Valery Petrykin

Preparation and characterization of citratoperoxotitanate barium compound for BaTiO3 synthesis

Abstract An orange precipitate has been obtained from the solution containing ammonium citratoperoxotitanate—(NH 4 ) 8 [Ti 4 (C 6 H 4 O 7 ) 4 (O 2 ) 4 ] and barium nitrate. The powder XRD pattern of this product corresponded to a new phase with the monoclinic unit cell a =18.345(1) A, b =11.0862(9) A, c =13.1003(9) A and β =117.133(4)°. Based on the TG-DTA, Raman spectroscopy and XRD analysis of the calcination product, we have concluded that the obtained compound is Ba[Ti(C 6 H 4 O 7 )O 2 ]·2H 2 O (Ba 4 [Ti 4 (C 6 H 4 O 7 ) 4 (O 2 ) 4 ]·8H 2 O, taking into account the structure of the complex anion). This compound has been demonstrated to be a convenient precursor for synthesis of BaTiO 3 by simple calcination in air at the reduced temperature of 630 °C.

Structural disorder and superconductivity suppression in NdBa2Cu3Oz (z∼7)

Abstract Structural features of a low-Tc nearly stoichiometric Nd1+xBa2−xCu3Oz phase (x⩽0.05, z∼6.9, a=3.897–3.899 A , b=3.902–3.908 A , c=11.707–11.719 A ) have been investigated for the first time by XRD, EXAFS structure refinement techniques and Raman spectroscopy. It has been found that the abnormal lattice parameters and the low superconductivity transition temperature (55 K) of such a phase are related to structural disorder in the Ba and Nd sites resulting in oxygen disordering which cannot be avoided by a standard oxidation procedure. It is shown that longer annealing at high temperatures is an important factor for reducing the oxygen disordering and hence enhancing superconducting properties after oxidation.

A simple and reproducible way to synthesize PrBa2Cu4O8 under 1 atm of oxygen by amorphous citrate method

Abstract A simple aqueous solution technique utilizing citric acid (CA) as a complexant was applied to prepare PrBa 2 Cu 4 O 8 (Pr124) at ambient oxygen pressure. Heating of a mixed-solution of CA, water (H 2 O) and ions of Pr, Ba and Cu with a molar ratio of [CA]:[H 2 O]:[Pr+Ba+Cu]=3.6:40:1 on a hot-plate set at 200°C produced a transparent amorphous citrate “gel” without any precipitation, which after heat-treatment at 450°C was converted to a precursor for Pr124. The Pr124 compound with only minor traces of BaCuO 2 , PrBaO 3 and CuO formed after heat-treating the precursor twice at 850°C for 60 h under 1 atm oxygen pressure. The amount of impurities in the citrate-derived Pr124 was approximately half of that in the best sample previously prepared by the more elaborate high-pressure technique.

Ferromagnetic and superconducting properties of pure RuCe0.5Eu1.5Sr2Cu2O10 samples prepared by polymerizable complex method

Abstract The polymerizable complex method was successfully applied for synthesis of RuCe 0.5 Eu 1.5 Sr 2 Cu 2 O 10 compound. To obtain single-phase samples one should control oxygen partial pressure and prevent Ru loss by conducting synthesis in the sealed volume. Prepared single-phase samples became ferromagnetic below 120 K and did not show bulk superconductivity down to 7 K. The additional annealing in oxygen at 500 °C, P O 2 =10 atm resulted in the R ( T ) dependence of more metallic character, however it did not increase superconducting phase fraction to a significant extent. On the other hand, preparation of Ru x Ce 0.5 Eu 1.5 Sr 2 Cu 2 O 10 samples and studies of their superconducting properties revealed that appearance of superconductivity in the case of Ru-1222 phases might be related to the Ru loss during high temperature synthesis.

Oxygen mobility in layered cuprates La2−xSrxCuO4−δ

Abstract Oxygen mobility in layered lanthanum cuprates La 2− x Sr x CuO 4− δ is studied by dynamic-thermal isotope exchange method and coulometric titration. Depending on the temperature, different preferred surface exchange reactions are realized. Such characteristics of bulk diffusion process as activation energy and diffusion coefficient are stated for the cuprates. Theoretical modeling of diffusion process in La 2 CuO 4 and LaSrCuO 3.5 phases is carried out using molecular dynamics simulations. The data obtained are compared to the ones previously obtained experimentally.

Raman spectroscopy as a unique tool for characterizing high-Tc superconducting oxides

Abstract We address our full attention to the various useful capabilities of the Raman scattering technique for characterizing high- T c superconducting oxides. The powerful aspect of Raman spectroscopy is demonstrated with regard to detection of traces of impurities as compared with the conventional X-ray diffraction technique. Oxygen disordering within CuO chains in Co-doped Y123 type materials is sensitively detected by the Raman scattering technique, the degree of which is proportional to the intensity of the disorder induced Raman mode above 550 cm −1 . The phonon Raman spectra of (Pb 2 Cu)Sr 2 Y 1− x Ca x Cu 2 O 8+ d , (Pb 2 Cu)Sr 0.9 La 1.1 CuO 6+ d and (Pb 0.5 Cu 0.5 )Sr 2 YCu 2 O 7+ d are investigated and phonon assignments compatible with the layer stacking sequences found in these Pb-based cuprates are proposed. Raman scattering can also be used as a means to detect insulator-to-metal transitions in various high- T c cuprates, an example of which is demonstrated for (Pb 2 Cu)Sr 2 Y 1− x Ca x Cu 2 O 8+ d ; the intensity for the B 1 g O Cu mode abruptly drops across the insulator-to-metal transition as a result of the increased metallic screening in the hole-introduced samples.

Structural disorder and degradation of superconductivity in the NdBa2Cu3Oz (z∼7) defects simulations

Abstract This short paper is an extension of our previous studies of low- T c stoichiometric NdBa 2 Cu 3 O z prepared by polymerizable complex method and via nitrates melting technique. The most recent results describing the effect of annealing time and temperature on the superconducting transition for the samples prepared from the fine precursor are presented. The defects simulation was applied to understand the previously unexplained structural data discovered by EXAFS.

Synthesis of high-purity (CaxLa1.00-x)(Ba1.75-xLa0.25+x)Cu3Oz tetragonal superconductor by an aqueous solution technique using citric acid

Abstract A simple aqueous solution technique utilizing citric acid as a complexant was applied to prepare (Ca x La 1.00− x )(Ba 1.75− x La 0.25+ x )Cu 3 O z . The combined results of X-ray diffraction and Raman scattering analysis confirmed the single-phase character of polycrystalline sample for the whole range of x from 0.0 to 1.0. The T c onset values, determined precisely from the complex magnetic susceptibility measurements, values increased from 40.6 K for x =0.0 to 78.7 K for x =0.4, remained almost constant at 80 K between x =0.5 and =0.7, and then decreased to 44.6 K for x =1.0.

Harmonic susceptibilities of polycrystalline (La1-xCax)(Ba1.75-xLa0.25+x)Cu3O7+d synthesized by amorphous metal-complex method

Abstract Single-phase tetragonal (La 1− x Ca x )(Ba 1.75− x La 0.25+ x )Cu 3 O 7+ d polycrystals with the composition range 0.1⩽ x ⩽1.0 were synthesized by the amorphous metal-complex method. The fundamental and the higher-harmonic complex susceptibilities χ n = χ n ′ −i χ n ′′ ( n =1, 2, 3, 5, and 7) were measured by means of Hartshorn bridge. The ac-magnetic field superimposed on the earths field was applied to the specimen. We observed nonzero values of χ n for all n in the temperature range of the superconducting transition. The observed χ n – T curves were analyzed by the Kim–Anderson critical-state model. We found that the pinning force density increases up to x =0.5 and decreases over x =0.5 with increasing the Ca content x , and is proportional to two kinds of the power of the elemental pinning force.

Phase diagrams, crystal growth and structural features of RE1+xBa2−xCu3Oz superconductors (RE = La, Nd, Pr, Sm, Eu)

Abstract Phase relations in REBaCuO systems and compositionally induced structural changes in RE 1+x Ba 2−x Cu 3 O z (RE = La, Pr, Nd, Sm, Eu) were studied. New “anomalous” examples of disordered low-T c “quasicubic” NdBa 2 Cu 3 O 7 and superstructural ordered orthorhombic Nd 2 BaCu 3 O 7 were analyzed on the base of behavior of CuO, NdO and NdCu polyhedra to explain physical properties of RE-containing superconducting materials.