<p>The present thesis concerns applications of first principles electronic structure calculations in conjunction with methods of statistical mechanics for simulations of transition metal alloys both in the bulk and at surfaces.</p><p>A fully relativistic generalization of the exact muffin-tin orbitals (EMTO) method has been developed. The method accurately takes into account spin-orbit coupling and allows one to calculate orbital polarization and magneto-crystalline anisotropy in magnetic systems as well as increasing the range of applicability of the EMTO method to heavy elements. A new direct-exchange Monte Carlo (DEMC) method has been proposed, which is capable to tackling effectively statistical simulations of surface segregations in disordered and ordered alloys.</p><p>The applications of relativistic methods include calculations of spin and orbital magnetization in iron-cobalt disordered and partially ordered alloys, as well as computation of the core-level shifts (CLS) in transition metal alloys. It has been found, that relativistic corrections are important for CLS calculations in 5-d metal alloys. Properties of a Ni monolayer deposited on a Cu surface have been studied. The monolayer is found to be unstable in the top layer, and its magnetization depends greatly on the surface orientation. Two distinct energy levels have been found to exist Co/Cu/Ni trilayers deposited on the (100) Cu surface, which correspond to a completely paramagnetic trilayer and the case when only Ni is paramagnetic.</p><p>Vacancy ordering in substoichometric titanium carbides TiC_x have been simulated. Existence of three ordered phases in the range of carbon concentration x=0.5 ÷1.0 has been revealed and a theoretical phase diagram has been proposed. Surface segregations have been calculated in disordered Ni₅₀Pt₅₀ and Ni₅₀Pd₅₀ as well as in ordered NiPt alloys. Segregation reversal has been observed in the Ni₅₀Pt₅₀ alloy with Pt segregation at the (111) surface and Ni segregation at the (110). In the ordered NiPt alloys segregation behaviour is found to be affected greatly by small deviations from the exact stoichiometric composition in bulk. Surface magnetization in PdV and MoV bcc alloys have been studied. It has been found, that in PdV alloys surface segregations suppress magnetic order at the surface, while in MoV alloys magnetization is substantially enhanced due to the segregation.</p>

The present thesis concerns applications of first principles electronic structure calculations in conjunction with methods of statistical mechanics for simulations of transition metal alloys both in the bulk and at surfaces. A fully relativistic generalization of the exact muffin-tin orbitals (EMTO) method has been developed. The method accurately takes into account spin-orbit coupling and allows one to calculate orbital polarization and magneto-crystalline anisotropy in magnetic systems as well as increasing the range of applicability of the EMTO method to heavy elements. A new direct-exchange Monte Carlo (DEMC) method has been proposed, which is capable to tackling effectively statistical simulations of surface segregations in disordered and ordered alloys. The applications of relativistic methods include calculations of spin and orbital magnetization in iron-cobalt disordered and partially ordered alloys, as well as computation of the core-level shifts (CLS) in transition metal alloys. It has been found, that relativistic corrections are important for CLS calculations in 5-d metal alloys. Properties of a Ni monolayer deposited on a Cu surface have been studied. The monolayer is found to be unstable in the top layer, and its magnetization depends greatly on the surface orientation. Two distinct energy levels have been found to exist Co/Cu/Ni trilayers deposited on the (100) Cu surface, which correspond to a completely paramagnetic trilayer and the case when only Ni is paramagnetic. Vacancy ordering in substoichometric titanium carbides TiCx have been simulated. Existence of three ordered phases in the range of carbon concentration x=0.5 ÷1.0 has been revealed and a theoretical phase diagram has been proposed. Surface segregations have been calculated in disordered Ni50Pt50 and Ni50Pd50 as well as in ordered NiPt alloys. Segregation reversal has been observed in the Ni50Pt50 alloy with Pt segregation at the (111) surface and Ni segregation at the (110). In the ordered NiPt alloys segregation behaviour is found to be affected greatly by small deviations from the exact stoichiometric composition in bulk. Surface magnetization in PdV and MoV bcc alloys have been studied. It has been found, that in PdV alloys surface segregations suppress magnetic order at the surface, while in MoV alloys magnetization is substantially enhanced due to the segregation.

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Abstract In this thesis oxidation of a pure and alloyed Cu surface was studied using density functional theory based calculations. The mechanism and energetics behind experimentally observed missing row reconstruction on a pure Cu(100) surface were studied. Examination of the formation of the missing row was approached by studying the profitableness of vacancy formation to the surface at oxygen coverages below and equal to 0.5 ML. However, an ideal surface was in all cases more favourable in energy than vacancy included structures. Therefore an additional Cu atom was added to the simulation cell to promote vacancy formation. A consequence of this on-surface Cu addition was the identifying of a new energetically favoured Cu-O-Cu chain formation. This newly identified structure is a possible transition state in the path from an ideal surface to a reconstructed one. Copper(I)oxide formation through sub-surface adsorption was examined on a reconstructed Cu(100) surface. At low O coverage on-surface adsorption was found to be much more favoured than adsorption on sub-surface sites. However, when coverage increased to 0.75 ML structures with both on-surface and sub-surface adsorbates possessed the lowest energy. Furthermore, the diffusion barriers to the sub-surface adsorption sites were low. Both these facts support the conclusion that the transition from the reconstructed missing row structure to the copper(I)oxide takes place without any large energy effort. The adsorption induced segregation of copper on a Ag/Cu(100) surface was studied by calculating the surface energies of slabs where the silver layer was positioned at different depths with varying O coverage. In addition, the segregation energies were also calculated with two Ag compositions. It was discovered that without oxygen adsorbates silver atoms lying on top of the surface were favoured but the surface segregation probability of Cu increases with increasing O coverage. The same calculation procedure was expanded to study the segregation probability of five additional alloy metals with a Cu surface. Without oxygen present Mg, Al and Zn possess a weak tendency to segregate to the surface, however, V and Cr prefer to stay in the bulk. The addition of oxygen adsorbates on the surface causes the segregation of all the dopants to the top of the surface. With the Al dopant the effect of the surface orientation and the oxygen coverage were also studied. The bonds between adsorbates and the surface were strongest with the densest surface, (111), and weakest with the most opened surface, (110). Furthermore, the enrichment probability was largest in the case of the (111) surface and lowest with the (110) surface. Probability increased with increasing oxygen coverage. / Tiivistelmä Tässä väitöstyössä on tutkittu puhtaan ja seostetun kuparipinnan hapettumista tiheysfunktionaaliteoriaan perustuvilla laskuilla. Kokeellisesti Cu(100) pinnan on havaittu rekonstruoituvan hapen vaikutuksesta. Rekonstruoitumisen aikana ideaalisesta pinnasta irtoaa joka neljäs pintakuparirivi ja muodostuu niin kutsuttu missing row- rakenne. Tämän rakenteen muodostumismekanismia ja muodostumiseen kuluvaa energiaa tutkittiin vertaamalla pintavakansillisten rakenteiden pintaenergioita vastaaviin ideaalipinnan arvoihin vaihtelevalla happipeitolla. Koska ideaalipinta oli kaikilla peittoasteilla energeettisesti suotuisampi kuin vakanssirakenne, yksittäisiä kupariatomeja lisättiin pinnalle stabiloimaan vakanssirakenteita. Kuparilisäyksen seurauksena löydettiin uusi energeettisesti edullinen Cu-O-Cu ketjurakenne. Tämä rakenne on mahdollinen ideaalisen ja rekonstruoituneen pinnan välinen siirtymätila. Kupari(I)oksidin muodostumista pinnanalaisen happiadsorption kautta tutkittiin rekonstruoituneella Cu(100) pinnalla. Pienillä happi-peittoasteilla pinnanpäällisen adsorption havaittiin olevan huomattavasti suotuisampaa kuin pinnanalaisen adsorption. Kuitenkin happi peiton kasvaessa 0,75 ML:iin rakenteet, joissa oli adsorbaatteja sekä pinnan alla että päällä, tulivat muita suotuisimmiksi. Myös lasketut energiavallit hapen diffuusiolle pinnalta pinnan alle olivat hyvin pieniä. Kaikki tulokset viittaavat siihen, että rekonstruoitunut Cu(100) pinta hapettuu helposti kupari(I)oksidiksi. Happiadsorption aikaansaamaa kuparin rikastumista (segregaatio) hopeakupariseosmetallin pinnalle tutkittiin laskemalla vaihtelevilla happipeitoilla pintaenergiat kuparipinnoille, joissa ensimmäinen, toinen ja kolmas kuparikerros oli korvattu hopealla. Myös segregaatioenergiat laskettiin kahdella eri seoskoostumuksella. Saatujen tulosten mukaan ilman happiadsoptiota hopea segregoituu pinnalle, kun taas hapen peittoasteen kasvaessa kupari rikastuu pinnalle. Samaa tutkimustapaa käytettiin myös tarkasteltaessa viiden muun seosmetallin segregaatiotodennäköisyyttä kuparipinnalla. Ilman pintahappea magnesiumilla, alumiinilla ja sinkillä oli heikko taipumus rikastua pinnalle, kun taas vanadium ja kromi pysyivät syvemmällä metallissa. Sen sijaan happea lisättäessä kaikki tutkitut seosmetallit pyrkivät siirtymään kohti pintaa. Myös hapen peiton sekä pintaorientaation vaikutusta segregaatioon tutkittiin hopeakuparipinnalla. Sekä happiadsorbaattien ja pinnan välinen sidos että segregaatiotodennäköisyys oli sitä voimakkaampaa, mitä tiheämmästä pinnasta oli kyse. Segregaatiotodennäköisyys kasvoi pinta-adsorption lisääntyessä.