Electronic structure and thermodynamic stability of double-layeredSrTiO3(001)surfaces:Ab initiosimulations

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) $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ surfaces: (i) SrO-terminated, (ii) $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated, and (iii) $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction of $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ recently suggested by Erdman et al. [Nature (London) 419, 55 (2002)]. A thermodynamic stability diagram obtained from first-principles calculations shows that regular $\mathrm{Ti}{\mathrm{O}}_{2}$- and SrO-terminated surfaces are the most stable. The stability regions of $(2\ifmmode\times\else\texttimes\fi{}1)$ DL $\mathrm{Ti}{\mathrm{O}}_{2}$- and DL SrO-terminated surfaces lie beyond the precipitation lines of SrO and $\mathrm{Ti}{\mathrm{O}}_{2}$ compounds and thus are less stable with respect to regular $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ surfaces. Analysis of the stability diagram suggests that Sr precipitation on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ 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.