We have studied the thermal expansion and magnetostriction of (Formula presented) ((Formula presented) 0.01, 0.015, and 0.02) single crystals under magnetic fields up to 15 T. The lattice distortion in the antiferromagnetic (AF) state, which is opposite to that in the dimerized (Formula presented) phase, exhibits unusual magnetic-field dependence for (Formula presented) where the AF state coexists with the (Formula presented) state. The lattice distortion due to AF ordering is rapidly suppressed and in its place, the lattice distortion due to the dimerization is enhanced by magnetic field up to (Formula presented) Anomalies in the thermal expansion and specific heat at (Formula presented) are very small at (Formula presented) in contrast with their clear anomalies in lower and higher magnetic fields. These indicate that the nature of the AF state is largely changed by the magnetic field and that the electronic state below and above (Formula presented) is not so different at (Formula presented) Such unusual behavior in a magnetic field is not observed in the AF state for (Formula presented) where there exists only AF ordering. © 1997 The American Physical Society.