Xavier Torrelles

High‐quality purely c‐axis oriented epitaxial thin films of the Aurivillius phase Sr₂Bi₄Ti₅O₁₈ with n = 5 (Sr,Bi)TiO₃ perovskite‐like layers, are grown on SrTiO₃ substrates by pulsed laser deposition. The highest crystalline quality is obtained with a 20 wt.% Bi‐excess target and average stacking order values in the proximity of the ideal value n = 5 are attained for an optimum deposition temperature of 650 °C. Scanning transmission electron microscopy reveals regions with n ranging from 4 to 6 around an average thickness of n = 5, in agreement with the X‐ray diffraction analysis. Interdigital electrodes are used to probe the in‐plane polarization and survey the electrocaloric properties. A maximum adiabatic temperature change of ΔT ∼ 0.95 °C for an electric field of 150 kV cm⁻¹ is observed at ≈135 °C. Larger values are expected at higher temperatures around the ferroelectric Curie temperature, T_C. Since T_C of Sr₂Bi₄Ti₅O₁₈ can be tuned by codoping, the findings pave the way toward a large electrocaloric effect at ambient temperature.

Layered perovskites of general formula AA' CuFeO 5 are characterized by the presence of spiral magnetic phases whose ordering temperatures T spiral can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at temperatures suitable for applications. Moreover, it has been proposed that the highest T spiral value that can be reached in this structural family ( ∼ 400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here eleven YBa 1 − x Sr x CuFeO 5 solid solutions ( 0 ≤ x ≤ 1 ), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization measurements, synchrotron x-ray and neutron powder diffraction we show that the spiral state with k s = ( 1 2 , 1 2 , 1 2 ± q ) is destabilized beyond a critical Sr content, being replaced by a fully antiferromagnetic state with ordering temperature T coll 2 ≥ T spiral and propagation vector k c 2 = ( 1 2 , 1 2 , 0 ) . Interestingly, both T spiral and T coll 2 increase with x with comparable rates. This suggests a common, disorder-driven origin for both magnetic phases, consistent with theoretical predictions. Published by the American Physical Society 2024

Owing to the importance of the single-wavelength anomalous diffraction (SAD) technique, the recently developed |ρ|-based phasing algorithm (S _M,|ρ|) incorporating the inner-pixel preservation (ipp) procedure [Rius & Torrelles (2021). Acta Cryst A77, 339–347] has been adapted to the determination of anomalous scattering substructures and its applicability tested on a series of 12 representative experimental data sets, mostly retrieved from the Protein Data Bank. To give an idea of the suitability of the data sets, the main indicators measuring their quality are also given. The dominant anomalous scatterers are either SeMet or S atoms, or metals/clusters incorporated by soaking. The resulting SAD-adapted algorithm solves the substructures of the test protein crystals quite efficiently.

The incorporation of the new peakness-enhancing fast Fourier transform compatible ipp procedure (ipp = inner-pixel preservation) into the recently published S_M algorithm based on |ρ| [Rius (2020). Acta Cryst A76, 489–493] improves its phasing efficiency for larger crystal structures with atomic resolution data. Its effectiveness is clearly demonstrated via a collection of test crystal structures (taken from the Protein Data Bank) either starting from random phase values or by using the randomly shifted modulus function (a Patterson-type synthesis) as initial ρ estimate. It has been found that in the presence of medium scatterers (e.g. S or Cl atoms) crystal structures with 1500 × c atoms in the unit cell (c = number of centerings) can be routinely solved. In the presence of strong scatterers like Fe, Cu or Zn atoms this number increases to around 5000 × c atoms. The implementation of this strengthened S_M algorithm is simple, since it only includes a few easy-to-adjust parameters.

The surface structure of fluoroapatite (0001) (FAp₀₀₀₁) under quasi-dry and humid conditions has been probed with surface X-ray diffraction (SXRD). Lateral and perpendicular atomic relaxations corresponding to the FAp₀₀₀₁ termination before and after H₂O exposure and the location of the adsorbed water molecules have been determined from experimental analysis of the crystal truncation rod (CTR) intensities. The surface under dry conditions exhibits a bulk termination with relaxations in the outermost atomic layers. The hydrated surface is formed by a disordered partially occupied H₂O layer containing one water molecule (33% surface coverage) adsorbed at each of the three surface Ca atoms, and is coupled with one OH group randomly bonded to each of the three topmost P atoms with a 33% surface coverage.

The (011) termination of rutile TiO₂ is reported to be particularly effective for photocatalysis. Here the structure of the interface formed between this substrate and water is revealed using surface X-ray diffraction. While the TiO₂(011) surface exhibits a (2x1) reconstruction in ultra-high vacuum, this is lifted in the presence of a multilayer of water at room temperature. This change is driven by the formation of Ti-OH at the interface, which has a bond distance of 1.93 ± 0.02 Å. The experimental solution is in good agreement with density functional theory and first principles molecular dynamics calculations. These results point to the important differences that can arise between the structure of oxide surfaces in ultra-high vacuum and technical environments and will ultimately lead to an atomistic understanding of the photocatalytic process of water splitting on TiO₂ surfaces.

Cobalt ferrite ultrathin films with the inverse spinel structure are among the best candidates for spin filtering at room temperature. High-quality epitaxial CoFe₂O₄ films about 4 nm thick have been fabricated on Ag(001) following a three-step method: an ultrathin metallic CoFe₂ alloy was first grown in coherent epitaxy on the substrate and then treated twice with O₂, first at room temperature and then during annealing. The epitaxial orientation and the surface, interface and film structure were resolved using a combination of low-energy electron diffraction, scanning tunnelling microscopy, Auger electron spectroscopy and in situ grazing-incidence X-ray diffraction. A slight tetragonal distortion was observed, which should drive the easy magnetization axis in-plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in situ resonant X-ray diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using FDMNES, an ab initio program already extensively used to simulate X-ray absorption spectroscopy, and shows that the Co ions are predominantly located on octahedral sites with an inversion parameter of 0.88 (5). Ex situ X-ray photoelectron spectroscopy gives an estimation in accordance with the values obtained through diffraction analysis.