The strong, electronically induced anomaly in the spectrum of the
longitudinal optical (LO) phonons propagating along the main axes of the
CuO2 plane is tentatively attributed to the oxygen-oxygen charge transfer
between the two oxygens in the plane. It is argued that this charge transfer
can be large and that it is strongly coupled to the zone boundary LO phonons.
The corresponding negative contribution to the free energy is quartic in the LO
phonon amplitude, making the LO phonon unstable through the first order phase
transition, with a concomitant domain structure.Comment: 3 pages, 2 figure