High-Entropy Pyrochlore A2B2O7 with Both Heavy and Light Rare-Earth Elements at the A Site

A novel class of high-entropy pyrochlore ceramics (HEPCs) with multiple heavy and light rare-earth elements at the A site were successfully synthesized via solid-state reaction. Both the XRD patterns and Raman spectroscopy demonstrated the single pyrochlore structure feature of seven kinds of HEPCs. Electron microscopic images revealed the typical morphology and the homogeneous distribution of all rare-earth elements. It can be concluded that the significance of configuration entropy in the HEPC system has promoted the tervalent lanthanide nuclides to form a single pyrochlore structure. This work is expected to provide guidance for the further design of high-entropy pyrochlore/fluorite ceramics.

Structural Stability of Pyrochlore Manganate In2Mn2O7 Under High Pressure

The pyrochlore manganate In2Mn2O7 is a very promising ferromagnetic semiconductor material, which has a good application prospect in spin transport due to its very low electron effective mass, high Curie temperature, and structural stability. In this paper, In2Mn2O7 with pyrochlore structure was successfully prepared by high temperature and high pressure combined with the sol–gel method, and the in situ high-pressure X-ray diffraction experiment was carried out on it. The results showed that the structure of In2Mn2O7 was very stable in the pressure range of 0–29.0 GPa, and its bulk modulus was given. This lays a foundation for the application of In2Mn2O7 in extreme environments.

Electrochemical Characterization of Novel Polyantimonic-Acid-Based Proton Conductors for Low- and Intermediate-Temperature Fuel Cells

The development of novel proton-conducting membrane materials for electrochemical power units, i.e., low temperature fuel cells (FCs), efficiently working up to 300 °C, is a critical problem related to the rapid shift to hydrogen energy. Polyantimonic acid (PAA) is characterized by high conductivity, sufficient thermal stability and can be regarded as a prospective proton-conducting material. However, the fabrication of bulk PAA-based membranes with high proton conductivity remains a challenging task. In the present work, for the first time, the authors report the investigation on proton conductivity of bulk PAA-based membranes in the temperature range 25–250 °C, both in dry air and in moisturized air. Using PAA powder and fluoroplastic as a binder, fully dense cylindrical membranes were formed by cold uniaxial pressing. The structures of the PAA-based membranes were investigated by SEM, EDX, XRD and Raman techniques. STA coupled with in situ thermo-XRD analysis revealed that the obtained membranes corresponded with Sb2O5·3H2O with pyrochlore structure, and that no phase transitions took place up to 330 °C. PAA-based membranes possess a high-grain component of conductivity, 5 × 10−2 S/cm. Grain boundary conductivities of 90PAA and 80PAA membranes increase with relative humidity content and their values change non-linearly in the range 25–250 °C.

Electrical Properties of Li+-Substituted Solid Solutions Based on Gd2Zr2O7

Solid solution $${\text{G}}{{{\text{d}}}_{{2 - x}}}{\text{L}}{{{\text{i}}}_{x}}{\text{Z}}{{{\text{r}}}_{2}}{{{\text{O}}}_{{7 - x}}}$$ with a pyrochlore structure is synthesized for the first time. The cationic composition is confirmed via chemical analysis and nuclear reactions. It is found that the stoichiometry with respect to lithium is retained up to 1100°C. The lattice parameter diminishes in the homogeneity range 0 ≤ x ≤ 0.30, while the free volume of migration grows. Introducing lithium into the Gd sublattice raises oxygen–ion conductivity, due to the emergence of oxygen vacancies and enhancement of their mobility. Maximum conductivity is reached for composition with х = 0.10 (~1 × 10−3 Ω−1 cm−1, 650°C). An assumption is made about the formation of associates of the type $${{\{ {\text{Li}}_{{{\text{Gd}}}}^{{''}} \cdot {\text{V}}_{{\text{o}}}^{{ \bullet \bullet }}\} }^{ \times }}$$ at high contents of the dopant (x = 0.30), accompanied by an increase in the activation energy of conductivity.

Spectral studies of bismuth-containing pyrochlores

Thermal stable solid solutions of titanates, niobates, and tantalates of bismuth with a pyrochlore structure doped with 3d-metal atoms were studied using XPS and NEXAFS spectroscopy. Based on spectral studies, it was shown that the manganese, cobalt, nickel and copper atoms in these solid solutions have mainly charge state +2 and iron atoms – charge state +3.

Synergy of diffraction and spectroscopic techniques to unveil the crystal structure of antimonic acid

The elusive crystal structure of the so-called ‘antimonic acid’ has been investigated by means of robust and state-of-the-art techniques. The synergic results of solid-state magic-angle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochlore-type structure of stoichiometric formula (H3O)1.20(7)H0.77(9)Sb2O6. Some protons belong to heavily delocalized H3O+ subunits, while some H+ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O–H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bond-valence energy landscape analysis. X-ray absorption spectroscopy results corroborate the structural data around Sb5+ ions at short-range order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Fabrication of Y2Ti2O7 Transparent Ceramic by Spark Plasma Sintering

Y2Ti2O7 transparent ceramic was fabricated by reactive sintering using spark plasma sintering at 1673 K for 2.7 ks. The sintered body exhibited a cubic pyrochlore structure and uniform microstructure with an average grain size of 2.9 μm. The transmittance reached 73% at a wavelength of 2000 nm after annealing at 1023 K for 21.6 ks.

Structural, electrical and magnetic properties of substituted pyrochlore oxide nanoparticles synthesized by the co-precipitation method

Five substituted pyrochlore nanooxides such as Nd1.9Ho0.1Zr1.8Ce0.2O7, La1.95Ce0.05Zr0.29Ce1.71O7, Y1.79Pr0.21Ru1.99Pr0.01O7, Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 were synthesized by coprecipitation method. These precursors were monitored by thermal studies (TGA-DTA). The prepared nanosized substituted pyrochlore oxides were characterized by EDS, XRD, SEM, TEM, d. c. electrical conductivity, Thermoelectric power, Hall effect measurement, dielectric properties and magnetization measurements. XRD confirmed the formation of a single phase crystalline substituted pyrochlores with a cubic nature of nanoparticles. All substituted compounds were adopted a stable pyrochlore structure with rA3+/rB4+ = 1.395 except La1.95Ce0.05Zr0.29Ce1.71O7 compound, which has rA3+/rB4+ = 1.175 indicate disorder pyrochlore structure (i.e. fluorite structure). The temperature dependence of d. c. electrical conductivity for all substituted pyrochlores exhibits two distinct slopes with a break. This discontinuity can be attributed to extrinsic to intrinsic semiconducting properties. The thermoelectric power and Hall effect measurements for all compounds were confirmed the p-type semiconductivity except Y1.79Pr0.21Ru1.99Pr0.01O7 compound and which showed n-type semiconductivity. The dielectric constant (ε’) and dielectric loss (tan δ) i. e dissipation factor decreases with an increase in frequencies and reaching constant at particular frequencies. The applied field dependence of magnetization curve at room temperature (300 K) for Nd1.9Ho0.1Zr1.8Ce0.2O7, Y1.79Pr0.21Ru1.99Pr0.01O7 and Dy1.9Yb0.1Mn1.93Cu0.07O7, showed hysteresis loop with a small kink around the origin and which can be attributed to small but definite ferromagnetic ordering along with significant paramagnetic and superparamagnetic components. The magnetization at 2K showed a clear hysteresis loop for Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 pyrochlores are soft (weak) ferromagnets.

Unveiling the crystal structure of ‘antimonic acid’: short- and long-range perspectives

The elusive crystal structure of the socalled “antimonic acid” has been investigated by means of robust and state-of-the-art techniques. The synergic results of solidstate magicangle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochloretype structure of stoichiometric formula (H3O)1.20(7)H0.77(9)Sb2O6. Some protons belong to heavily delocalized H3O+ subunits, while some H+ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O − H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bondvalence energy landscape analysis. Xray absorption spectroscopy results corroborate the structural data around Sb5+ ions at shortrange order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Thermodynamic Functions of Terbium Hafnate

The molar heat capacity of terbium hafnate with a pyrochlore structure was measured by relaxation and adiabatic calorimetry. The smoothed values of the molar heat capacity were used to calculate the thermodynamic functions in the temperature range 6–330 K. The general form of the Schottky anomaly was determined.