The field of high temperature superconductors (HTSC) isstill controversial. The difficulty to explain the high Tcvalues as well as other anomalous superconductingand normal state properties of the HTSCs necessitatesfurther studies and search for empirical relations in the largeamount of collected data.The major part of this thesis is devoted to the study ofcharge neutral doping with equal amounts of Ca and Pr/Th in theYBa2Cu3O7-δfamily of high temperature superconductors.The strong effect on Tcin spite of preserved charge concentration hasbeen investigated by neutron diffraction and calculations ofbond valence sums, µSR experiments and measurements of Tcand the upper critical field from electricalresistivity. In the first part of the thesis, some results fromthe literature have been examined with an attempt to summarizetrends and empirical relations. In addition some exploratorywork searching for new superconductors in the hydrides isreported.
The valence of Pr in the Y-123 family is anothercontroversial issue. Results from literature are presentedsuggesting a valence change from Pr4+at low Pr content to mostly Pr3+at high Pr content. At low doping levels Cacompensates for the electron doping by Pr and isolates a stronglinear depression of Tc. The linear depression rate increases linearlywith increasing ion radius of the rare earth host whereas theparabolic Tcdependence on hole concentration is not affected.For Ca-Th doping the linear depression rates are even higherand suggested to be due to electronic disorder. For both chargeneutral dopings the structural changes with doping are smallwith an increasing planar copper to apical oxygen (Cu2-O4)distance as the most distinct one.
High temperature superconductors, electrical conductivity,structural studies, neutron powder diffraction, X-ray powderdiffraction, muon spin rotation, YBa2Cu3O7-δ, substitution studies, hole concentration,charge neutral doping, disorder.
Fysiska institutionen , 1998. , 127 p.