  We report the results of Cu and Cl nuclear magnetic resonance experiments
(NMR) and thermal expansion measurements in magnetic fields in the coupled
dimer spin system TlCuCl3. We found that the field-induced antiferromagnetic
transition as confirmed by the splitting of NMR lines is slightly
discontinuous. The abrupt change of the electric field gradient at the Cl
sites, as well as the sizable change of the lattice constants, across the phase
boundary indicate that the magnetic order is accompanied by simultaneous
lattice deformation.
