Cation termination at ion‐polished and chemically etched (001)YBa2Cu3O7crystal surfaces: An ion channeling study

Local cation order approaching ideal bulk termination was found for cleaned (001) surfaces of YBa2Cu3O7 crystals by ion‐channeling surface‐peak analysis. Surfaces etched in dilute CH3OH:Br2 and ultrasonically agitated in CH3OH to dislodge the Br2 appear terminated preferentially by the Cu‐O ‘‘double‐plane’’ sequence, containing less than 1/2‐cation monolayer of surface disorder. Planar polishing with ion beams incident at a glancing angle on a rotating crystal leaves residual disorder as low as ∼8 A, damage which is readily removed by a light chemical etch.