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 BaCuSi2O6 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 similar to 100 K. This lattice modulation is characterized by a resolution-limited reduced wavevector q(IC)=(0,0.129 +/- 0.001,0) (reciprocal lattice units) referred to the orthorhombic lattice, and its second and third 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.