Flexoelectricity in antiferroelectrics

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO and AgNbO) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity

Control of lateral composition distribution in graded films of soluble solid systems A1-xBx by partitioned dual-beam pulsed laser deposition

Altres ajuts: AGAUR agency (project 2017SGR)Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries aswell as for high-throughputmaterials exploration.We herein propose amethod to prepare epitaxial lateral compositionally-graded films using a dual-beampulsed laser deposition (PLD)method with two targets separated by a partition. Tuning the ambient pressure and the partition-substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1-x)PbTiO-xPbZrO and (1-x)LaMnOLaSrMnO films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD

Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging

The large electrocaloric coupling in PbZrO allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response.The authors acknowledge financial support to ICN2, which is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706). The authors also acknowledge the support of Plan Nacional (MINECO, Grant Nos. MAT2016-77100-C2-1-P and BES-2016-077392), as well as the Agencia Estatal de Investigacion (Grant No. PID2019-108573GB-C21). R.F. and E.D. thank the Luxembourg National Research Fund (FNR) for funding part of this research through the projects CAMELHEAT/C17/MS/11703691/Defay. This work was also supported in part by the Spanish Ministry of Science, Innovation and Universities under the HIPERCELLS project (RTI2018-098392-B-I00), the Regional Government of the Generalitat de Catalunya under Grant Nos. 2017 SGR 1384 and 2017 SGR 00579. This work was also supported by the National Science Centre, Poland, within the Project No. 2016/21/B/ST3/02242

Origin of the Large Negative Electrocaloric Effect in Antiferroelectric PbZrO3

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electro-cooling of -3.5 K to an electro-heating of +5.5 K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.Comment: Article (17 pages) and supplemental material (12 pages) present in .pdf fil

p-Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics

The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm–3, while hole mobilities were found to be μh = 7–10 cm2/(V s) (in the 680–850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics

Electromechanical and electrocaloric properties of antiferroelectric PbZrO3

Els antiferroelèctrics són materials no polars que, sota un camp elèctric, canvien a una fase ferroelèctrica (polar), mostrant una característic cicle d' histèresi doble de polarització-voltatge. Actualment aquests materials s'estan estudiant intensament, tant des del punt de vista fonamental, per entendre quin és l'origen d'aquest comportament, com des del punt de vista pràctic, ja que la seva resposta funcional és útilper a l'emmagatzematge d'energia electrostàtica i prometedora per a aplicacions en la transducció electromecànica i la refrigeració electrocalórica. L'interès de l'efecte electrocalòric (ECE) dels antiferroeléctrics prové de la gran magnitud de les seves respostes ECE i de la seva anòmala resposta ECE negativa, per la qual el material es refreda quan s'aplica un voltatge, i s'escalfa quan se'l retira. En aquesta tesi investigo els efectes flexoelèctrics, electrocalòrics i de dinàmica de transició dels antiferroelèctrics. L'interès en aquestes dues propietats concretes és múltiple. Des d'un punt de vista fonamental, la flexoelectricitat és rellevant perquè s'ha proposat com a explicació de la disposició antiparalela dels dípols i, per tant, de l'origen de la antiferroelectricidad. Alhora, el desplaçament antiparal·lel dipolar és a la base d'un dels dos models teòrics que competeixen entre si i que intenten explicar l'efecte electrocalórico anòmal dels antiferroeléctrics. Les conclusions de la meva recerca s'extrapol·len directament a una discussió sobre com s'han d'entendre físicament els antiferroeléctrics: des d'una perspectiva estructural, com a desplaçaments antiparal·lels dels ions a la xarxa? O des d'una visió energètica entre les fases antiferroelèctrica i ferroelèctrica? Els nostres resultats suggereixen que aquesta última és la visió més acurada. Passant dels fonaments a les posibles aplicacions, aquest treball també explora la magnitud, la dinàmica i el rang de temperatura d'aplicabilitat de l'efecte electrocalóric anòmal dels antiferroelèctrics. Per acabar, presento també resultats sobre el creixement de pel·lícules primes antiferroelèctriques de PbZrO3 i la caracterització de les seves funcionalitats elèctriques i electromecàniques.Los antiferroeléctricos son materiales no polares que, bajo un campo eléctrico, cambian a una naturaleza ferroeléctrica (polar), mostrando así la característica histéresis doble de polarización-voltaje. Actualmente se están estudiando intensamente, tanto desde el punto de vista fundamental, para investigar cuál es el origen y el comportamiento de su respuesta funcional, como desde el punto de vista práctico, ya que son adecuados para el almacenamiento de energía electrostática y prometedores para aplicaciones en la transducción electromecánica y la refrigeración electrocalórica. El interés del efecto electrocalórico (ECE) de los antiferroeléctricos proviene de la gran magnitud de sus respuestas ECE y de su exótica respuesta ECE negativa, en la que el material se enfría cuando se aplica un step de voltaje, y se calienta cuando se retira. En esta tesis investigo los efectos flexoeléctricos y electrocalóricos y de dinámica de los antiferroeléctricos. El interés de estas dos propiedades específicas es múltiple. Desde un punto de vista fundamental, la flexoelectricidad es relevante ya que se ha propuesto como explicación de la disposición antiparalela de los dipolos y, por tanto, del origen de la antiferroelectricidad. A su vez, el desplazamiento antiparalelo dipolar está en la base de uno de los dos modelos que compiten entre sí y que pretenden explicar el efecto electrocalórico anómalo de los antiferroeléctricos. Las conclusiones se extrapolan directamente a una discusión sobre cómo deben entenderse físicamente los antiferroeléctricos: ¿desde una perspectiva estructural, como desplazamientos antiparalelos de los iones en la red? ¿O desde una visión energética entre las fases antiferroeléctrica y ferroeléctrica? Nuestros resultados sugieren que esta última es la visión más útil. Pasando de los fundamentos a las aplicaciones potenciales, el presente trabajo también explora la magnitud, la dinámica y el rango de temperatura de aplicabilidad del efecto electrocalórico anómalo de los antiferroeléctricos. Finalmente, presento resultados preliminares sobre el crecimiento de películas delgadas antiferroeléctricas de PbZrO3 y sus funcionalidades eléctricas y electromecánicas.Antiferroelectrics are non-polar materials which, under an electric field, switch to a ferroelectric (polar) nature, thus displaying the characteristic double-loop polarization-voltage hysteresis. They are currently being studied intensively, both from the fundamental point of view, to investigate what is the origin and behaviour of their functional response, and also from a practical point of view, as they are suitable for electrostatic energy storage and promising for applications in electromechanical transduction and electrocaloric cooling. The interest the electrocaloric effect (ECE) of antiferroelectrics stems from the large magnitude of their ECE responses and their exotic negative ECE response, whereby the material cools down when a voltage step is applied, and warms up when it is removed. In this thesis I take a look on the flexoelectric and electrocaloric effects and dynamics of antiferroelectrics. The interest in these two specific properties is manyfold. From a fundamental point of view, flexoelectricity is relevant as it has been proposed as an explanation for the antiparallel arrangement of dipoles and therefore for the origin of antiferroelectricity. In turn, the antiparallel dipole arrangement is at the basis of one of two competing models that aim to explain the anomalous electrocaloric effect of antiferroelectrics. The conclusions extrapolate directly onto a discussion about how antiferroelectrics should be physically understood: from a structural perspective, as antiparallel displacements of ions in the lattice? Or from an energetic view between the antiferroelectric and ferroelectric phases? Our results suggest the latter is the more useful view. Going from fundamentals to potential applications, the present work also explores the magnitude, dynamics and temperature range of applicability of the anomalous electrocaloric effect of antiferroelectrics. Finally, I present preliminary results on the growth of antiferroelectric PbZrO3 thin films and their electrical and electromechanical functionalities

Flexoelectricity in antiferroelectrics

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO and AgNbO) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity

Pulsed laser deposition of epitaxial non-doped PbTiO3 thin films from Pbo-TiO2 mosaic targets

PbTiO3 (PTO) suffers from difficulty in preparing high-density robust bulk ceramics, which in turn has been a bottleneck in thin films growth with physical vapor deposition (PVD) methods. In the present work, we prepared non-doped PTO thin films by a pulsed laser deposition (PLD) method with either a single PTO target or a mosaic target consisting of PbO and TiO2 pie-shaped pieces. On the PTO single target, laser irradiation caused selective ablation of Pb, resulting in Tirich cone-shaped pillar structure on the surface, whereas the irradiated surface of PbO and TiO2 pieces was smoother. Epitaxial PTO films deposited on SrTiO3 (001) substrates from the pie-chart targets with PbO:TiO2 areal ratio from 3:5 to 5:3 resulted in composition, crystallinity, flatness, and ferroelectric properties almost independent of the areal ratio. The averaged composition of each film was close to stoichiometric, suggesting a compositional self-control mechanism. For growing epitaxial and high-quality non-doped PTO films, a PbO-TiO2 pie-chart target is advantageous in easiness of handling and stable surface structure.This research was supported by Spanish Ministry of Competitiveness (projects MAT2016-77100-C2-1-P and PID2019-108573GB-C21) and the AGAUR agency (project 2017SGR). ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706)

Giant negative electrocaloric effect with antiferroelectrics

Resumen del trabajo presentado al 4th Scientific Meeting of BNC-b Students (JPhD), celebrado en Bellatera (España) del 6 al 7 de junio de 2019.Peer reviewe

Enhancing the intrinsic p-type conductivity of the ultra-wide bandgap Ga2O3 semiconductor

International audienceWhile there are several n-type transparent semiconductor oxides (TSO) for optoelectronic applications (e.g. LEDs, solar cells or display TFTs), their required p-type counterparts oxides are known to be more challenging. For the time being, the n-type TSO with the largest bandgap (~5eV) is Ga2O3 that holds the promisse of extending the light transparency further into the deep ultraviolet. In this work, it is demonstrated that strongly compensated Ga2O3 is also the intrinsic (or native) p-type TSO with the largest bandgap for any reported p-type TSO (e.g. NiO, SnO, delafossites, oxychalcogenides). The achievement of hole mobility in excess of 10 cm 2 /Vs and (high temperature) free hole concentrations in the ~10 17 cm-3 range challenges the current thinking about achieving p-type conductivity in Ga2O3 being "out of question". The results presented in this paper therefore further clarify that p-type Ga2O3 is possible, although more research must be conducted to determine what are the real Ga2O3 prospects for solar blind bipolar optoelectronics and ultrahigh power electronics based in p-n homojunctions