Reversible tuning of the collapsed tetragonal phase transition in CaF e2 A s2 by separate control of chemical pressure and electron doping

Single crystals of Ca(Fe1-xRux)2As2(0≤x≤0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2(0≤y≤0.2) have been synthesized and studied with respect to their structural, electronic, and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x≤0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At xcr=0.023 a first-order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near xcr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027, we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y ≥ 0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.