Magnetic Phase Separation in the Oxypnictide Sr₂Cr_1.85Mn_1.15As₂O₂

Layered Sr2M3As2O2-type oxypnictides are composed of tetrahedral M2Pn2 and square planar MO2 layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr2Cr3–xMnxAs2O2 solid solution, Sr2Cr2MnAs2O2 has been synthesized. The compound crystallizes in the I4/mmm tetragonal space group with a refined stoichiometry of Sr2Cr1.85Mn1.15As2O2. The M(2) site within the M2Pn2 slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below TN(M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO2 layers is fully occupied by Cr, and antiferromagnetic order is observed below TN(M1) = 200 K. Along c, there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr2M3As2O2 phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds

Localized Spin Dimers and Structural Distortions in the Hexagonal Perovskite Ba₃CaMo₂O₉

Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba3CaMo2O9. Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo2O9 dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba3SrMo2O9 analogue, in which the Mo2O9 dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba3CaMo2O9 and Ba3SrMo2O9 have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo5+ hexagonal perovskites

Imaging Dye Aggregation in MK-2, N3, N749, and SQ‑2 dye···TiO₂ Interfaces That Represent Dye-Sensitized Solar Cell Working Electrodes

Dye-sensitized solar cells (DSSCs) are a strong contender for next-generation photovoltaic technology with niche applications as solar-powered windows. The performance of a DSSC is particularly susceptible to the dye sensitizer, which is adsorbed onto the surface of a wide-band-gap semiconductor such as TiO2, to form the working electrode. The nature by which such surfaces are sensitized stands to influence the resulting dye···TiO2 interfacial structure and thence the operational performance of the DSSC working electrode. In particular, a nanoscopic understanding of the sensitization process would ultimately help to improve DSSC device function. In this study, atomic force microscopy (AFM) is used to image the nanoscopic formation of dye···TiO2 interfacial structures. This employs, as case studies, four well-known DSSC dyes adsorbed onto amorphous TiO2 substrates: two ruthenium-based dyes, N3 and the Black Dye (N749); and two organic dyes, the thiophenylcarbazole, MK-2, and the zwitterionic squaraine, SQ-2. We discover that all four dyes present some form of aggregation upon sensitization of TiO2, whose spatial distributions show distinct nanoaggregate particle characteristics. These particle clusters of N749, N3, and MK-2 are found to assemble in lines of nanoaggregates, while clusters of SQ-2 dye chromophores distribute themselves randomly on the amorphous TiO2 substrates. This nanoparticle structural assembly persists even when these dye···TiO2 interfaces are fabricated using hundred-fold diluted dye sensitization concentrations. The formation of dye aggregates in N749 is further studied as a function of dye sensitization time. This tracks the pattern formation of aggregates of N749 and reveals that dye aggregation begins within the first hour and has completed within a 5 h period. The large expanse of dye nanoaggregates observed shows that dye···dye interactions are much more important than previously envisaged, while the nature of their spatial distribution can be related to different aggregation modes of the dye molecules. These nanostructural features will undoubtedly impact the performance of DSSCs

Terahertz Faraday Rotation of SrFe₁₂O₁₉ Hexaferrites Enhanced by Nb Doping

The magneto-optical and dielectric behavior of M-type hexaferrites as permanent magnets in the THz band is essential for potential applications like microwave absorbers and antennas, while are rarely reported in recent years. In this work, single-phase SrFe12–xNbxO19 hexaferrite ceramics were prepared by the conventional solid-state sintering method. Temperature dependence of dielectric parameters was investigated here to determine the relationship between dielectric response and magnetic phase transition. The saturated magnetization increases by nearly 12%, while the coercive field decreases by 30% in the x = 0.03 composition compared to that of the x = 0.00 sample. Besides, the Nb substitution improves the magneto-optical behavior in the THz band by comparing the Faraday rotation parameter from 0.75 (x = 0.00) to 1.30 (x = 0.03). The changes in the magnetic properties are explained by a composition-driven increase of the net magnetic moment and enhanced ferromagnetic exchange coupling. The substitution of the donor dopant Nb on the Fe site is a feasible way to obtain multifunctional M-type hexaferrites as preferred candidates for permanent magnets, sensors, and other electronic devices

Counting the Acid Sites in a Commercial ZSM‑5 Zeolite Catalyst

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized

Site-Selective d¹⁰/d⁰ Substitution in an <i>S</i> = ¹/₂ Spin Ladder Ba₂CuTe_1–<i>x</i>W_<i>x</i>O₆ (0 ≤ <i>x</i> ≤ 0.3)

Isovalent nonmagnetic d10 and d0 B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B′ cations in A2B′B″O6 double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d10 or d0. Furthermore, the competition generated by introducing mixtures of d10 and d0 cations can drive the material into the realms of exotic quantum magnetism. Here, Te6+ d10 was substituted by W6+ d0 in the hexagonal perovskite Ba2CuTeO6, which possesses a spin ladder geometry of Cu2+ cations, creating a Ba2CuTe1–xWxO6 solid solution (x = 0–0.3). We find W6+ is almost exclusively substituted for Te6+ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, Jrung and Jleg, interactions. Modeling the magnetic susceptibility data shows the d0 orbitals modify the relative intraladder interaction strength (Jrung/Jleg) so the system changes from a spin ladder to isolated spin chains as W6+ increases. This further demonstrates the utility of d10 and d0 dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures