Different response of the crystal structure to isoelectronic doping inBaFe2(As1−xPx)2and(Ba1−xSrx)Fe2As2

Abstract

Superconductivity up to 30 K in charge neutrally doped ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ has been ascribed to chemical pressure caused by the shrinking unit cell. But the latter induces no superconductivity in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$ in spite of the same volume range. We show that the spin-density-wave (SDW) state of ${\text{BaFe}}_{2}{\text{As}}_{2}$ becomes suppressed in ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates ${z}_{\text{As}}$ and ${z}_{\text{P}}$. High-resolution x-ray diffraction experiments with ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$. 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 ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ 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 $x\ensuremath{\approx}0.3$, which corrects the unreasonable linear decrease in the so-called pnictide height.