Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO3 crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO3 at 544 degrees C, where LaAlO3 changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral R(3) over barc$ low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}(pseudocubic) planes of twin domains were measured by SFM on the surface of a macroscopic (100)(cubic) cut crystal plate. The misorientation angle omega(100) between {100} twins is 0.195(8)degrees, while {110} twinning gives an angle of omega(100)=0.276(7)degrees. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the R(3) over barc$ phase as alpha(1)=90.0973(40)degrees and alpha(2)=90.0975(30)degrees, respectively. The XRD result for this rhombohedral angle is alpha= 90.096(1)degrees. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T-c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps. (C) 1998 Elsevier Science B.V.