Avoided quantum criticality and magnetoelastic coupling in BaFe _2-xNi_xAs₂

We study the structural and magnetic orders in electron-doped BaFe _2-xNi_xAs₂ by high-resolution synchrotron x-ray and neutron scatterings. Upon Ni doping x, the nearly simultaneous tetragonal-to-orthorhombic structural (T_s) and antiferromagnetic (T_N) phase transitions in BaFe₂As₂ are gradually suppressed and separated, resulting in T_s>T_N with increasing x, as was previously observed. However, the temperature separation between T_s and T_N decreases with increasing x for x≥0.065, tending toward a quantum bicritical point near optimal superconductivity at x≈0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.0882;0.104, resulting in a finite value of T_N≈T_c+10 K above the superconducting dome around x≈0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.