Stoichiometric cerium oxide nanoparticles with different sizes, ranging between 2 and 12 nm, were prepared by a rational design of the synthetic concept, based on a special hydrothermal procedure. This tuning in particle size was achieved by the decoupling of nucleation and growth, as well as the suitable variation of synthesis temperature and time. In contrast to previous studies utilizing a trivalent cerium compound, we used a tetravalent cerium precursor which results in a cerium oxide powder consisting of single particles with low microstrain and low concentration of bulk defects and, hence, an almost ideal stoichiometry (CeO2.0) in the particles' interior. Owing to these features, the obtained nanoparticles were used as an ideal material for the investigation of the relationship between the particle size and the oxygen storage capacity (OSC). The OSC increased with decreasing particle size, which is expected, as this parameter corresponds to the exchange of oxygen at the surface. By contrast, as a key result, we found that the so-called complete OSC (OSCc) increases with the particle size, which is counterintuitive, as this parameter should not be dependent on the particles' dimension. Furthermore, these particles allowed for a quantitative description of the Raman redshift of the F2g mode of CeO2 in terms of solely the particle size, using theoretical calculations based on the phonon confinement model.

Since there are still research interests in the physical properties of quasi-binary thermoelectric Mg2X1-xYx alloys, with X, Y = Si, Ge, Sn, we present an ab initio analysis that yields the relative formation energy and effective masses of the conduction bands, in the whole compositional range x. We base our calculations on the full-relativistic Korringa, Kohn and Rostocker (KKR) Green's functions formalism within the coherent potential approximation (CPA). Formation energies, measured relative to the end Mg2X compounds, show no excess energy for the Mg2Si-Mg2Ge substitution thus indicating a complete solubility. In contrast, concave and asymmetric formation energies for intermediate compositions in the Mg2X-Mg2Sn alloys manifest a miscibility gap. With this basis, we compute and discuss the crossing of the conduction bands observed in n-type Mg2X1-xSnx materials. We present direction- and band-dependent effective masses using a generalized single parabolic band effective mass approximation to discuss anisotropic effects, to interpret available experimental and theoretical data, and to predict intermediate and not yet published transport parameters on these alloys.

Mg2X1-xYx thermoelectric materials and their pseudobinary related alloys, for X, Y = Si,Ge,Sn, have been the subject of intense research activity. Studies have revealed their electronic nature, but important properties are still unclear or missing in the case of intermediate solid solutions. Within the CPA-KKR formalism, in the full-relativistic description, we observe stronger deviations from Vegard's law as the lattice constant difference increases. We compute the Bloch spectral density function to map a local-parabolic band structure in the alloys, whenever the disorder-induced broadening is small. We trace nonlinear trends for the indirect and direct band gaps, spin-orbit coupling and the crossing between the low-lying conduction bands observed in the Mg2X1-3Snx systems. Our computations show that the broadening of the heavy- and light-hole bands is the smallest, but, in either case, we discuss the trends and anisotropy effects of band- and direction-dependent effective masses.

The existence of an intermediate phase of tin oxide was first reported in 1882. However, its stoichiometry and its crystal structure have been dubious and heavily debated ever since, despite a multitude of structural investigations. We show that Raman spectroscopy combined with ab initio theory offers a viable alternative for structure determination in cases where diffraction studies are inconclusive. We unambiguously identify the intermediate phase as Sn3O4 and rule out the other likely candidate, Sn2O3. We assign the one-phonon Raman signals of Sn3O4 to the mode symmetries of the corresponding point group C-2h and confirm that P2(1)/c with 14 atoms per unit cell is the space group of Sn3O4.

We present density functional theory (DFT) calculations on phonon dispersions, phonon density of states, and thermodynamic quantities for the three copper oxide phases Cu2O, Cu4O3, and CuO. For monoclinic CuO we consider the correct antiferromagnetic ground state. Sound velocities for the acoustic phonon branches and Debye temperatures are calculated and are found to be in good agreement with experiment. We further show how the method for the treatment of dipole-dipole interactions in dynamical matrices of Gonze and Lee [Phys. Rev. B 55, 10355 (1997)] may be incorporated in the real-space (direct) method for interatomic force constants (FCs). The role of the long-ranged dipole-dipole interactions in the phonon dispersion is discussed. Based on this method, we outline a perturbationlike scheme to compute first-order derivatives of the phonon mode frequencies with respect to the wave vector which can be used to compute velocities of sound.

The Raman spectrum of crystalline Cu2O taken off resonance is reproducible and independent of the growth method and conditions employed. But, in contrast to most other crystalline materials, the Raman spectrum of Cu2O is dominated by infrared active and silent lattice modes rather than by the only Raman allowed phonon mode. We show that this unusual behavior is most likely caused by the presence of copper vacancies in the so-called split configuration, a point defect particular to Cu2O. The reduction of symmetry due to the presence of point defects may lift the Raman selection rules and may introduce Raman activity for phonon modes that are Raman forbidden in the case of perfect crystal symmetry. Based on this group theoretical consideration, we predict the angle dependence of the Raman intensities of all Cu2O one-phonon modes at k = 0 for rotation about the (100) direction caused by the presence of various intrinsic point defects. Of all intrinsic defects in question, only the presence of the copper vacancy in the split configuration introduces Raman activity for all Cu2O extended phonon modes observed in experiment and is consistent with the angle-dependent measurements. Our study underlines the special role of the split vacancy in Cu2O.

We analyze the influence of the Mg concentration on several important properties of the band structure of Zn1-xMgxO alloys in wurtzite structure using ab initio calculations. For this purpose, the band structure for finite concentrations is defined in terms of the Bloch spectral density, which can be calculated within the coherent potential approximation. We investigate the concentration dependence of the band gap and the crystal-field splitting of the valence bands. The effective electron and hole masses are determined by extending the effective mass model to finite concentrations. We compare our results with experimental results and other calculations.

We perform density functional theory calculations to determine equilibrium lattice parameters of wurtzite Zn1−xMgxO alloys for Mg concentrations x ranging from 0 to 31.25 %. We use the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for the exchange correlation functional. For the lattice constants a and c we find a deviation from Vegard’s law and a constant unit cell volume independent of the Mg concentration.