Principal spectra describing magnetooptic permittivity tensor in cubic crystals

We provide unified phenomenological description of magnetooptic effects being linear and quadratic in magnetization. The description is based on few principal spectra, describing elements of permittivity tensor up to the second order in magnetization. Each permittivity tensor element for any magnetization direction and any sample surface orientation is simply determined by weighted summation of the principal spectra, where weights are given by crystallographic and magnetization orientations. The number of principal spectra depends on the symmetry of the crystal. In cubic crystals owning point symmetry we need only four principal spectra. Here, the principal spectra are expressed by ab-initio calculations for bcc Fe, fcc Co and fcc Ni in optical range as well as in hard and soft x-ray energy range, i.e. at the 2p- and 3p-edges. We also express principal spectra analytically using modified Kubo formula

Vibrational properties and the stability of the KCuF3 phases

We report theoretical investigations of the lattice dynamics of KCuF3. Our calculations are based on the generalized gradient approximation and parametrization of Perdew–Burke–Ernzerhof to the density functional theory corrected for on-site Coulomb interaction (GGA + U). Vibrations of the KCuF3 lattice are studied within the harmonic approximation. Energetic stability of tetragonal and orthorhombic polymorphic structures of KCuF3 is analyzed. Our results show that the orthorhombic polymorph is energetically not preferred. The Raman and infrared-active phonon modes in two distinct tetragonal polymorphs of KCuF3 are discussed with respect to the available experimental data. A detailed examination of the phonon densities of states in both tetragonal polymorphic structures of KCuF3 is provided together with discussion on similarities and differences between the vibrational dynamics of two distinct tetragonal lattices of the KCuF3 system.Web of Science2511art. no. 11540

Phonon spectrum, thermal expansion and heat capacity of UO2 from first-principles

We report first-principles calculations of the phonon dispersion spectrum, thermal expansion, and heat capacity of uranium dioxide. The so-called direct method, based on the quasiharmonic approximation, is used to calculate the phonon frequencies within a density functional framework for the electronic structure. The phonon dispersions calculated at the theoretical equilibrium volume agree well with experimental dispersions. The computed phonon density of states (DOSs) compare reasonably well with measured data, as do also the calculated frequencies of the Raman and infrared active modes including the LO/TO splitting. To study the pressure dependence of the phonon frequencies we calculate phonon dispersions for several lattice constants. Our computed phonon spectra demonstrate the opening of a gap between the optical and acoustic modes induced by pressure. Taking into account the phonon contribution to the total free energy of UO2 its thermal expansion coefficient and heat capacity have been computed from first-principles. Both quantities are in good agreement with available experimental data for temperatures up to about 500 K.Web of Science4261-311410

Anharmonicity and structural phase transition in the Mott insulator Cu2_2P2_2O7_7

Ab initio investigations of structural, electronic, and dynamical properties of the high-temperature $\beta$ phase of copper pyrophosphate were performed using density functional theory. The electronic band structure shows the Mott insulating state due to electron correlations in copper ions. By calculating phonon dispersion relations, the soft mode at the A point of the Brillouin zone was revealed, showing the dynamical instability of the $\beta$ phase at low temperatures. The double-well potential connected with the soft mode is derived and the mechanism of the structural phase transition to the $\alpha$ phase is discussed. The self-consistent phonon calculations based on the temperature-dependent effective potential show the stabilization of the $\beta$ phase at high temperatures, due to the anharmonic effects. The pronounced temperature dependence and the large line width of the soft mode indicate an essential role of anharmonicity in the structural phase transition

Influence of carbon on energetics, electronic structure, and mechanical properties of TiAl alloys

We present first-principles calculations of carbon-doped TiAl alloys. The effect of carbon on the structural, electronic, and elastic behavior of the gamma phase (L1(0) structure) of TiAl is investigated. The calculated enthalpy of formation at zero temperature indicates that carbon atoms favor to occupy rather interstitial than substitutional positions. The computed solubility of carbon in the stoichiometric gamma phase is very low, in agreement with experimental findings. However, it is significantly enhanced for the Ti-rich alloy and when located inside Ti-6 octahedra. Mechanical properties such as Cauchy pressure, elastic anisotropy, Young's modulus, as well as Pugh and Poisson ratios of stoichiometric and off-stoichiometric compositions are analyzed as a function of carbon concentration and its location. As a general trend, we obtain that below a concentration of 3 at.%, carbon plays a minor role in changing the ductile behavior of gamma-TiAl. A slight increase in ductility is found in the Ti-rich gamma alpha phase if either located in the Ti-plane (Ti4Al2 octahedral site) or in a Ti-6 octahedra.Web of Science237art. no. 07304

Anomalous bond softening mediated by strain-induced Friedel-like oscillations in a BC2N superlattice

The crystal structure of BC2N and the origin of its superhardness remain under constant debate, hindering its development. Herein, by evaluating the x-ray diffraction pattern, the thermodynamic stability at normal and high pressures of a series of BC2N candidates, the (111) BC2N2x2 superlattice (labeled R2u-BC2N) is identified as the realistic crystal structure of the experimentally synthesized BC2N. We further reveal that the strain-induced Friedel-like oscillations dominates the preferable slip systems of R2u-BC2N by drastically weakening the heterogenous bonds across the slip plane and thus leads to its ultralow dislocation slip resistance, which originates from the metallization triggered by the reduction in energy separation between bonding and antibonding interactions of the softened bonds. Our results rule out R2u-BC2N as the intrinsic superhard material surpassing c-BN, whereas the experimentally determined extreme hardness can be attributed to the nanocrystalline grains glued by interfacial amorphous carbon which provides a strong barrier for plastic deformation. These findings provide a view of the longstanding issue of the possible structure of experimentally observed BC2N, and establish a mechanism underlying the strain-driven electronic instability of superlattice structures, providing guidance towards rational design of superhard materials.Web of Science1066art. no. L06010