Processing and characterization of an Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 high-entropy Li–garnet electrolyte

In this paper, we demonstrated the processing of Li₇La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ (LLZNTH) high-entropy Li–garnet with promising properties for lithium batteries. We first synthesized the LLZNTH Li–garnet powders which have a single cubic garnet phase (space group: $\text{◂⋅▸}Ia\overline{3}d$; No. 230) without any secondary phases as well as uniform elements distributions. The prepared powders were further densified to a relative density of ∼94% with well-crystallized grains and good contact with the neighboring grains. Minimal grain growth can be observed in the sintering time range from 8 to 20 h, which is likely due to the sluggish effects of high-entropy compounds. The sample also maintains the cubic garnet phase along with uniform elements distribution after sintering. Electrochemical characterizations indicate that the densified sample has an adequate ionic conductivity of 4.67 × 10⁻⁴ S cm⁻¹ at room temperature, a low activation energy of 0.25 eV, and a low electronic conductivity in the order of 10⁻⁸ S cm⁻¹. The significance of designing high-entropy electrolyte is further discussed.     

Phase pure rhombohedral R3¯m$R\bar{3}m$ AgCoO2 delafossite: Rietveld,selected area electron diffraction,X-ray photoemission spectroscopy,and magnetic studies

A p-type rhombohedral AgCoO₂ (S.G. $R\overline{3}m$) reported to form only at pressures greater than 300 MPa at 773 K, is stabilized in pure phase at 0.2 MPa oxygen pressure. Analysis of powder X-ray diffraction pattern by Rietveld and selected area electron diffraction patterns confirms the purity of rhombohedral phase. Core-shell X-ray photoemission spectroscopy studies show that Ag ions are in +1 oxidation state with Co ions largely in +3 state. AgCoO₂ is paramagnetic undergoing super-paramagnetic transition at 44 K. The magnetic moment of Co is 0.001 μB which corresponds to low spin (LS) Co³⁺ along with a small fraction of LS Co⁴⁺ ions.     

Microstructure characterization of high-pressure induced texture in fully densified zirconium diboride ceramics

Fully dense oriented monolithic zirconium diboride (ZrB₂) ceramics with <0001> fiber texture were fabricated using high-pressure spark plasma sintering. The <0001> fiber texture is proposed to be triggered by grain rotation, as (0001) <$11\overline{2}0$> basal $⟨a⟩$ slip of ZrB₂ was activated during sintering. The high temperature, uniaxial high pressure, and uniform starting particles play a vital role in the texture formation of ZrB₂ ceramics. The result provides visible evidence for the occurrence of plastic deformation during high-pressure-assisted sintering.     

Improved microwave dielectric characteristics in CaHf1−xTixO3 ceramics

In the present work, effects of Ti-substitution for Hf upon microwave dielectric characteristics for CaHfO₃ ceramics were investigated. CaHf_1−xTi_xO₃ solid solution in space group Pnma was determined in the present ceramics with x = 0–0.075, and only minor amount of secondary phases of CaHfO₃ (R$\overline{3}c$, 1.3 wt%) and Ca₂Hf₇O₁₆ (R$\overline{3}$, 0.8 wt%) was detected. The significantly improved microwave dielectric characteristics were achieved with Ti-substitution in CaHfO₃ ceramics. With increasing x, the temperature coefficient of resonant frequency τ_f was substantially tuned from negative to positive through zero, which was caused by the decrease of oxygen octahedral tilting, and the Qf value was apparently improved simultaneously and reached the maximum of 43670 GHz at x = 0.025, while the dielectric constant ε_r increased monotonously. The remarkable improvement of Qf was attributed to the enhancement of octahedral framework rigidity and the decreased nonuniform distribution of inner chemical bonding in crystal based on the change of chemical bond parameters calculated by the Phillips–Van Vechten–Levine bond theory, which was further confirmed by the higher frequency and narrower width of stretch mode of octahedron (i.e., A_1g(O) near 730 cm⁻¹) in Raman spectra. An interesting phenomenon was found in titanium-containing perovskite ceramics that they had consistent trend of τ_f changing with tolerance factor, which was helpful to design temperature-stable perovskite microwave dielectric ceramics. The best combination of microwave dielectric characteristics was achieved at x = 0.035:  ε_r = 27.2, Qf = 42 870, and τ_f = +3 ppm/°C. For practical applications of the present ceramics, further improvement of Qf is required, and it is expected to be achieved through microstructure optimization.     

Energetics and structure of the B-type solid solution in the Nd2O3–Y2O3 system

The system Nd₂O₃–Y₂O₃ contains solid-solution phases with several different structures. Single-phase B-type (Nd_1−xY_x)₂O₃ solid solutions in the range of $0.2\le x\le 0.5$ were formed at 1873 K and retained on cooling to ambient temperature. They showed a linear composition dependence of lattice parameters, enabling extrapolation to x = 0 and 1 for comparison with the structures of the stable endmembers. A positive enthalpy of formation from A-type Nd₂O₃ and C-type Y₂O₃ determined using oxide melt solution calorimetry indicated entropic stabilization of the B-type phase. A positive interaction parameter for mixing in the B-type solid solution, Ω $=$ 47.46 ± 4.04 kJ/mol, was obtained by fitting the data using a regular solution model. This value is significantly more positive than that obtained by previous phase diagram analysis using the CalPhaD approach. Exsolution into two B-type phases is predicted to occur below 1427 K, making it unimportant for the equilibrium phase relations that will be dominated by other structures at low temperature.     

Adlayer formation on C-plane (0001) and R-plane (11¯${\rm{\bar{1}}}$02) Al2O3 surfaces

Adlayers on C-plane (0001) and R-plane (1$\overline{1}$02) terminated surfaces of corundum phase aluminum oxide were synthesized by annealing mixtures of two oxide powders, aluminum oxide with an additive. Using high-angle annular dark field scanning transmission electron microscopy, the adsorbed layers were characterized, and image simulations aided interpretation of the results. The adlayers were pseudomorphic, one atomic layer thick, and with a fractional site occupancy. Atomic positions of the adlayer atoms relaxed and changed relative to the bulk structure, where there is evidence that the magnitude of the relaxation is sensitive to the ionic radius of the adsorbate. The pseudomorphic adlayer structure is formed for different elements including, but not limited to, the lanthanides (i.e., Ge, Ba, and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm).