Tetragonal-Orthorhombic Structural Modulation at Low Temperature inLa2−xBaxCuO4

Below ∼60 K ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x\mathrm{Cu}{\mathrm{O}}_4 (0.10\mathrm{}\le x\le 0.15)$ reveals superstructure reflections consistent with the tetragonal space group $\frac{P{4}_2}{\mathrm{ncm}}$ [low temperature tetragonal (LTT)]. However, twins, characteristic of the related orthorhombic phase [low temperature orthorhombic (LTO)], persist in this material. Electron microscopy shows that the LTT superstructure reflections originate from the twin-boundary region. This suggests a phase mixture of orthorhombic and tetragonal with a gradual change from LTO (space group $\mathrm{Bmab}$) in the interior of the twin domains, via the $\mathrm{Pccn}$, to LTT at the twin boundary. This is modeled as a spatial modulation, the tipping angle of Cu${\mathrm{O}}_6$ octahedra being an order parameter.