Structural, Vibrational, and Electronic Study of Sb2S3 at High Pressure

Antimony trisulfide (Sb2S3), found in nature as the mineral stibnite, has been studied under compression at room temperature from a joint experimental and theoretical perspective. X-ray diffraction and Raman scattering measurements are complemented with ab initio total-energy, lattice-dynamics, and electronic structure calculations. The continuous changes observed in the volume, lattice parameters, axial ratios, bond lengths, and Raman mode frequencies as a function of pressure can be attributed to the different compressibility along the three orthorhombic axes in different pressure ranges, which in turn are related to the different compressibility of several interatomic bond distances in different pressure ranges. The structural and vibrational properties of Sb2S3 under compression are compared and discussed in relation to isostructural Bi2S3 and Sb2Se3. No first-order phase transition has been observed in Sb2S3 up to 25 GPa, in agreement with the stability of the Pnma structure in Bi2S3 and Sb2Se3 previously reported up to 50 GPa. Our measurements and calculations do not show evidence either for a pressure-induced second-order isostructural phase transition or for an electronic topological transition in Sb2S3.This work has been performed under financial support from Spanish MINECO under Projects MAT2013-46649-C4-2/3-P and MAT2015-71070-REDC. This publication is fruit of "Programa de Valoracion y Recursos Conjuntos de I+D+i VLC/CAMPUS" and has been financed by the Spanish Ministerio de Educacion, Cultura y Deporte, as part of "Programa Campus de Excelencia Internacional" through Projects SP20140701 and SP20140871. These experiments were performed at BL04-MSPD beamline at ALBA Synchrotron with the collaboration of ALBA staff. Supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster. J.A.S. acknowledges financial support through Juan de la Cierva fellowship.Ibáñez, J.; Sans-Tresserras, JÁ.; Popescu, C.; López-Vidrier, J.; Elvira-Betanzos, J.; Cuenca Gotor, VP.; Gomis, O.... (2016). Structural, Vibrational, and Electronic Study of Sb2S3 at High Pressure. Journal of Physical Chemistry C. 19(120):10547-10558. https://doi.org/10.1021/acs.jpcc.6b01276S10547105581912

X-ray Absorption Spectroscopy Study of TiO2–xThin Films for Memory Applications

Metal–insulator–metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2–x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2–x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr, and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2–x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and extended X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers, and Debye–Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti–O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti4+ with a slight content of Ti3+. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600 °C. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms