Pressure Renormalization for Premelting Phenomena

We develop the pressure renormalization of a phonon potential for atoms interacting anharmonically, starting from a fully dynamical Hamiltonian description in pseudoharmonic approximation. We derive a generalized equation for the correlation function of atom–atom displacement across a thin film, which under certain conditions reduces to the one previously discussed in the literature. Our theory yields expressions for a series of melting temperatures in terms of the microscopic parameters of a layered system using Lindemann's argument. The global instability temperature is obtained due to the distribution of displacement correlations in the surface region while the local melting transition is recognized as a process of surface premelting which expands into the stable bulk with increasing temperature. The pressure dependencies of instability and premelting temperatures are mainly discussed.

A THEORETICAL STUDY OF LATTICE INSTABILITIES IN LaCuO

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.