Low-Temperature Heat Capacity and Phonon Kinetics in Some Rare-Earth Pentaphosphate Single Crystals and Glasses

Journal of Experimental and Theoretical Physics - The influence of the Schottky low-energy excitations related to the presence of rare-earth metals on the thermodynamic and kinetic characteristics...     

Thermal conductivity of systems with a gap in the phonon spectrum

An original theoretical model for describing the low-temperature thermal conductivity in systems with a region of forbidden values(a gap) in the phonon spectrum is proposed. The model is based on new experimental results on the temperature dependence of the phonon diffusion coefficient in nanoceramics and dielectric glasses which showed a similar anomalous behavior of the diffusion coefficient in these systems that may be described under the assumption of a gap in the phonon spectrum. In this paper, the role of the gap in low-temperature behavior of the thermal conductivity, κ(T), is analyzed. The plateau in the temperature dependence of the thermal conductivity is shown to correlate with the position and the width of the gap. The temperature dependence of thermal conductivity of such systems when changing the scattering parameters related to various mechanisms is studied. It is found that the umklapp process(U-processes) involving low-frequency short-wavelength phonons below the gap forms the behavior of the temperature dependence of thermal conductivity in the plateau region. A comparison of the calculated and experimental results shows considerable possibilities of the model in describing the low-temperature thermal conductivity in glass-like systems.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

Resonant scattering of nonequilibrium phonons (<Emphasis Type="Italic">λ</Emphasis><Subscript>ph</Subscript> = 10–50 nm) in nanostructured ceramics based on YSZ + Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites

Specific features of the transport of weakly nonequilibrium thermal phonons (λ _ph = 10–50 nm) in nanoscale ceramics at a transition from micro-to nanosizes have been investigated. On the basis of the model of spherical shells randomly distributed in space and modeling grain boundaries, whose elastic properties differ from the elastic properties of grains, features of the phonon spectrum in the wavelength range λ _ph ～ R _g have been studied. The conditions leading to the occurrence of a gap in the phonon spectrum of nanoscale materials are analyzed. It is shown that the position of the top gap edge in the phonon spectrum is determined to a large extent by the structure of phase boundaries, while the presence of inclusions (pores, other phases) with characteristic sizes smaller than that of grains of the main ceramic material shifts the gap to high frequencies in the phonon spectrum. Temperature dependences of the diffusion coefficient of nonequilibrium phonons near the top gap edge in the phonon spectrum have been measured for multiphase ceramics based on YSZ + 14.3% Al₂O₃ composites.     

Nonlocal effects in diffusive propagation of heat pulses in systems with trapping centers of nonequilibrium phonons

The propagation of heat pulses in systems with defects as trapping centers of nonequilibrium phonons is investigated theoretically. Among these defects are point defects involving two-level systems (TLSs) of different nature. It is demonstrated that, in addition to the principal signal, one more signal can be detected by the bolometer due to reemission of the nonequilibrium TLS energy at a certain ratio of relaxation times in the phonon and TLS subsystems. The temperature and concentration dependences of the time of signal arrival at the bolometer are analyzed. The results of theoretical investigations are compared with experimental data on the propagation of weakly nonequilibrium thermal phonons in solid solutions of the Y_3?x Er_xAl₅O₁₂ rare-earth yttrium aluminum garnets.     

Microscopic mechanism of stability in yttria-doped zirconia

The relaxed configurations of yttria-stabilised zirconia (YSZ) between 3 and 10 mol % of Y₂O₃ were modeled using the pseudopotential technique. In the displacive limit of a double-well potential model, the vibrational mode corresponding to the soft phonon in pure c-ZrO₂ was calculated for each Y₂O₃ composition. These anharmonic vibrations, associated with the stabilization of YSZ, were investigated within the self-consistent phonon approximation making obtainable the fine structure in spectral density. In studying the phonon dynamics, we use the displacement probability density, which can quantify very accurately the transition temperature necessary for stabilizing the YSZ cubic phase.     

Propagation of phonons in nanocrystalline ZrO2: Y2O3 ceramics

Phonon transfer in yttrium-oxide (Y₂O₃) stabilized ZrO₂ ceramics is studied experimentally in the range of helium temperatures (1.7–3.8 K). A model of the structure of the intergranular layer in the ceramic is considered, which explains the temperature dependence of the phonon diffusion coefficient, makes it possible to determine the intergranular layer parameters (density, velocity of sound, and thickness), and gives an idea about its structure. Scattering of injected phonons from resonance vibrations of nanoceramic grains is discussed.     

Effect of phonon decay processes on the formation of a phonon nonequilibrium signal in crystals with two two-level subsystems

The effect of phonon decay on the characteristic propagation time and shape of a phonon nonequilibrium signal in disordered crystals, including crystals containing inelastic phonon scattering centers, is studied theoretically. Attention is focused on slow processes, which are typical of yttrium-aluminum garnet solid solutions and erbium-doped aluminates. It is shown that the temperature dependence of the arrival time of a phonon nonequilibrium signal in these systems can be governed, to a considerable extent, by phonon-phonon interactions. The results of the theoretical studies are compared with experimental data on the propagation of weakly nonequilibrium thermal phonons in solid solutions of rare-earth yttrium-aluminum garnets and aluminates.