Twin structures in tetragonal $SrTiO3:$ The ferroelastic phase transition and the formation of needle domains

Direct optical observation of the twin domains in ferroelastic strontium titanite at $8 K<T<105 K$ show that twin boundaries in the bulk do not change at temperatures below 103.4 K. At temperatures between 103.4 K and $Tc=105.65 K$ the contrast between the birefringences of two adjacent domains is too weak to allow the visual observation of the domain wall. The temperature independent, coarse domain structure in the bulk of the crystal is in strong contrast with a fine domain structure and strain relaxations near the crystal surface. No indication for wall flip motion was found for repeated, slow cooling, and heating through the transition point.