Dailto Silva

Lattice location and thermal stability of implanted nickel in silicon studied by on-line emission channeling

We have studied the lattice location of implanted nickel in silicon, for different doping types (n, n+, and p+). By means of on-line emission channeling, 65Ni was identified on three different sites of the diamond lattice: ideal substitutional sites, displaced bond-center towards substitutional sites (near-BC), and displaced tetrahedral interstitial towards anti-bonding sites (near-T). We suggest that the large majority of the observed lattice sites are not related to the isolated form of Ni but rather to its trapping into vacancy-related defects produced during the implantation. While near-BC sites are prominent after annealing up to 300–500 °C, near-T sites are preferred after 500–600 °C anneals. Long-range diffusion starts at 600–700 °C. We show evidence of Ni diffusion towards the surface and its further trapping on near-T sites at the Rp/2 region, providing a clear picture of the microscopic mechanism of Ni gettering by vacancy-type defects. The high thermal stability of near-BC sites in n+-type Si, ...

A Clinopyroxene-Orthopyroxene-Plagioclase Symplectite Formed by Garnet Breakdown in Granulite Facies, Guaxupé, Minas Gerais, Brazil

Abstract In mafic granulites, garnet can form by reactions such as Opx + Pl = Cpx + Grt + Qtz; Opx + Pl = Grt + Qtz. As a result of isothermal decompression (ITD), garnet can then break down to a characteristic orthopyroxene-plagioclase symplectite. Mafic, iron-rich garnet-pyroxene granulite from the Guaxupe Massif has symplectite that formed by near-isothermal decompression, as a consequence of uplift of the granulite facies terrane. This symplectite was found to consist of vermicular clinopyroxene-orthopyroxene-plagioclase, with clinopyroxene clearly growing from the garnet that is breaking down, modal amounts of clinopyroxene being less than orthopyroxene. Electron probe analyses show clear differences between core (Cpx 1 ), rim, and symplectite clinopyroxene (Cpx 2 ). Considering also the presence of magnetite in the symplectite texture, garnet breakdown is thought to be better represented by a reaction such as Cpx 1 + Grt + O 2 = Cpx 2 + Opx + Pl +Mt + Qtz.

Geochemistry of tourmalines associated with iron formation and quartz veins of the Morro da Pedra Preta Formation, Serra do Itaberaba Group (São Paulo, Brazil)

Tourmalines of intermediate schorl-dravite composition occur in iron formation (including metachert and tourmalinites), metasediments, calc-silicate and metabasic/intermediate rocks of the Morro da Pedra Preta Formation, a volcanic-sedimentary sequence of the Serra do Itaberaba Group (northeast of Sao Paulo City, southeastern Brazil). The Morro da Pedra Preta Formation is crosscut by quartz veins that contain both intermediate schorl-dravite and an alkali-deficient, Cr-(V-)bearing tourmaline, in which the occupancy of the X-site is ϑ0.51Ca0.33Na0.15, characterizing it as intermediate to foitite and magnesiofoitite end-members. Mg# values for this tourmaline are higher than those for intermediate schorl-dravite. Raman spectroscopy also confirms the presence of two groups of tourmalines. Stable isotope data indicate sediment waters as fluid sources, rather than fluids from magmatic/post-magmatic sources. Delta18O compositions for tourmalines, host metachert, and quartz veins are similar, showing that fluid equilibration occurred during crystallization of both quartz and tourmaline. Syngenetic, intermediate schorl-dravite tourmalines were formed under submarine, sedimentary-exhalative conditions; amphibolite-grade metamorphism did not strongly affect their compositions. Younger tourmalines of compositions intermediate to foitite and magnesiofoitite reflect the composition of the host rocks of quartz veins, due to fluid percolation along faults and fractures that caused leaching of Cr (and V) and the crystallization of these alkali-deficient, Cr-(V-)bearing tourmalines.

Minority anion substitution by Ni in ZnO

We report on the lattice location of implanted Ni in ZnO using the β− emission channeling technique. In addition to the majority substituting for the cation (Zn), a significant fraction of the Ni atoms occupy anion (O) sites. Since Ni is chemically more similar to Zn than it is to O, the observed O substitution is rather puzzling. We discuss these findings with respect to the general understanding of lattice location of dopants in compound semiconductors. In particular, we discuss potential implications on the magnetic behavior of transition metal doped dilute magnetic semiconductors.

Precise lattice location of substitutional and interstitial Mg in AlN

The lattice site location of radioactive 27Mg implanted in AlN was determined by means of emission channeling. The majority of the 27Mg was found to substitute for Al, yet significant fractions (up to 33%) were also identified close to the octahedral interstitial site. The activation energy for interstitial Mg diffusion is estimated to be between 1.1 eV and 1.7 eV. Substitutional Mg is shown to occupy ideal Al sites within a 0.1 A experimental uncertainty. We discuss the absence of significant displacements from ideal Al sites, in the context of the current debate, on Mg doped nitride semiconductors.

Turmalinitos vulcanogênicos da formação Morro da Pedra Preta do Grupo Serra do Itaberaba (SP): petrografia, composição química da turmalina e implicações metalogenéticas

Turmalinitos associados a anfibolitos, metachert, formacao ferrifera, metatufos, rochas metassedimentares, metavulcanoclasticas e calciossilicaticas sao encontrados na Formacao Morro da Pedra Preta, sequencia metavulcano-sedimentar basal do Grupo Serra do Itaberaba (SP - Brasil). A deformacao da rocha, que e formada por leitos ricos em turmalina alternados com leitos ricos em quartzo, a presenca de clastos do tipo rip-up e a quimica de rocha total semelhante a das rochas metassedimentares comprovam a origem sin-sedimentar dos turmalinitos. Analises quimicas da turmalina por microssonda eletronica mostram que, para a turmalina dos turmalinitos associados a metatufos, formacao ferrifera, metachert e rochas metavulcanoclasticas e metassedimentares, as composicoes sao intermediarias a serie schorlita-dravita, enquanto para os turmalinitos associados a anfibolitos, metatufos e a rochas calciossilicaticas, as composicoes sao intermediarias a serie dravitauvita. As primeiras apresentam conteudos baixos de Ca e elevados de Al e Na, com variacoes nos teores de Fe e Mg (serie schorlita-dravita), e as ultimas sao caracterizadas por teores baixos de Na e Al e altos de Mg e Ca, contendo adicionalmente fluor. A componente schorlita da turmalina dos turmalinitos situados em porcoes proximais aos centros de atividade vulcano-exalativa de fundo oceânico e indicativa do potencial para ouro da Formacao Morro da Pedra, o que contrasta com as condicoes requeridas para a formacao de depositos de sulfetos macicos (fluidos hipersalinos e associacao com turmalinitos magnesianos). A turmalina dos varios turmalinitos da Formacao Morro da Pedra Preta nao e tipicamente magnesiana e sua composicao foi mais fortemente influenciada pela composicao das rochas circundantes do que por aquela dos fluidos hidrotermais, especialmente no caso em que o turmalinito se depositou em uma posicao intermediaria a distal em relacao ao centro de atividade vulcano-exalativa.

Publisher Correction: Geochemical constraints on the Hadean environment from mineral fingerprints of prokaryotes

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Geochemical constraints on the Hadean environment from mineral fingerprints of prokaryotes

The environmental conditions on the Earth before 4 billion years ago are highly uncertain, largely because of the lack of a substantial rock record from this period. During this time interval, known as the Hadean, the young planet transformed from an uninhabited world to the one capable of supporting, and inhabited by the first living cells. These cells formed in a fluid environment they could not at first control, with homeostatic mechanisms developing only later. It is therefore possible that present-day organisms retain some record of the primordial fluid in which the first cells formed. Here we present new data on the elemental compositions and mineral fingerprints of both Bacteria and Archaea, using these data to constrain the environment in which life formed. The cradle solution that produced this elemental signature was saturated in barite, sphene, chalcedony, apatite, and clay minerals. The presence of these minerals, as well as other chemical features, suggests that the cradle environment of life may have been a weathering fluid interacting with dry-land silicate rocks. The specific mineral assemblage provides evidence for a moderate Hadean climate with dry and wet seasons and a lower atmospheric abundance of CO2 than is present today.

UTILIZAÇÃO DA ESTRUVITA FORMADA NA PRECIPITAÇÃO QUÍMICA DA AMÔNIA COMO FONTE DE NUTRIENTES PARA PLANTAS

Resumo O projeto teve como objetivo avaliar o potencial da estruvita na liberação de nutrientes essenciais para o desenvolvimento de plantas. Para isso, foi instalado um experimento em casa de vegetação localizada na Faculdade de Engenharia Agrícola. A cultura utilizada foi a alface lisa, cultivar Regiane. Após colhidas, a parte aérea foi separada da radicular, pesadas e colocadas cada parte em sacos para serem levadas à estufa, a análise de massa fresca da parte aérea (MFPA) e número de folhas (NF) maiores ou iguais a 6 cm. Concluiu-se que o tratamento que recebeu apenas estruvita com o dobro da dose recomendada de P apresentou, em média, o maior número de folhas e uma maior produção de fitomassa com tendência de aumento de produção conforme a dosagem de estruvita era aumentada. Além disso, o tratamento que recebeu estruvita+carnalita com o dobro da dose recomendada de P apresentou resultados próximos ao do adubo comercial NPK 10:10:10.

Emission Channeling with Short-Lived Isotopes (EC-SLI) at CERN’s ISOLDE Facility

We give an overview on the historical development and current program for lattice location studies at CERN’s ISOLDE facility, where the EC-SLI (Emission Channeling with Short-Lived Isotopes) collaboration maintains several setups for this type of experiments. We illustrate that the three most decisive factors for the success of the technique are access to facilities producing radioactive isotopes, position-sensitive detectors for the emitted decay particles, and reliable simulation codes which allow for quantitative analysis.