Structural properties of BaFe1.8Ni0.2As2 under pressure

► The high-pressure behaviors of BaFe1.8Ni0.2As2 were investigated by X-ray diffraction. ► The lattice parameter a exhibits an anomalous compression with increasing pressure. ► The lattice parameter c drastically decreases with increasing pressure. ► The Fe-magnetic state plays a crucial role in the lattice collapse of the system.