A model of lanthanum orthoniobate which possesses a ferroelastic tetra
gonal-monoclinic phase transition is proposed. It contains only one pa
rticle per unit cell, but it is constructed consistently with symmetry
changes at the phase transition. The model parameters are chosen to r
eproduce the bare soft mode, degree of deformation of the tetragonal u
nit cell to a monoclinic one, and the phase transition temperature. Th
e ferroelastic system with free boundary conditions was simulated by t
he molecular dynamics technique, and the second order phase transition
was reproduced. The studied annealing process shows formation of the
stripe lenticular domain pattern, which has been interrupted by the ap
pearance of a temporary band of perpendicularly oriented lenticular do
mains. The maps contain W'-type domain walls whose orientations are fi
xed only by interplay of potential parameters and not by symmetry elem
ents. The simulated domain pattern has the same features as those obse
rved by transmission electron microscopy.