A. Ya. Fishman

Random fields in disordered magnetics with Jahn-Teller ions

Abstract The character of distribution of random fields acting on a two-fold degenerate orbital E-term is investigated. The distribution functions for deformational and Coulomb fields caused by point defects are found. Random fields of exchange type on Jahn-Teller ions in crystals with a spinel structure are also examined. Estimates of distribution width for different fields are performed. The influence of random fields on the structure phase transition temperature and also on the behaviour of specific heat, elastic module and magnetic anisotropy of crystals is analysed.

Thermophysical properties of Ti-5Al-5V-5Mo-3Cr-1Zr titanium alloy

The thermophysical properties of the Ti-5Al-5V-5Mo-3Cr-1Zr titanium alloy in a wide range of temperatures from room temperature to 1000°C have been studied by the methods of differential scanning calorimetry, the laser flash method, and dilatometry. The obtained data on heat capacity, thermal diffusivity, and thermal expansion have been used for calculating coefficient of thermal conductivity. The sequence and temperatures of structural transformations during heating of the alloy have been established. It has been shown that the studied alloy possesses a coefficient of thermal conductivity that is 3.5–4 times smaller than that of pure titanium.

Structural phase transformation and limited miscibility in Jahn–Teller systems

Immiscibility of crystal systems with Jahn–Teller (JT) ions is studied at low temperatures. Phase transformations under which concentrated JT systems are separated into phases with a high and low concentration of JT ions with structural JT transformations are analyzed. A dependence of the phase diagram topology on the ratio between the cooperative interaction energy in the JT subsystem and the intensity of random crystal fields acting on the JT ions is established. The possibility of low-temperature three-phase equilibrium of two cubic and a tetragonal phase with different concentrations of JT centers is demonstrated for a high degree of substitutional-type disorder.

Grain boundary self-diffusion of tracer 18O atoms in nanocrystalline oxide LaMnO3 + δ

Diffusion coefficients of tracer 18O atoms at boundaries of nanograins of LaMnO3 + δ oxide have been measured in the temperature range of 400–500°C. The samples of the nanocrystalline oxide are prepared with the use of the shockwave loading method. The concentration profile of the tracer atoms after diffusion annealing is measured with the use of the nuclear microanalysis method. The activation energies of the grain boundary diffusion amounts to about 2 eV and the boundary width is ∼0.05 nm. The measured coefficients of the grain boundary diffusion at 500°C exceed the corresponding coefficients of the volume diffusion by seven orders of magnitude.

Effect of mechanical activation on the morphology and structure of hydroxyapatite

We have studied the effect of grinding in planetary mills on the phase composition, morphology, and water content of hydroxyapatite powder. The results indicate that milling for even relatively short times, which reduces the average particle size by a factor of 2, causes the monetite present in the unmilled powder to disappear and reduces the crystallite size of the hydroxyapatite. The fraction of nanoparticles in the powder is then 98% and remains constant during further milling. Milling for longer times leads to hydroxyapatite amorphization. For an average size of large particles R ≥ 1 μm, the surface area of the particles per unit volume, E (cm−1), is determined only by R (E ∼ 1/R).

Diffusion in Concentrated Jahn–Teller Systems

Abstract The effect of strontium on the diffusion coefficients D of oxygen tracers in the La 1− y Sr y MnO 3− δ , temperature dependence of the oxygen tracer diffusion coefficient in La 1−y Sr y MnO 3−δ (y=0.025) were determined. The D values were measured using 18 O tracers and the nuclear microanalysis method. Analysis of the concentration dependences of the diffusion coefficients D was made using the approach, which takes into account variation of the number of orbitally degenerate (JT) states of 3d-ions during non-isovalent alloying.

Features of the thermal expansion of dense nanoceramic CuO

The thermal expansion of samples of high-density nanoceramic CuO is investigated. The nanoceramic is obtained by the method of loading coarse-grained copper oxide by spherically convergent shock waves. Anomalous behavior of the thermal expansion coefficient α of the oxide is observed in the temperature region below 50K (α⩽0). The appearance of these anomalies is explained by the presence of orientationally degenerate tunneling centers and/or low-temperature phase separation.

Low-temperature properties of orientationally degenerate (OH)− centers in proton-conducting oxides

It is shown that the presence of proton-associated dipole centers (OH)− can cause glass-like properties to appear at low temperatures in the oxides ABO3−y doped with cations with a lower valence. These properties are due to the splitting of the orientationally degenerate states of (OH)− dipolar centers as a result of proton tunneling and random crystal fields. It is established that deuterium or tritium substituted for hydrogen can cause unusual and anomalously large isotopic effects in the contributions of degenerate centers to the thermodynamic properties and the absorption of elastic and electromagnetic waves.

Orientationally degenerate (OH)− centers in perovskite-type proton-conducting oxides: Low-temperature thermodynamics and H/D isotope effects

It is shown that proton-associated dipole centers (OH)− can provide significant, glasslike contributions to the low-temperature properties of perovskite-type proton conducting oxides, such as ABO3−y, doped with cations of lower valence. These contributions result from the splitting of the orientationally degenerate states of dipole centers by random crystal fields and proton tunneling. It is demonstrated that H/D (or H/T) substitution leads to large and abnormal isotope effects for the properties caused by (OH)− degenerate states.

Phase transformations of the decomposition type in systems with orbital degeneracy

The immiscibility phase diagrams of systems exhibiting the cooperative Jahn–Teller effect are investigated. It is found that the Jahn–Teller mechanism of interaction between ions is capable of describing the main topologically different types of immiscibility phase diagrams in quasibinary systems.