Crystalline Structure and Vacancy Ordering across a Surface Phase Transition in Sn/Cu(001)

We report a surface X-ray diffraction study of the crystalline structure changes and critical behavior across the (3√2 × √2)R45° → (√2 × √2)R45° surface phase transition at 360 K for 0.5 monolayers of Sn on Cu(100). The phase transition is of the order-disorder type and is due to the disordering of the Cu atomic vacancies present in the low temperature phase. Two different atomic sites for Sn atoms, characterized by two different heights, are maintained across the surface phase transition.This work was funded by the Spanish MINECO under Grants FIS2011-23230 and MAT2014-52477-C5-5-P. E.G.M. and P.S. acknowledge financial support from MINECO through the “Maria de Maeztu” Programme for Units of Excellence in R&D (MDM-2014-0377).Peer Reviewe

New Energy Calibration of the CMAM 5MV Tandem Accelerator

Abstract Ion accelerators are fundamental in the ongoing research on materials for future energy sources, being the primary tool for understanding the behaviour of different classes of materials (functional, structural, diagnostic) under e.g. the intense radiation expected in fission reactors or the critical thermal operational conditions in IV generation fission reactors. The relevance of ion accelerators research extends straightforwardly to the modification and analysis of materials to be used in future developments of diverse non-nuclear sources like photovoltaic, fuel batteries, etc. From the analytical point of view, the energy of the accelerated ion needs, in many cases, to be known with a precision higher than e.g. the width of reaction resonances that are used for controlling either the yield of a reaction or the penetration depth of the ion, imposing a calibration of the accelerator terminal voltage. This paper reports on the new energy calibration performed for the 5 MV CMAM tandem accelerator

Elastic properties of B-C-N films grown by N2-reactive sputtering from boron carbide targets

The following article appeared in Journal of Applied Physics 114.21 (2013): 213508 and may be found at http://scitation.aip.org/content/aip/journal/jap/114/21/10.1063/1.4837655Boron-carbon-nitrogen films were grown by RF reactive sputtering from a B4C target and N2 as reactive gas. The films present phase segregation and are mechanically softer than boron carbide films (a factor of more than 2 in Young's modulus). This fact can turn out as an advantage in order to select buffer layers to better anchor boron carbide films on substrates eliminating thermally induced mechanical tensions.This work has been supported by Spanish MINECO under contracts MAT2009-08786 and MAT2012-37276- C03-01 as well as by the Madrid Regional Government though contract S2009/MAT-1756

Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed

Surface phase diagram and temperature induced phase transitions of Sn/Cu(100)

Room temperature deposition of Sn on Cu(100) gives rise to a rich variety of surface reconstructions in the submonolayer coverage range. In this work, we report a detailed investigation on the phases appearing and their temperature stability range by using low-energy electron diffraction and surface X-ray diffraction. Previously reported reconstructions in the submonolayer range are p(2 x 2) (for 0.2 ML), p(2 x 6) (for 0.33 ML), (3 root 2 x root 2)R45 degrees (for 0.5 ML), and c(4 x 4) (for 0.65 ML). We find a new phase with a ((-4)(0) (2)(4)) structure for a coverage of 0.45 ML. Furthermore, we analyze the temperature stability of all phases. We find that two phases exhibit a temperature induced reversible phase transition: the (3 root 2 x root 2)R45 degrees phase becomes (root 2 x root 2)R45 degrees phase above 360 K, and the new ((-4)(0) (2)(4)) phase becomes p(2 x 2) also above 360 K. The origin of these two-phase transitions is discussed. (c) 2005 Elsevier B.V. All rights reserved

Surface phase transition and electronic structure of c(5 root 2 x root 2)R45 degrees-Pb/Cu(100)

The c(5 root 2 x root 2)R45 degrees-Pb/Cu(100) surface phase is investigated by means of angle resolved ultraviolet photoemission and low energy electron diffraction in the temperature range between 300 and 550 K. We identify and characterize a temperature-induced surface phase transition at 440 K from the room temperature e(5 root 2 x root 2) R45 degrees phase to a (root 2 x root 2)R45 degrees structure with split superstructure spots. The phase transition is fully reversible and takes place before the two-dimensional melting of the structure at 520 K. The electronic structure of the split (root 2 x root 2)R45 degrees phase is characterized by a metallic free-electron like surface band. This surface band is backfolded with c(5 root 2 x root 2)R45 degrees periodicity phase at room temperature, giving rise to a surface band gap at the Fermi energy. We propose that a gain in electronic energy explains in part the stability of the c(5 root 2 x root 2)R45 degrees phase. (c) 2006 Elsevier B.V. All rights reserved

Elastic properties of B-C-N films grown by N2-reactive sputtering from boron carbide targets

Boron-carbon-nitrogen films were grown by RF reactive sputtering from a B4C target and N2 as reactive gas. The films present phase segregation and are mechanically softer than boron carbide films (a factor of more than 2 in Young's modulus). This fact can turn out as an advantage in order to select buffer layers to better anchor boron carbide films on substrates eliminating thermally induced mechanical tensions. © 2013 AIP Publishing LLC.This work has been supported by Spanish MINECO under contracts MAT2009-08786 and MAT2012-37276-C03-01 as well as by the Madrid Regional Government though contract S2009/MAT-1756.Peer Reviewe