ANALYSIS OF THE INSTABILITY AND PHASE TRANSITION OF THE COMPOUND La₂CuO₄

The phase transition for La₂CuO₄ changing from I4/mmm to Cmca with cell-doubling has been analysed within the frame work of Landau's theory. It is a second order transition via softening of the optical breathing mode, coupling with a slight lattice distortion arising from the softening elastic mode based on electron-phonon interaction.