Raimund Podloucky

This work presents a study of the thermodynamic stabilities of the thorium boron carbides including a revised version of the Th-B binary system: (i) a maximum of 1 at.% Th dissolves in beta boron, (ii) the “hectoboride ThB66(O)” is found at ThB60, (iii) the B-rich boundary of the hexaboride Th1-x□xB6 (□ denotes a vacancy) extends according to its homogeneity region 0≤x≤0.22, (iv) novel thorium diboride with AlB2-type (a = 0.310(1) nm, c = 0.402(1) nm) forms in a peritectic reaction at ~2200°C. Based on DFT derived energies of formation, thermodynamic phase equilibria in the binary system Th-B have been calculated i.e. phase relations have been defined for Th-B up to the melting range.For all ternary Th-B-C compounds, density functional theory calculations were made within the pseudopotential approach of VASP utilizing a general gradient approximation for the exchange correlation functional. A metaGGA approach was then applied to all compounds in their equilibrium structures, which significantly lowered the enthalpies of formation for ThC and ThC2, whereas it had only a small impact on ThB4. Atomic volumes and charges were computed by the concept of Bader and distinctly ionic features are derived consisting of Th+2, B+1, B-½, C-1, C-2 and C-3 ions. All compounds are, however, of metallic character.

This work presents a study of the thermodynamic stabilities of the thorium boron carbides including a revised version of the Th-B binary system: (i) a maximum of 1 at.% Th dissolves in beta boron, (ii) the “hectoboride ThB66(O)” is found at ThB60, (iii) the B-rich boundary of the hexaboride Th1-x□xB6 (□ denotes a vacancy) extends according to its homogeneity region 0≤x≤0.22, (iv) novel thorium diboride with AlB2-type (a = 0.310(1) nm, c = 0.402(1) nm) forms in a peritectic reaction at ~2200°C. Based on DFT derived energies of formation, thermodynamic phase equilibria in the binary system Th-B have been calculated i.e. phase relations have been defined for Th-B up to the melting range.For all ternary Th-B-C compounds, density functional theory calculations were made within the pseudopotential approach of VASP utilizing a general gradient approximation for the exchange correlation functional. A metaGGA approach was then applied to all compounds in their equilibrium structures, which significantly lowered the enthalpies of formation for ThC and ThC2, whereas it had only a small impact on ThB4. Atomic volumes and charges were computed by the concept of Bader and distinctly ionic features are derived consisting of Th+2, B+1, B-½, C-1, C-2 and C-3 ions. All compounds are, however, of metallic character.

Traditional thermoelectric materials rely on low thermal conductivity to enhance their efficiency but suffer from inherently limited power factors. Innovative pathways to optimize electronic transport are thus crucial. Here, we achieve ultrahigh power factors in Ni ₃ Ge-based systems through an unconventional thermoelectric materials design principle. When overlapping flat and dispersive bands are engineered to the Fermi level, charge carriers can undergo intense interband scattering, yielding an energy filtering effect similar to what has long been predicted in certain nanostructured materials. Via a multistep DFT-based screening method developed here, we find a family of L1 ₂ -ordered binary compounds with ultrahigh power factors up to 11 mW m ⁻¹ K ⁻² near room temperature, which are driven by an intrinsic phonon-mediated energy filtering mechanism. Our comprehensive experimental and theoretical study of these intriguing materials paves the way for understanding and designing high-performance scattering-tuned metallic thermoelectrics.

Density functional theory (DFT) calculations for deriving enthalpies of formation [Formula: see text] H for ferromagnetic MnX (X [Formula: see text] P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal [Formula: see text] and orthorhombic [Formula: see text]. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT[Formula: see text]U approach which requires [Formula: see text] as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of [Formula: see text] H for MnP and MnAs in both structures whereby the result for MnP [Formula: see text] agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding [Formula: see text] H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT[Formula: see text]U ansatz was employed for all compounds and elemental Mn. The values for [Formula: see text] ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of [Formula: see text] H. As a reference for pure Mn the [Formula: see text]-Mn phase was taken with [Formula: see text] eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader's concept resulting in ionicities of Mn less than [Formula: see text].

Phase equilibria of the system Ce–Zn–Si have been determined for the isothermal section at 600 °C for 1-Ce₇Zn₂₁(Zn_1−xSi_x)₂ (unique type; 0.45 ≤ x ≤ 0.99), τ₂-Ce(Si_1−xZn_x)₂ (AlB₂-type; 0.36 ≤ x ≤ 0.73), τ₅-CeZn(Zn_1−xSi_x)₂ (CeNiSi₂-type; 0.68 ≤ x ≤ 0.76), τ₆-CeZn₂(Si_1−xZn_x)₂ (ThCr₂Si₂-type; 0.25 ≤ x ≤ 0.30) and τ₇-Ce₃₇Zn₄₈Si₁₅ (structure unknown). Whereas τ₁, τ₂ and τ₅ are stable at 600 and 800 °C, the phases τ₆, τ₇ are unstable at 800 °C. Atom site distribution in the crystal structures of τ₅, τ₆ and {La,Ce}₇Zn₂₁(Zn_1−xGe_x)₂ have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce}₇Zn₂₁[Zn_1−xSi(Ge)_x]₂ suggest a hypothetical binary phase “{La,Ce}₇Zn₂₃”. The stabilizing effect of Ge in Ce₇Zn_23−xGe_x has been elucidated from density functional theory (DFT) calculations discussing the electronic structure in terms of the density of states (DOS) and defining enthalpies of formation for Ce₇Zn_23−xGe_x (x = 0, 0.5, 2) as well as for several neighbouring binary Ce–Zn phases. A Schultz–Scheil diagram for the solidification behaviour in the (Zn,Si)-rich part of the diagram was constructed from DTA measurements in closed silica crucibles along with partial isothermal sections determined in the temperature range from 400 to 900 °C. The phases τ₅ and τ₆ both form in degenerate ternary peritectic reactions: L + CeSi₂,β-Ce₂Zn₁₇ ⇔ τ₅ at 865 ± 5 °C and L + τ₅,CeZn₁₁ ⇔ τ₆ at 695 ± 5 °C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ₅-CeZn(Zn_1−xSi_x)₂ revealed Kondo lattice behavior with ferromagnetic ordering below T_C = 4.4 K, whereas susceptibility and specific heat studies of τ₆-CeZn₂(Zn_0.28Si_0.72)₂ revealed Curie–Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie–Weiss fits of τ₅ (2.50μ_B) and τ₆ (2.34μ_B) reveal a ground state close to trivalent Ce.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

The aim of our first-principles study is to model dilute Fe-Cu alloys by including vibrational properties for the determination of phase boundaries. Single-atom and pairwise defects of Cu in bcc Fe and Fe in fcc Cu are considered in terms of structurally relaxed supercells, for which the corresponding total energies are calculated by a density-functional theory (DFT) approach. Based on DFT total energy differences strongly nonbonding substitutional formation energies for all defects are derived. For determining the entropy of mixing, a mechanical statistical model is designed, which is also able to include pairwise and larger defects. By the same DFT approach and within the harmonic model vibrational properties and, in particular, the temperature-dependent vibrational free energies are calculated. Based on these results a vibrational free energy for each defect species is obtained, which in combination with the corresponding DFT formation energies and the entropy of mixing is added up to a grand canonical potential. Minimizing this potential as a function of defect concentrations yields the desired free energies as a function of concentration and temperature, from which the phase boundaries are derived. Analyzing our results we find that the vibrational entropies are determining the temperature dependency of the vibrational free energies. We find, that including the vibrational free energy via the vibrational entropy is very important for explaining the rather wide miscibility range of mixing Cu to bcc Fe as observed experimentally. On the Cu-rich side of the Fe-Cu system including the vibrational entropy narrows the miscibility range.