Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 (ΔH f = -150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 (ΔH f = -191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous (ΔHf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 ([Delta]Hf =-150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 ([Delta]Hf =-191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous ([Delta]Hf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 ([Delta]Hf =-150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 ([Delta]Hf =-191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous ([Delta]Hf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Effects of Temperature and Pressure on the Magnetic Properties of La 1– x

For La1–xPrxCoO3cobaltites (x¼0, 0.1, 0.2, and 0.3), the dependence ofmagnetic susceptibilityχðTÞis studied in the temperature range 5–400 K. Also,the crystal structure of these cobaltites is investigated, and the effect ofpressure up to 2 kbar on their susceptibility is measured atT¼78, 150, and300 K. The specificdependenciesχðTÞandthelargenegativepressureeffectare assumed to arise from Co3þions contribution to the total susceptibilityevaluated using La1 xPrxAlO3as a reference system. The obtained experi-mental data on temperature and pressure effects in magnetism are analyzedwithin a two-level model with energy gapΔbetween the ground state of thesystem with zero spin of Co3þions and the excited higher-spin state. In thismodel, magnetism of Co3þions is determined by the temperature-inducedpopulation of the excited state, and magnitude of the pressure effect is gov-erned by the volume dependence ofΔ. The results of the analysis, supple-mented by the theoretical calculations of the electronic structures of LaCoO3and PrCoO3, indicate significant increase inΔwith decrease in the unit cellvolume both under hydrostatic pressure and by substituting La with Pr having asmaller ionic radius.publishe

Magnetic and structural properties of La1−xGdx CoO3 compounds

We present a study of magnetic and structural properties of La1−xGdxCoO3 compounds with x = 0, 0.2, 0.5 and 1. The Gd term in magnetic susceptibility obeys the Curie-Weiss law with antiferromagnetic interaction between the magnetic moments of Gd3+ ions. The value of the paramagnetic Curie temperature Θ is proportional to Gd content and for GdCoO3 it reaches Θ ' −5.5 K. The Co contribution, χCo(T), derived by subtracting the Gd term from the total susceptibility, shows a non-monotonous behavior which is originated by the temperature induced transition of Co3+ ions from the ground low spin state to a higher spin state separated by an energy ∆. The revealed strong increase in ∆ with increasing Gd content is presumably due to the manifestation of a chemical pressure upon replacing La with Gd having a smaller ionic size.publishe

Effects of temperature and pressure on the magnetic properties of La1–xPrxCoO3

For La1–xPrxCoO3cobaltites (x¼0, 0.1, 0.2, and 0.3), the dependence ofmagnetic susceptibilityχðTÞis studied in the temperature range 5–400 K. Also,the crystal structure of these cobaltites is investigated, and the effect ofpressure up to 2 kbar on their susceptibility is measured atT¼78, 150, and300 K. The specificdependenciesχðTÞandthelargenegativepressureeffectare assumed to arise from Co3þions contribution to the total susceptibilityevaluated using La1 xPrxAlO3as a reference system. The obtained experi-mental data on temperature and pressure effects in magnetism are analyzedwithin a two-level model with energy gapΔbetween the ground state of thesystem with zero spin of Co3þions and the excited higher-spin state. In thismodel, magnetism of Co3þions is determined by the temperature-inducedpopulation of the excited state, and magnitude of the pressure effect is gov-erned by the volume dependence ofΔ. The results of the analysis, supple-mented by the theoretical calculations of the electronic structures of LaCoO3and PrCoO3, indicate significant increase inΔwith decrease in the unit cellvolume both under hydrostatic pressure and by substituting La with Pr having asmaller ionic radius.publishe