  We propose that a valence-bond-solid (VBS) order can be stabilized in certain
two-dimensional antiferromagnets due to spin-lattice coupling. In contrast to
the VBS state of the Affleck-Kennedy-Lieb-Tesaki (AKLT) type in which the spin
$2S$ and the lattice coordination $z$ must be commensurate, the spin-lattice
coupling-induced VBS state can occur when $2S$ is not an integer multiple of
$z$. As a concrete example, S=2 spins on the triangular network with $z=6$ is
discussed. Within the Schwinger boson mean-field theory it is shown that the
ground state is given by the $\sqrt{3}\times\sqrt{3}$ modulation of the valence
bond amplitudes for sufficiently strong spin-lattice coupling. Using the
corresponding AKLT wave function, we work out the excitation spectrum for this
state within the single-mode approximation. The calculated spectrum should
provide a new type of collective mode which is distinct from the spin wave
excitations of the magnetically ordered ground state.
