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. 
E. C. Samulon, Z. Islam, S. E. Sebastian, P. B. Brooks, M. K. McCourt   Jr., J. Ilavsky, I. R. Fisher