@article{rak_brenner_2020, title={Exchange interactions and long-range magnetic order in the (Mg,Co,Cu,Ni,Zn)O entropy-stabilized oxide: A theoretical investigation}, volume={127}, ISSN={["1089-7550"]}, DOI={10.1063/5.0008258}, abstractNote={The magnetic structure of the entropy-stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O has been investigated using first-principles methods in combination with Monte Carlo (MC) simulations. Similar to other transition metal oxides with the rock salt structure, such as CoO and NiO, the dominant interaction in this entropic oxide is the antiferromagnetic (AFM) superexchange interaction that takes place between second nearest neighbor cations. This superexchange interaction is responsible for the long-range type-II antiferromagnetic order observed in the material, with ferromagnetic (111) planes coupled antiferromagnetically in the (111) direction. The Néel temperature (TN) is evaluated via MC simulation, where the entropic oxide is modeled by a lattice of randomly distributed strengths of magnetic exchanges obtained from the binary and ternary oxides. The composition dependence of TN suggests that the material becomes paramagnetic when the concentration of nonmagnetic species exceeds 84%. The comparison between the theoretical results and the available experimental data indicates that the magnetic interactions in the entropic oxide can be predicted from magnetic exchange parameters calculated in the binary and ternary oxides.}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rak, Zs and Brenner, D. W.}, year={2020}, month={May} }
 @article{rak_brenner_2019, title={Effect of water chemistry on the composition of oxides formed on stainless steel surfaces in light water reactors}, volume={526}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2019.151773}, abstractNote={Results of first-principles calculations have been combined with thermochemical data to evaluate the free energies of formation and the equilibrium composition of Cr-containing nickel ferrite spinel in contact with aqueous solution. Under conditions of solid-liquid equilibrium, with temperature, pH, and concentration of aqueous cations (Ni2+, Fe2+/3+, Cr3+) that are characteristic to operating light water reactors (LWRs), Cr is incorporated at the octahedral site of the spinel up to 12–14 at. %. The effects of temperature, pH, and concentration of aqueous species on the spinel composition are discussed.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Rak, Zs and Brenner, D. W.}, year={2019}, month={Dec} }
 @article{lim_rak_braun_rost_kotsonis_hopkins_maria_brenner_2019, title={Influence of mass and charge disorder on the phonon thermal conductivity of entropy stabilized oxides determined by molecular dynamics simulations}, volume={125}, ISSN={["1089-7550"]}, DOI={10.1063/1.5080419}, abstractNote={It is shown using classical molecular dynamics simulations that phonon scattering from disorder in the interatomic forces introduced by charge transfer and not from mass disorder is needed to explain the thermal conductivity reduction experimentally measured that accompanies the addition of a sixth cation to the entropy stabilized oxide J14 [(Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5]. The simulations were performed on five entropy-stabilized oxides, J14, and J14 plus Sc, Sn, Cr, or Ge in equi-molar cation proportions. Comparing the simulation results to predictions from the Bridgman equation using properties from the simulations suggests that despite phonon scattering from disorder in both atomic forces and mass, the thermal conductivity for these systems is still above an analytical limit for an amorphous structure.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Lim, M. and Rak, Zs. and Braun, J. L. and Rost, C. M. and Kotsonis, G. N. and Hopkins, P. E. and Maria, J. -P. and Brenner, D. W.}, year={2019}, month={Feb} }
 @article{kleppinger_pershin_rak_mandal_2019, title={Investigation on origin of Ru-induced deep-level defects in 4H-SiC epilayer based Schottky diodes by DLTS and theoretical calculations}, volume={11114}, ISSN={["1996-756X"]}, DOI={10.1117/12.2530177}, abstractNote={We present a dual experimental and computational studies on ruthenium (Ru) induced point defects in wide bandgap semiconductor 4H-silicon carbide (4H-SiC) which is of high interest in alpha, x-ray, and low energy gamma spectroscopy due to Ru’s high weighted metal work function of 4.76 eV which forms a high barrier Schottky contact with low leakage current. We first measured the activation energies and concentrations of deep levels in RF sputtered Ru/n-4H-SiC Schottky diodes annealed at 950°C using deep level transient spectroscopy (DLTS) and identified two deep level defects at Ec – (0.89 ± 0.03) eV and Ec - (1.98 ± 0.03) eV which appear unique to Schottky diodes with Ru. In order to correlate these defects theoretically, we then calculated the formation energies and transition levels of Ru induced point defects in 4H-SiC at charge states [-2, 2] for substitutions and [-2,+4] for interstitials using the projector augmented wave method (PAW) with both PBE and hybrid pseudopotentials on a 3 x 3 x 1 supercell. We found two transition levels which correlate very well with our experimental DLTS results. The transition (-1/0) for Ru substituted into the cubic silicon site at Ev + 2.39 eV and the transition (-1/0) for Ru placed in interstitial site with tetrahedral symmetry to carbon at Ev + 1.23 eV respectively.}, journal={HARD X-RAY, GAMMA-RAY, AND NEUTRON DETECTOR PHYSICS XXI}, author={Kleppinger, Joshua W. and Pershin, Yuriy and Rak, Zsolt and Mandal, Krishna C.}, year={2019} }
 @article{rak_brenner_2019, title={Negative Surface Energies of Nickel Ferrite Nanoparticles under Hydrothermal Conditions}, volume={2019}, ISSN={["1687-4129"]}, DOI={10.1155/2019/5268415}, abstractNote={The formation of nickel ferrite (NiFe2O4) nanoparticles under hydrothermal conditions has been modeled using a method that combines results of first-principle calculations, elements of aqueous thermochemistry, and experimental free energies of formation. The calculations predict negative formation energies for the (111) surfaces and positive free energies for the formation of bulk nickel ferrite. Based on classical nucleation theory, the combination of the negative surface and positive bulk energies yields thermodynamically stable nickel ferrite nanoparticles with sizes between 30 and 150 nm in the temperature range of 300 to 400 K under alkaline conditions. The surface and bulk energetics as well as the stability of the nickel ferrite nanoparticle as a function of temperature and pH are discussed.}, journal={JOURNAL OF NANOMATERIALS}, author={Rak, Zs. and Brenner, D. W.}, year={2019}, month={Oct} }
 @article{rak_maria_brenner_2018, title={Evidence for Jahn-Teller compression in the (Mg, Co, Ni, Cu, Zn)O entropy-stabilized oxide: A DFT study}, volume={217}, ISSN={["1873-4979"]}, DOI={10.1016/j.matlet.2018.01.111}, abstractNote={The local atomic configuration and the electronic structure of the octahedral Cu2+ centers in the entropy-stabilized oxides (ESOs), (MgCoNiZn)1−xCuxO (x = 0.13, 0.2, 0,.26), have been investigated using density functional theory. The calculated Cu–O bond lengths combined with the analysis of the electronic density of states indicate that ∼10% of the CuO6 octahedra in ESOs undergo Jahn-Teller (JT) compression. This unexpected behavior is likely related to the structural constraints imposed by the rocksalt lattice and reflects the competition between the regular octahedral environment preferred by the cations in the system and the JT distortion imposed by the Cu-centers. The orientation of the distortions appears to be random, in agreement with recent experimental results.}, journal={MATERIALS LETTERS}, author={Rak, Zs. and Maria, J. -P. and Brenner, D. W.}, year={2018}, month={Apr}, pages={300–303} }
 @article{rak_brenner_2018, title={First-principles investigation of diffusion and defect properties of Fe and Ni in Cr2O3}, volume={123}, number={15}, journal={Journal of Applied Physics}, author={Rak, Z. and Brenner, D. W.}, year={2018} }
 @article{rak_brenner_2017, title={Ab initio investigation of the surface properties of austenitic Fe-Ni-Cr alloys in aqueous environments}, volume={402}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2017.01.048}, abstractNote={The surface energetics of two austenitic stainless steel alloys (Type 304 and 316) and three Ni-based alloys (Alloy 600, 690, and 800) are investigated using theoretical methods within the density functional theory. The relative stability of the low index surfaces display the same trend for all alloys; the most closely packed orientation and the most stable is the (111), followed by the (100) and the (110) surfaces. Calculations on the (111) surfaces using various surface chemical and magnetic configurations reveal that Ni has the tendency to segregate toward the surface and Cr has the tendency to segregate toward the bulk. The magnetic frustration present on the (111) surfaces plays an important role in the observed segregation tendencies of Ni and Cr. The stability of the (111) surfaces in contact with aqueous solution are evaluated as a function of temperature, pH, and concentration of aqueous species. The results indicate that the surface stability of the alloys decrease with temperature and pH, and increase slightly with concentration. Under conditions characteristic to an operating pressurized water reactor, the Ni-based alloy series appears to be of better quality than the stainless steel series with respect to corrosion resistance and release of aqueous species when in contact with aqueous solutions.}, journal={APPLIED SURFACE SCIENCE}, author={Rak, Zs. and Brenner, D. W.}, year={2017}, month={Apr}, pages={108–113} }
 @article{rost_rak_brenner_maria_2017, title={Local structure of the MgxNixCoxCuxZnxO(x=0.2) entropy-stabilized oxide: An EXAFS study}, volume={100}, ISSN={["1551-2916"]}, DOI={10.1111/jace.14756}, abstractNote={Abstract}, number={6}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Rost, Christina M. and Rak, Zsolt and Brenner, Donald W. and Maria, Jon-Paul}, year={2017}, month={Jun}, pages={2732–2738} }
 @article{rak_rost_lim_sarker_toher_curtarolo_maria_brenner_2016, title={Charge compensation and electrostatic transferability in three entropy-stabilized oxides: Results from density functional theory calculations}, volume={120}, number={9}, journal={Journal of Applied Physics}, author={Rak, Z. and Rost, C. M. and Lim, M. and Sarker, P. and Toher, C. and Curtarolo, S. and Maria, J. P. and Brenner, D. W.}, year={2016} }
 @article{rak_o'brien_shin_andersson_stanek_brenner_2016, title={Theoretical assessment of bonaccordite formation in pressurized water reactors}, volume={474}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2016.02.016}, abstractNote={The free energy of formation of bonaccordite (Ni2FeBO5) as a function of temperature has been calculated using a technique that combines first principles calculations with experimental free energies of formation of aqueous species. The results suggest that bonaccordite formation from aqueous metal ions (Ni2+ andFe3+) and boric acid is thermodynamically favorable at elevated temperature and pH that have been predicted to exist at the CRUD-clad interface in deposits thicker than 60 μm.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Rak, Zs and O'Brien, C. J. and Shin, D. and Andersson, A. D. and Stanek, C. R. and Brenner, D. W.}, year={2016}, month={Jun}, pages={62–64} }
 @article{rak_o'brien_brenner_andersson_stanek_2016, title={Understanding the Atomic-Level Chemistry and Structure of Oxide Deposits on Fuel Rods in Light Water Nuclear Reactors Using First Principles Methods}, volume={68}, ISSN={["1543-1851"]}, DOI={10.1007/s11837-016-2102-z}, number={11}, journal={JOM}, author={Rak, Zs. and O'Brien, C. J. and Brenner, D. W. and Andersson, D. A. and Stanek, C. R.}, year={2016}, month={Nov}, pages={2912–2921} }
 @article{brenner_lu_christopher j. o'brien_bucholz_rak_2015, title={A particle assembly/constrained expansion (PACE) model for the formation and structure of porous metal oxide deposits on nuclear fuel rods in pressurized light water reactors}, volume={457}, ISSN={["1873-4820"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84915820676&partnerID=MN8TOARS}, DOI={10.1016/j.jnucmat.2014.11.061}, abstractNote={A new model is proposed for the structure and properties of porous metal oxide scales (aka Chalk River Unidentified Deposits (CRUD)) observed on the nuclear fuel rod cladding in Pressurized Water Reactors (PWR). The model is based on the thermodynamically-driven expansion of agglomerated octahedral nickel ferrite particles in response to pH and temperature changes in the CRUD. The model predicts that porous nickel ferrite with internal {1 1 1} surfaces is a thermodynamically stable structure under PWR conditions even when the free energy of formation of bulk nickel ferrite is positive. This explains the pervasive presence of nickel ferrite in CRUD, observed CRUD microstructures, why CRUD maintains its porosity, and variations in porosity within the CRUD observed experimentally. This model is a stark departure from decades of conventional wisdom and detailed theoretical analysis of CRUD chemistry, and defines new research directions for model validation, and for understanding and ultimately controlling CRUD formation.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Brenner, Donald W. and Lu, Shijing and Christopher J. O'Brien and Bucholz, Eric W. and Rak, Zsolt}, year={2015}, month={Feb}, pages={209–212} }
 @article{rak_bucholz_brenner_2015, title={Defect formation in aqueous environment: Theoretical assessment of boron incorporation in nickel ferrite under conditions of an operating pressurized-water nuclear reactor (PWR)}, volume={461}, ISSN={["1873-4820"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84925834410&partnerID=MN8TOARS}, DOI={10.1016/j.jnucmat.2015.03.038}, abstractNote={A serious concern in the safety and economy of a pressurized water nuclear reactor is related to the accumulation of boron inside the metal oxide (mostly NiFe2O4 spinel) deposits on the upper regions of the fuel rods. Boron, being a potent neutron absorber, can alter the neutron flux causing anomalous shifts and fluctuations in the power output of the reactor core. This phenomenon reduces the operational flexibility of the plant and may force the down-rating of the reactor. In this work an innovative approach is used to combine first-principles calculations with thermodynamic data to evaluate the possibility of B incorporation into the crystal structure of NiFe2O4, under conditions typical to operating nuclear pressurized water nuclear reactors. Analyses of temperature and pH dependence of the defect formation energies indicate that B can accumulate in NiFe2O4 as an interstitial impurity and may therefore be a major contributor to the anomalous axial power shift observed in nuclear reactors. This computational approach is quite general and applicable to a large variety of solids in equilibrium with aqueous solutions.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Rak, Zs. and Bucholz, E. W. and Brenner, D. W.}, year={2015}, month={Jun}, pages={350–356} }
 @article{rak_brenner_2015, title={Interplay of electronic structure and unusual development in crystal structure of YbAuIn and Yb3AuGe2In3}, volume={95}, ISSN={["1478-6443"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84936847188&partnerID=MN8TOARS}, DOI={10.1080/14786435.2015.1052859}, abstractNote={First-principles calculations within the DFT are employed to investigate the relationship between the electronic structure and the unexpected features of the hexagonal cell parameters of YbAuIn and . Calculations indicate that YbAuIn is an intermediate valent system with one Yb 4f state pinned to the Fermi level, while is closer to integer valency with all Yb 4f states occupied. Structural relaxations performed on LaAuIn and LuAuIn analogues reveal that expansion of the c-parameter in is attributable to larger size of the divalent Yb compared with intermediate valent Yb.}, number={20}, journal={PHILOSOPHICAL MAGAZINE}, author={Rak, Zs. and Brenner, D. W.}, year={2015}, month={Jul}, pages={2167–2174} }
 @article{christopher j. o'brien_rak_brenner_2014, title={Calculated Stability and Structure of Nickel Ferrite Crystal Surfaces in Hydrothermal Environments}, volume={118}, ISSN={["1932-7447"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84896339695&partnerID=MN8TOARS}, DOI={10.1021/jp5002308}, abstractNote={A comprehensive theoretical investigation of nickel ferrite (NiFe2O4) surfaces is undertaken to understand the structure and stability of nanocrystallites that would be present under conditions of hydrothermal synthesis (HTS). In particular, the focus is on conditions characteristic to an operating pressurized water nuclear reactor (PWR). Solid–liquid equilibrium is assumed between bulk nickel ferrite and the aqueous environment saturated with respect to nickel ferrite. A theoretical framework is developed in which the surface energies are evaluated in terms of concentrations of aqueous metal cations, pH, temperature, and pressure. The energies of the bare and water terminated surfaces are calculated and discussed. Surfaces that have more metal cations exposed are found to be more stable. Water adsorption on the nickel ferrite surfaces is an exothermic process, with the magnitude of exothermicity decreasing as a function of temperature. At temperatures relevant to operating PWRs, the energy gain due to wa...}, number={10}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Christopher J. O'Brien and Rak, Zs and Brenner, Donald W.}, year={2014}, month={Mar}, pages={5414–5423} }
 @article{o'brien_rak_bucholz_brenner_2014, title={First principles calculations predict stable 50 nm nickel ferrite particles in PWR coolant}, volume={454}, ISSN={["1873-4820"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84906355696&partnerID=MN8TOARS}, DOI={10.1016/j.jnucmat.2014.07.049}, abstractNote={Thermodynamic calculations that combine experimental data with the results of first principles calculations yield negative free energies for {1 1 1} surfaces of nickel ferrite for the temperature, pressure and ion concentrations typical of Pressurized Light Water Reactor (PWR) coolant. When combined with a positive bulk free energy of formation, the negative surface energies predict that thermodynamically-stable octahedral nickel ferrite particles with diameters of ∼50 nm should be present in PWR coolant during operation. These particles would not be removed by mixed bed demineralizers and would be below the filter pore sizes typically used in Chemical and Volume Control Systems. The calculations also predict that these particles are not thermodynamically stable in coolant under ambient conditions. Based on these results it is proposed that solvated nickel ferrite particles, which are predicted to be stable and likely long-lived in PWR primary coolant, contribute to the nucleation of metal oxide scale on PWR fuel rod cladding and that conventional methods for purifying the primary coolant may be ineffective in removing these species.}, number={1-3}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={O'Brien, C. J. and Rak, Zs. and Bucholz, E. W. and Brenner, D. W.}, year={2014}, month={Nov}, pages={77–80} }
 @article{rak_o'brien_brenner_2014, title={First-principles investigation of boron defects in nickel ferrite spinel}, volume={452}, ISSN={["1873-4820"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903142361&partnerID=MN8TOARS}, DOI={10.1016/j.jnucmat.2014.05.031}, abstractNote={The accumulation of boron within the porous nickel ferrite (NiFe2O4, NFO) deposited on nuclear reactor fuel rods is a major technological problem with important safety and economical implications. In this work, the electronic structure of nickel ferrite spinel has been investigated using first-principles methods, and the theoretical results have been combined with experimental data to analyze B incorporation into the spinel structure of NFO. Under thermodynamic solid–solid equilibrium between NFO and atomic reservoirs of Ni and Fe, our calculations predict that the incorporation of B into the NFO structure is unfavorable. The main factors that limit B incorporation are the narrow stability domain of NFO and the precipitation of B2O3, Fe3BO5, and Ni3B2O6 compounds as secondary phases. The B incorporation energies depend sensitively on the electron chemical potential (EF) and the charge state of the defect. In n-type NFO, the most stable defect is the Ni vacancy VNi2- while in p-type material lowest the formation energy belongs to the interstitial B occupying a tetrahedrally coordinated site BT2+. Because of these limiting conditions it is more thermodynamically favorable for B to form secondary phases with Fe, Ni and O (e.g. B2O3, Fe3BO5, and Ni3B2O6) than it is to form point defects in NFO.}, number={1-3}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Rak, Zs and O'Brien, C. J. and Brenner, D. W.}, year={2014}, month={Sep}, pages={446–452} }
 @inproceedings{rák_o’brien_brenner_2014, title={First-principles investigation of boron incorporation into CRUD under Pressurized Water Reactor conditions}, volume={1709}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84924370814&partnerID=MN8TOARS}, DOI={10.1557/opl.2014.748}, abstractNote={ABSTRACT}, booktitle={Materials Research Society Symposium Proceedings}, author={Rák, Z. and O’Brien, C.J. and Brenner, D.W.}, year={2014} }
 @article{guo_rak_tavakoli_becker_ewing_navrotsky_2014, title={Thermodynamics of thorium substitution in yttrium iron garnet: comparison of experimental and theoretical results}, volume={2}, ISSN={["2050-7496"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907789784&partnerID=MN8TOARS}, DOI={10.1039/c4ta03683b}, abstractNote={The thermodynamic stability of Th-doped yttrium iron garnet (Y3Fe5O12, YIG) as a possible actinide-bearing material has been investigated using calorimetric measurements and first-principles electronic-structure calculations.}, number={40}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Guo, X. and Rak, Zs. and Tavakoli, A. H. and Becker, U. and Ewing, R. C. and Navrotsky, A.}, year={2014}, month={Oct}, pages={16945–16954} }
 @article{nelson_rak_abrecht-schmitt_becker_ewing_2014, title={Three New Silver Uranyl Diphosphonates: Structures and Properties}, volume={53}, ISSN={["1520-510X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84896370524&partnerID=MN8TOARS}, DOI={10.1021/ic401897n}, abstractNote={The hydrothermal reaction of uranium trioxide and methylenediphosphonic acid in the presence of silver nitrate resulted in the formation of three new uranyl coordination polymers: AgUO2[CH2(PO3)(PO3H)] (Ag-1), [Ag2(H2O)1.5]{(UO2)2[CH2(PO3)2]F2}·(H2O)0.5 (Ag-2), and Ag2UO2[CH2(PO3)2] (Ag-3). All consist of uranyl pentagonal bipyramids that form two-dimensional layered structures. Ag-1 and Ag-3 possess the same structural building unit, but the structures are different; Ag-3 is formed through edge-sharing of F atoms to form UO5F2 dimers. The pH and silver cation have significant effects on the structure that is synthesized. Raman spectra of single crystals of Ag-1, Ag-2, and Ag-3 reveal v1 UO2(2+) symmetric stretches of 816 and 829, 822, and 802 cm(-1), respectively. Electronic structure calculations were performed using the projector augmented wave (PAW) method with density functional theory (DFT) to gain insight into the nature of bonding and electronic characteristics of the synthesized compounds. Herein, we report the syntheses, crystal structures, Raman spectroscopy, and luminescent behavior of these three compounds.}, number={6}, journal={INORGANIC CHEMISTRY}, author={Nelson, Anna-Gay D. and Rak, Zsolt and Abrecht-Schmitt, Thomas E. and Becker, Udo and Ewing, Rodney C.}, year={2014}, month={Mar}, pages={2787–2796} }
 @article{rak_ewing_becker_2013, title={Electronic structure and thermodynamic stability of uranium-doped yttrium iron garnet}, volume={25}, ISSN={["1361-648X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887587342&partnerID=MN8TOARS}, DOI={10.1088/0953-8984/25/49/495502}, abstractNote={The electronic and thermodynamic properties of yttrium iron garnet (Y3Fe5O12, YIG), as a possible uranium-bearing phase, have been investigated using first-principles and semi-empirical methods. The electronic structures of pure and U-doped YIG were calculated and compared in order to obtain a fundamental understanding of the incorporation mechanism and stability of U in a YIG matrix. Uranium at the A-site is in 4 + oxidation state, acting as a single donor and introducing a localized defect state in the band gap. The ionic relaxations show U at the A-site is an off-center impurity. At the B-site, uranium is in 5 + oxidation state giving rise to two localized defect states in the middle of the band gap. At thermodynamic equilibrium the incorporation of U is limited by (i) the relatively narrow stability domain of the host YIG and (ii) the precipitation of uranium oxides as secondary phases. Under Y-rich growth conditions, YIG is unstable with respect to competing phases such as the iron oxides, Y2O3 and YFeO3. Under O-rich conditions, the incorporation U is obstructed by the formation of uranium-oxide precipitates. Under Fe-rich growth conditions, the formation energies of UY (U at the A-site) and UFe (U at the B-site) become negative for 0 ≤ EF ≤ 0.62 eV and 0 ≤ EF ≤ 0.77 eV, respectively, indicating that U might be incorporated in p-type YIG.}, number={49}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Rak, Zs and Ewing, R. C. and Becker, U.}, year={2013}, month={Dec} }
 @article{rak_ewing_becker_2013, title={Ferric garnet matrices for immobilization of actinides}, volume={436}, ISSN={["1873-4820"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873616579&partnerID=MN8TOARS}, DOI={10.1016/j.jnucmat.2013.01.290}, abstractNote={The electronic structure of the Pu- and Np-containing Ca3(Ti,Zr,Hf)2Fe23+SiO12 garnet series has been investigated using ab initio methods within density functional theory (DFT, GGA + U). The calculations provide a fundamental understanding of the incorporation mechanism and stability of Pu and Np in the garnet structure. The detailed analysis of the electronic densities of states (DOS), band structures and charge density distributions confirm that electrons are transferred from the actinides to the surrounding Fe atoms. This occurs through the double exchange coupling of the actinide f shell and the NN Fe d shells and stabilizes the actinide at the A-site, when the spins of the f and d shells are anti-parallel. The presence of Fe is crucial, since it accommodates the extra valence electrons introduced by the actinides, and this electron transfer lowers the total energies of the actinide-doped structure. Comparing the incorporation energies at the A- and B-site, both Pu and Np clearly prefer the A-site, provided that there are sufficient Fe atoms to facilitate the charge transfer. The calculated incorporation energies suggest that ferric garnet is promising material for actinide immobilization.}, number={1-3}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Rak, Zs and Ewing, R. C. and Becker, U.}, year={2013}, month={May}, pages={1–7} }
 @article{o’brien_rák_brenner_2013, title={Free energies of (Co, Fe, Ni, Zn) Fe 2 O 4 spinels and oxides in water at high temperatures and pressure from density functional theory: results for stoichiometric NiO and NiFe2O4surfaces}, volume={25}, ISSN={0953-8984 1361-648X}, url={http://dx.doi.org/10.1088/0953-8984/25/44/445008}, DOI={10.1088/0953-8984/25/44/445008}, abstractNote={A set of effective chemical potentials (ECPs) are derived that connect energies of (Co, Fe, Ni, Zn)Fe2O4 spinels and oxides calculated at 0 K from density functional theory (DFT) to free energies in high temperature and pressure water. The ECPs are derived and validated by solving a system of linear equations that combine DFT and experimental free energies for NiO, ZnO, Fe2O3, Fe3O4, FeO(OH), CoFe2O4, ZnFe2O4, NiFe2O4 and H2O. To connect to solution phase chemistry, a set of ECPs are also derived for solvated Ni2+, Zn2+, Fe2+ and Fe3+ ions using an analogous set of linear equations and the solid ECPs. The ECPs are used to calculate free energies of low index stoichiometric surfaces of nickel oxide (NiO) and nickel ferrite (NiFe2O4) in water as a function of temperature from 300 to 600 K at a pressure of 155 bar. Surface denuding at high temperatures is predicted, the implications of which for the formation of oxide corrosion products on heat transfer surfaces in light-water nuclear reactors are discussed.}, number={44}, journal={Journal of Physics: Condensed Matter}, publisher={IOP Publishing}, author={O’Brien, C J and Rák, Z and Brenner, D W}, year={2013}, month={Oct}, pages={445008} }
 @article{rak_ewing_becker_2013, title={Hydroxylation-induced surface stability of AnO(2) (An = U, Np, Pu) from first-principles}, volume={608}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870423082&partnerID=MN8TOARS}, DOI={10.1016/j.susc.2012.10.002}, abstractNote={The relative stabilities of clean and hydroxylated surfaces of three actinide dioxides (AnO2, An = U, Np, Pu) have been investigated using first-principles methods within the DFT + U framework. In the case of the clean surfaces, the calculated surface energies are consistently the lowest for the (111) surface for all three AnO2 compositions, followed by the (110) and (100) surface energies. In the case of UO2, for instance, the calculated surface energies are 0.78, 1.05, and 1.47 J/m2 for the (111), (110), and (100) surfaces, respectively, reinforcing the well-established surface energy trend for metal-dioxides: (111) < (110) < (100). Dissociated water, at one monolayer coverage, is adsorbed preferentially onto the (100) surface for all three AnO2 systems. In the case of UO2 the water adsorption energy on the (100) surface (− 1.34 J/m2) is almost four times higher than the adsorption energy on the (111) surface (− 0.35 J/m2), and almost twice as large as the adsorption energy on the (110) surface (− 0.77 J/m2). Similar trend in the adsorption energies is observed for both NpO2 and PuO2. As a result, the relative stability of the hydroxylated AnO2 (An = U, Np, Pu) surfaces changes to (100) < (110) < (111). The effects of the geometric relaxations on the clean and hydroxylated surfaces are discussed.}, journal={Surface Science}, author={Rak, Zs and Ewing, R. C. and Becker, U.}, year={2013}, pages={180–187} }
 @article{rak_ewing_becker_2011, title={Ab initio investigation of actinide incorporation in Ti, Zr, Hf, and Sn-containing garnet structure}, volume={241}, journal={Abstracts of Papers of the American Chemical Society}, author={Rak, Zsolt and Ewing, Rodney C. and Becker, Udo}, year={2011} }
 @article{shuller_renock_rak_fernando_dzulkifli_ewing_becker_2011, title={Actinide adsorption onto mineral surfaces: A quantum-mechanical investigation}, volume={242}, journal={Abstracts of Papers of the American Chemical Society}, author={Shuller, Lindsay and Renock, Devon and Rak, Zsolt and Fernando, Sandy and Dzulkifli, Izyan and Ewing, Rodney and Becker, Udo}, year={2011} }
 @article{becker_shuller_ewing_rak_2011, title={Actinide incorporation into uranyl alteration phases and garnets}, volume={242}, journal={Abstracts of Papers of the American Chemical Society}, author={Becker, Udo and Shuller, Lindsay C. and Ewing, Rodney C. and Rak, Zsolt}, year={2011} }
 @article{wang_rak_zhang_ewing_becker_2011, title={Electronic structure and energetics of tetragonal SrCuO2and its high-pressure superstructure phase}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80755127291&partnerID=MN8TOARS}, DOI={10.1088/0953-8984/23/46/465503}, abstractNote={First-principles calculations have been used to investigate the electronic structure and energetics of the simple tetragonal SrCuO2 (P4/mmm) and its high-pressure tetragonal superstructure (P4/mmm). Based on the calculations, the high-pressure phase is metastable as compared with the low pressure tetragonal phase, with an energy difference of 0.13 eV per SrCuO2 formula unit. The energy barrier to the transition from the superstructure to the simple tetragonal structure is 0.24 eV at 7 GPa; thus, high temperatures are required to synthesize the latter. Among the possible structural configurations resulting from the partially occupied oxygen site in the superstructure phase, the most stable structure has a space group , reduced from that of the simple tetragonal structure P4/mmm. The detailed analysis of the electronic band structures of the simple tetragonal and superstructure phases suggests that the out-of-plane buckling of the O atoms in the superstructure leads to significant decrease in the O p–Cu d orbital overlap, allowing the energy of the system to be lowered, which is necessary for the structural stability. An understanding of the electronic structure and energetics of the high-pressure superstructure phase and its relation to the simple tetragonal phase provides a basis for exploring the physical properties of the infinite layer, high-TC superconductor.}, number={46}, journal={Journal of Physics-Condensed Matter}, author={Wang, Jianwei and Rak, Zsolt and Zhang, Fuxiang and Ewing, Rodney C. and Becker, Udo}, year={2011}, pages={465503} }
 @article{rak_mahanti_mandal_fernelius_2011, title={Electronic structure of Cd, In, Sn substitutional Defects in GaSe}, volume={994}, DOI={10.1557/PROC-0994-F03-10}, abstractNote={Abstract}, journal={In, Sn substitutional Defects in GaSe', <i>MRS Proceedings}, author={Rak, Zsolt and Mahanti, Subhendra D. and Mandal, Krishna C. and Fernelius, Nils C.}, year={2011} }
 @article{rak_ewing_becker_2011, title={First-principles investigation of Ca-3(Ti, Zr, Hf, Sn)(2)Fe2SiO12 garnet structure for incorporation of actinides}, volume={83}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79961113423&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.83.155123}, abstractNote={The structure, electronic structure, and bonding properties of Ca{sub 3} (Ti,Zr,Hf,Sn){sub 2} Fe{sub 2} SiO{sub 12} garnets have been investigated using ab initio electronic structure methods within density functional theory. The incorporation mechanisms of U into different sites of garnet with different compositions have been investigated. In order to describe the behavior of the localized Fe d and U f states, we employed the local spin density approximation (LSDA)+U method. The theoretical unit cell parameters and (cation-O) bond lengths for the four compositions are in good agreement with available experimental data. The calculated densities of states of all garnet compositions investigated are quite similar, suggesting only minor differences in the cation-O interactions. The analysis of the charge density using Bader’s scheme suggests that the (Hf-O) bond has the most ionic character, while the (Ti-O) bond is more covalent. This result provides some insight into the radiation response of the garnet structure. Based on total energy calculations, U preferentially occupies the octahedral site in the garnet structure. The lowest incorporation energy occurs when U substitutes for Sn in the octahedral site. In order to accommodate U at the dodecahedral site (i.e., substituting for Ca), some partial electron transfer from the Umore » to the Fe ions is necessary, making U incorporation in this site unfavorable.« less}, number={15}, journal={Physical Review B}, author={Rak, Zs and Ewing, R. C. and Becker, U.}, year={2011} }
 @article{rak_ewing_becker_2011, title={Role of iron in the incorporation of uranium in ferric garnet matrices}, volume={84}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80455129666&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.84.155128}, abstractNote={The structural relaxations and electronic structure of U-containing Ca₃(Ti,Zr,Hf,Sn)₂(Fe₂Si)O₁₂ garnet systems have been investigated using ab initio methods within density functional theory (DFT) in the generalized gradient approximation with a Hubbard correction U (GCA + U). The calculations provide a fundamental understanding of the role of Fe in the incorporation and stability of U in the garnet structure. The atomic relaxations around U are controlled by a delicate balance between the Coulomb interactions among the ions and the size effect of the large U atom. The relaxation pattern indicates that when U occupies the A site, a charge transfer occurs from U to its nearest-neighbor (NN) Fe atom. This is further verified by the detailed analysis of the electronic band structures and charge density distribution. The double exchange coupling of the U f and the NN Fe d shells via the transfer of electrons lowers the energy of the system when the spins of the f and d shells are antiparallel. The incorporation energy of U at the A site (substituting Ca) increases dramatically with the decrease in the number of Fe atoms in the neighboring tetrahedral sites. The presence of Fe is crucial, since it accommodates the extra valencemore » electrons introduced by U and the electron transfer allows the lowering of the total energy of the structure. Comparing the incorporation energies at the A and B site (octahedral site), U clearly prefers the A site, provided that there are sufficient Fe atoms in its vicinity to facilitate the charge transfer.« less}, number={15}, journal={Physical Review B}, author={Rak, Zs. and Ewing, R. C. and Becker, U.}, year={2011} }
 @article{rak_mahanti_mandal_fernelius_2010, title={Defect-induced rigidity enhancement in layered semiconductors}, volume={150}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77954218210&partnerID=MN8TOARS}, DOI={10.1016/j.ssc.2010.04.011}, abstractNote={We discuss the mechanism responsible for the observed improvement in the structural properties of In doped GaSe, a layered material of great current interest. Formation energy calculations show that by tuning the Fermi energy, In can substitute for Ga or can go as an interstitial charged defect$(\text{In}_{\text{i}}^{\text{3+}})$. We find that $\text{In}_{\text{i}}^{\text{3+}}$ dramatically increases the shear stiffness of GaSe, explaining the observed enhancement in the rigidity of In doped p-GaSe. The mechanism responsible for rigidity enhancement discussed here is quite general and applicable to a large class of layered solids with weak interlayer bonding.}, number={27-28}, journal={Solid State Communications}, author={Rak, Zs. and Mahanti, S. D. and Mandal, Krishna C. and Fernelius, N. C.}, year={2010}, pages={1200–1203} }
 @article{rak_mahanti_mandal_fernelius_2010, title={Doping dependence of electronic and mechanical properties of GaSe1-xTex and Ga1-xInxSe from first principles}, volume={82}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78149246709&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.82.155203}, abstractNote={The electronic and mechanical properties of the hexagonal, layered GaSe doped with Te and In have been studied using first-principles pseudopotential method within density-functional theory. The calculated elastic constants of the end compounds GaSe and InSe compare well with the available experimental and theoretical values. As we go from GaSe to InSe, the elastic constants ${\text{C}}_{13}$, ${\text{C}}_{33}$, and ${\text{C}}_{44}$ increase while ${\text{C}}_{11}$ and ${\text{C}}_{12}$ decrease, suggesting that the crystal becomes stiffer in the direction perpendicular to the atomic layers and the softer in the direction parallel to the layers, as more substitutional In is incorporated in GaSe. The electronic structure and the formation energies of several defects and simple defect complexes are discussed and the calculated charge transition levels are compared to available experimental data. We demonstrate that In doping may play an important role in the observed enhancement in the structural properties of GaSe. Depending on the Fermi energy, In can either substitute for Ga $({\text{In}}_{\text{Ga}})$ or occupy an interstitial position as a triply charged defect $({\text{In}}_{i}^{3+})$. While the substitutional In does not change significantly the electronic and mechanical properties of the host, we find that the shear stiffness of GaSe is considerably increased when In is incorporated as charged interstitial impurity.}, number={15}, journal={Physical Review B}, author={Rak, Zs. and Mahanti, S. D. and Mandal, Krishna C. and Fernelius, N. C.}, year={2010} }
 @article{rak_ewing_becker_2010, title={Electronic structure of Ti, Zr, Hf and Sn containing garnets - Materials for immobilization of actinides}, volume={74}, number={12}, journal={Geochimica Et Cosmochimica Acta}, author={Rak, Zsolt and Ewing, Rodney C. and Becker, Udo}, year={2010}, pages={A845} }
 @article{wu_francisco_zsolt rak_mahanti_kanatzidis_2009, title={ChemInform Abstract: Synthesis, Magnetism and Electronic Structure of YbNi2-xFexAl8(x = 0.91) Isolated from Al Flux.}, volume={40}, DOI={10.1002/chin.200911014}, abstractNote={Abstract}, number={11}, journal={ChemInform}, author={Wu, Xiuni and Francisco, Melanie and Zsolt Rak, T.Bakas and Mahanti, S.D. and Kanatzidis, Mercouri G.}, year={2009} }
 @article{rak_mahanti_mandal_fernelius_2009, title={Electronic structure of substitutional defects and vacancies in GaSe}, volume={70}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-59049094224&partnerID=MN8TOARS}, DOI={10.1016/j.jpcs.2008.10.022}, abstractNote={We have investigated the nature of defect states associated with substitutional impurities (Cd, In, Sn) and both Ga and Se vacancies in GaSe using ab initio electronic structure methods within density functional theory. These calculations were done using supercell model allowing for internal atomic relaxation. Binding energies (BEs) of defects obtained in this model are compared with effective mass approximation results. Significant central cell corrections are present for most of the defects. This is consistent with charge densities associated with the defect states that show clearly their strongly localized nature. Because of the difficulties associated with LDA/GGA in giving the correct band gap in semiconductors, we have only compared the acceptor BEs with available experiments. Our theoretical results agree well with the experiment for CdGa and VGa. The fundamental role played by the Ga dimers in the formation of defect states is discussed.}, number={2}, journal={Journal of Physics and Chemistry of Solids}, author={Rak, Zs. and Mahanti, S. D. and Mandal, Krishna C. and Fernelius, N. C.}, year={2009}, pages={344–355} }
 @article{tomic_rak_veazey_malliakas_mahanti_kanatzidis_tessmer_2009, title={Scanning tunneling microscopy study of the CeTe3 charge density wave}, volume={79}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-62149152365&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.79.085422}, abstractNote={We have studied the nature of the surface charge distribution in ${\text{CeTe}}_{3}$. This is a simple cleavable layered material with a robust one-dimensional incommensurate charge density wave (CDW). Scanning tunneling microscopy (STM) has been applied on the exposed surface of a cleaved single crystal. At 77 K, the STM images show both the atomic lattice of surface Te atoms arranged in a square net and the CDW modulations oriented at $45\ifmmode^\circ\else\textdegree\fi{}$ with respect to the Te net. Fourier transform of the STM data shows Te square lattice peaks and peaks related to the CDW oriented at $45\ifmmode^\circ\else\textdegree\fi{}$ to the lattice peaks. In addition, clear peaks are present, consistent with subsurface structure and wave-vector mixing effects. These data are supported by electronic structure calculations, which show that the subsurface signal most likely arises from a lattice of Ce atoms situated $2.53\text{ }\text{\AA{}}$ below the surface Te net.}, number={8}, journal={Physical Review B}, author={Tomic, A. and Rak, Zs. and Veazey, J. P. and Malliakas, C. D. and Mahanti, S. D. and Kanatzidis, M. G. and Tessmer, S. H.}, year={2009} }
 @article{rak_mahanti_mandal_fernelius_burger_franks_james_2008, title={Theoretical studies of defects states in GaSe and GaTe}, volume={7079}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57549108317&partnerID=MN8TOARS}, DOI={10.1117/12.796229}, abstractNote={We have investigated the electronic structures of bulk GaSe and GaTe as well as the nature of defect states associated with substitutional impurities and vacancies in GaSe and GaTe. These calculations were done using ab initio density functional theory and supercell models. We find that the Ga-Ga dimers play an important role in the formation of defect states. Analysis of the charge densities and the band structures associated with the defect states indicates that they are strongly localized. Theoretical results are in good agreement with experiment for CdGa and VGa in GaSe and for VGa in GaTe. The effect of spin-orbit interaction on the band structure of GaTe has been investigated; it is found that the top valence bands at the Γ-point shift up in energy by ~ 0.1 eV due to the mixing of Te px-py and pz bands.}, journal={Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X}, author={Rak, Zsolt and Mahanti, S. D. and Mandal, Krishna C. and Fernelius, N. C. and Burger, A and Franks, LA and James, RB}, year={2008} }
 @inbook{rak_mahanti_mandal_fernelius_ashok_chevallier_kiesel_ogino_2007, title={Electronic structure of Cd, In, Sn substitutional defects in GaSe}, volume={994}, booktitle={Semiconductor Defect Engineering-Materials, Synthetic Structures and Devices II}, author={Rak, Zsolt and Mahanti, Subhendra D. and Mandal, Krishna C. and Fernelius, Nils C. and Ashok, S and Chevallier, J and Kiesel, P and Ogino, T}, year={2007}, pages={73–78} }
 @inproceedings{rak_mahanti_mandal_fernelius_2007, title={Electronic structure of Cd, In, Sn substitutional defects in GaSe}, volume={994}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-45749109334&partnerID=MN8TOARS}, booktitle={Materials Research Society Symposium Proceedings}, author={Rak, Z. and Mahanti, S.D. and Mandal, K.C. and Fernelius, N.C.}, year={2007}, pages={73–78} }