Suchergebnisse

Under the terms of the Creative Commons Attribution 3.0 Unported License.-- Trabajo presentado a la 49th European High Pressure Research Group Conference (EHPRG), celebrada en Mumbai (India) del 25 al 30 de septiembre de 2011.-- et al. ; High-pressure Raman scattering measurements have been carried out in ZnGa2Se4 for both tetragonal defect chalcopyrite and defect stannite structures. Experimental results have been compared with theoretical lattice dynamics ab initio calculations and confirm that both phases exhibit different Raman-active phonons with slightly different pressure dependence. A pressure-induced phase transition to a Raman-inactive phase occurs for both phases; however, the sample with defect chalcopyrite structure requires slightly higher pressures than the sample with defect stannite structure to fully transform into the Raman-inactive phase. On downstroke, the Raman-inactive phase transforms into a phase that could be attributed to a disordered zincblende structure for both original phases; however, the sample with original defect chalcopyrite structure compressed just above 20 GPa, where the transformation to the Raman-inactive phase is not completed, returns on downstroke mainly to its original structure but shows a new peak that does not correspond to the defect chalcopyrite phase. The pressure dependence of the Raman spectra with this new peak and those of the disordered zincblende phase is also reported and discussed. © 2013 AIP Publishing LLC. ; This study was supported by the Spanish government MEC under Grants No. MAT2010-21270-C04-01/03/04, by MALTA Consolider Ingenio 2010 project (CSD2007- 00045), and by the Vicerrectorado de Investigación y Desarrollo of the Universitat Politécnica de Valencia (UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11). ; Peer Reviewed

et al. ; Order-disorder phase transitions induced by thermal annealing have been studied in the ordered-vacancy compound ZnGa2Se4 by means of Raman scattering and optical absorption measurements. The partially disordered as-grown sample with tetragonal defect stannite (DS) structure and I N42m space group has been subjected to controlled heating and cooling cycles. In situ Raman scattering measurements carried out during the whole annealing cycle show that annealing the sample to 400 °C results in a cation ordering in the sample, leading to the crystallization of the ordered tetragonal defect chalcopyrite (DC) structure with I N4 space group. On decreasing temperature the ordered cation scheme of the DC phase can be retained at ambient conditions. The symmetry of the Raman-active modes in both DS and DC phases is discussed and the similarities and differences between the Raman spectra of the two phases emphasized. The ordered structure of annealed samples is confirmed by optical absorption measurements and ab initio calculations, that show that the direct bandgap of DC-ZnGa2Se4 is larger than that of DS-ZnGa 2Se4. © 2013 IOP Publishing Ltd. ; This study was supported by the Spanish government MEC under grants MAT2010-21270-C04-01/03/04 and MAT2010-19837-C06-06, by MALTA Consolider Ingenio 2010 project (CSD2007-00045), and by the Vicerrectorado de Investigación y Desarrollo of the Universitat Politècnica de Valencia (UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11). ; Peer Reviewed

An ab initio study of β−As2Te3 (R¯3m symmetry) at hydrostatic pressures shows that this compound is a trivial small band-gap semiconductor at room pressure that undergoes a quantum topological phase transition to a 3D topological Dirac semimetal around 2 GPa. At higher pressures, the band gap reopens and again decreases above 4 GPa. Our calculations predict an insulator-metal transition above 6 GPa due to the closing of the band gap, with strong topological features persisting between 2 and 10 GPa with Z4=3 topological index. By investigating the lattice thermal conductivity (κL), we observe that close to room conditions κL is very low, either for the in-plane and the out-of-plane axis, with 0.098 and 0.023Wm−1K−1, respectively. This effect occurs due to the presence of two low-frequency optical modes, namely Eu and Eg, which increase the phonon-phonon scattering rate. Therefore, our results suggest that ultralow lattice thermal conductivities, which enable highly efficient thermoelectric materials, can be engineered in systems that are close to a structural instability derived from phonon Kohn anomalies. At higher pressures, the values of the in- and out-of-plane thermal conductivities not only increase in magnitude, but also approximate in value as the layered character of the compound decreases. ; This research was supported by the European Union Horizon 2020 research and innovation programme under Marie Sklodowska-Curie Grant No. 785789-COMEX and Project No. NORTE-01-0145-FEDER-022096, Network of Extreme Conditions Laboratories (NECL), financed by FCT and cofinanced by NORTE 2020, through the programme Portugal 2020 and FEDER. Authors also thank the financial support of the Generalitat Valenciana under Project PROMETEO 2018/123-EFIMAT and of the Agencia Española de Investigación under Projects No. MAT2016-75586-C4-2/4-P, No. FIS2017-2017-83295-P, and No. PID2019-106383GB-C42, as well as the MALTA Consolider Team research network under Project No. RED2018-102612-T. Additionally, authors acknowledge the computer resources at MareNostrum with technical support provided by the Barcelona Supercomputing Center (QCM-2019-1-0032/37). ; Peer reviewed

57th European High Pressure Research Group Meeting on High Pressure Science and Technology (EHPRG2019) in Prague, 1-6 september 2019 ; Metavalent or resonant bonding is a recently proposed new class of bonding that is intermediate between p-type covalent bonding and metallic bonding. It characterizes a new family of materials known as "incipient metals" [1]. Metavalent bonding occurs in materials where there is a deficiency of valence electrons in the unit cell to form a large number of bonds, such as in octahedrally-coordinated rocksalt-related structures occurring in GeTe, SnTe, PbSe, PbTe, Sb2Te3, Bi2Te3, Bi2Se3, Sb, Bi and AgSb2Te4. The main characteristics of metavalent bonding are: i) a much higher cation coordination than that assumed with the 8-N rule; ii) very high Born effective charge and optical dielectric constant as compared to typical covalent materials; iii) high mode Grüneisen parameters of phonons and low wavenumbers of optical phonons as compared to typical covalent materials; and iv) a moderately high electrical conductivity caused by thesmall bandgap which stems from the partial delocalization of electrons between several bonds ; This work has been financially supported by Spanish MINECO under projects FIS2017-83295-P and MAT2016-75586-C4-1/2/3-P and by Generalitat Valenciana under project PROMETEO/2018/123-EFIMAT. Supercomputer time has been provided by the Red Española de Supercomputación (RES) and the MALTA cluster. JAS acknowledges "Ramón y Cajal" fellowship (RYC-2015-17482) and E.L.d.S acknowledges Marie Sklodowska-Curie grant No. 785789-COMEX from European Union's Horizon 2020 research and innovation programme. We also thank ALBA synchrotron for funded experiments.