The effect of antimony doping on the transport and magnetic properties of Ce(O/F)FeAs

Abstract

Oxypnictides of types CeOFeAs_{1 - x}Sb_x, CeO_0.9F_0.1FeAs_{1 - x}Sb_x and CeO_0.8F_0.2FeAs_{1 - x}Sb_x have been synthesized in order to understand the effective change in electron doping by the twin effects of F-incorporation and the variation in Fe-As bonding by substitution of a larger ion (Sb) at the As site. All the compounds crystallize in a tetragonal ZrCuSiAs-type structure (space group: P4/nmm). Both the lattice parameters (a and c) increase with the increase in antimony substitution due to the larger ionic size of antimony. CeOFeAs_{1 - x}Sb_x (x = 0.1, 0.2 and 0.3) compounds were found to be semimetallic, like the parent CeOFeAs. Substitution of Sb in CeOFeAs_{1 - x}Sb_x results in a shift of the temperature of the resistance anomaly corresponding to the structural phase transition. CeO_0.9F_0.1FeAs_{1 - x}Sbx ('x' = 0, 0.05, 0.1 and 0.15) was superconducting with a maximum transmission temperature (T_c) of 43.17 K for 'x' = 0.1, which is higher than the antimony-free CeO_0.9F_0.1FeAs (T_c = 38 K). Further increase in antimony (x > 0.15) results in the suppression of superconductivity. Similarly, Sb doping in optimally electron doped CeO_0.8F_0.2FeAs superconductor leads to the lowering of the superconducting transition temperature ('x' = 0.1) and further increase in antimony substitution ('x' > 0.1) leads to a loss of superconductivity. CeO_0.8F_0.2FeAs_0.9Sb_0.1 shows a very high value of the upper critical field H_c2(0) (137 T) as compared to Sb-free CeO_0.8F_0.2FeAs. In this paper, we discuss the first example among electron-doped oxypnictide superconductors where the T_c increases as a result of a decrease in chemical pressure (an increase in lattice parameters).