Electronic structure and thermodynamic stability of double-layered SrTi O3 (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) SrTi O3 (001) surfaces: (i) SrO-terminated, (ii) Ti O2 -terminated, and (iii) (2×1) reconstruction of Ti O2 -terminated SrTi O3 (001) recently suggested by Erdman [Nature (London) 419, 55 (2002)]. A thermodynamic stability diagram obtained from first-principles calculations shows that regular Ti O2 - and SrO-terminated surfaces are the most stable. The stability regions of (2×1) DL Ti O2 - and DL SrO-terminated surfaces lie beyond the precipitation lines of SrO and Ti O2 compounds and thus are less stable with respect to regular SrTi O3 (001) surfaces. Analysis of the stability diagram suggests that Sr precipitation on SrTi O3 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.