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1. Flux‐mediated synthesis and photocatalytic activity of NaNbO 3 particles

   https://doi.org/10.1111/jace.16765  

   Hamilton, Adam M. ; O'Donnell, Shaun ; Zoellner, Brandon ; Sullivan, Ian ; Maggard, Paul A. ( September 2019 , Journal of the American Ceramic Society)

   Using molten‐salt synthetic techniques, NaNbO₃(Space groupPbcm; No. 57) was prepared in high purity at a reaction time of 12 hours and a temperature of 900°C. All NaNbO₃products were prepared from stoichiometric ratios of Nb₂O₅and Na₂CO₃together with the addition of a salt flux introduced at a 10:1 molar ratio of salt to NaNbO₃, that is, using the Na₂SO₄, NaF, NaCl, and NaBr salts. A solid‐state synthesis was performed in the absence of a molten salt to serve as a control. The reaction products were all found to be phase pure through powder X‐ray diffraction, for example, with refined lattice constants ofa = 5.512(5) Å,b = 5.567(3) Å, andc = 15.516(8) Å from the Na₂SO₄salt reaction. The products were characterized using UV‐Vis diffuse reflectance spectroscopy to have a bandgap size of ~3.5 eV. The particles sizes were analyzed by scanning electron microscopy (SEM) and found to be dependent upon the flux type used, from ~<1 μm to >10 μm in length, with overall surface areas that could be varied from 0.66 m²/g (for NaF) to 1.55 m²/g (for NaBr). Cubic‐shaped particle morphologies were observed for the metal halide salts with the set of exposed (100)/(010)/(001) crystal facets, while a truncated octahedral morphology formed in the sodium sulfate salt reaction with predominantly the set of (110)/(101)/(011) crystal facets. The products were found to be photocatalytically active for hydrogen production under UV‐Vis irradiation, with the aid of a 1 wt% Pt surface cocatalyst. The platinized NaNbO₃particles were suspended in an aqueous 20% methanol solution and irradiated by UV‐Vis light (λ > 230 nm). After 6 hours of irradiation, the average total hydrogen production varied with the particle morphologies and sizes, with 753 µmol for Na₂SO₄, 334 µmol for NaF, 290 µmol for NaCl, 81 µmol for NaBr, and 249 µmol for the solid‐state synthesized NaNbO₃. These trends show a clear relationship to particle sizes, with smaller particles showing higher photocatalytic activity in the order of NaF > NaCl > NaBr. Furthermore, the particle morphologies obtained from the Na₂SO₄flux showed even higher photocatalytic activity, though having a relatively similar overall surface area, owing to the higher activity of the (110) crystal facets. The apparent quantum yield (100 mW/cm²,λ = 230 to 350 nm, pH = 7) was measured to be 3.7% for NaNbO₃prepared using the NaF flux, but this was doubled to 6.8% when prepared using the Na₂SO₄flux. Thus, these results demonstrate the powerful utility of flux synthetic techniques to control particle sizes and to expose higher‐activity crystal facets to boost their photocatalytic activities for molecular hydrogen production.

    

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2. Perovskite Na-ion conductors developed from analogous Li 3x La 2/3−x TiO 3 (LLTO): chemo-mechanical and defect engineering

   https://doi.org/10.1039/d1ta04252a  

   Lin, Yu-Ying ; Gustafson, William J. ; Murray, Shannon E. ; Shoemaker, Daniel P. ; Ertekin, Elif ; Krogstad, Jessica A. ; Perry, Nicola H. ( September 2021 , Journal of Materials Chemistry A)

   Na-ion conducting solid electrolytes can enable both the enhanced safety profile of all-solid-state-batteries and the transition to an earth-abundant charge-carrier for large-scale stationary storage. In this work, we developed new perovskite-structured Na-ion conductors from the analogous fast Li-ion conducting Li 3 x La 2/3− x TiO 3 (LLTO), testing strategies of chemo-mechanical and defect engineering. Na x La 2/3−1/3 x ZrO 3 (NLZ) and Na x La 1/3−1/3 x Ba 0.5 ZrO 3 (NLBZ) were prepared using a modified Pechini method with varying initial stoichiometries and sintering temperatures. With the substitution of larger framework cations Zr 4+ and Ba 2+ on B- and A-sites respectively, NLZ and NLBZ both had larger lattice parameters compared to LLTO, in order to accommodate and potentially enhance the transport of larger Na ions. Additionally, we sought to introduce Na vacancies through (a) sub-stoichiometric Na : La ratios, (b) Na loss during sintering, and (c) donor doping with Nb. AC impedance spectroscopy and DC polarization experiments were performed on both Na 0.5 La 0.5 ZrO 3 and Na 0.25 La 0.25 Ba 0.5 ZrO 3 in controlled gas environments (variable oxygen partial pressure, humidity) at elevated temperatures to quantify the contributions of various possible charge carriers (sodium ions, holes, electrons, oxygen ions, protons). Our results showed that the lattice-enlarged NLZ and NLBZ exhibited ∼19× (conventional sintering)/49× (spark plasma sintering) and ∼7× higher Na-ion conductivities, respectively, compared to unexpanded Na 0.42 La 0.525 TiO 3 . Moreover, the Na-ion conductivity of Na 0.5 La 0.5 ZrO 3 is comparable with that of NaNbO 3 , despite having half the carrier concentration. Additionally, more than 96% of the total conductivity in dry conditions was contributed by sodium ions for both compositions, with negligible electronic conductivity and little oxygen ion conductivity. We also identified factors that limited Na-ion transport: NLZ and NLBZ were both challenging to densify using conventional sintering without the loss of Na because of its volatility. With spark plasma sintering, higher density can be achieved. In addition, the NLZ perovskite phase appeared unable to accommodate significant Na deficiency, whereas NLBZ allowed some. Density functional theory calculations supported a thermodynamic limitation to creation of Na-deficient NLZ in favor of a pyrochlore-type phase. Humid environments generated different behavior: in Na 0.25 La 0.25 Ba 0.5 ZrO 3 , incorporated protons raised total conductivity, whereas in Na 0.5 La 0.5 ZrO 3 , they lowered total conductivity. Ultimately, this systematic approach revealed both effective approaches and limitations to achieving super-ionic Na-ion conductivity, which may eventually be overcome through alternative processing routes. 

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3. Spin Transitions and Compressibility of ε‐Fe 7 N 3 and γ′‐Fe 4 N: Implications for Iron Alloys in Terrestrial Planet Cores

   https://doi.org/10.1029/2020JB020660  

   Lv, Mingda ; Liu, Jiachao ; Zhu, Feng ; Li, Jie ; Zhang, Dongzhou ; Xiao, Yuming ; Dorfman, Susannah M. ( November 2020 , Journal of Geophysical Research: Solid Earth)

   Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe₇N₃and γ′‐Fe₄N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe₇N₃and γ′‐Fe₄N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe₇N₃by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe₄N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yieldsV₀ = 83.29 ± 0.03 (ų),K₀ = 232 ± 9 GPa,K₀′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe₇N₃above the spin transition completion pressure, andV₀ = 54.82 ± 0.02 (ų),K₀ = 152 ± 2 GPa,K₀′ = 4.0 ± 0.1 for γ′‐Fe₄N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe₃S, Fe₃P, Fe₇C₃, and Fe₃C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe₃S, Fe₃P, and Fe₃C induces elastic stiffening, whereas that of Fe₇C₃induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth's and planetary cores.

    

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4. Enhanced Piezoelectric, Ferroelectric, and Electrostrictive Properties of Lead‐Free (1‐ x )BCZT‐( x )BCST Electroceramics with Energy Harvesting Capability

   https://doi.org/10.1002/smll.202300549  

   Baraskar, Bharat G. ; Kolekar, Yesappa D. ; Thombare, Balu. R. ; James, Ajit. R. ; Kambale, Rahul C. ; Ramana, C. V. ( May 2023 , Small)

   Next‐generation electronics and energy technologies can now be developed as a result of the design, discovery, and development of novel, environmental friendly lead (Pb)‐free ferroelectric materials with improved characteristics and performance. However, there have only been a few reports of such complex materials’ design with multi‐phase interfacial chemistry, which can facilitate enhanced properties and performance. In this context, herein, novel lead‐free piezoelectric materials (1‐x)Ba_0.95Ca_0.05Ti_0.95Zr_0.05O₃‐(x)Ba_0.95Ca_0.05Ti_0.95Sn_0.05O₃, are reported, which are represented as (1‐x)BCZT‐(x)BCST, with demonstrated excellent properties and energy harvesting performance. The (1‐x)BCZT‐(x)BCST materials are synthesized by high‐temperature solid‐state ceramic reaction method by varyingxin the full range (x= 0.00–1.00). In‐depth exploration research is performed on the structural, dielectric, ferroelectric, and electro‐mechanical properties of (1‐x)BCZT‐(x)BCST ceramics. The formation of perovskite structure for all ceramics without the presence of any impurity phases is confirmed by X‐ray diffraction (XRD) analyses, which also reveals that the Ca²⁺, Zr⁴⁺, and Sn⁴⁺are well dispersed within the BaTiO₃lattice. For all (1‐x)BCZT‐(x)BCST ceramics, thorough investigation of phase formation and phase‐stability using XRD, Rietveld refinement, Raman spectroscopy, high‐resolution transmission electron microscopy (HRTEM), and temperature‐dependent dielectric measurements provide conclusive evidence for the coexistence of orthorhombic + tetragonal (Amm2+P4mm) phases at room temperature. The steady transition ofAmm2crystal symmetry toP4mmcrystal symmetry with increasingxcontent is also demonstrated by Rietveld refinement data and related analyses. The phase transition temperatures, rhombohedral‐orthorhombic (T_R‐O), orthorhombic‐ tetragonal (T_O‐T), and tetragonal‐cubic (T_C), gradually shift toward lower temperature with increasingxcontent. For (1‐x)BCZT‐(x)BCST ceramics, significantly improved dielectric and ferroelectric properties are observed, including relatively high dielectric constantε_r≈ 1900–3300 (near room temperature),ε_r≈ 8800–12 900 (near Curie temperature), dielectric loss, tanδ≈ 0.01–0.02, remanent polarizationP_r≈ 9.4–14 µC cm⁻², coercive electric fieldE_c≈ 2.5–3.6 kV cm⁻¹. Further, high electric field‐induced strainS≈ 0.12–0.175%, piezoelectric charge coefficientd₃₃≈ 296–360 pC N⁻¹, converse piezoelectric coefficient ≈ 240–340 pm V⁻¹, planar electromechanical coupling coefficientk_p≈ 0.34–0.45, and electrostrictive coefficient (Q₃₃)_avg≈ 0.026–0.038 m⁴C⁻²are attained. Output performance with respect to mechanical energy demonstrates that the (0.6)BCZT‐(0.4)BCST composition (x= 0.4) displays better efficiency for generating electrical energy and, thus, the synthesized lead‐free piezoelectric (1‐x)BCZT‐(x)BCST samples are suitable for energy harvesting applications. The results and analyses point to the outcome that the (1‐x)BCZT‐(x)BCST ceramics as a potentially strong contender within the family of Pb‐free piezoelectric materials for future electronics and energy harvesting device technologies.

    

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5. Enabling coherent BaZrO 3 nanorods/YBa 2 Cu 3 O 7−x interface through dynamic lattice enlargement in vertical epitaxy of BaZrO 3 /YBa 2 Cu 3 O 7−x nanocomposites

   https://doi.org/10.1088/1361-6668/ac4aff  

   Wu, Judy Z ; Ogunjimi, Victor ; Sebastian, Mary Ann ; Zhang, Di ; Jian, Jie ; Huang, Jijie ; Zhang, Yifan ; Gautam, Bibek ; Haugan, Timothy ; Wang, Haiyan ( February 2022 , Superconductor Science and Technology)

   Abstract One-dimensional c -axis-aligned BaZrO 3 (BZO) nanorods are regarded as strong one-dimensional artificial pinning centers (1D-APCs) in BZO-doped YaBa 2 Cu 3 O 7− x (BZO/YBCO) nanocomposite films. However, a microstructure analysis has revealed a defective, oxygen-deficient YBCO column around the BZO 1D-APCs due to the large lattice mismatch of ∼7.7% between the BZO (3a = 1.26 nm) and YBCO (c = 1.17 nm), which has been blamed for the reduced pinning efficiency of BZO 1D-APCs. Herein, we report a dynamic lattice enlargement approach on the tensile strained YBCO lattice during the BZO 1D-APCs growth to induce c -axis elongation of the YBCO lattice up to 1.26 nm near the BZO 1D-APC/YBCO interface via Ca/Cu substitution on single Cu-O planes of YBCO, which prevents the interfacial defect formation by reducing the BZO/YBCO lattice mismatch to ∼1.4%. Specifically, this is achieved by inserting thin Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7− x (CaY-123) spacers as the Ca reservoir in 2–6 vol.% BZO/YBCO nanocomposite multilayer (ML) films. A defect-free, coherent BZO 1D-APC/YBCO interface is confirmed in transmission electron microscopy and elemental distribution analyses. Excitingly, up to five-fold enhancement of J c ( B ) at magnetic field B = 9.0 T// c -axis and 65 K–77 K was obtained in the ML samples as compared to their BZO/YBCO single-layer (SL) counterpart’s. This has led to a record high pinning force density F p together with significantly enhanced B max at which F p reaches its maximum value F p,max for BZO 1D-APCs at B // c -axis. At 65 K, the F p,max ∼158 GN m −3 and B max ∼ 8.0 T for the 6% BZO/YBCO ML samples represent a significant enhancement over F p,max ∼ 36.1 GN m −3 and B max ∼ 5.0 T for the 6% BZO/YBCO SL counterparts. This result not only illustrates the critical importance of a coherent BZO 1D-APC/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO 1D-APCs. 

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