Carlos Frontera

Anion order in heteroanionic compounds may induce electrical dipoles and the formation of noncentrosymmetric polar structures. The majority of perovskite oxynitrides are centrosymmetric, but some of them show high dielectric permittivities and ferroelectric response resulting from local anion order. Here, we report a study of polar, partially anion-ordered Nd2AlO3N and Sm2AlO3N n = 1 Ruddlesden–Popper oxynitrides, prepared by solid-state reaction in flowing N2 at 1500 °C. They crystallize in the noncentrosymmetric I4mm space group with parameters a = 3.703314(3) and c = 12.545382(14) Å for Nd2AlO3N and a = 3.674477(5) and c = 12.40414(3) Å for Sm2AlO3N. Neutron powder diffraction data of Nd2AlO3N indicate that oxygen occupies the equatorial sites 4b of the AlO5N octahedra, whereas both anions occupy two crystallographically independent 2a axial X2 and X3 sites, with different O/N populations (68/32 for X2 and 32/68 for X3) and a larger bond distance to the Al atom for the oxygen rich site. Dielectric permittivities of 260 and 150 are measured for Nd2AlO3N and Sm2AlO3N, respectively, and second-harmonic generation spectroscopy and scanning transmission electron microscopy furnish complementary evidence of the polar nature of both compounds. These results represent the first experimental demonstration of acentric functionalities in a Ruddlesden–Popper oxynitride.

Prussian blue analogues (PBAs) are widely investigated as positive electrode materials for sodium-ion batteries. Amongst them, FeFe and FeMn based materials are prticularly interesting, and several chemical suppliers have recently begun offering commercial-grade materials. Here, we perform a comprehensive structural, spectroscopic, and electrochemical comparison of two commercially available MnFe–PW (Prussian White) samples and one FeFe–PB (Prussian Blue) analogues. Despite nominal compositional similarity, the MnFe–PW samples show markedly different electrochemical behaviour when tested in high loading electrodes which aligns with differences in microstructure and impurity levels. Operando synchrotron FTIR and operando X-ray diffraction have been performed to get further insight on the fate of water molecules during cell operation, which was found to be significantly different for MnFe–PW and FeFe–PB.

In this work, we investigate how the crystallographic growth direction influences spin current transmission in antiferromagnetic (AF) NiO thin films. By manipulating epitaxial growth, we explored the spin transport characteristics in La_2/3Sr_1/3MnO₃/NiO/Pt heterostructures grown on top of (001)- and (111)-oriented SrTiO₃ substrates, varying the NiO barrier thickness (t_NiO). Spin currents were generated via spin pumping (SP), and detection was done by the inverse spin Hall effect (ISHE). X-ray diffraction and high-resolution electron microscopy techniques confirmed high-quality epitaxial films with nearly atomically sharp interfaces and similar dislocation distributions, irrespective of the growth direction. Nevertheless, epitaxially engineered (111) heterostructures exhibited superior spin transport properties, including lower magnetic damping (α), longer spin diffusion lengths (λ_Sd), and higher spin mixing conductance (g^↑↓). The temperature dependence of the ISHE voltage signal (V_ISHE) also showed orientation-dependent behavior: while (001)-oriented samples followed a monotonic trend, (111)-oriented samples displayed a peak that shifted to higher temperatures with increasing t_NiO, associated with the emergence of AF ordering. Moreover, (111)-oriented samples demonstrated notable spin current amplification at room temperature, peaking at t_NiO ≈ 1 nm before decaying quasi-exponentially, indicative of spin diffusion-mediated conduction. Although the spin diffusion length in (111)-oriented samples was roughly double that of their (001)-oriented counterparts, it was still too short to be explained by angular momentum transport by mobile antiferromagnons through NiO. Instead, these findings point to a mechanism involving magnetic correlations and short-range thermal magnons. The superior spin transport properties and the enhanced spin conduction in (111)-oriented samples are primarily attributed to a synergistic combination of interfacial and dynamic effects, a more favorable Néel vector alignment and distinct interface symmetry, which can enhance spin-Hall effects or enable different spin textures. Overall, this study underscores the pivotal role of the Néel vector and crystallographic orientation in AF spin transport, providing valuable insights for the design and optimization of spintronic devices.

This study reports on the epitaxial growth and structural characterization of ultrathin NiO‐films deposited by magnetron sputtering on La_2/3Sr_1/3MnO₃ (LSMO) films grown on SrTiO₃ (STO) substrates with (001)‐ and (111)‐orientations. X‐ray diffraction and atomic‐force microscopy show that all NiO layers are single‐phase, face‐centered pseudo‐cubic, atomically smooth, root‐main‐square (RMS) surface roughness <0.15 nm, and form abrupt interfaces with LSMO. High‐resolution reciprocal‐space maps reveal that the films are largely relaxed, but exhibit a slight compressive distortion, yielding unit‐cell volumes larger than bulk NiO. Despite a nominal ≈7% lattice mismatch, aberration‐corrected scanning transmission electron microscopy uncovers an array of misfit dislocations at the NiO/LSMO interface that help to accommodate strain allowing epitaxial growth of NiO layers. On (001)‐oriented samples, the four antiferromagnetic T‐domains are oblique to the sample plane, while on the (111) case, one lies in‐plane. This in‐plane domain shows greater spacing between ferromagnetic (111) planes due to unit cell distortion. This structural domain splitting can influence magnetic order and spin transmission efficiency, highlighting crystallographic orientation as a key factor in designing high‐performance spintronic devices.

Eu ²⁺ ₃ Eu ³⁺ Ta ₂ ON ₅ is an anion deficient oxynitride with square pyramidal coordination for Ta that shows ferromagnetic order of Eu ²⁺ ions below 20 K, with superexchange interactions enhanced by nitride-reinforced hybridization of Eu–(N/O)–Eu bonds.

The new double perovskite oxynitride Ba₂MnWO_4.42N_1.58 has been obtained by topochemical ammonolysis at 700 °C of B-site ordered Ba₂MnWO₆ using a high NH₃ flow rate of 600 cm³/min. The relatively low synthesis temperature hinders the cation mobility, allowing the order of Mn and W cations of the precursor oxide to be unperturbed in the oxynitride. Synchrotron X-ray diffraction, electron diffraction, and neutron diffraction indicate that Ba₂MnWO_4.42N_1.58 crystallizes in the Fm3̅m space group with a larger parameter of 8.2434(5) Å compared to Ba₂MnWO₆ [8.20337(1) Å]. Magnetic susceptibility measurements show that Ba₂MnWO_4.42N_1.58 orders antiferromagnetically below T _N = 3.8 K, and the observed Curie-Weiss temperature θ_CW = -70(3) K indicates a frustrated Mn lattice with frustration parameter f = |θ_CW|/T _N ≈ 18, which is significantly larger than for the oxide (f ≈ 7.2). The substitution of oxide by the nitride anion induces the oxidation of Mn²⁺ to Mn^3+/4+ and a subsequent decrease of the paramagnetic effective moment from 6.28 to 5.16 μ_B/f.u. The observation in the oxynitride of a frustration parameter which is larger than twice that of the oxide precursor is rationalized as caused by the relative enhancement of the nearest neighbors' magnetic interactions (J ₁) Mn-(O/N)-(O/N)-Mn at 90°, compared to next-nearest neighbors' interactions at 180° (J ₂) Mn-(O/N)-W-(O/N)-Mn, due to the smaller electronegativity of nitrogen compared to oxygen that increases the covalency of bonding. These results expand the chemical and structural diversity of complex transition metal oxynitrides and provide a new route to tailor spin frustration in transition metal double perovskites.

FullProfAPP is a software tool for data processing, refinement and visualization of large collections of powder diffraction patterns. Featuring an intuitive graphical user interface, it seamlessly facilitates a variety of tasks. These include conducting full-profile phase searches, sequential and high-throughput Rietveld refinements, and managing background (and peak) detection. FullProfAPP also provides convenient interaction with crystallographic databases and supports the visualization and export of high-quality pixel and vector graphics depicting the refinement results, among other functionalities. FullProfAPP wraps around the refinement program FullProf [Rodríguez-Carvajal (1993), Physica B, 192, 55–69] and offers the flexibility of user-defined workflows by accessing and editing FullProf's input files and triggering its execution as necessary. FullProfAPP is distributed as open-source software and is presently available for Windows and Linux operating systems.