Stabilization of the tetragonal structure in (Ba_1-xSr_x)CuSi₂O₆

We present a structural analysis of the substituted system (Ba1-xSrx)CuSi2O6, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with low-temperature neutron and synchrotron powder diffraction, room-temperature, and cryogenic high-resolution NMR, as well as magnetic- and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x=0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x=0.1 and magnetic measurements prove that the Cu dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einstein-condensation (BEC) of triplons, within the tetragonal crystal structure of I41/acd. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.