  Controlling the surface structure on the atomic scale is a major difficulty
for most transition metal oxides; this is especially true for the ternary
perovskites. The influence of surface stoichiometry on the atomic structure of
the SrTiO$_3$(001) surface was examined with scanning tunneling microscopy,
low-energy electron diffraction, low-energy He$^+$ ion scattering (LEIS), and
X-ray photoelectron spectroscopy (XPS). Vapor deposition of 0.8 monolayer (ML)
strontium and 0.3 ML titanium, with subsequent annealing to 850 {\deg}C in 4
$\times$ 10$^{-6}$ mbar O$_2$, reversibly switches the surface between c(4
$\times$ 2) and (2 $\times$ 2) reconstructions, respectively. The combination
of LEIS and XPS shows a different stoichiometry that is confined to the top
layer. Geometric models for these reconstructions need to take into account
these different surface compositions.
