Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi${}_2$O${}_6$ in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below $\sim$100 K. This lattice modulation is characterized by a resolution limited wave vector {\bf q}${}_{IC}$=(0,$\sim$0.13,0) and its 2${}^{nd}$ and 3${}^{rd}$ harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.