Synthesis and Characterization of Superconducting Ca₁_-NaFFeAs

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca_0.86Na_0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 - 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca_0.89Na_0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.