PHASE-TRANSITIONS AT A SRTIO3 SURFACE WITH REGULAR STEPS

We studied surface structure and lattice parameters of surfaces with r egular steps (which are present because the planar surface is tilted b y 7 degrees to crystallographic plane (100) of SrTiO3) using the RHEED technique at temperatures T from 5 to 300 K. The lattice parameters a (parallel to) and a(perpendicular to), measured in plane parallel and normal to the edges of the regular steps, from 300 K down to 150 K alm ost coincide, then bifurcate, and a(perpendicular to) bends downward; the relative deviation saturates at roughly 6%. Thus, our measurements show at least one strong transition at T about 105 K and weaker ones at lower temperatures. We think that these transitions are surface-pro moted transformations closely related to the weak transitions in bulk strontium titanate, which are ascribed to changes in the electrical su bsystem. The emerging electrical polarization is directed mainly norma l to the step edges.