Electronic structure and thermodynamic stability of double-layered SrTiO₃(001) surfaces: Ab initio simulations

Using the B3PW hybrid exchange-correlation functional within density-functional theory and employing Gaussian-type basis sets, we calculated the atomic and electronic structures and thermodynamic stability of three double-layered (DL) SrTiO₃(001) surfaces: (i) SrO-terminated, (ii) TiO₂-terminated, and (iii) (2×1) reconstruction of TiO₂-terminated SrTiO₃(001) recently suggested by Erdman et al. Nature (London) 419 55 (2002)]. A thermodynamic stability diagram obtained from first-principles calculations shows that regular TiO₂- and SrO-terminated surfaces are the most stable. The stability regions of (2×1) DL TiO₂- and DL SrO-terminated surfaces lie beyond the precipitation lines of SrO and TiO₂ compounds and thus are less stable with respect to regular SrTiO₃(001) surfaces. Analysis of the stability diagram suggests that Sr precipitation on SrTiO₃ surface never occurs. Our simulations show a substantial increase of Ti-O covalency on the DL surfaces as compared to the regular surfaces, which are themselves more covalent than the crystalline bulk. The implications of our calculated results for recent experimental observations are discussed.