Effect of 'in-plane' contraction on the (001) surface of the model perovskite SrTiO3

Reflection high-energy electron diffraction in the temperature range 8-300 K revealed several surface structural transformations accompanied with a strong 'in-plane' lattice parameter growth on the (001) surface of the model perovskite SrTiO3 single crystal. Rather may occurring at sharp temperatures, these transformations are observed to extend over temperature intervals separated from each other by temperature zones where no structural changes occur and the crystal lattice is relaxed. In such relaxed zones the equilibrium lattice parameters were identified and analyzed as a function of penetration depth. A strong 'in-plane' contraction of the crystal lattice in the first surface layers was experimentally revealed at low temperatures and theoretically confirmed by hybrid DFT calculations. Along with such an 'in-plane' contraction of the (001) surface at low temperatures we have found that the thermal expansion on the surface is much higher than in the bulk which results in approaching of the surface 'in-plane' and the bulk lattice parameters at room temperature. This contradicts the general assumption that surface 'in-plane' and bulk lattice parameters are identical.