  $BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model
material for studying Bose-Einstein condensation (BEC) of magnons in high
magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be
grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth
method with enhanced oxygen partial pressure, have the same tetragonal
structure ($I4_1/acd$) at room temperature. The mixed system shows no
structural phase transition, so that the tetragonal structure is stable down to
low temperatures. The oxygen partial pressure acts as control parameter for the
growth process. A detailed understanding of the crystal structure depending on
the oxygen content will enable the study of the spin dynamics of field-induced
order states in this model magnetic compound of high current interest with only
one type of dimer layers, which shows the same distance between the Cu atoms,
in the structure.
