Surface Structural Phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$*

Surface structural phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) are investigated using Low Energy Electron Diffraction (LEED-IV). The surface structure and phases are compared to the bulk in this layered perovskite material. Bulk phases are generated from rotational and tilt distortions of the RuO$_{6}$ octahedral. While the surface emphasizes the system's instability against the rotational distortion, the tilt distortion is stabilized. Surface relaxation interferes with the RuO$_{6}$ tilt resulting in a lower metal-to-insulator transition (MIT) temperature (x = 0.1) and a lower tetragonal-to-orthorhombic phase transition temperature (0.2 $<$ x $<$ 0.5). In addition, structural transitions accompanying the bulk MIT are nonexistent on the surface for x = 0.1 and superstructure reflections are evident for x = 0.5 indicating a shift in the quantum critical point at the surface.