Multiferroism Induced by Spontaneous Structural Ordering in Antiferromagnetic Iron Perovskites

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE1.2Ba1.2Ca0.6Fe3O8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO4-tetrahedra (T) and FeO6-octahedra (O) results in a polar crystal structure. The layered structure consists of the stacking sequence of RE/Ca-RE/Ca-Ba-RE/Ca layers in combination with the TOOT sequence in a unit cell. A polar moment of 33.0 μC/cm2 for the Gd-oxide (23.2 μC/cm2 for the Tb one) is determined from the displacements of the cations, mainly Fe, and oxygen atoms along the b-axis. These oxides present antiferromagnetic ordering doubling the c-axis, and the magnetic structure in the Tb compound remains up to 690 K, which is one of the highest transition temperatures reported in Fe perovskites

On the Origin of the Spontaneous Formation of Nanocavities in Hexagonal Bronzes (W,V)O3

[EN] Hexagonal (W,V)O3−x oxides of high thermal stability have been synthesized hydrothermally through the intermediate products Nax(W,V)O3·zH2O and (NH4)0.33−x(W,V)O3−y. The obtained crystals show nanostructured surface via the formation of a dense population of polyhedral nanocavities self-distributed along particular crystallographic directions. Nanocavities present a regular size that ranges from 5 to 10 nm in both length and width. The synthesis process involves a significant topotactic relationship between the as-synthesized product and the desired final product and this relationship is suggested as the origin of the observed surface nanostructure. The comparison of our results with observations in different solids has allowed us to suggest that the formation of nanocavities is an extensive spontaneous process when materials are obtained by the chemical reactions of solids leading to products with defined crystallographic orientation with respect to the original compound. The characterization provides evidence regarding the potential relevance of nanocavities in the functional properties of the resulting solids.Authors acknowledge the financial support from DGICYT in Spain through projects MAT2010-19837-C06-05 and CTQ2012-37925-C03-1. Authors are also grateful to the Centro de Microscopia Electronica (UCM) for facilities.García-González, E.; Soriano Rodríguez, MD.; Urones-Garrote, E.; López Nieto, JM. (2014). On the Origin of the Spontaneous Formation of Nanocavities in Hexagonal Bronzes (W,V)O3. Dalton Transactions. 43:14644-14652. https://doi.org/10.1039/C4DT01465KS14644146524

Ternary erbium chromium sulfides : structural relationships and magnetic properties

Single crystals of four erbium-chromium sulfides have been grown by chemical vapor transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er(3)CrS(6) octahedral sites are occupied exclusively by Cr(3+) cations, leading to one-dimensional CrS(4)(5-) chains of edge-sharing octahedra, while in Er(2)CrS(4), Er(3+), and Cr(2+) cations occupy the available octahedral sites in an ordered manner. By contrast, in Er(6)Cr(2)S(11) and Er(4)CrS(7), Er(3+) and Cr(2+) ions are disordered over the octahedral sites. In Er(2)CrS(4), Er(6)Cr(2)S(11), and Er(4)CrS(7), the network of octahedra generates an anionic framework constructed from M(2)S(5) slabs of varying thickness, linked by one-dimensional octahedral chains. This suggests that these three phases belong to a series in which the anionic framework may be described by the general formula [M(2n+1)S(4n+3)](x-), with charge balancing provided by Er(3+) cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er(4)CrS(7), Er(6)Cr(2)S(11), and Er(2)CrS(4) may thus be considered as the n = 1, 2, and infinity members of this series. While Er(4)CrS(7) is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er(3)CrS(6) (T(C)(Cr) = 30 K; T(C)(Er) = 11 K) and Er(2)CrS(4) (T(N)(Cr) = 42 K, T(N)(Er) = 10 K) whereas Er(6)Cr(2)S(11) exhibits ordering of the chromium sub-lattice only (T(N) = 11.4 K)

Kinetic Transition during Ferrite Growth in Fe-C-Mn Medium Carbon Steel

The kinetics of austenite-to-ferrite diffusional transformation in a medium carbon Fe-C-Mn steel was calculated based on classical nucleation and growth theory coupled with CALPHAD multi-component thermodynamics. The description of the growth rate of proeutectoid ferrite includes a time-dependence due to the carbon enrichment in the remaining austenite. The experimental slower kinetics, especially a stagnating behavior at the later stage, has been successfully reproduced when a transition from initial paraequilibrium (PE) to local equilibrium negligible partition (LENP) conditions at austenite:ferrite (γ/α) interface was assumed. This transition is allowed when the velocity of moving γ/α interface is slow enough to be compared with Mn diffusivity, which leads to built up a Mn spike in the interface. This assumption is consistent with a series of scanning transmission electron microscopy (STEM) analyses for Mn and C which indicates that initial unpartitioned Mn ferrite growth is replaced by partitioned growthfinancial support of Japan Science and Technology Agency (JST). The authors gratefully acknowledge the support of the Spanish Ministry of Science and Innovation for funding this research under the contract MAT2007 – 63873. J. Cornide acknowledges the Spanish Ministerio de Ciencia e Inovación for financial support in the form of PhD research grant (FPI)Peer reviewe

Continuous hardening during isothermal aging at 723 K (450 °C) of a precipitation hardening stainless steel

The isothermal aging behavior of a cold-rolled precipitation hardening stainless steel has been studied at 723 K (450 °C) for holding times up to 72 hours. The precipitation hardening has been investigated using microhardness Vickers (Hv), thermoelectric power (TEP) measurements, and tensile testing. Microhardness compared to TEP measurements is more sensitive to detect the initial stages of aging. Two precipitation regimes have been observed: the first one related to the formation of Cu-clusters for aging times below 1 hour and a second one associated with formation of Ni-rich precipitates. The results show that the material exhibits an outstanding continuous age strengthening response over the aging time investigated, reaching a hardness of 710 ± 4 H and an ultimate tensile strength (σ) of 2.65 ± 0.02 GPa after 72 hours. Engineering stress-plastic strain curves reveal that the strength increases and the ductility decreases as the aging time increases. However, after prolonged holding times (24–72 hours) and, although small, a rise in both the strength and the total elongation is observed. The precipitation kinetics can be well predicted over the entire range of aging times by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Finally, a reliable linear hardness-yield strength correlation has been found, which enables a rapid evaluation of the strength from bulk hardness measurements.The authors would like to acknowledge the financial support from the Ministerio de Economía y Competitividad (Project No. MAT2010-19522). Carola Celada-Casero would like to thank the financial support from the Consejo Superior de Investigaciones Científicas (CSIC) in the form of a JAE-predoc Grant, cofunded by the European Social Fund

Interplay between humidity, temperature and electrical response of a conductivity sensor based on a La2LiNbO6 double perovskite

The La2LiNbO6 perovskite has been prepared in the polycrystalline form by a solid state reaction. Structural characterization by means of monochromatic X-ray and neutron powder diffraction (XRD and ND) and Rietveld refinement showed that the crystal structure belongs to the group of 1 : 1 B-site rock-salt ordered double perovskites with the most common tilting system amongst them being a-a-b+ (S. G. P21/n, a = 5.61612(3), b = 5.76645(2), c = 7.94107(4) degrees A, b = 90.276(2)degrees). Scanning Transmission Electron Microscopy (STEM) evidences that there is no cross-substitution between Li and Nb and that a remaining portion of lanthanum is randomly located in the projected positions of lithium. Impedance spectroscopy has been used to analyse the electrical-response properties of the materials. Conductivity is strongly dependent on the relative humidity (RH), changing by about 3 orders of magnitude between 25 and 90% RH. However, no conductivity increase with change in RH% is observed when the lateral surfaces of the sensor are covered with paraffin. This confirms that adsorption of water by the sample plays a crucial role in modulating the conduction mechanism. La2LiNbO6 also exhibits a very good durability, reproducibility, response time, hysteresis and dynamic linearity to be considered as a promising sensing material for a practical humidity sensor.This work has been supported by Projects funded by the regional government (Comunidad de Madrid through MATERYENER3CM S2013/MIT-2753) and the Spanish Government, (MICINN through MAT2016-78362-C4-3R and MAT2016-78362-C4-4R). V.D.N. thanks the University Carlos III of Madrid for the "8220;Cátedras de Excelencia UC3M-Santander"; (Chair of Excellence UC3M-Santander)

Strengthening by intermetallic nanoprecipitation in Fe-Cr-Al-Ti alloy

The strengthening mechanism observed during ageing at temperatures of 435 and 475 °C in the oxide dispersion strengthened (ODS) Fe-Cr-Al-Ti system has been investigated. Atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM) analyses determined that the alloy undergoes simultaneous precipitation of Cr-rich (α′ phase) and nanoscale precipitation of TiAl-rich intermetallic particles (β′ phase). APT indicated that the composition of the intermetallic β′ phase is FeAlTiCr, and the evolving composition of α′ phase with ageing time was also determined. The results obtained from HRTEM analyses allow us to confirm that the β′ precipitates exhibit a cubic structure and hence their crystallography is related to the Heusler-type FeAlTi (L2) structure. The strengthening could be explained on the basis of two hardening effects that occur simultaneously: the first is due to the α-α′ phase separation through the modulus effect, and the second mechanism is due to the interaction of nanoscale β′ particles with dislocations.PM 2000™ is a trademark of Plansee GmbH. LEAP® is a registered trademark of CAMECA Instruments Inc. CC and JC acknowledge financial support to Spanish Ministerio de Economia y Competitividad (MINECO) through in the form of a Coordinate Project (MAT2013-47460-C5-1-P). Atom probe tomography (MKM) was supported through a user project supported by ORNL's Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.Peer Reviewe