  We show that the introduction of frustration into the spin-1/2
two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via
a next-nearest neighbor exchange interaction can lead to a Peierls-like
transition, from a tetragonal to an orthorhombic phase, when the spins are
coupled to adiabatic phonons. The two different orthorhombic ground states
define an Ising order parameter, which is expected to lead to a finite
temperature transition. Implications for ${\rm Li_2VOSiO_4}$, the first
realization of that model, will be discussed.
