single crystals*

The compound URu₂Si₂ has attracted much interest due to its “hidden order” (HO) phase, whose order parameter has not been definitively determined after more than three decades of research on this compound. The substitution of isoelectronic Fe for Ru in URu2Si2 produces a transition from the HO phase to a large moment antiferromagnetic (LMAFM) phase, similar to what occurs under the application of pressure. This has been attributed to the “chemical pressure” associated with the substitution of the smaller Fe atom for Ru, which increases with Fe concentration. The substitution of isoelectronic Os for Ru also produces a transition from the HO to the LMAFM phase with increasing Os concentration. Since the substitution of Os, which is larger than Ru, generates a negative chemical pressure, this suggests other factors must be involved in the case of Os (e.g., spin-orbit coupling, increased hybridization due to increased spatial extent from Os d-orbitals). In this talk, we present the T vs x phase diagram, based on electrical resistivity, specific heat, magnetization, and thermal expansion measurements as a function of temperature and pressure, for Czochralski grown URu_2-xOs_xSi₂ single crystals.