Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide (original) (raw)
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Phonon spectrum, thermal expansion and heat capacity of UO2 from first-principles
Journal of Nuclear Materials, 2012
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 (DOS) compare reasonably well with measurement 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 ab initio computed. Both quantities are in good agreement with available experimental data for temperatures up to about 500 K.
Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide
Physical Review B, 2013
We present a study of the structural phase transition and the mechanical and thermodynamic properties of UO 2 by means of the local density approximation (LDA) + U approach. A phase transition pressure of 40 GPa is obtained from theory at 0 K, and agrees well with the experimental value of 42 GPa. The pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of the groundstate fluorite phase are predicted. The phonon spectra of both the ground state fluorite structure and high-pressure cotunnite structure calculated by the supercell approach show that the cotunnite structure is dynamically unstable under ambient pressure. Based on the imaginary mode along the-X direction and soft phonon mode along the-Z direction, a transition path from cotunnite to fluorite has been identified. We calculate the lattice vibrational energy in the quasiharmonic approximation using both first-principles phonon density of state and the Debye model. The calculated temperature dependence of lattice parameter, entropy, and specific heat agrees well with experimental observations in the low temperature domain. The difference of the Gibbs free energy between the two phases of UO 2 has predicted a boundary in the pressure-temperature phase diagram. The solid-liquid boundary is approximated by an empirical equation using our calculated elastic constants.
Infrared dielectric anisotropy and phonon modes of rutile TiO2
Journal of Applied Physics, 2013
Spectroscopic ellipsometry in the infrared spectral range is used for comprehensive analysis of the anisotropic dielectric response of sapphire. We determine the ordinary and extraordinary infrared complex dielectric functions as well as all infrared-active phonon modes of single crystal ␣-Al 2 O 3 for wavelengths from 3 to 30 m. Data were acquired from high-symmetry orientations of a-plane and c-plane surfaces cut from bulk crystals. A simple classification scheme is developed, which allows identification of the total reflection bands for p-and s-polarized light in anisotropic materials with multiple phonon branches. We employ a factorized form of the dielectric function for superior best-fit calculation of the infrared ellipsometry spectra adjusting frequencies and damping parameters of the transverse and longitudinal phonon modes with A 2u and E u symmetry separately. A generalized Lowndes condition for the damping parameters is derived and found satisfied for the A 2u and E u branches. Excellent agreement with phonon mode literature values is obtained, and improper use of selection rules reported previously for calculation of the sapphire dielectric functions is revised ͓Harman, Ninomiya, and Adachi, J. Appl. Phys. 76, 8032 ͑1994͔͒. The dielectric function model will become useful for infrared ellipsometry investigation of multiple-layer structures grown on ␣-Al 2 O 3 substrates such as group-III nitride heterostructures. PRB 61 8189 INFRARED DIELECTRIC ANISOTROPY AND PHONON . . .
Elastic constants, phonon density of states, and thermal properties of UO2
Physical Review B, 2011
The elastic properties and phonon density of states of UO 2 have been studied by first-principles spin-polarized electronic-structure calculations in both the local density approximation (LDA) and the generalized-gradient approximation (GGA) for the experimentally determined antiferromagnetic spin configuration. Calculations have also been done both with and without Hubbard corrections (LDA + U and GGA + U). The elastic properties and phonon density of states are in very good agreement with experimental measurements when the Hubbard correction is included. The elastic constants and low-frequency (acoustic mode) phonons are in reasonably good agreement with experiment for all the different calculations. However, when Hubbard corrections are not included, the high-frequency phonons are pushed to lower frequencies and the optical phonons are significantly underestimated. The melting temperature is approximated by using an empirical equation, which uses elastic constants as input parameters, and is in good agreement with experiment. The first-principles calculations are also used to obtain the specific heat and entropy within the harmonic approximation at finite temperatures. It is shown that harmonic approximation is valid up to room temperature. The Debye temperature is estimated using two different methods. The predicted values are in excellent agreement with experimental results. It is shown that inclusion of the spin-orbit interaction does not significantly alter either the elastic or thermal properties.
Optical conductivity and infrared-active phonons inRuSr2GdCu2O8
Physical Review B
The reflection spectra of polycrystalline RuSr 2 GdCu 2 O 8 are reported in the frequency range 25-5000 cm Ϫ1 and temperatures between 10 and 300 K. The material exhibits a metallic behavior over the whole temperature range. No superconductivity related features were detected in the low frequency infrared response despite the observed zero resistance below T c ϭ32 K. A rather low reflection level at frequencies above 800 cm Ϫ1 suggests a strongly overdamped free charge carrier response. Six of the seven A 2u phonon modes, predicted by group theoretical analysis, were observed experimentally and assigned to the definite c-axis eigenmodes by a comparison of the experimental frequencies with the results of lattice dynamical calculations. One of the CuO 2 plane-oxygen related modes exhibits pronounced anomalous softening upon decreasing temperature in the normal state.
Thin Solid Films, 2013
The SrCu 2 O 2 material is a p-type transparent conductive oxide. A theoretical study of the SrCu 2 O 2 crystal is performed with a state of the art implementation of the Density Functional Theory. The simulated crystal structure is compared with available X-ray diffraction data and previous theoretical modeling. Density Functional Perturbation Theory is used to study the vibrational properties of the SrCu 2 O 2 crystal. A symmetry analysis of the optical phonon eigenvectors at the Brillouin zone center is proposed. The Raman spectra simulated using the derivatives of the dielectric susceptibility, show a good agreement with Raman scattering experimental results.
Physical review. B, Condensed matter, 1991
Comparison of photoinduced local modes (PILM's) of insulating YBa2CusOs s and Tl2Ba2Caq Gd Cu208 with Raman spectra of their metallic counterparts suggests that carrier injection anomalously increases the frequencies of phonons involving s-axis apex-oxygen motion in both the dilute (insulator) and concentrated (metallic) limit. In La2Cu04, we observe PILM s whose frequencies suggest a correspondence with the "ghost modes" observed in neutron scattering [Rietschel et al. , Physica C 162-164, 1705 (1989)]. These data, together with Raman data on apex-oxygen anharmonicity, enable us to discuss the vibronic mode couplings involved in polaron formation in the three materials.
Symmetry of phonon, magnetic, and spin-phonon excitations inGdSr2RuCu2O8single crystals
Physical Review B, 2001
We present a polarized Raman scattering study of GdSr 2 RuCu 2 O 8 single crystals. In this compound, the RuO 6 octahedra are rotated around the c axis that in turn gives some Raman activity of the oxygen (O Ru) vibrations at 265 cm Ϫ1 (A 1g), 411 cm Ϫ1 (B 2g) and 607 cm Ϫ1 (B 1g). These vibrations, as well as those of the apical oxygen of RuO 6 at 654 cm Ϫ1 (A 1g) and the Cu-plane oxygen at 318 cm Ϫ1 (B 2g), clearly respond to the onset of magnetic ordering at T m Ϸ140 K. In particular, the 265 cm Ϫ1 mode hardens anomalously with decreasing temperature below T m. Notably, the corresponding phonon line is absent in the Raman spectra of GdSr 2 NbCu 2 O 8 (Nb 5ϩ :4d 0), a compound isomorphic to GdSr 2 RuCu 2 O 8 (Ru 5ϩ :4d 3). We argue that the eigenvector of the 265 cm Ϫ1 phonon ͑rotational mode͒ facilitates efficient modulation of the Ru(t 1g)-O Ru (p) bands. This modulation gives both Raman scattering strength to the 265 cm Ϫ1 mode and strong spin-phonon coupling.
Physical review, 2001
We have systematically investigated the totally symmetric Raman-scattering spectra of Sr 2 RuO 4 crystals in the temperature region between room temperature and 6 K. The electron-phonon interaction for the apical oxygen vibration has been experimentally investigated by a line-shape analysis based on the Fano interference model. It has been found that the electron-phonon interaction strongly depends on not only the polarization geometry but also the phonon-propagating direction. The phonon, which travels along the RuO 2 plane in the (a,a) geometry, shows the strongest electron-phonon interaction. The intensity of the broad background due to the magnetic excitation has been suppressed in the energy region less than 300 cm Ϫ1 at low temperature. Furthermore, the anisotropic formation of the spin gap has been also observed in excitation spectra with the propagation vector parallel to the RuO 2 plane below 60 cm Ϫ1 at 7 K. These anisotropic properties have been obtained for a crystal with a high superconductivity transition temperature of 1.4 K. The sample quality is very important, since such an anisotropic property has not been observed in the Raman spectra for samples with T c ϭ0.97 K. The observed magnetic excitation suggests that the magnetic correlation is also important in understanding the mechanism of superconductivity of Sr 2 RuO 4 .