Epitaxial ferroelectric hafnia stabilized by symmetry constraints

Ferroelectric memories experienced a revival in the last decade due to the discovery of ferroelectricity in HfO$_2$-based nanometer-thick thin films. These films exhibit exceptional silicon compatibility, overcoming the scaling and integration obstacles that impeded perovskite ferroelectrics' use in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films remains debated with no single factor identified that could stabilize the ferroelectric phase thermodynamically. Here, supported by density functional theory (DFT) high-throughput (HT) calculations that screen a broad range of epitaxial conditions, we demonstrate conclusively that specific epitaxial conditions achievable with common substrates such as yttria-stabilized zirconia (YSZ) and SrTiO$_3$ can favor the polar Pca2$_1$ phase thermodynamically over other polar phases such as R3m and Pmn2$_1$ and nonpolar P2$_1$/c phase. The substrate's symmetry constraint-induced shear strain is crucial for the preference of Pca2$_1$. The strain-stability phase diagrams resolve experiment-theory discrepancies and can guide the improvement of ferroelectric properties of epitaxial hafnia thin films

Análisis de la evolución de las exportaciones de españa a China, (2001-2014) = Spanish exports to China, (2001-2014)

El proceso de reforma que China inició hace más de treinta años ha convertido al país, no sólo en el primer exportador, sino también en el segundo importador mundial de mercancías. Centrándonos en este segundo aspecto, este trabajo pretende realizar un análisis general de la evolución y composición de las exportaciones españolas a China durante todo el periodo que abarca su reforma económica, poniendo énfasis en los cambios experimentados a partir del año 2001, momento en el que China se adhiere a la Organización Mundial de Comercio y sus relaciones comerciales aumentan considerablemente. Para ello, con el objetivo de contextualizar el análisis posterior, en la primera parte del trabajo realizaremos un repaso de las medidas más significativas de la reforma económica china y de las relaciones económicas e institucionales entre China y España. En la segunda parte realizaremos un estudio descriptivo – analítico, por productos, de las exportaciones españolas a China durante el periodo 2001-2014 utilizando distintos niveles de desagregació

Giant polarization in super-tetragonal ferroelectric thin films through interphase strain

Strain engineering has emerged as a powerful tool to enhance the performance of known functional materials. Here we demonstrate a general and practical method to obtain super-tetragonality and giant polarization using interphase strain. We use this method to create an out-of-plane–to–in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO3), compared to 1.065 in bulk. These thin films with super-tetragonal structure possess a giant remanent polarization, 236.3 microcoulombs per square centimeter, which is almost twice the value of known ferroelectrics. The super-tetragonal phase is stable up to 725°C, compared to the bulk transition temperature of 490°C. The interphase-strain approach could enhance the physical properties of other functional materials.PostprintPeer reviewe

Anion-induced robust ferroelectricity in sulfurized pseudo-rhombohedral epitaxial BiFeO3 thin films via polarization rotation

Polarization rotation caused by various strains, such as substrate and/or chemical strain, is essential to control the electronic structure and properties of ferroelectric materials. This study proposes anion-induced polarization rotation with chemical strain, which effectively improves ferroelectricity. A method for the sulfurization of BiFeO3 thin films by introducing sulfur anions is presented. The sulfurized films exhibited substantial enhancement in room-temperature ferroelectric polarization through polarization rotation and distortion, with a 170% increase in the remnant polarization from 58 to 100.7 μC cm−2. According to first-principles calculations and the results of X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy, this enhancement arose from the introduction of S atoms driving the re-distribution of the lone-pair electrons of Bi, resulting in the rotation of the polarization state from the [001] direction to the [110] or [111] one. The presented method of anion-driven polarization rotation might enable the improvement of the properties of oxide materials.This work was supported by the National Key Research and Development Program of China (2018YFA0703700, 2017YFE0119700, 2021YFA1400300 and 2018YFA0305700), the National Natural Science Foundation of China (21801013, 51774034, 22271309, 11721404, 11934017, 12261131499, and 51961135107), the Fundamental Research Funds for the Central Universities (FRF-IDRY-19-007 and FRF-TP-19-055A2Z), the National Program for Support of Top-notch Young Professionals, the Young Elite Scientists Sponsorship Program by CAST (2019-2021QNRC), the Beijing Natural Science Foundation (Z200007), and the Chinese Academy of Sciences (XDB33000000). This research used the resources of the Beijing Synchrotron Radiation Facility (1W1A and 4B9B beamlines) of the Chinese Academy of Science.Peer reviewe

Ferroelectricity in layered bismuth oxide down to 1 nanometer

Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution deposition. We observed a standard ferroelectric hysteresis loop down to a thickness of ~1 nanometer. The thin films with thicknesses that range from 1 to 4.56 nanometers possess a relatively large remanent polarization from 17 to 50 microcoulombs per square centimeter. We verified the structure with first-principles calculations, which also pointed to the material being a lone pair-driven ferroelectric material. The structure design of the ultrathin ferroelectric films has great potential for the manufacturing of atomic-scale electronic devices.This work was supported by the National Key Research and Development Program of China (2018YFA0703700, 2017YFE0119700, and 2020YFA0406202), the National Natural Science Foundation of China (21801013, 51774034, 51961135107, 62104140, 12175235, 22090042, 12074016, 11704041, and 12274009), the Fundamental Research Funds for the Central Universities (FRF-IDRY-19-007 and FRF-TP-19-055A2Z), the National Program for Support of Top-notch Young Professionals, the Young Elite Scientists Sponsorship Program by CAST (2019-2021QNRC), and Lingang Laboratory Open Research Fund (grant LG-QS-202202-11). Use of the Beijing Synchrotron Radiation Facility (1W1A beamlines, China) of the Chinese Academy of Sciences is acknowledged. Y.-W.F. acknowledges the support of Masaki Azuma’s group during his stay at the Tokyo Institute of Technology. Y.L. acknowledges the support of the Beijing Innovation Team Building Program (grant no. IDHT20190503), the Beijing Natural Science Foundation (Z210016), the Research and Development Project from the Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2022SX-TD001), and the General Program of Science and Technology Development Project of Beijing Municipal Education Commission (KM202110005003).Peer reviewe

Adsorption of Perchlorate from Water Using Calcined Iron-Based Layered Double Hydroxides

In this study, a new calcined iron-based layered double hydroxide material was synthesized to adsorb perchlorate from water. The MgFe-CO 3 layered double hydroxides (MgFe-LDH) were prepared at pH 9-10 and with different molar Mg/Fe ratios (2-5) using a co-precipitation method, and then calcined at a temperature range of 300-900°C. Results showed that the best synthesis conditions to maximize the perchlorate adsorption capacity of the calcined MgFe-CO 3 layered double hydroxide (MgFe-CLDH) were the calcination temperature of 550°C and [Mg]/[Fe]=3. Furthermore, the adsorbents and their adsorption products were characterized by X-ray, FT-IR and thermogravimetric analyses. The layered double hydroxide structures in the adsorbent were lost during calcination at 550°C, but were reconstructed subsequent to adsorption of perchlorate, indicating that the \u27memory effect\u27 appeared to play an important role in perchlorate adsorption. The perchlorate adsorption pattern was well described by the pseudo-second-order kinetic model, while the Freundlich isotherm provided the best fitting of adsorption isotherms with the experimental data at chemical equilibrium at 25°C. Indeed, an initial solution pH of 4-10, a higher adsorbent dose, and a lower initial perchlorate concentration typically favored the removal of perchlorate from water. Furthermore, co-existing anions, including PO 4 3-, SO 4 2-, Cl - and NO 3 -, inhibited the perchlorate adsorption, to different degrees. At 25°C, MgFe-3 CLDH=1.33g/L (calcination temperature=550°C and [Mg]/[Fe]=3), and the initial solution pH of 4-10, 2000μg/L of perchlorate was almost all adsorbed within 720min. This study demonstrated that the new calcined iron-based layered double hydroxide was a promising adsorbent for control of the perchlorate pollution in water