Segregation of alloying elements to intrinsic and extrinsic stacking faults in γ'-Ni3Al via first principles calculations

First principles calculations are used to investigate the segregation behaviour of Co, Cr, Re, Mo and W to intrinsic and extrinsic stacking faults in c0-Ni3Al. It is shown that the change in stacking fault energy depends on local alloying concentration and is related to subtle changes in the electronic structure of the alloying elements and adjacent nickel atoms. The results are consistent with observed stacking fault segregation in commercial superalloys and in particular the behaviour of Co and Cr.Support for this work was provided by the EPSRC/ Rolls-Royce Strategic Partnership. The calculations were per- formed using the high performance computing facilities at the University of Cambridge and the UK national facility ARCHER. Access to the latter was obtained via the UKCP con- sortium and funded by EPSRC grant EP/K014560/1.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S1359646215000743

Hybrid density functional calculations of the defect properties of ZnO:Rh and ZnO:Ir

We report density functional calculations of the atomic and electronic structure of the spinel phases ZnRh2O4 and ZnIr2O4 as well as crystalline ZnO lightly doped (1 at.%) with Rh and Ir ions using the B3LYP hybrid functional. Calculations for the spinels show band gaps (∼3 eV) and lattice parameters (∼2% difference) in reasonable agreement with experimental data. Incorporation of the transition metals into ZnO induces local distortions in the lattice and the appearance of metal d levels in the low gap region and near the conduction band minimum, with a d-d splitting larger than 2 eV, which helps maintain transparency in the material. Addition of a hole to the simulation cell of both spinels and doped ZnO leads to charge localization in the neighbourhood of Rh/Ir accompanied by local lattice deformations to form a small polaron which may lead to low hole mobility. We calculate polaron diffusion barriers in the spinels and obtain values around 0.02-0.03 eV. These very low barrier energies suggest that at high Rh/Ir concentration hole conduction occurs mainly by the band conduction mechanism at room temperature. We also develop models of the amorphous spinels by means of classical molecular dynamics simulations, and observe a marked reduction in the coordination number of Rh/Ir, from 6 to 4, in the amorphous phase, which may reduce transparency in these materials.Financial support for this work is provided by the European Commission through contract No. NMP3-LA-2010-246334 (ORAMA).This is the originally submitted version of the article, and does not include any of the changes arising from peer-review. The final, peer-reviewed and edited version of the article is available at http://www.sciencedirect.com/science/article/pii/S0040609013013059

Point defect formation in M₂AlC (M = Zr,Cr) MAX phases and their tendency to disorder and amorphize

First principles calculations are performed on Zr₂AlC and Cr₂AlC MAX phases to compare their ability to accommodate point defects under irradiation. Interatomic bonding is stronger in Cr₂AlC than Zr₂AlC but contrary to expectation Zr₂AlC exhibits higher vacancy and antisite pair formation energies. However, interstitials and Frenkel defects are generally more difficult to form in Cr₂AlC. The results are attributed to the mixed covalent/ionic/metallic nature of the bonding. Detailed comparison of all the energies suggests that the preferred defects in Zr₂AlC and Cr₂AlC are the V_Al+Al_i Frenkel and Cr_Al+Al_Cr antisite respectively. Thus the potential response of the two phases to irradiation is different and taking account of other competing defects it is suggested that Zr₂AlC is less susceptible to amorphization.The calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the CaFFE consortium and funded by EPSRC under Grant No. EP/M018768/1

Identifying early stage precipitation in large-scale atomistic simulations of superalloys

A method for identifying and classifying ordered phases in large chemically and thermally disordered atomistic models is presented. The method uses Steinhardt parameters to represent local atomic configurations and develops probability density functions to classify individual atoms using naïve Bayes. The method is applied to large molecular dynamics simulations of supersaturated Ni-20 at% Al solid solutions in order to identify the formation of embryonic γ'-Ni3Al. The composition and temperatures are chosen to promote precipitation, which is observed in the form of ordering and is found to occur more likely in regions with above average Al concentration producing 'clusters' of increasing size. The results are interpreted in terms of a precipitation mechanism in which the solid solution is unstable with respect to ordering and potentially followed by either spinodal decomposition or nucleation and growth.Support for this work was provided by Rolls Royce plc and EPSRC Doctoral Training Grants EP/J500380/1 and EP/K503009/1

Effect of trivalent dopants on local coordination and electronic structure in crystalline and amorphous ZnO

Density functional theory calculations are used to investigate the structure and binding energies of clusters formed between oxygen vacancies and trivalent dopant atoms (indium, gallium and aluminium) substituted into zinc oxide. Our results show that indium atoms form stable nearest neighbour pairs with oxygen vacancies, while gallium and aluminium atoms associate with them at next nearest neighbour sites. Using a combination of classical molecular dynamics and Reverse Monte Carlo methods, models of amorphous indium zinc oxide at different compositions up to 25 at.% indium are created. Analysis of these models indicates that, in contrast with the trend observed in the crystal phase, indium does not tend to be undercoordinated in the amorphous phase. The value of the band gap obtained for the amorphous compositions is smaller than that of crystalline undoped ZnO by about 0.8 eV and is largely independent of the indium concentration. Electron effective masses calculated in all the amorphous models decrease with increasing amount of indium due to the larger dispersion of the In-dominated conduction bands. This trend is compared to resistivity measurements on amorphous indium zinc oxide which also decrease with increasing indium concentration.Financial support for this work is provided by the European Commission through contract No. NMP3-LA-2010-246334 (ORAMA).This is the original submitted version of the article. It does not incorporate changes arising from peer-review. The final peer-reviewed version of the article has been published in Thin Solid Films as 'Effect of trivalent dopants on local coordination and electronic structure in crystalline and amorphous ZnO' and is available at http://www.sciencedirect.com/science/article/pii/S004060901301002X

Prediction of subgap states in Zn- and Sn-based oxides using various exchange-correlation functionals

We present a density-functional-theory analysis of crystalline and amorphous Zn- and Sn-based oxide systems which focuses on the electronic defect states within the band gap. A comparison of these electronic levels reveals that the hybrid functionals PBE0, HSE06, or B3LYP agree with a self-interaction corrected (SIC) local-density-approximation functional on occupied defect levels when similar treatments of the self-interaction are considered. However, for unoccupied levels, the hybrid functionals and the SIC approach lead to very different predictions. We show that a prerequisite for the determination of the energetic position of subgap states in these oxides is that a functional needs to predict correctly the electronic band structure over a wide energy range and not just close to the band gap. We conclude that for accurate defect levels, an adequate treatment of the self-interaction problem is required especially in the presence of nearby metal-metal interactions.Financial support for this work was provided by the European Commission through Contract No. NMP3-LA-2010-246334 (ORAMA). The calculations at Cambridge were performed using the High Performance Computing Facility, Darwin, and also the UK national high performance computing service ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC Grant No. EP/K014560/1.Phys. Rev. B 90, 195142 – Published 21 November 2014 ©2014 American Physical Society, http://dx.doi.org/10.1103/PhysRevB.90.19514

Synthesis, crystal structure, magnetic and electronic properties of the caesium-based transition metal halide Cs<inf>3</inf>Fe<inf>2</inf>Br<inf>9</inf>

The diversity of halide materials related to important solar energy systems such as CsPbX3 (X = Cl, Br, I) is explored by introducing the transition metal element Fe. In particular a new compound, Cs3Fe2Br9 (space group P6_3/mmc with a = 7.5427(8) and c = 18.5849(13) {\AA}), has been synthesized and found to contain 0D face-sharing Fe2Br9 octahedral dimers. Unlike its isomorph, Cs3Bi2I9, it is black in color, has a low optical bandgap of 1.65 eV and exhibits antiferromagnetic behavior below TN = 13 K. Density functional theory calculations shed further light on these properties and also predict that the material should have anisotropic transport characteristics

[Am]Mn(HPOO): a new family of hybrid perovskites based on the hypophosphite ligand

A family of five hybrid ABX3 perovskites has been synthesized using hypophosphite (H2POO)- as the X-site ion. These compounds adopt the general formula [Am]Mn(H2POO)3, where Am = guanidinium (HUA), formamidinium (FA), imidazolium (IM), triazolium (TRZ), and dabconium (DAB). We explore the diverse structural and phase transition behavior of these materials through single-crystal diffraction measurements and demonstrate contrasting magnetism in two of the phases, Am = GUA and FA, that arises from structural distortions. The results show that hypophosphite perovskites offer a promising platform for generating new functional materials.Y.W., F.B., and A.K.C. gratefully thank the Ras Al Khaimah Center for Advanced Materials for financial support. S.S. gratefully thanks the Winston Churchill Foundation of the USA for financial support. R.M. thanks SERB, New Delhi for a JC Bose Fellowship. DFT calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the MCC consortium and funded by EPSRC under Grant No. EP/L000202/1. Magnetic measurements were carried out using the Advanced Materials Characterization Suite, funded by EPSRC Strategic Equipment Grant EP/M000524/1

Anomalous diffusion of single metal atoms on a graphene oxide support

Recent studies of single-atom catalysts open up the prospect of designing exceptionally active and environmentally efficient chemical processes. The stability and durability of such catalysts is governed by the strength with which the atoms are bound to their support and their diffusive behaviour. Here we use aberration-corrected STEM to image the diffusion of single copper adatoms on graphene oxide. We discover that individual atoms exhibit ano malous diffusion as a result of spatial and energetic disorder inherent in the support, and interpret the origins of this behaviour to develop a physical picture for the surface diffusion of single metal atoms

The Signaller's Dilemma: A Cost–Benefit Analysis of Public and Private Communication

Understanding the diversity of animal signals requires knowledge of factors which may influence the different stages of communication, from the production of a signal by the sender up to the detection, identification and final decision-making in the receiver. Yet, many studies on signalling systems focus exclusively on the sender, and often ignore the receiver side and the ecological conditions under which signals evolve.We study a neotropical katydid which uses airborne sound for long distance communication, but also an alternative form of private signalling through substrate vibration. We quantified the strength of predation by bats which eavesdrop on the airborne sound signal, by analysing insect remains at roosts of a bat family. Males do not arbitrarily use one or the other channel for communication, but spend more time with private signalling under full moon conditions, when the nocturnal rainforest favours predation by visually hunting predators. Measurements of metabolic CO(2)-production rate indicate that the energy necessary for signalling increases 3-fold in full moon nights when private signalling is favoured. The background noise level for the airborne sound channel can amount to 70 dB SPL, whereas it is low in the vibration channel in the low frequency range of the vibration signal. The active space of the airborne sound signal varies between 22 and 35 meters, contrasting with about 4 meters with the vibration signal transmitted on the insect's favourite roost plant. Signal perception was studied using neurophysiological methods under outdoor conditions, which is more reliable for the private mode of communication.Our results demonstrate the complex effects of ecological conditions, such as predation, nocturnal ambient light levels, and masking noise levels on the performance of receivers in detecting mating signals, and that the net advantage or disadvantage of a mode of communication strongly depends on these conditions