45 research outputs found
Structure of chalcogenide glasses by neutron diffraction
5 pages. Proceedings International Workshop Non-crystalline Solids, Gijon (Spain).International audienceThe purpose of this work is to study the change in the structure of the Ge-Se network upon doping with Ag. The total structure factor S(Q) for two samples has been measured by neutron diffraction using the two-axis diffractometer dedicated to structural studies of amorphous materials, D4, at the Institut Laue Langevin. We have derived the corresponding radial distribution functions for each sample and each temperature, which gives us an insight about the composition and temperature dependence of the correlation distances and coordination numbers in the short-range. Our results are compatible with the presence of both GeSe4/2 tetrahedra and Se-Se bonds. The Ag atoms are linked to Se in a triangular environment. Numerical simulations allowing the identification of the main peaks in the total pair correlation functions have complemented the neutron diffraction measurements
Raman spectroscopy of GeSe and AgGeSe thin films
The structural properties of Agy(Ge0.25Se0.75)1-y thin films (y=0, 0.07, 0.10, 0.15, 0.20 and 0.25 at. fraction) were studied. The films were prepared by pulsed laser deposition using bulk glass targets of the studied ternary system and deposited onto microscope slides. Their amorphous structures were confirmed by XRD (X-ray Diffraction). The effect of silver content on films structures was analysed by Raman spectroscopy. Typical Raman vibration modes were observed in the Ge0.25Se0.75 binary film: Ge-Se corner-sharing tetrahedra mode (CS) at 199 cm-1, edge sharing tetrahedra mode (ES) at 217 cm-1, and SeSe rings and chains mode at 255-265 cm-1 (CM). In the Agy(Ge0.25Se0.75)1-y ternary thin films, the same modes were observed but with a red shift and an intensity reduction in the ES and CM bands.Fil: Conde Garrido, Juan Manuel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de TecnologĂas y Ciencias de la IngenierĂa "Hilario FernĂĄndez Long". Universidad de Buenos Aires. Facultad de IngenierĂa. Instituto de TecnologĂas y Ciencias de la IngenierĂa ; Argentina. Universidad de Buenos Aires. Facultad de Ingenieria. Departamento de Fisica. Laboratorio de SĂłlidos Amorfos; ArgentinaFil: Piarristeguy, A.. Universidad de Buenos Aires. Facultad de Ingenieria. Departamento de Fisica. Laboratorio de SĂłlidos Amorfos; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de TecnologĂas y Ciencias de la IngenierĂa "Hilario FernĂĄndez Long". Universidad de Buenos Aires. Facultad de IngenierĂa. Instituto de TecnologĂas y Ciencias de la IngenierĂa ; ArgentinaFil: Le Parc, R.. Centre National de la Recherche Scientifique; Francia. Universite Montpellier II; FranciaFil: Ureña, Maria Andrea.Fil: Fontana, Marcelo.Fil: Arcondo, B..Fil: Pradel, A.. Universite Montpellier Ii; Franci
Utilisation des microscopies en champ proche pour la caractérisation électrique de verres hétérogÚnes
Les systĂšmes Ge(As)-Se(S) prĂ©sentent un large domaine de vitrification favorable Ă
lâĂ©tude dâun grand nombre de propriĂ©tĂ©s et de leurs corrĂ©lations avec la structure et/ou
la composition. En particulier, les propriétés électriques des verres sont fortement
modifiĂ©es par lâaddition de mĂ©taux alcalins ou argent (Li+, Na+,
Ag+). Les études combinées de la conduction électrique par spectroscopie
dâimpĂ©dance complexe (CIS) et de la microstructure par microscopie Ă force Ă©lectrique
(EFM) nous ont permis de caractériser les hétérogénéités électriques qui existent dans de
nombreux verres chalcogĂ©nures contenant de lâargent. Si les verres
(Ag2S)x(GeS)60(GeS2)40-x sont homogĂšnes,
les verres sulfures
(Ag2S)x(GeS2)100-x et
(Ag2S)x(As2S3)100-x ainsi que les
verres séléniures
Agx(Ge0,25Se0,75)100-x
prĂ©sentent des hĂ©tĂ©rogĂ©nĂ©itĂ©s. De telles hĂ©tĂ©rogĂ©nĂ©itĂ©s sont la signature Ă©lectrique dâune
démixtion. La séparation de phases et le seuil de percolation qui en découle permettent
dâexpliquer lâĂ©norme saut de conductivitĂ© (~6â8 ordres de grandeur) Ă©lectrique observĂ©
dans ces matériaux. La caractérisation électrique de chacune des micro(nano)phases
présentes dans le verre
Agx(Ge0,25Se0,75)100-x a
été réalisée en combinant deux techniques de microscopie en champ proche : la microscopie
Ă force Ă©lectrique â electric force microscopy (EFM) et la microscopie Ă force atomique
conductrice â conducting atomic force microscopy (C-AFM). La permittivitĂ© relative des
deux phases change avec la teneur en argent. De plus, la sensibilité de la technique C-AFM
permet de mettre en Ă©vidence, avec lâaugmentation de la teneur en argent, une augmentation
du courant de quelques pico-ampÚres dans la phase riche en argent. Ce résultat montre que
lâaugmentation de conductivitĂ© des verres Ag-Ge-Se dans la rĂ©gion de forte conductivitĂ©
(x > 8â10 %at.) rĂ©sulte dâune augmentation de
la conductivitĂ© de la phase riche en argent et non dâune augmentation de la quantitĂ© de
cette phase qui aurait une composition et une conductivité constantes
Phase separation and ionic conductivity: an Electric Force Microscopy investigation of silver chalcogenide glasses
International audienc
Homogeneousâinhomogeneous models of Agx(Ge0.25Se0.75)100âx bulk glasses
International audienc
Towards accurate models for amorphous GeTe: Crucial effect of dispersive van der Waals corrections on the structural properties involved in the phase-change mechanism
International audienceThe effect of van der Waals dispersion correction in combination with density functional theory is investigated on a canonical amorphous phase-change material. Density functional theory (DFT), using the generalized gradient approximation, usually fails to reproduce the structure of amorphous tellurides, which manifests by an overestimation of the interatomic bond distances, and particularly the Ge-Te one involved in local geometries (tetrahedral or defect octahedral). Here, we take into account dispersion forces in a semiempirical way and apply such DFT simulations to amorphous GeTe. We obtain a substantial improvement of the simulated structure factor and pair-correlation function, which now reproduce the experimental counterparts with an unprecedented accuracy, including on a recent partial contribution from anomalous x-ray scattering and from x-ray absorption. A detailed analysis of the corresponding structures indicates that the dispersion correction reduces the Ge-Te bond length, increases the fraction of tetrahedral germanium, and reduces the presence of heteropolar so-called fourfold ABAB rings. Given that these structural features have been stressed to be central for the understanding of the phase-change mechanism, the present results challenge our current understanding of the crystal to amorphous transformation at play
Nanoscale intrinsic heterogeneities in Ag-Ge-Se glasses and their correlation with physical properties
Quantitative assessment of network depolymerization in archetypal superionic glasses and its relationship with ion conduction: A case study on Na2SâGeS2
International audienc