= Fe, Os) systems*

We investigated the effect of applied pressure and chemical substitution on the hidden order (HO) and antiferromagnetic (AFM) phases in URu₂Si₂ and report electrical resistivity measurements of single crystals of URu_2−xM_xSi₂ (M = Fe, Os) under pressure. A transition from the HO phase to the AFM phase in URu₂Si₂ occurs at a critical pressure of 1.5 GPa. The AFM phase is also induced upon substitution of Fe or Os ions for Ru at ambient pressure. We observed that as x is increased in URu_2−xFe_xSi₂, less external pressure is required to induce the AFM phase. This is explained by the reduction in unit cell volume owing to the substitution of smaller Fe ions for Ru, which may be interpreted as a chemical pressure. We also observed that as y is increased in URu_2−yOs_ySi₂, less external pressure is required to induce the AFM phase. This is contrary to the expectation that larger pressures would be required to induce the AFM phase in a system biased with a negative chemical pressure owing to the substitution of larger Os ions for Ru. This suggests that effects other than volume change may be responsible for the HO to AFM transition.