Rearrangement Of Oxygen Atoms Research Articles

Electron microscopic investigations of neutron irradiated YBa 2Cu 3O 7-x high temperature superconducting single crystals

There has been several recent explorations of HTSC materials particularly YBCO elucidating enhancements of critical current density ‘Jc’ as a result of fast neutron irradiation (∼1 MeV, fluence 10 16/cm 2) (see e.g. Van Dover et al 1989). Although the enhancements in Jc are known to result from the flux pinning effects arising from the neutron irradiation created defects, the nature of these defects have remained unknown. The present communication elucidates the details of these defects. Employing the technique of transmission electron microscopy in normal and high resolution modes, the nature of the defects have been uncovered. It has been shown that the fast neutron irradiation creates defects corresponding to thermal and displacement spikes. In the thermal spike region the defects correspond to local removal of chain containing CuO layers and consequent creation of linear defect stripe regions with a characteristic of width of ∼ 13.6 Å. After the defect stripe, the displacement spike corresponding to removal and rearrangement of oxygen atoms from CuO chain containing CuO layers are produced. These defects consist of linearly arranged clustures (∼ 100 to 300 Å ). The effectiveness of these defects particularly the defect stripes as flux pinning centres has been outlined.

Heat capacity measurement of α-Ti-O solid solution doped with vanadium and aluminium from 320 to 910K

Heat capacities of titanium-oxygen alloys, TiO 0.215, TiO 0.306, TiO 0.409 and TiO 0.466, and those of doped alloys, (Ti 1-yV y) ~0.2 (y = 0.016 and 0.048) and (Ti 1-yAl y)O ~0.2 (y = 0.03 and 0.05), were measured from 320 to 910K by adiabatic scanning calorimetry. Two kinds of heat capacity anomalies were observed for all samples except the O/Ti composition of 0.466. The anomaly at higher temperatures is assigned to be due to an order-disorder rearrangement of oxygen atoms. Another anomaly obtained at lower temperatures depends on cooling condition, indicating that this anomaly is due to a nonequilibrium phenomenon. The enthalpy and entropy changes due to the order-disorder transition for α-Ti-O solid solution obtained from this experiment are compared with theoretical values. The transition temperature, and enthalpy and entropy changes due to order-disorder transition decrease with increasing dopant contents of vanadium and aluminium, indicating that arrangement of oxygen atoms at lower temperatures may be partially ordered by the interaction between doped metal and oxygen atoms.

Heat capacity of vanadium-oxygen alloy

Heat capacities of vanadium-oxygen alloys with various compositions, VO 0.0834, VO 0.1127, VO 0.1245, and VO 0.1296, were measured from 320 to 920K by adiabatic scanning calorimetry. A heat capacity anomaly due to order-disorder rearrangement of oxygen atoms was observed for all the compositions. The transition temperatures from α′ to β phase were found to be 780, 791, 786, and 768K for VO 0.0839, VO 0.1127, VO 0.1245, and VO 0.1296, respectively. The transition temperatures from β′ to β were also observed to be 665 and 660K for VO 0.1245 and VO 0.1296, respectively, but they shifted to lower temperatures in repeated measurements. The excess heat capacity due to order-disorder transition was obtained by assuming that the heat capacity can be expressed as the sum of a harmonic term of lattice vibration, a dilational term, an electronic term, and an anharmonic term of lattice vibration. The entropy changes due to the transition for VO 0.0834, VO 0.1127, VO 0.1245, VO 0.1296 were determined from the excess heat capacities to be 1.90, 2.88, 2.82, and 2.88 J K −1 mole −1, respectively, values which were explained by calculating the entropy changes due to the order-disorder rearrangement of oxygen atoms in the superstructures of VO x alloys. From the O/V dependence of the transition temperature and entropy change, the most stable composition of the α′ phase was thought to be V 48O 5.