Phase transitions and properties of epitaxial perovskite films

We have studied the effects of strain and substrate constraint on structural phase transitions of perovskite films. In a model system, SrTiO3 (STO), the phase transition temperature Tc for the cubic-tetragonal transition is determined by monitoring the superlattice peaks associated with rotations of TiO6 octahedra. It is found that Tc is a strong function of in-plane strain, which in turn depends on film thickness, buffer layer thickness, and lattice mismatch between films and substrates. The measured strain-temperature phase diagram for epitaxial STO films is qualitatively consistent with theory, however the increase in Tc is much larger than predicted. A second effect is that substrate clamping changes the nature of the phase transition. The STO lattice is tetragonal at all temperatures, thus the tetragonality of the STO unit cells is no longer a good secondary order parameter. The internal degree of freedom, symmetry, is decoupled from the external degree of freedom, shape. In some cases, new phases result that are impossible in a free crystal. ^ Several other representative film systems are also studied. In the first-order ferroelectric transitions in BaTiO3 films and metal-insulator transitions in NdNiO3 films, it is found that the in-plane lattice parameters are clamped by the substrates, while out-of-plane lattice parameters vary to accommodate the spontaneous polarization or the volume change, respectively, at the phase transitions. Again substrate clamping effect plays an important role for epitaxial film systems. However, in SrRuO3 films, we observe lattice relaxation and domain formation at a tetragonal to orthorhombic transition. In this case, the internal strain energy dominates the substrate clamping. Taken together, the energetics of phase transitions in epitaxial films can be classified. ^ We have successfully observed charge ordering (stripes) in La2- xSrxNiO 4 films for the first time. The peaks representing phases with charge stripes are present in unstrained thick films. In highly strained multilayer samples, the stripe peaks are suppressed. Studying the detail of stripe suppression will give new insight into the nature of self-assembly of highly correlated electrons.