Stabilization of the tetragonal structure in (Ba$_{1-x}$Sr$_{x}$)CuSi$_{2}$O$_{6}$

We present a structural analysis of the substituted system (Ba$_{1-x}$Sr$_{x}$)CuSi$_{2}$O$_{6}$, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, room-temperature and cryogenic highresolution 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-Einsteincondensation (BEC) of triplons, within the tetragonal crystal structure of $I\,4_{1}/acd$. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.