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1. Magnetoelectric Properties of Aurivillius-Layered Perovskites

   https://doi.org/10.3390/cryst14040299  

   Veenachary, Vadla; Ramana, Eskilla Venkata; Narendra_Babu, Simhachalam; Puli, Venkata Sreenivas; Saha, Sujoy; Srinivasan, Gopalan; Prasad, G; Prasad, N V (April 2024, Crystals)

   In the present work, we have synthesized rare-earth ion modified Bi4−xRExTi2Fe0.7Co0.3O12−δ (RE = Dy, Sm, La) multiferroic compounds by the conventional solid-state route. Analysis of X-ray diffraction by Rietveld refinement confirmed the formation of a polycrystalline orthorhombic phase. The morphological features revealed a non-uniform, randomly oriented, plate-like grain structure. The peaks evident in the Raman spectra closely corresponded to those of orthorhombic Aurivillius phases. Dielectric studies and impedance measurements were carried out. Asymmetric complex impedance spectra suggested the relaxation of charge carriers belonging to the non-Debye type and controlled by a thermally activated process. Temperature-dependent AC conductivity data showed a change of slope in the vicinity of the phase transition temperature of both magnetic and electrical coupling natures. Based on the universal law and its exponent nature, one can suppose that the conduction process is governed by a small polaron hopping mechanism but significant distortion of TiO6 octahedral. The doping of the A-sites with rare-earth element ions and changes in the concentrations of Fe and Co ions located on the B-sites manifested themselves in saturated magnetic hysteresis loops, indicating competitive interactions between ferroelectric and canted antiferromagnetic spins. The magnetic order in the samples is attributed to pair-wise interactions between adjacent Fe3+–O–Fe3+, Co2+/3+–O–Co3+/2+, and Co2+/3+–O–Fe3+ ions or Dzyaloshinskii–Moriya interactions among magnetic ions in the adjacent sub-lattices. As a result, enhanced magnetoelectric coefficients of 42.4 mV/cm-Oe, 30.3 mV/cm-Oe, and 21.6 mV/cm-Oe for Bi4−xDyxTi2Fe0.7Co0.3O12−δ (DBTFC), Bi4−xLaxTi2Fe0.7Co0.3O12−δ (LBTFC), and Bi4−xSmxTi2Fe0.7Co0.3O12−δ (SBTFC), respectively, have been obtained at lower magnetic fields (<3 kOe). The strong coupling of the Aurivillius compounds observed in this study is beneficial to future multiferroic applications. 

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2. High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium( iii ) metallocenium single-molecule magnets

   https://doi.org/10.1039/c8sc03907k  

   Randall McClain, K.; Gould, Colin A.; Chakarawet, Khetpakorn; Teat, Simon J.; Groshens, Thomas J.; Long, Jeffrey R.; Harvey, Benjamin G. (November 2018, Chemical Science)

   A series of dysprosium( iii ) metallocenium salts, [Dy(Cp iPr4R ) 2 ][B(C 6 F 5 ) 4 ] (R = H ( 1 ), Me ( 2 ), Et ( 3 ), iPr ( 4 )), was synthesized by reaction of DyI 3 with the corresponding known NaCp iPr4R (R = H, iPr) and novel NaCp iPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy–C distances and larger Cp–Dy–Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy–C distance decreases and the Cp–Dy–Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2–4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature of 62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm −1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s −1 . Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature Dy III metallocenium single-molecule magnets. 

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3. Domain-wall magnetoelectric coupling in multiferroic hexagonal YbFeO3 films

   https://doi.org/10.1038/s41598-023-28365-x  

   Li, Xin; Yun, Yu; Thind, Arashdeep Singh; Yin, Yuewei; Li, Qiang; Wang, Wenbin; N’Diaye, Alpha T.; Mellinger, Corbyn; Jiang, Xuanyuan; Mishra, Rohan; et al (January 2023, Scientific Reports)

   Abstract Electrical modulation of magnetic states in single-phase multiferroic materials, using domain-wall magnetoelectric (ME) coupling, can be enhanced substantially by controlling the population density of the ferroelectric (FE) domain walls during polarization switching. In this work, we investigate the domain-wall ME coupling in multiferroic h-YbFeO₃thin films, in which the FE domain walls induce clamped antiferromagnetic (AFM) domain walls with reduced magnetization magnitude. Simulation according to the phenomenological theory indicates that the domain-wall ME effect is dramatically enhanced when the separation between the FE domain walls shrinks below the characteristic width of the clamped AFM domain walls during the ferroelectric switching. Experimentally, we show that while the magnetization magnitude remains same for both the positive and the negative saturation polarization states, there is evidence of magnetization reduction at the coercive voltages. These results suggest that the domain-wall ME effect is viable for electrical control of magnetization. 

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4. Local structure analysis and structure mining for design of photocatalytic metal oxychloride intergrowths

   https://doi.org/10.1039/D2TA05663A  

   Chatterjee, Kaustav; Magnard, Nicolas Pierre; Mathiesen, Jette K.; Jensen, Kirsten M.; Skrabalak, Sara E. (November 2022, Journal of Materials Chemistry A)

   In this work, the local structures of durable, high-activity Bi 4 TaO 8 Cl–Bi 2 GdO 4 Cl intergrowth photocatalysts that were prepared in a molten flux are determined by pair distribution function analysis of X-ray total scattering data and correlated to their photocatalytic performance. This system gives understanding to how the local structure of photocatalysts can be manipulated controllably through incorporation of rigid and flexible layers via intergrowth formation to achieve high activity. This analysis revealed that the local symmetry and distortion of the [TaO 6 ] octahedra introduced through intergrowth formation and dictated by intergrowth stoichiometry correlate with their photocatalytic activity. That is, the greater the Ta–O–Ta bond angles, the higher the photocatalytic activity of a given intergrowth for the oxygen evolution reaction. Moreover, greater tilting of the [TaO 6 ] octahedra is associated with a larger band gap. This analysis was coupled with a structure mining approach to model the intergrowth structure by building supercells for refinement of the X-ray diffraction data. This analysis found that Ta- and Gd-domains are separated within the intergrowths, with large Gd-domains separated by small Ta-domains at high Gd% and the opposite for high Ta%. Taken together with Williamson–Hall analysis, our results highlight that the local structure of layered materials can be modulated through strain engineering enabled by the selection of rigid and flexible intergrowth layers, providing a new design pathway to high performance photocatalysts. 

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5. Electronic and nanoscale structures of metal oxyhalide intergrowth photocatalysts

   https://doi.org/10.1063/5.0159761  

   Chatterjee, Kaustav; Stanley, Jared; Dravid, Vinayak P; dos_Reis, Roberto; Skrabalak, Sara E (October 2023, Applied Physics Letters)

   Multimetal oxyhalide intergrowths show promise for photocatalytic water splitting. However, the relationships between intergrowth stoichiometry and their electronic and nanoscale structures are yet to be identified. This study investigates Bi4TaO8Cl–Bi2GdO4Cl intergrowths and demonstrates that stoichiometry controls the tilting of [TaO6] octahedra, influencing the bandgap of the photocatalyst and its valence and conduction band positions. To determine how the [TaO6] octahedral tilting in the intergrowths manifests as a function of intergrowth stoichiometry, we investigated changes in crystal symmetry by analyzing features arising at the higher order Laue zone (HOLZ) of convergent-beam electron diffraction patterns. Higher Ta content intergrowths displayed a more intense outer HOLZ ring compared to lower Ta content intergrowths, indicating transformation from P21cn (orthorhombic) to P4/mmm (tetragonal). This finding suggests that more distortion occurs along the ⟨001⟩ directions of the crystal than the ⟨100⟩ and ⟨010⟩ directions. This variation directly impacts the electronic structure, affecting both conduction and valence band energy levels. By combining ultraviolet photoelectron spectroscopy, UV-visible diffuse reflectance spectroscopy, and electron energy loss spectroscopy, the absolute band positions of the intergrowths were determined. Agreement between the bandgaps obtained via ensemble and nanoscale measurements indicates nanoscale homogeneity of the electronic structure. Overall, the integrated approach establishes that the bandgap energy increases with increasing Ta content, which is correlated with the crystal symmetry and [TaO6] octahedral tilting. Broadly, the modular nature of intergrowths provides building block layers to tune octahedral tilting within perovskite layers for manipulation of optoelectronic properties. 

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