Different response of the crystal structure to isoelectronic doping in BaFe2(As1-xPx)2 and (Ba1-xSrx)Fe2As2

Superconductivity up to 30 K in charge neutrally doped BaFe2(As1-xPx)(2) has been ascribed to chemical pressure caused by the shrinking unit cell. But the latter induces no superconductivity in (Ba1-xSrx)Fe2As2 in spite of the same volume range. We show that the spin-density-wave (SDW) state of BaFe2As2 becomes suppressed in BaFe2(As1-xPx)(2) by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates z(As) and z(P). High-resolution x-ray diffraction experiments with BaFe2(As1-xPx)(2) single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in (Ba1-xSrx)Fe2As2. Since the Fe-As bond length is a gauge for the magnetic moment, our results show why the SDW is suppressed by P doping, but not by Sr doping. Only the Fe-P interaction increases the width of the iron 3d bands, which destabilizes the magnetic SDW ground state. The simultaneous contraction of the Fe-As bonds is rather a consequence of the vanishing magnetism. Ordered structure models of BaFe2(As1-xPx)(2) obtained by density-functional theory calculations agree perfectly with the single-crystal x-ray structure determinations. The contraction of the Fe-As bonds saturates at doping levels above x approximate to 0.3, which corrects the unreasonable linear decrease in the so-called pnictide height.