We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin-1/2 linear-chain compound ${\mathrm{CuGeO}}_3$. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a remarkable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition temperature of 14 K.

• Received 28 January 1994

DOI:https://doi.org/10.1103/PhysRevB.50.1278