“…Actually, the influence of Ca doping in Y-123 has been widely studied from the point of view of its crystal structure both for polycrystalline material and for single crystals not grown with the whisker technique (Bö ttger et al, 1997;Chen et al, 2000;Stoyanova-Ivanova et al, 2006;Hijar et al, 1995). In principle, Ca should preferably substitute Y since the ionic radius of Ca 2+ (r = 1.12 Å ) is much closer to that of Y 3+ (r = 1.019 Å ) than to that of Ba 2+ (r = 1.42 Å ), and indeed X-ray structure refinements of single crystals have shown that at low Ca concentrations (< 11%) Y 3+ ions are substituted, while at higher concentrations Ca 2+ also replaces Ba 2+ (Bö ttger et al, 1997).…”