A THEORETICAL STUDY OF LATTICE INSTABILITIES IN LaCuO

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Abstract
Starting from experimental findings of structural phase transitions in LaCuO a microscopical model is introduced to describe the softening of the tilting mode associated with CuO 6-octahedra rotations. The interaction of the soft mode with the acoustic phonons is taken into account. Using the standard method of double-time retarded Green functions the equations of motion are solved within a pseudoharmonic approximation. With the help of experimental data we calculate the potential parameters of oxygen by moving in the tilting mode, the sound velocity step at the transition point I4/ mmm → Cmca and the value of the soft-mode splitting in the orthorhombic phase. The corresponding free energy of our model is investigated in order to describe the transition Cmca → P4 2/ ncm . The Sr,Ca,Ba-doping and the exchange of Cu by Co, Ni are discussed within the presented model approach.

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