role in the emission of Mn4+ in La5/3MgTaO6 because Mn4+—which is located in the non-inversion symmetry site—is activated for such an emission. In addition, the newly Mn-activated pyrochlore-type Mg2LaTaO6 phosphor exhibited green emission under UV excitation at 254 ​nm owing to the oxide ion deficiency of Mg2LaTaO6. Based on the features of the oxide-deficient pyrochlore-type structure, we infer that Mn2+ may be present in the six-coordinate octahedral site with high distortion in pyrochlore-type Mg2LaTaO6.

Fabrication, characterisation, and structure of Nd2Zr2O7 microbelts prepared by electrospinning method

• A novel Nd 2 Zr 2 O 7 microbelts. • Electrospinning technology. • A pyrochlore-type crystal structure. • A feasible formation mechanism. Nd 2 Zr 2 O 7 microbelts were successfully prepared by simple electrospinning paired with calcination. The composition, microstructure, and structure of the as-prepared Nd 2 Zr 2 O 7 microbelts were systematically characterized through a combination of characterisations, such as XRD, SEM, TEM, FT-IR, and TG-DSC techniques. Results indicated that the as-prepared Nd 2 Zr 2 O 7 microbelts had a pyrochlore-type crystal structure and uniform size with about 10 μm thickness. Moreover, a feasible formation mechanism of the Nd 2 Zr 2 O 7 microbelts was discussed. The present procedure can be expanded to develop other rare-earth zirconate materials with belt structure.

Hydrothermal synthesis of lanthanide ruthenate single crystals

A series of lanthanide ruthenates were synthesized as large single crystals using a high temperature hydrothermal method (600–650 °C, 200 MPa). In all cases, the lanthanide sesquioxides (Ln2O3) and RuO2 were used as starting materials in various ratios. A range of products containing ruthenium in a variety of oxidation states were isolated, including LnRuO3, Ln2Ru2O7, Ln2RuO5(OH), and Ln5Ru2O12, depending on the size of the rare earth and the relative ratio of starting materials. In most cases, the products were formed in good yield as high-quality single crystals. This approach provides a route to single crystals of the perovskite and pyrochlore structure types to enable advanced physical properties measurements. The study also introduces the Ln2RuO5(OH) structure type for Ru5+, and significantly expands the phase space for Ln5Ru2O12 to the later lanthanides, also as well-formed single crystals. Careful analyses of diffraction data suggest that the materials contain minimal site disorder or nonstoichiometry.

Phase equilibria in the ZrO2–HfO2–Sm2O3 system at 1500 °С

The phase equilibria in the ternary system ZrO 2 –HfO 2 –Sm 2 O 3 were studied by X-ray diffraction and microstructural analyses. The formation of new phases in the ZrО 2 –HfO 2 –Sm 2 O 3 system at 1500 °С was not observed. It is established that in the studied system at 1500°C the fields of solid solutions based on tetragonal (T) modification of ZrО 2 , monoclinic (M) modifications of HfO 2 , and monoclinic (B) modifications of Sm 2 O 3 , and ordered phase with the structure of pyrochlore-type (Py) Ln 2 Zr 2 O 7 (Ln 2 Hf 2 O 7 ) are present. The boundaries of the phase fields and the parameters of the unit cells of the formed phases are determined. The studied isothermal cross section of the ZrО 2 –HfO 2 –Sm 2 O 3 system is characterized by the formation of continuous series of cubic solid solutions based on a phase with a structure of the pyrochlore-type Sm 2 Zr 2 O 7 (Sm 2 Hf 2 O 7 ) and a structure of the fluorite type F-ZrO 2 (HfO 2 ). It is established that two regions of homogeneity of cubic solid solutions are formed in the determined system. The existence of these regions of homogeneity is due to the rupture of the solubility of the F-ZrO 2 (HfO 2 ) phase in the region of the existence of an ordered phase with a pyrochlore-type structure Sm 2 Zr 2 O 7 (Sm 2 Hf 2 O 7 ). It is established that the solid solution based on a cubic modification with fluorite-type structure exists in equilibrium with all phases observed in the system. Isothermal section of the phase diagram of the system ZrO 2 –HfO 2 –Sm 2 O 3 at 1500°C has been characterized by the presence of one three-phase (F + T + M), as well as five two-phase (B + F, two - F + Py, M + F, T + F) regions.

Polymorphic transformations of NaYGeO4 olivine under high pressure and high temperature

Two new high-pressure modifications of sodium yttrium germanate NaYGeO4 are described. A reversible pressure-induced phase transition of olivine-type NaYGeO4 to new phases has been studied in the pressure range of 4–10 GPa at temperatures from 400 to 1000°C. It was shown that NaYGeO4 olivine is stable at 400–600°C up to 10 GPa. A high-pressure polymorph of NaYGeO4 with NaNdSiO4 crystal structure is formed at 800°C starting from 6 GPa. Along with this polymorph, another new high-pressure modification of NaYGeO4 with NaSmSiO4 crystal structure was found in small amount. In addition, a yet undescribed polymorph of Y2Ge2O7 with pyrochlore-type structure was isolated at 8 GPa and 800°C. The crystal structures of the new phases were determined by X-ray powder diffraction. The results of DFT calculations confirmed the experimental crystallography and gave a preliminary insight into the relative thermodynamic stability and electronic properties for all three new compounds.