Three-Dimensional (3D) Microporous Iron Silicate with an Imandrite Type of Structure.

Small-pore iron silicate MS-1 (Minho-Sofia, solid number 1) with a 3D porous system, an analogue of the rare mineral imandrite, has been synthesized and characterized. This material is the lowest framework density iron silicate, one of the most siliceous (Si/Fe = 6) iron silicates, the first iron cyclosilicate achieved at hydrothermal conditions, and the only synthetic iron-based member of the lovozerite mineral group.

Contactless doping characterization of [Formula: see text] using acceptor Cd probes.

Finding suitable p-type dopants, as well as reliable doping and characterization methods for the emerging wide bandgap semiconductor [Formula: see text]-[Formula: see text] could strongly influence and contribute to the development of the next generation of power electronics. In this work, we combine easily accessible ion implantation, diffusion and nuclear transmutation methods to properly incorporate the Cd dopant into the [Formula: see text]-[Formula: see text] lattice, being subsequently characterized at the atomic scale with the Perturbed Angular Correlation (PAC) technique and Density Functional Theory (DFT) simulations. The acceptor character of Cd in [Formula: see text]-[Formula: see text] is demonstrated, with Cd sitting in the octahedral Ga site having a negative charge state, showing no evidence of polaron deformations nor extra point defects nearby. The possibility to determine the charge state of Cd will allow assessing the doping type, in particular proving p-type character, without the need for ohmic contacts. Furthermore, a possible approach for contactless charge mobility studies is demonstrated, revealing thermally activated free electrons for temperatures above [Formula: see text] 648 K with an activation energy of 0.54(1) and local electron transport dominated by a tunneling process between defect levels and the Cd probes at lower temperatures.

Tailoring Negative Thermal Expansion via Tunable Induced Strain in La-Fe-Si-Based Multifunctional Material.

Zero thermal expansion (ZTE) composites are typically designed by combining positive thermal expansion (PTE) with negative thermal expansion (NTE) materials acting as compensators and have many diverse applications, including in high-precision instrumentation and biomedical devices. La(Fe1-x,Six)13-based compounds display several remarkable properties, such as giant magnetocaloric effect and very large NTE at room temperature. Both are linked via strong magnetovolume coupling, which leads to sharp magnetic and volume changes occurring simultaneously across first-order phase transitions; the abrupt nature of these changes makes them unsuitable as thermal expansion compensators. To make these materials more useful practically, the mechanisms controlling the temperature over which this transition occurs and the magnitude of contraction need to be controlled. In this work, ball-milling was used to decrease particles and crystallite sizes and increase the strain in LaFe11.9Mn0.27Si1.29Hx alloys. Such size and strain tuning effectively broadened the temperature over which this transition occurs. The material's NTE operational temperature window was expanded, and its peak was suppressed by up to 85%. This work demonstrates that induced strain is the key mechanism controlling these materials' phase transitions. This allows the optimization of their thermal expansion toward room-temperature ZTE applications.

Group Theory Analysis to Study Phase Transitions of Quasi-2D Sr3Hf2O7.

We present an ab-initio study performed in the framework of density functional theory, group-subgroup symmetry analysis and lattice dynamics, to probe the octahedral distortions, which occur during the structural phase transitions of the quasi-2D layered perovskite Sr3Hf2O7 compound. Such a system is characterized by a high-temperature I4/mmm centrosymmetric structure and a ground-state Cmc21 ferroelectric phase. We have probed potential candidate polymorphs that may form the I4/mmm → Cmc21 transition pathways, namely Fmm2, Ccce, Cmca and Cmcm. We found that the band gap widths increase as the symmetry decreases, with the ground-state structure presenting the largest gap width (∼5.95 eV). By probing the Partial Density of States, we observe a direct relation regarding the tilts and rotations of the oxygen perovskite cages as the transition occurs; these show large variations mostly of the O p-states which contribute mostly to the valence band maximum. Moreover, by analyzing the hyperfine parameters, namely the Electric Field Gradients and asymmetric parameters, we observe variations as the transition occurs, from which it is possible to identify the most plausible intermediate phases. We have also computed the macroscopic polarization and confirm that the Cmc21 phase is ferroelectric with a value of spontaneous polarization of 0.0478 C/m2. The ferroelectricity of the ground-state Cmc21 system arises due to a second order parameter related to the coupling of the rotation and tilts of the O perovskite cages together with the Sr displacements.

Tailoring the Anodic Hafnium Oxide Morphology Using Different Organic Solvent Electrolytes.

Highly ordered anodic hafnium oxide (AHO) nanoporous or nanotubes were synthesized by electrochemical anodization of Hf foils. The growth of self-ordered AHO was investigated by optimizing a key electrochemical anodization parameter, the solvent-based electrolyte using: Ethylene glycol, dimethyl sulfoxide, formamide and N-methylformamide organic solvents. The electrolyte solvent is here shown to highly affect the morphological properties of the AHO, namely the self-ordering, growth rate and length. As a result, AHO nanoporous and nanotubes arrays were obtained, as well as other different shapes and morphologies, such as nanoneedles, nanoflakes and nanowires-agglomerations. The intrinsic chemical-physical properties of the electrolyte solvents (solvent type, dielectric constant and viscosity) are at the base of the properties that mainly affect the AHO morphology shape, growth rate, final thickness and porosity, for the same anodization voltage and time. We found that the interplay between the dielectric and viscosity constants of the solvent electrolyte is able to tailor the anodic oxide growth from continuous-to-nanoporous-to-nanotubes.

Printed Flexible µ-Thermoelectric Device Based on Hybrid Bi2Te3/PVA Composites.

Inorganic-polymer composites have become promising materials to be processed by printing technologies because of their unique properties that allow the fabrication of flexible wearable electronics at reduced manufacturing costs. In the present work, a complete methodological process of assembling a flexible microthermoelectric generator based on inorganic-polymer materials is presented. The used microparticles were prepared by a top-down approach beginning with a previously prepared material by solid-state reaction and later scaled down through the use of ball milling. It was found that the necessity to proceed with a chemical treatment with HCl to reduce Bi2O3 present on the surface of the microparticle leads to a power factor (PF) of 2.29 µW K-2 m-1, which is two times higher than that of the untreated sample. On the fabrication of flexible inorganic-organic thermoelectric thick films based on Bi2Te3 microparticles (<50 µm) and the poly(vinyl alcohol) (PVA) polymer with different thicknesses ranging from 11 to 265 µm and with different Bi2Te3 weight percentages (wt %), we found that PVA allowed achieving a homogeneous dispersion of the parent inorganic thermoelectric materials, while still maintaining their high performance. The best produced ink was obtained with 25 wt % of PVA and 75 wt % of chemically treated Bi2Te3 micropowder with a Seebeck coefficient of -166 µV K-1 and a PF of 0.04 µW K-2 m-1. For this optimized concentration, a flexible thermoelectric device was fabricated using n-type thermoelectric inks, which constitutes a major advantage to be applied in printing techniques because of their low curing temperature. The device architecture was composed of 10 stripes with 0.2 × 2.5 cm2 each in a one-leg configuration. This prototype yielded a power output up to ∼9 µW cm-2 with a 46 K temperature gradient (Δ T), and the results were combined with numerical simulations showing a good match between the experimental and the numerical results. The thermoelectric devices studied in this work offer easy fabrication, flexibility, and an attractive thermoelectric output for specific power requirements such as for environmental health monitoring.

Room temperature synthesis of Bi25FeO39 and hydrothermal kinetic relations between sillenite- and distorted perovskite-type bismuth ferrites.

The time-dependent study at mild hydrothermal conditions revealed the kinetic relation between structurally unrelated bismuth ferrites. We report the synthetic conditions that disclose the in situ transformation of Bi25FeO39 into BiFeO3 by only extending the time for crystallization. We also demonstrate how a careful investigation of the early stages of multiferroics crystallization provides cheap and easy room-temperature access to the important sillenite-type of structure of Bi25FeO39. The crystal structure analysis by the Rietveld method and magnetization measurements provided detailed structure and magnetic relations between the synthesis conditions and the properties of the run product. The BiFeO3 antiferromagnetic behavior with a small ferromagnetic component arising from the particle's surface uncompensated spins is ascertained.

Layered transition metal carboxylates: synthesis, structural aspects and observation of multi-step magnetic transition through phase diagram.

Two new layered transition metal carboxylate frameworks, [Co3(L)2(H2O)6]·2H2O () and [Ni3(L)2(H2O)6]·2H2O () (L = tartronate anion or hydroxymalonic acid), have been synthesized and characterized by X-ray single crystal analysis. Both compounds have similar 2D structures. In both compounds there are two types of metal centers where one center is doubly bridged by the alkoxy oxygen atoms through µ2-O bridging to form a 1D infinite chain parallel to the crystallographic b-axis with the corners shared between the metal polyhedra. Magnetic susceptibility measurements revealed the existence of antiferromagnetic short range correlations between Co(Ni) intra-chain metal centers (with exchange constants JCo = -22.6 and JNi = -35.4 K). At low temperatures, long range order is observed in both compounds at Néel temperatures of 11 (for ) and 16 (for ) K, revealing that other exchange interactions, rather than the intra-chain ones, play a role in these systems. Whereas compound has an antiferromagnetic ground state, compound exhibits a ferromagnetic component, probably due to spin canting. Isothermal magnetization data unveiled a rich phase diagram with three metamagnetic phase transitions below 8 K in compound .