27 research outputs found

    Yodatos bajo condiciones extremas: Transiciones de fase inducidas por la presión, propiedades estructurales, vibracionales y electrónicas

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    La estructura cristalina de los yodatos metálicos juega un papel importante en la respuesta SHG, así como también lo hacen la estructura de bandas electrónica y la energía de bandgap de la misma. Por ello, en esta tesis doctoral se estudia en profundidad y se presentan y analizan resultados de la estructura cristalina a alta presión, las vibraciones atómicas y la estructura de bandas electrónica de cuatro yodatos metálicos. Estos son el yodato de hierro, Fe(IO3)3, el yodato de cobalto, Co(IO3)2, el yodato de zinc, Zn(IO3)2, y yodato de magnesio Mg(IO3)2. Estos materiales han sido estudiados por medio de difracción de rayos X a alta presión (HPXRD), dispersión Raman a alta presión (HPRS), espectroscopia infrarroja por transformada de Fourier a alta presión (HPFTIR), absorción óptica a alta presión (HPOA) y cálculos de primeros principios empleando la teoría del funcional de la densidad. En el trabajo se presentan resultados sobre los efectos de la presión sobre el LEP estereoquímicamente activo en el yodo y la estructura cristalina, observándose fenómenos como el aumento de la coordinación del yodo con el oxígeno inducida por la presión, las transiciones de fase isoestructurales inducidas por la presión y la expansión de la distancia de enlace yodo-oxígeno inducida por la presión. Diversas características comunes de los espectros Raman de los compuestos estudiados también se observan a presión ambiente y a alta presión. Algunos modos Raman-activos muestran un comportamiento de ablandamiento (softening) bajo presión que está íntimamente relacionado con el cambio de la estructura cristalina bajo presión. Los cambios en la estructura de bandas electrónica, combinados con los cambios de la estructura cristalina bajo compresión, nos llevan a establecer dos reglas fenomenológicas útiles para el diseño de yodatos metálicos de energía de banda prohibida ancha.The crystal structure of metal iodates plays an important role in the SHG response, as well as the electronic band structure and the bandgap energy of it. So here in this doctoral thesis, the high-pressure crystal structure, atomic vibration and electronic band structure of four metal iodates, Fe(IO3)3, Co(IO3)2, Zn(IO3)2 and Mg(IO3)2, are studied and reported, by means of high-pressure X-ray diffraction (HPXRD), high-pressure Raman scattering (HPRS), high pressure synchrotron-based Fourier Transform Infrared spectroscopy (HPFTIR), high pressure optical absorption (HPOA) and first-principle calculation. The effects of pressure on the stereochemically active LEP in iodine and crystal structure were reported, like the pressure-induced oxygen coordination increase of iodine, the pressure-induced isostructural phase transition and pressure-induced expansion of iodine-oxygen bond distance. The common features of the Raman spectra are also observed at ambient and high pressure. Some Raman-active modes show a soften behavior under pressure which is consistence with the change of the crystal structure under pressure. The electronic band structure changes, combined with the changes of crystal structure under compression, lead us to conclude two useful instructions for designing wide-bandgap energy metal iodates

    High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms

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    This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpcc.0c06541.[EN] We report high-pressure Raman spectroscopy studies of Fe(IO3)(3) up to nearly 21 GPa that have been interpreted with the help of density functional theory calculations, which include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region, there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a threefold coordination to an almost sixfold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair so that likely a real sixfold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong nonlinear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.This work was supported by the Spanish Ministry of Science, Innovation and Universities, the Spanish Research Agency (AEI), the European Fund for Regional Development (ERDF, FEDER) under grants MAT2016-75586-C4-1/2/3-P, PID2019-106383GB-C41/42/43, and RED2018-102612-T (MALTA Consolider-Team Network), and the Generalitat Valenciana under grant Prometeo/2018/123 (EFIMAT). A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025).Liang, A.; Rahman, S.; Rodriguez-Hernandez, P.; Muñoz, A.; Manjón, F.; Nenert, G.; Errandonea, D. (2020). High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms. The Journal of Physical Chemistry C. 124(39):21329-21337. https://doi.org/10.1021/acs.jpcc.0c06541S21329213371243

    Pressure-Induced Phase Transition Versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite

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    The crystal structure of CH3NH3PbBr3 perovskite has been investigated under high-pressure by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm-3m->Im-3->Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21, but the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments and visual observations and by the fact that changes induced by pressure up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy which is discussed using the structural information obtained from X-ray diffraction.Comment: 15 pages, 4 figure

    Pressure-induced band-gap increase in hydrated Ca(IO3)2 [Jamieson Award Introduction and Lecture]

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    The search for the next generation of nonlinear optical (NLO) materials has led to the synthesis and characterization of many new metal iodates [1,2]. However, finding an ideal NLO crystal with a high laser damage threshold, excellent thermal stability, a wide transparency window, and a large second-harmonic generation (SHG) response remains a challenge. This work is inspired by our previous high-pressure studies on metal iodates and the finding of the general inverse correlation between the bandgap of metal iodates (which only contain non-transition or closed-shell metal) and the distance of average I-O covalent bonds [3]. In this study [4], we report on the first instance of a metal iodate, hydrated Ca(IO3)2, with an opened bandgap achieved by applying external pressure. The bandgap increased from 4.52 eV to 4.92 eV without saturation at increasing pressure (Figure 1). This is explained by: ) molecular orbital (MO) diagrams established from theoretical calculations of the density of state (DOS) and crystal orbital overlap population (COOP), and) the pressure dependence of the bond distance of the primary, short, strong I-O covalent bonds and secondary, long, weak I···O halogen bonds, as obtained from powder X-ray diffraction measurements. In addition, two reversible isostructural phase transitions were observed in the pressure range of 6.6–8.0 and 13.0–15.5 GPa, characterized by nonlinear changes in the bandgap energy, crystal lattice parameters, and the occurrence of extra peaks and peak splitting in the Raman spectra

    Pressure-induced phase transition and band gap decrease in semiconducting β-Cu2V2O7

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    The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu2V2O7, finding it undergoes a pressure-induced phase transition to γ-Cu2V2O7 below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu2V2O7 has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu2V2O7 could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.The authors acknowledge financial support from the Spanish Research Agency (AEI) and Spanish Ministry of Science and Investigation (MCIN) under projects PID2019106383GBC41/ C43/C44 (DOI: 10.13039/501100011033), and projects PGC2018-101464−B-I00 and PGC2018-097520-A-I00 (cofinanced by EU FEDER funds). The authors acknowledge financial support from the MALTA Consolider Team network, under project RED2018-102612-T. R.T. acknowledges funding from the Spanish Ministry of economy and competitiveness (MINECO) via the Juan de la Cierva Formación program (FJC2018-036185-I). J.G.P. thanks the Servicios Generales de Apoyo a la Investigación (SEGAI) at the University of La Laguna. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025, and the authors would also like to thank them for funding under the Grant Prometeo/2018/123 (EFIMAT). The authors also thank ALBA synchrotron light source for funded experiment under proposal numbers 2020074389 and 2020074398 at the MSPD-BL04 beamline

    Pressure-induced band-gap energy increase in a metal iodate

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    [EN] A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely Ca(IO3)2 center dot H2O, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pres-sure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6-8.0 and 13.0-15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.This study was supported by the MALTA ConsoliderTeam network (Project No. RED2018-102612-T), financed by MINECO/AEI/0.13039/501100003329,the I+D+i Project No.PID2019-106383GB-41/42 financed by MCIN/AEI/10.13039/501100011033, as well as by the Projects No. PROMETEO CIPROM/2021/075 (GREEN-MAT) and No. MFA/2022/007 financed by GeneralitatValenciana. This study forms part of the Advanced Materials programme and was supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and by Generalitat Valenciana. A.L. and D.E. thank the Generalitat Valenciana for Ph.D. Fellowship No. GRISOLIAP/2019/025.R.T. and D.E. thank the Generalitat Valenciana for Postdoctoral Fellowship No.CIAPOS/2021/20.The authors also thank ALBA synchrotron light source for the experiment funded under Proposal No. AV-2021095390at the MSPD-BL04 beamline. J.S. and K.V. would like to acknowledge IIT Hyderabad for providing computational facilities. J.S. would like to acknowledge CSIR for his Ph.D.fellowship. G.V. would like to acknowledge the Institute of Eminence, University of Hyderabad (UoH-IoE-RC3-21-046)for funding and CMSD, University of Hyderabad, forproviding computational facilities. The authors also thank the Tirant supercomputer (Universitat de Valencia) for providing computational resources.Liang, A.; Shi, L.; Turnbull, R.; Manjón, F.; Ibáñez, J.; Popescu, C.; Jasmin, M.... (2022). Pressure-induced band-gap energy increase in a metal iodate. PHYSICAL REVIEW B-CONDENSED MATTER. 106(23):1-9. https://doi.org/10.1103/PhysRevB.106.235203191062

    Pressure-induced band-gap energy increase in a metal iodate

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    A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely Ca(IO3)2 center dot H2O, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pres-sure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6-8.0 and 13.0-15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.This study was supported by project MALTA Consolider Team network (RED2018‐102612‐T), financed by MINECO/AEI/10.13039/501100003329, I+D+i project PID2019‐106383GB‐41/42 financed by MCIN/AEI/10.13039/501100011033; as well as by projects PROMETEO CIPROM/2021/075 (GREENMAT) and MFA/2022/007 financed by Generalitat Valenciana. A.L. and D.E. thank the Generalitat Valenciana for the Ph.D. Fellowship No. GRISOLIAP/2019/025. R.T. and D.E. thank the Generalitat Valenciana for the postdoctoral Fellowship No. CIAPOS/2021/20. The authors also thank ALBA synchrotron light source for funded experiment under proposal number AV-2021095390 at the MSPD-BL04 beamline

    Structural, vibrational and electronic properties in the glass-crystal transition of thin films Sb₇₀Te₃₀ doped with Sn

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    Antimony-telluride based phase-change materials doped with Sn have been proposed to be ideal materials for improving the performance of phase-change memories. It is well known that Sb₇₀Te₃₀ thin films show a sharp fall in the electrical resistance in a narrow temperature range when heating. Therefore, it is interesting to study the effect of adding Sn into this composition. In this work, undoped and Sn-doped SbeTe thin films of composition Snₓ[Sb₀.₇₀Te₀.₃₀]₁₀₀₋ₓ, with x = 0.0, 2.5, 5.0 and 7.5 at. %, have been obtained by pulsed laser deposition. Their electrical resistance has been measured while heating from room temperature to 650 K. A sharp fall in the electrical resistance, associated to the glass-crystal transition, has been detected in all the samples within a narrow temperature range. The onset temperature of this transformation increases with the Sn content. Both as-obtained and thermally-treated films have been structurally characterized by X-ray and by Raman spectroscopy. We have compared the results among these compositions in terms of the identified crystallization products, transformation onset temperatures, transformation temperature ranges and amorphous/crystallized electrical resistance ratio. We have found that the frequency of the Raman modes decreases with Sndoping. Finally, in order to study the electronic structure and to determine the band gap, the frequencies of the allowed Raman modes and the vibration directions of the Sb₇₀Te₃₀ compound, Density Functional Theory based ab initio calculations have been performed as a function of the Sn concentration.Facultad de Ciencias ExactasInstituto de Física La Plat

    2D Cu(I)‑I Coordination Polymer with Smart Optoelectronic Properties and Photocatalytic Activity as a Versatile Multifunctional Material

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    This work presents two isostructural Cu(I)-I 2-fluoropyrazine (Fpyz) luminescent and semiconducting 2D coordination polymers (CPs). Hydrothermal synthesis allows the growth of P-1 space group single crystals, whereas solvent-free synthesis produces polycrystals. Via recrystallization in acetonitrile, P21 space group single crystals are obtained. Both show a reversible luminescent response to temperature and pressure. Structure determination by single-crystal X-ray diffraction at 200 and 100 K allows us to understand their response as a function of temperature. Applying hydrostatic/uniaxial pressure or grinding also generates significant variations in their emission. The high structural flexibility of the Cu(I)-I chain is significantly linked to the corresponding alterations in structure. Remarkably, pressure can increase the conductivity by up to 3 orders of magnitude. Variations in resistivity are consistent with changes in the band gap energy. The experimental results are in agreement with the DFT calculations. These properties may allow the use of these CPs as optical pressure or temperature sensors. In addition, their behavior as a heterogeneous photocatalyst of persistent organic dyes has also been investigatedThanks to Micro and Nanotechnology Institute CNM-CSIC for SEM images. Thanks to the SCXRD laboratory of the Interdepartmental Research Service and to Servicios Generales de Apoyo a la Investigación (SEGAI) at La Laguna University. This work has been supported by MCINN/AEI/ 10.13039/ 5011000011033 under the National Program of Sciences and Technological Materials, PID2019-108028GB-C22, PID2019- 106383GB-C41/C44, and TED2021-131132B-C22. Thanks to Gobierno d e Canarias and EU-FEDER (grant: ProID2020010067). This study forms part of the Advanced Materials program and was supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and by Generalitat Valenciana (grant MFA/2022/007 and PROMETEO CIPROM/2021/075-GREENMAT). A.L. (R.T.) and D.E. thank the Generalitat Valenciana for the Ph.D. (Postdoctoral) Fellowship No. GRISOLIAP/2019/025 (CIAPOS/2021/20). J.C.G. and R. W. acknowledge the support from the Spanish Ministry of Science and Innovation (RTI2018-097508-B-I00, PID2021-128313OB-I00, TED2021- 131018B-C22) and the Regional Government of Madrid through projects NMAT2D-CM (S2018/NMT-4511). J.C.G. acknowledges support from the Regional Government of Madrid through “Proyectos Sinérgicos de I + D” (grant Y2018/NMT-5028 FULMATEN-CM) and NANOCOV-CM (REACT-UE). IMDEA Nanociencia acknowledges support from the Severo Ochoa Programme for Centres of Excellence in R&D (MINECO, grant CEX2020-001039-S

    Joint experimental and theoretical study of PbGa2S4 under compression

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    [EN] The effect of pressure on the structural, vibrational, and optical properties of lead thiogallate, PbGa2S4, crystallizing at room conditions in the orthorhombic EuGa2S4-type structure (space group Fddd), is investigated. Results from X-ray diffraction, Raman scattering, and optical-absorption measurements at high pressure beyond 20 GPa are reported and compared not only to ab initio calculations, but also to the related compounds ¿ ¿ -Ga2S3, CdGa2S4, and HgGa2S4. Evidence of a partially reversible pressure-induced decomposition of PbGa2S4 into a mixture of Pb6Ga10S21 and Ga2S3 above 15 GPa is reported. Thus, our measurements and calculations show a route for the high-pressure synthesis of Pb6Ga10S21, which is isostructural to the stable Pb6In10S21 compound at room pressure.This study forms part of the Advanced Materials programme and was supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and by Generalitat Valenciana through projects MFA/2022/024 (ARCANGEL), PROMETEO CIPROM/2021/075 (GREENMAT) and MFA/2022/007 (MATGREEN) and from the Spanish Ministerio de Ciencia e Innovacion and Agencia Estatal de Investigacion (MCIN/AEI/10.13039/501100011033) under grant No. PID2019-106383GB-41/42/43, PID2021-125927NB-C21, and RED2022-134388-T (MALTA-Consolider Team network). T.G.-S. thanks Universitat Politecnica de Valencia for the support through the program "Ayudas para la recualificacion del profesorado universitario", financial support provided by Ministerio de Universidades, funding from the European Union-Next generation EU. T. G.-S. and V. P. C.-G. thanks Primeros proyectos de investigacion 2022 (PAID-06-22), en el marco de ayudas del Vicerrectorado de Investigacion de la Universitat Politecnica de Valencia. The authors also thank the ALBA synchrotron light source for providing beamtime under proposal number 2021085226. H. H. O. and S. G. P. acknowledge PRACE for awarding access to the Fenix Infrastructure resources at CINECA, which are partially funded from the European Union's Horizon 2020 research and innovation programme through the ICEI project under the grant agreement No. 800858. We also acknowledge the computer resources of the Centro de Supercomputacion de Castilla y Leon (SCAYLE)".García-Sánchez, TM.; Gallego-Parra, S.; Liang, A.; Rodrigo-Ramon, JL.; Muñoz, A.; Rodriguez-Hernandez, P.; Gonzalez-Platas, J.... (2023). Joint experimental and theoretical study of PbGa2S4 under compression. Journal of Materials Chemistry C. 11(34):11606-11619. https://doi.org/10.1039/D3TC02288A1160611619113
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