Probing Phosphorus Solubility and Its Effect on Critical Temperature (Tc) in the Helical Superconducting Magnet RbEuFe4As4–xPx

RbEuFe4As4 exhibits a rare combination of high-temperature superconductivity and noncollinear magnetism, originating from the FeAs and Eu layers, respectively. In order to fine-tune its superconducting and magnetic properties, we studied P-doped RbEuFe4As4–xPx. To determine the value of x for which RbEuFe4As4–xPx exists, members of the series have been synthesized in the form of relatively large-sized single crystals by a high-temperature flux growth technique using RbAs flux as well as polycrystalline powder by direct combination reactions between RbFe2As2 and EuFe2As2–xPx. The flux crystal growth reactions indicate that RbEuFe4As4–xPx forms with the values of x ≤ 0.12, which is corroborated by the Rietveld refinement analysis on the products obtained from direct combination reactions. The solubility limit exists primarily due to the preferential site As(2) occupation by P in the RbEuFe4As4 crystal structure. As x increases in RbEuFe4As4–xPx, magnetization measurements reveal that the superconducting critical transition temperature (Tc) decreases, while the magnetic ordering temperature (TN) remains almost unchanged in comparison to undoped RbEuFe4As4.